Taming harmful bursts and heat flux in high-confinement tokamak plasmas

Domain: physics
Published: 2025-12-05
Editorial outcome: major_revision

Abstract
A major challenge in tokamak fusion research is first-wall erosion caused by steady heat loads and sudden energy bursts known as edge-localized modes. Divertor detachment reduces steady-state heat flux, while resonant magnetic perturbations can suppress these instabilities. However, integrating the two has been difficult because they require conflicting operating conditions. Here we demonstrate simultaneous achievement of resonant magnetic perturbations mitigated small edge-localized modes and impurity seeded partial divertor detachment in plasmas with an ITER-similar shape on the DIII-D tokamak. Experiments and simulations show that resonant magnetic perturbations facilitate detachment by redistributing particles, lowering the core density and increasing the scrape-off layer density, thereby reducing the amount of injected gas required. Cooling-gas injection eliminates the secondary heat-flux peak created by three-dimensional magnetic lobes, while edge cooling weakens the plasma response to the applied magnetic fields. These advances illustrate a viable pathway for integrating edge stability control with power exhaust in future fusion reactors.

Introduction
The pursuit of thermonuclear fusion as an energy source, demonstrated through magnetically confined plasmas, stands as the primary goal of ITER1. This endeavor encompasses generating a high-temperature, high-pressure ‘burning’ plasma and showcasing its sustained operation to validate power extraction capabilities2–4. However, these plasmas pose challenges: the nuclear fusion products bring heat to the plasma edge, imposing continuous heat load on the plasma-facing components (PFC) and reducing their lifespan5. Simultaneously, high thermal confinement (H-mode) operation is susceptible to edge-localized modes (ELMs), releasing transient damaging bursts of heat and particles into the reactor chamber6. Protecting the PFC necessitates managing both steady-state and transient heat loads.

Divertor detachment, achieved by creating a radiative plasma boundary to shield the divertor target from direct plasma heat flux, offers promise for controlling steady-state heat loads4,7–11. Divertor detachment generally occurs at high density, typically above fG = 0.512. In this paper, fG denotes the Greenwald fraction, defined as the ratio of the pedestal-top electron density (ne,ped) to the Greenwald density limit (nG)13. This condition is achieved by injecting extra gas or impurity into the plasma to reduce the plasma temperature in the divertor. Consequently, atomic processes are intensified enough to detach plasma from the divertor target plate. For the control of ELMs, two main strategies are pursued. The first is to exploit natural no-ELM or small-ELM regimes, including the improved energy confinement regime (I-mode)14–16, the quiescent H-mode (QH-mode)17–20, the enhanced Dα (EDA) H-mode regime21,22, small-ELM regimes23,24, including grassy ELMs25, the quasi-continuous exhaust (QCE) regime26–28, and the X-point radiator (XPR)29. While promising, QH-mode and I-mode have not yet demonstrated compatibility with detachment. Others, such as EDA H-mode, grassy ELMs/QCE27, and the XPR29, have achieved detachment, but often require challenging conditions—such as high collisionality, strong shaping, or a high edge safety factor (q95)—which can compromise confinement quality. The second strategy employs external actuators to actively control ELMs. Among these, the externally applied non-axisymmetric resonant magnetic perturbation (RMP)30 is demonstrated to be a robust, reliable, and promising method to fully suppress/mitigate large type-I ELMs30–35, showing flexibility across a wide operational space36. It is thought to trigger magnetic islands at the edge of the plasma to limit the growth of the edge transport barrier37–39. Accordingly, RMP has been planned for one of the primary ELM control schemes in ITER40.

Despite its efficacy, RMP-driven ELM suppression typically increases steady-state heat flux41 and operates effectively only at low pedestal density with fG < 0.342,43 in current devices. ITER is expected to operate with a simultaneous dissipative, partially detached regime in combination with RMP ELM control to protect its PFC44, which involves a combination of high gas throughout and impurity seeding. To date, the integration of RMP ELM control with divertor detachment has not been demonstrated in recent devices, facing three primary challenges: (1) RMP is found to prevent achieving divertor detachment in tokamaks45–47, contrary to observations in stellarators where RMP facilitates detachment48–50. These conflicting outcomes challenge the compatibility of RMP with divertor detachment, questioning the underlying physics. (2) RMP introduces 3D lobes and heat flux striations in the scrape-off layer (SOL)51–53. These generate an additional, challenging second peak heat flux52 that is resistant to detachment, as depicted in 3D simulations for ITER54. (3) A substantial discrepancy in operational density hinders the concurrent attainment of RMP-driven ELM suppression and detachment55–58. Efforts to increase density for detachment during RMP-driven ELM suppression have consistently led to ELM reappearance55,57,58. Consequently, there is a pressing need to explore the integration of RMP ELM control with divertor detachment. This exploration holds crucial significance in advancing our comprehension of the underlying physics and its implications for ITER.

We report advances in resolving this core-edge integration challenge: the demonstration of a high-confinement (H98 ~ 1.1), RMP-driven small ELM regime robustly sustained under partially detached divertor conditions. This is achieved in DIII-D by optimizing RMP alongside D₂ puffing and impurity seeding. Our work proves that an active, tunable actuator like RMP—the primary ELM control scheme for ITER—can be used to simultaneously tame edge instabilities and facilitate a dissipative divertor, a combination essential for future reactors. RMP is found to facilitate detachment by enhancing transport, which lowers the pedestal density while raising the SOL density, thereby reducing the fueling threshold for detachment. Furthermore, we show that gas puffing eliminates resilient, lobe-induced secondary heat flux peaks before detachment onset. Once partial detachment is established, the outward pedestal shift with higher collisionality reduces edge bootstrap current, thereby weakening the plasma response to the applied RMP. These findings are robustly supported by both SOLPS-ITER and EMC3-EIRENE simulations, confirming the underlying mechanisms. Taken together, these findings demonstrate a controllable path toward integrating ELM control, high confinement, and divertor detachment—an essential advance for ITER and future fusion reactors.

Results
Integration of RMP ELM control with partial divertor detachment
The DIII-D experiments are conducted in the upper single null (USN) divertor configuration with ITER-similar shape (ISS) plasmas, as shown in Fig. 1 for the poloidal cross-section. Key plasma parameters include a plasma current Ip = 1.27 MA, toroidal field Bt = 1.7 T, major and minor radii R/a = 1.7 m/0.6 m, edge safety factor at the 95% of poloidal magnetic flux surface is q95 = 3.5, neutral beam injected (NBI) power and torque are approximately 6 MW and 4 Nm, respectively. The normalized pressure βN ranges from 2.5 to 3.0, with conferment enhancement factors H98 between 1.1 and 1.3. Divertor detachment is induced by deuterium (D2) puffing and/or nitrogen (N2) impurity seeding through main-chamber gas valves, as indicated in Fig. 1. Notably, recent DIII-D studies have demonstrated that detachment onset occurs at lower density in the USN configuration with a more closed divertor compared to the lower single null (LSN) configuration with an open divertor12. This is attributed to enhanced divertor neutral compression, reduced neutral leakage, and increased carbon concentration in the divertor region12,59–61. The in-vessel 3D coils are configured to generate RMP with toroidal mode number n = 3, as illustrated in Fig. 1 for the coils and magnetic perturbations. The upper and lower row coil currents are independently controlled, enabling even or odd parity configuration. “Even” parity refers to in-phase current between the upper and lower coil rows, while “odd” parity denotes reversed-phase currents. Even parity is generally more resonant with plasma at low q95 and low collisionality62 and is the default configuration used in this study unless stated otherwise.

Figure 2 presents DIII-D discharge 193119, which demonstrates the simultaneous achievement of RMP-mitigated small ELMs with divertor detachment. Initially, D2 and N2 puffing (Fig. 2b) without RMP results in increased core radiation power between 2.5 and 3 s (Fig. 2c). During this phase, the N2 puffing rate is kept constant—based on detachment feedback control from prior discharges—while D2 puffing gradually increases. Transient divertor detachment occurs between 2.8 and 3 s (Fig. 2h) as the pedestal density exceeds ne,ped > 5.7 × 1019 m−3 (Fig. 2g), but large ELMs appear simultaneously (Fig. 2e), with energy losses up to ΔW/WMHD ~ 7% (Fig. 2f), leading to divertor reattachment. At 3.0 s, activating even parity n = 3 RMP at 6 kA leads to a drop in both the pedestal and separatrix densities (40% reduction in ne,ped, named density pump-out), mitigation of ELM frequency to fELM ~ 50-80 Hz with smaller size (ΔW/WMHD ~ 2–3%), and a 15% reduction in normalized beta βN (Fig. 2d). However, this early phase also corresponds to divertor reattachment (Fig. 2h). Partial detachment is re-established at 3.75 s as the density increases, even though pedestal and separatrix densities remain lower than in the initial detachment phase (2.8–3 s), suggesting that RMP facilitates detachment at reduced core fueling. During this period (3.75–4.2 s), type-I ELMs persist with decreasing frequency (as low as 50 Hz) and slightly increasing size (ΔW/WMHD up to 3%), consistent with previous observations in DIII-D55 and KSTAR58. At 4.2 s, compared to even parity, switching the RMP to odd parity (5.6 kA) leads to a marked improvement except similar level of density pump-out: detachment is sustained, ELMs are strongly mitigated with frequency fELM ~ 200-600 Hz and energy losses reduced to ΔW/WMHD ~ 0.3-0.5%, and the inter-ELM peak heat flux q⊥ measured by IRTV drops to 0.5 MW/m2 (Fig. 2f). In addition to moderate pedestal collisionality ν*e ≈ 1.2–1.5 (Fig. 2d), pedestal stability analysis confirms that unstable intermediate-n (n = 15–25) Peeling-ballooning modes (PBMs) are responsible for these small ELMs (Supplementary Fig. 1), differing from type-II or type-III ELMs63,64. Here, ν*e = 0.012 ne Zeff qR/ε3/2Te2 is the electron collisionality using the Sauter definition65. Importantly, compared to even-parity RMP, odd-parity results in recovery of stored energy and βN by ~10% (Fig. 2d), despite increased core radiation power. Additionally, the outer divertor electron temperature, measured by Langmuir probes, drops below 5 eV (Fig. 2h), and the second peak heat flux commonly associated with RMP-produced lobes is not observed throughout the discharge.

The influence of RMP current and injected gas species on ELM mitigation during partial divertor detachment is systematically examined in Fig. 3. The trends in ELM frequency (fELM, Fig. 3a), ELM-induced energy loss (ΔW/WMHD, Fig. 3b), and inter-ELM peak heat flux (q⊥, Fig. 3c) on the outer divertor target are presented as functions of the applied n = 3 RMP current. In the absence of RMP and detachment, the plasma exhibits natural type-I ELMs at ~35 Hz with energy losses of ~4.5% and a peak divertor heat flux of q⊥ = 1.8 MW/m2. Pure D2 puffing without RMP (IRMP = 0) induces partial ELM mitigation, increasing fELM to 80 Hz while reducing ΔW/WMHD to 2.4% and q⊥ to ~1 MW/m2. Interestingly, D2 puffing alone leads to mixed ELMs with increased ELM frequency before detachment and transiting to lower frequency pure type-I ELMs during detachment. Even parity n = 3 RMP (2–6 kA) further mitigates the ELMs during detachment, raising fELM up to 200 Hz and decreasing ΔW/WMHD to 1%, but no change in q⊥ (1.1 MA/m2). In contrast, N2 seeding—while effective at achieving detachment at lower density and higher temperature—results in weaker ELM mitigation under even parity n = 3 RMP, yielding lower fELM (~50 Hz), higher ELM energy loss (ΔW/WMHD = 3%), and higher q⊥ = 1.3 MW/m2, as seen from the blue star in Fig. 3a–c. The data point for D2 plus N2 seeding under even parity represents discharges with both stationary and nonstationary ELMs (193119 in Fig. 2). Odd parity n = 3 RMP with combined D2 and N2 fueling show to be the most effective approach to achieve detachment with small ELMs, marking more than eight- and four-times reduction compared to natural ELMs, respectively. The ELM energy loss ΔW/WMHD is about 0.3–0.5% as shown in Fig. 3b (also Fig. 2f), while the uncertainty for energy loss calculated from fast equilibrium is large due to the low amplitude. This value is close to the expected requirement for ITER small ELM44, i.e., 0.15% for 15 MA, or 0.3% for 7.5 MA, which are to avoid monoblock (MB) top surface and toroidal gap edge melting, though a detailed evaluation and assessment for the ELM energy fluence is required and dedicated for future exploration study.

The concurrent realization of divertor detachment and RMP-mitigated small-ELM regime is underpinned by several synergistic mechanisms, illustrated in Fig. 2, or the following sections. Crucially, RMPs reduce the required gas fueling rate and density inside the separatrix for detachment onset, namely detachment facilitation, while also lowering the heat flux to the divertor. The resulting small ELMs are not merely a mitigation of large ELMs or RMP-driven small ELMs at high density attached condition30,33, but represent a stable, high-frequency small ELMs regime achieved under the partially detached conditions that are essential for a reactor. In turn, gas puffing suppresses the secondary peak heat flux typically induced by RMPs. This specific combination of a high-frequency small-ELM regime with high confinement (H98 > 1) at a relatively low q95, all within a detached divertor, is a crucial finding. It demonstrates a concrete pathway to integrating core performance with divertor survivability, directly addressing a key challenge for ITER. These interactions are explored in detail in the following sections.

Divertor detachment facilitation by RMP
To systematically document the impact of RMP on the dynamics of the detachment process, dedicated experiments are conducted by employing pure D2 puffing with a gradually increasing rate. This methodology allows us to probe the edge plasma response across a continuous range of densities, which is essential for identifying the underlying physics mechanism that would be impossible to capture with an abrupt gas puffing. Figure 4a–f shows a representative comparison demonstrating how n = 3 RMP facilitates divertor detachment in DIII-D. The discharges analyzed are #193106 (without RMP) and #193107 (with 4 kA even parity n = 3 RMP). This gradually increasing gas puffing prevents abrupt changes in density and ELM dynamics, and the gas puffing waveform is identical for both cases. Even parity n = 3 RMP is turned on earlier when the density is lower, based on the comparable density pump-out with odd parity as introduced in Fig. 2.

In discharge #193106, D2 puffing begins at t = 2 s (Fig. 4a), steadily raising the upper divertor neutral pressure (Fig. 4a) and plasma density (Fig. 4e) and increasing the ELM frequency up to ~130 Hz (Fig. 4b). The core radiation gradually increases from 1 to 1.6 MW, and the power across the separatrix (Psep, Fig. 4c) slightly decreases. Partial divertor detachment is observed at t = 4.4 s, indicated by a sharp drop (~30 ms) in the divertor electron temperature Te,div to below 5 eV (Fig. 4d, blue dashed line) and a jump in the divertor neutral pressure Pdiv. In the experiments, we have found that the divertor detachment happens at a very short timescale (~30 ms) as indicated in the Te,div evolution, which is much shorter than the energy confinement time (~150 ms). At this onset, the D2 puffing rate is ΦD2 = 6.71 × 1021 s−1, with a normalized pedestal density of fG = 0.48 (ratio of pedestal density to Greenwald limit density). Following detachment, the ELM frequency decreases to 80 Hz, and the core radiation power increases to ~2.5 MW.

Discharge #193107 is identical in setup to #193106 except for the application of n = 3 RMP with 4 kA current beginning at t = 2 s (Fig. 4a). The RMP initially induces a density pump-out and ELM mitigation, as reflected in the pedestal density (ne,ped) and Dα emission (Fig. 4e and b). The applied RMP also leads to higher core radiation power compared to #193106. Partial detachment occurs earlier at t = 4.2 s, with a reduced puffing rate of ΦD2 = 6 × 1021 s−1 and fG = 0.4—approximately 11% and 17% lower than in #193106, respectively. While the power across the separatrix (Psep in Fig. 4c) at the time of detachment onset is the same as #193106 within measurement uncertainty, indicating earlier detachment is not due to lower Psep. Notably, RMP results in a lower separatrix density but higher upstream SOL density at ψN = 1.04 across both attached and detached phases from 3.2 to 5 s (Fig. 4f, detailed profile evolution in Supplementary Fig. 2). During detachment, the ELM frequency increases significantly to ~200 Hz, in contrast to the decreasing trend seen in #193106. Despite these differences, the core radiation power during detachment is similar to that in #193106.

A parametric scan of RMP current from 2 to 6 kA reveals a clear correlation between RMP and detachment facilitation. This RMP current scan enables comparison of edge profiles at different RMP amplitudes, allowing us to assess the impact of magnetic perturbations on detachment onset with similar fueling and heating conditions. Figure 4g, h show TS measurements of electron density and temperature profiles at the detachment onset for various RMP current levels. Compared to #193106, increasing RMP current causes a more pronounced density pump-out, yielding lower density inside the separatrix, but higher SOL density (Fig. 4g). This increased SOL density from TS is also consistently observed in the reflectometry (Fig. 4i), which has 80° difference in the 3D phase space compared to TS location, which mean this raised SOL density is not toroidally localized as observed in ASDEX-Upgrade66 tokamak during RMP ELM suppression. Crucially, it is well understood that this raised upstream SOL density aids in divertor detachment by enhanced impurity radiation and momentum loss67,68. Concurrently, the pedestal-top temperature rises by 20–40% (Fig. 4h). This scan clearly demonstrates, for the first time, enhanced radial transport from the pedestal to the SOL induced by RMP during detached conditions, providing systematic evidence that supports earlier, limited observations55.

Figure 4i, j summarizes the dependence of detachment onset density and D2 gas puffing rate on the RMP current. With RMP in the range of 2–6 kA, the line-averaged density at detachment onset drops by 10–30%, pedestal density by 10–15%, separatrix density by 10–20%, while the SOL density at ψN = 1.04 increases by 30–110% from both TS and reflectometry. This pronounced density redistribution is consistent with enhanced particle transport, though a contribution from the modified recycling due to 3D magnetic topology cannot be ruled out. Correspondingly, the required gas puffing rate ΦD2 decreases by ~10% for IRMP = 2–4 kA. However, at higher RMP current (5–6 kA), the required puffing rate approaches that of #193106. Similarly, the reduction in detachment onset density becomes saturated from the pedestal to the core region for IRMP ≥ 4 kA.

Further analysis reveals that the non-monotonic dependence of ΦD2 on RMP current arises because an overly strong RMP has counterproductive outcomes. While it successfully exhausts more thermal particles—lowering the detachment onset density—this effect also demands more fueling and degrades the degree of detachment (DoD)67. The stronger particle exhaust is evident from the increased density pump-out and a more degraded plasma performance (βN falls from 2.2 at 4 kA to 1.8 at 6 kA). Consequently, this greater thermal particle loss requires more fueling to achieve detachment. Furthermore, the stronger RMP leads to a lower DoD67, which decreases from 5 (without RMP) to 3.8 at 4 kA RMP, and falls below 3 above 5 kA RMP, as shown in Fig. 4j—a trend mirrored by the divertor neutral pressure. For moderate RMP currents (2–4 kA), the lower DoD is attributed to a lower extrapolated ion flux at a reduced core density (Fig. 4i), as the measured ion and heat fluxes are lower than reference discharge #193106.

In summary, moderate RMPs facilitate detachment by enhancing particle transport: this reduces the density inside the separatrix while raising it in the SOL, thereby lowering the gas puffing for detachment. However, an overly strong RMP degrades this effect, ultimately reducing the DoD even though the detachment onset density is lower. This finding is supported by comparable experiments in the EAST tokamak (Supplementary Fig. 3), which also showed a reduction in detachment onset density with n = 1 RMP and pulsed Neon seeding, though a similar reduction in the impurity puffing rate was not confirmed.

2D and 3D simulations show consistent detachment facilitation and heat flux mitigation by RMP
SOLPS-ITER simulations with a two-dimensional (2D) boundary qualitatively reproduce the experimental observations by incorporating RMP-enhanced transport effects. These simulations compute the background plasma by solving the 2D Braginskii equations, while the neutral distribution is calculated using a kinetic neutral transport code. Turbulence-driven radial transport is represented through ad-hoc cross-field transport coefficient profiles. Neoclassical transport effects are not included, as all relevant particle drifts (E×B, B×∇B) are activated in the modeling. The cross-field particle and thermal diffusivities are iteratively tuned to match TS measurements of electron density and temperature profiles at the LFS midplane. Figure 5a, b present the SOLPS-ITER derived particle (D⊥) and thermal (χe) diffusivity profiles for #193106 without RMP and #193108 with 5 kA RMP at the time of detachment onset (see Fig. 4g, h). A noticeable increase in both cross-field particle diffusivity (D⊥) and thermal diffusivity (χe) is observed around the separatrix for the RMP case. Specifically, the elevated χe in the region 0.98 < ψN < 1 is a direct consequence of the flattened temperature gradient there. This finding aligns with the established understanding that RMP-driven magnetic islands near the separatrix act to flatten (not shown in Fig. 4h) the local Te profile38. For discharge #193108, a reduction in stored energy indicates that ~0.5 MW of additional power is exhausted to the SOL due to the applied RMP. In the simulations, the separatrix density at the LFS midplane is scanned to determine the detachment threshold for both RMP and non-RMP cases. By incorporating the transport coefficients from Fig. 5a, b, and the additional 0.5 MW of SOL power for the RMP case, SOLPS-ITER simulations predict the detachment onset density, shown in Fig. 5c as Jsat versus ne,sep. The stars denote the threshold ne,sep required to achieve detachment, defined by a target electron temperature below 10 eV. Comparing discharges #193106 and #193108, the simulations predict a 19% reduction in the detachment onset density due to RMP, in good agreement with the 21% reduction observed experimentally. These simulations again confirm that the RMP-induced enhancement in the cross-field transport helps to increase the SOL density and facilitate detachment by enabling more efficient plasma cooling.

EMC3-EIRENE simulations with a fully 3D boundary geometry support the experimental findings described above. EMC3-EIRENE69,70 solves a coupled set of fluid equations for plasma particle, parallel momentum, and energy, along with kinetic neutral particle transport for a 3D magnetic field configuration. The magnetic perturbation field, comprising both vacuum and plasma response, is calculated using the MARS-F71 code and superimposed on the 2D axisymmetric equilibrium field from discharge #193022 (Fig. 4g, h) to form the magnetic geometry for EMC3-EIRENE. The plasma backgrounds from EMC3-EIRENE are used as inputs for the ERO2.072,73 simulations, which self-consistently account for the carbon impurity erosion, transport, and radiation. These simulations are conducted with 4 MW of power in the SOL region, using constant cross-field diffusivities (D⊥,D = D⊥,C = 0.3 m2s−1, χi = χe = 1 m2s−1), in accordance with Fig. 5a, b at the separatrix. The use of constant coefficients is a standard and practical approach for these 3D simulations, chosen to ensure numerical stability and convergence. This assumption allows us to qualitatively isolate the effect of the RMP itself without the added complexity of spatially varying transport. A deuterium plasma with primary carbon impurity content is assumed, and a 4% carbon erosion yield is adopted based on SOLPS-ITER modeling for DIII-D74. The D2 puffing rate and the RMP current are scanned to qualitatively compare with the features observed in the experiment.

Figure 6a shows the simulated LFS midplane electron density profiles for IRMP = 0, 4, and 6 kA, using a D2 gas puffing rate of ΦD2 = 6.4 × 1021 s−1, which is insufficient to trigger detachment for these three cases. Comparing to the case without RMP, simulations with 4 and 6 kA RMP exhibit reduced density inside the separatrix (pump-out) and increased SOL density, the higher RMP current lead to stronger SOL density enhancement—highlighting the role of RMP in enhancing cross-field transport. Additionally, the simulations show toroidal variations in the SOL density caused by 3D effects, while the overall SOL density remains substantially higher than in the no-RMP case. These results qualitatively agree with experimental TS measurements shown in Fig. 4g, although it should be noted that the linear plasma response model used here does not include the nonlinear RMP penetration effects at the pedestal-foot38,75.

Figure 6b, c show the simulated heat flux on the outer divertor target for the case without RMP and IRMP = 6 kA, respectively, with a higher D2 gas puffing rate of ΦD2 = 7.7 × 1021 s−1 to sustain detachment. At the outer target, the case without RMP remains marginal to detach, with still high enough target heat flux and Te,div near 10 eV (Fig. 6b). The application of 6 kA RMP leads to partial detachment, with strongly mitigated target heat flux and a maximum Te,div of ~2 eV (Fig. 6c). These simulations again align well with the experimental results in Fig. 4, affirming the beneficial impact of RMP enhanced cross-field transport on divertor detachment.

Revisiting LSN divertor experimental results46 reinforces the observation that effective RMP-induced transport enhancement (density pump-out) facilitates detachment (Supplementary Fig. 4). The key involves activating RMP at lower density to trigger magnetic island38,76 and increase turbulence transport66,77 at the plasma edge, thereby inducing density pump-out. This is notably more difficult to achieve at higher density in both tokamak38,75 and stellarator78. Together, these experimental and numerical results suggest that RMP-facilitated detachment is a general phenomenon, contingent on the ability of RMP to effectively enhance edge plasma transport.

RMP mitigates heat flux during detachment without second heat flux peak
In addition to facilitating detachment, both experimental data and EMC3-EIRENE simulations reveal further benefits from combining RMP with divertor detachment. Figure 6d shows the simulated heat flux on the outer divertor target under attached conditions with 6 kA RMP and a low D2 puffing rate (ΦD2 = 1.3 × 1020 s−1). A distinct secondary peak in heat flux appears alongside the primary peak, a known concern for ITER when integrating RMP-based ELM control with detachment54. However, EMC3-EIRENE simulations demonstrate that this secondary peak disappears once the gas puffing rate is increased sufficiently to achieve detachment, as shown in Fig. 6c. This suppression of the secondary peak is corroborated by DIII-D experiment. Langmuir probe measurements of the outer divertor target heat flux for discharge #193106 (without RMP) and #193108 (5 kA RMP) are presented in Fig. 6e, f, respectively. In #193108, a secondary peak appears around 2 s, decreasing progressively with increasing gas puffing and plasma density (see Fig. 4a–f). This peak vanishes entirely at approximately 3 s—well earlier than the onset of detachment—and this trend is consistently observed in all discharges with applied RMP. Contrary to our observations, in the ASDEX-Upgrade experiments51, the lobes disappeared only under strong detachment.

Another notable effect is the further reduction of the primary heat flux during detachment when RMP is applied. As shown in Fig. 6c (with RMP), the peak heat flux is significantly weaker compared to the case without RMP in Fig. 6b. Experimental results reinforce this finding: during detachment, the primary heat flux drops by more than a factor of two with RMP (Fig. 6f vs. Fig. 6e). In #193106 (without RMP), the peak shifts outward and reduces to ~2 MW/m² after detachment onset. In contrast, #193108 (5 kA RMP) sees further mitigation of the peak to below 1 MW/m², attributed to increased SOL density. These findings underscore the role of RMP in not only aiding detachment but also mitigating heat loads on the divertor.

RMPs are known to generate magnetic island chains near the separatrix, which overlap to form magnetic stochasticity38 and lobe structures79 at the plasma edge. Figure 7a illustrates the perturbed magnetic topology caused by RMP in DIII-D discharge #193022 under both attached and detached conditions. The MARS-F80 and FLARE54 codes are used to compute the linear plasma response and field line tracing, respectively, revealing 2–3% edge stochasticity and lobes within ψN = 0.98–1. These 3D structures induce a 3D distribution of edge plasma parameters81 and enhance radial and parallel transport at the plasma edge38,54, thereby facilitating divertor detachment. The resulting edge 3D plasma distribution is evident in the CIII emission simulated by the EMC3-EIRENE, as shown in Fig. 7b, where lobe structures are prominent in the attached condition but vanish during detachment. Notably, since the E×B drift effect is not included in the EMC3-EIRENE simulations, detachment in the inner divertor is weaker than in the outer divertor, as evident by relatively stronger CIII emission and a target temperature of around 6 eV.

Experimentally, the existence of these lobes is confirmed by CIII impurity emission imaging. Figure 7c presents the 2D distribution of CIII emission from a tangential camera (TTV) at the same toroidal location and time as the simulations in Fig. 7a. Bright emission peaks, marked by arrows, appear on the HFS in both attached and detached phases, aligning with the ψN = 1 flux surface from the simulation (red curves). The TTV observation is consistent with the EMC3-EIRENE simulation during the attached condition but differs during detachment, which is likely because the detachment in the simulation is deeper compared to the experiment (partial detachment). During detachment, the peak impurity emission at the outer divertor target diminishes, while the X-point emission remains prominent. This behavior reflects target cooling and a redistribution of radiative losses, with a relatively enhanced role near the X-point. While the MHD model predicts lobe structures on both the HFS and LFS, experimental CIII emission shows pronounced lobes on the HFS only. On the outer divertor leg, lobes are not clearly visible due to the limited viewing angle of the TTV camera.

Decreasing RMP response in detached plasma
Simultaneous full RMP ELM suppression and detachment remain elusive in DIII-D experiments due to significantly reduced plasma response in high collisionality plasma. Previous modeling showed reduced plasma response at higher collisionality in attached plasmas.

Figure 8a shows the derived profiles of edge bootstrap current density for DIII-D discharge #193108 from low to high density/pedestal collisionality. Discharge #193108, as shown in Fig. 4g and h, was performed with increased RMP current (5 kA) under otherwise similar conditions to discharge #193106. Compared to low pedestal collisionality (υ*e ≈ 0.2), it exhibits over a 50% reduction in pedestal bootstrap current during high collisionality detached conditions. The degraded pedestal gradient at high collisionality is thought to be caused by the electron temperature gradient (ETG) mode82. The edge kink response, essential for ELM suppression, is thought to be amplified by the edge pedestal bootstrap current83. Therefore, a substantial decrease in the plasma response is anticipated with a significant reduction in the edge bootstrap current, as illustrated in Fig. 8b for the HFS poloidal magnetic response calculated by the ideal MHD code GPEC84. Here, the phase difference between the upper and lower row coil currents is performed, and the poloidal magnetic perturbation at the HFS midplane is shown because of its crucial role in ELM suppression85,86. Figure 8b reveals more than a 70% decrease in the HFS magnetic response with increased pedestal collisionality from υ*e ≈ 0.2 to 1.8. Nonlinear two-fluid MHD simulations via the TM1 code38 predict over 15 kA RMP current for full ELM suppression in detached plasma, surpassing the capability of 3D systems in DIII-D (maximum 6 kA). Additionally, Fig. 8b highlights a continuous ~40° phase shift for maximum HFS magnetic response, underscoring the importance of 3D configuration optimization with changing pedestal conditions87.

Figure 8c–e displays the radial magnetic response, denoted as δBr, on the plasma control surface for RMP in even parity with υ*e ≈ 0.2 (Fig. 8c), and both even (Fig. 8d) and odd (Fig. 8e) parity for detached conditions with υ*e ≈ 1.8. For υ*e ≈ 0.2 and even parity RMP (Fig. 8c), the patterns in δBr align well with the field line pitch depicted by the black curves, indicating effective resonance of the applied RMP with the plasma edge. However, for υ*e ≈ 1.8 and even parity RMP (Fig. 8d), the magnetic field strongly decreases and misaligns with the field line pitch at the LFS midplane. For υ*e ≈ 1.8 and odd parity RMP (Fig. 8e), the magnetic response still aligns with the field line pitch, although the amplitude is significantly reduced. Additionally, the magnetic response with components resonating at the pedestal-top is stronger for odd parity compared to even parity, consistent with observations in Fig. 2 that odd parity RMP leads to stronger ELM mitigation. These observations and analysis underscore the sensitivity of plasma response with plasma collisionality86, particularly the change in the pedestal structure when transiting from attached to detached divertor.

Methods
DIII-D tokamak
DIII-D91 National Fusion Facility is a tokamak research device operated by General Atomics in San Diego, California, for the U.S. Department of Energy. DIII-D is the largest magnetic fusion research facility in the U.S. The DIII-D program is pioneering the science and innovative technology that will enable the development of nuclear fusion as the next-generation energy source. The discharges presented in this paper have the major and minor radius of 1.7 and 0.6 m, respectively, the toroidal magnetic field Bt = 1.7 T at the magnetic axis, the plasma current Ip = 1.27 MA, 6 MW heating power, and 4.5 Nm torque are injected by using the neutral beam injection (NBI), edge safety factor q95 = 3.5, βN = 2-3, H98 = 1.1–1.3. DIII-D is equipped with two sets of 3D coils: one is the ex-vessel coils, consisting of 6 saddle coils at the low field side midplane called C-coils, and another one is the in-vessel coils, consisting of 12 saddle coils with an upper and lower row of 6 coils called I-coils. In our experiments, the C-coils are used to compensate the n = 1 intrinsic error field, and the I-coils are used to generate n = 3 RMP with the upper row and lower row coil current controlled separately.

EAST tokamak
The Experimental Advanced Superconducting Tokamak (EAST)2 is a superconducting tokamak magnetic fusion energy facility constructed and operated by the Institute of Plasma Physics, Chinese Academy of Sciences, in Hefei, China. EAST is aiming to conduct fundamental physics and engineering research on advanced tokamak fusion reactors with a steady, safe, and high-performance, to provide a scientific base for experimental reactor design and construction, and to promote the development of plasma physics and related disciplines and technologies. The referenced discharge has the plasma major radius of 1.85 m, minor radius of 0.45 m, and the toroidal magnetic field Bt = 2.25 T at the magnetic axis, the plasma current Ip = 0.45 MA with q95 ~ 5.2, βN = 1.6, H98 = 1 heated by 1.3 MW from the lower hybrid wave and 1.3 MW from NBI. The divertors in EAST have been upgraded to full tungsten. EAST equips 16 in-vessel saddle coils with an upper and lower row of 8 coils each. In our experiments, the 3D coils are used to generate n = 1 RMP with the upper row and lower row coil current controlled separately.

Diagnostics for kinetic profile and divertor parameters
In DIII-D, the profiles of electron temperature and density from the core to the SOL are measured by the multi-pulsed high-resolution Thomson Scattering system92,93. The profiles of ion temperature, the carbon density, and rotation are measured by the charge-exchange recombination spectroscopy system94 for C6+ particles. To obtain well-resolved profiles, the data are averaged over 100 ms within 70–95% of the ELM period. The pedestal height is obtained from hyperbolic tangent fits with edge profiles. The heat flux on the upper divertor target is measured by one tangential camera of the fast infrared television camera (IRTV), and the peak heat flux on the outer divertor target is derived from the IRTV data. The tangential TV viewing of the upper divertor is used to measure the C3+ and N2+ impurity emission. In both DIII-D and EAST, an array of Langmuir probes mounted on the divertor target is used to measure plasma parameters on the divertor target, including the heat flux, ion saturation current, and electron temperature.

Kinetic equilibrium reconstruction
By using the OMFIT integrated modeling framework95, a multi-step workflow is used to add pressure and plasma current density constraints in the equilibrium reconstruction to improve the accuracy of the reconstructed equilibrium. The workflow (for one iteration) includes three parts: profile fitting based on the existing equilibrium, power balance calculation for total pressure and plasma current components, and equilibrium reconstruction with pressure and current density constraints. Usually, two or three iterations would be sufficient to produce high-quality equilibria for a transport study. In the power balance part, NUBEAM96 is used for NBI-driven current and fast ion pressure calculation, and bootstrap current is derived from the measured pressure profiles based on the Sauter model65. Then TRANSP97 is employed to create the total pressure by combining thermal pressure and fast ion pressure together, and provide total plasma current density by considering external driven current, bootstrap current, and calculated Ohmic current by solving poloidal field diffusion. The equilibrium reconstruction part is performed by the EFIT98 code.

Determination of divertor detachment
Divertor detachment is accompanied by an abrupt drop of the target temperature to below 5–10 eV, and a roll-over in the ion saturation current measured by Langmuir probes, an abrupt increase in the neutral pressure measured by a pressure gauge, and radiation power moves towards the X-point measured by a bolometer. All these indications are consistently observed in DIII-D by other diagnostics, including bolometer and tangential TV. In EAST, the indications from Langmuir probes are used to determine the onset of detachment. The degree of detachment (DoD) is defined as the ratio between the extrapolated target ion flux (obtained by scaling with core density) and the measured ion flux at the divertor target. A commonly used formula67 is

DoD = Γtarget,extrapolated / Γtarget,measured

where Γtarget,extrapolated is target ion flux expected from attached scaling using core line-averaged density, Γtarget,measured is the ion flux measured by Langmuir probes at the divertor target. In the experiment, the ion flux near the outer strike point (separatrix) is used to derive DoD. Usually, 2 < DoDpeak < 10 is thought of as partial detachment.

Uncertainty and error bar definition
The error bars presented in this study represent different sources of uncertainty depending on the measurement type. Specifically, for the electron density of the separatrix (ne,sep) in Fig. 2h, as well as ne,ped, ne,sep and ne,sol in Fig. 4f measured by Thomson scattering (TS), the error bars reflect the systematic uncertainties associated with the diagnostic system itself93. In contrast, the error bars shown in Figs. 3, 4g–j, and 5a, b originate from statistical uncertainties due to time averaging of the measured data. These distinctions are maintained to accurately represent the nature of the uncertainty in each dataset.

SOLPS-ITER simulation
Drift-dependent modeling of the experimental discharges was performed for a better understanding of the physics involved, using the state-of-the-art boundary plasma code package, SOLPS-ITER99, the coupled version of the 2D multi-fluid transport code B2.5100, and the 3D kinetic neutral transport code EIRENE101. The B2.5 fluid code computes the background plasma by solving the 2D Braginskii equations and provides the solutions to EIRENE. Then EIRENE computes and returns the source and sink terms for plasma particles, momentum, and energy due to plasma-neutral interactions. The simulations were carried out for a deuterium (D) plasma with carbon (C) wall and targets. Simulated particles in the modeling include ions (D+, C+, C+2, C+3, C+4, C+5, C+6), atoms (D, C) and molecules (D2). A complete set of atomic and molecular reactions is accounted for in the modeling, including neutral-neutral collisions. All particle drifts, E×B, B×∇B, viscosity, and the associated currents are activated in the modeling.

EMC3-EIRENE simulation
EMC3-EIRENE is a numerical code used to model the plasma boundary. It is a coupling between the plasma fluid code EMC3 and the neutral kinetics EIRENE code. EMC369 is a Monte Carlo 3D plasma fluid code that solves a set of reduced Braginskii fluid equations for particles, parallel momentum, and energies for electrons and ions. The code models parallel electron and ion heat conductivity using classical assumptions. Perpendicular transport is determined by coefficients for anomalous particle transport (D⊥) and anomalous electron and ion heat transport (χ⊥), which are free model parameters. EIRENE101 is a kinetic edge transport code that solves the transport equations for neutral atoms and molecules, accounting for collisional processes. The code calculates ionization sources, momentum sources/losses, and energy sources/losses due to atomic/molecular processes, such as charge exchange and ionization. EMC3-EIRENE models impurity transport using a fluid approach, causing energy losses to the main plasma through excitation and ionization. The trace fluid approach assumes that impurities only cause small density perturbations and impact the main plasma species through ionization and excitation via an energy loss term in the energy balance equation.

Plasma response calculation
GPEC code84 is used to calculate the ideal MHD perturbed equilibrium and the magnetic perturbations. For a given kinetic equilibrium and 3D coil configuration, GPEC calculates perturbatively the total (vacuum plus ideal MHD kink response) 3D magnetic response of the plasma to the 3D coils in full toroidal shape geometry. In this paper, the poloidal magnetic perturbation at the HFS midplane is used to quantify the change of magnetic response according to previous studies85,86. The MARS-F code71 is used to calculate the magnetic perturbations generated by 3D coils. It solves the single fluid, perturbed MHD equations in full toroidal geometry.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Please see an attached PDF.", "reviewer_2": "The reviewer would like to thank the authors for a well written contribution with clearly presented information. There are\nsome new additions to our pool of observations regarding RMP ELM suppression and connections to SOL transport and\ndetachment. However, at several points key pieces of information are missing, that are essential to interpret the experimental\ndata and to support the claims that the authors make.\nTo address specifically the points requested by the journal:\n• The quality of the data appears technically sound and appropriately analysed. However, information is missing. In this\nsense, it has not been presented in sufficient detail.\n• The potential significance of the results could be high. However, in its present form, this conclusion cannot be drawn.\nKey results: For ITER, having an external “control” knob that would enable us to suppress ELMs while maintaining a stably\nattached divertor in Q=10 operation with a high-collisionality pedestal bottom and low-collisionality pedestal top is indeed\nattractive. It is possible, these results contribute to the development of such a tool set, but there is insufficient information to\nassess if this is indeed the case, or if the observations are a slightly new variation on previously observed phenomena. The\nReviewer suspects that while there is good material here for a Nuclear Fusion paper, the results are not sufficiently novel for\na Nature Communication. Detailed comments are given below.\nIn this work, the authors claim:\n1) N=3 RMP facilitates divertor detachment in DIII-D by reducing the density inside the separatrix and increasing the SOL\ndensity. They dub access to detachment at reduced core density “facilitation” of detachment.\n2) The second heat flux peak on the HFS due to RMP induced lobes disappears before detachment.\n3) By optimizing RMP configuration and impurity seeding, successfully achieved RMP-mitigated small ELMs and divertor\ndetachment in DIII0-D\nPage 2,\nIt would be worthwhile to note here that the Greenwald fraction you are referring to (I assume) is an edge Greenwald fraction\n– pedestal top? When referring to Greenwald fraction for which ELM ELM-suppression / mitigation is observed to be\neffectively, a good reference would be Fig 4c of Paz-Soldan 2024 (submitted)\nchrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://arxiv.org/pdf/2403.03693.pdf\nITER is expected to operate at relatively high SOL density, or density at the LCFS. Regarding the presence of heat flux\nlobes in the presence of RMPs and the inability to maintain detachment and RMP-suppression, one can also consider to\ninclude the ASDEX Upgrade results:\nD. Brida NF 57 116006 2017\nA. Kallenbach NF 61 016002 2021\nThe former is particularly interesting for the authors with respect to Claim 2, as it demonstrated the removal of the RMP-\ninduced lobe-heat fluxes in detached conditions in both L- and H-mode plasmas.\nOn page 2 the manuscript states, “We report that, for the first time, divertor detachment facilitation by RMP is observed in H-\nmode plasmas in DIII-D and reproduced in EAST.” With this statement, I expect to see in the subsequent pages, a clear\nexperimental demonstration that applying RMPs makes it easier to detach the divertor. And based on the connected\nstatement in the abstract, “The detachment facilitation correlates with the density pump-out magnitude, consistent with SOL-\nPS simulations.” I expect to be shown that with increasing RMP magnitude (higher and higher SOL density due to increasing\npump-out) it gets easier and easier to detach the divertor. Unfortunately, this is not shown.\nWe detach tokamak divertors by applying either D or impurity seeding to increase the radiation and parallel power loss in the\nSOL, removing the incident power before it reaches the target surfaces. Hence, if applying RMPs ‘facilitates’ detachment, I\nexpect to see a figure showing that less D or impurity needed to be puffed to reach a detached condition, compared to the\ncase without RMPs. I expect to see that as I increase the RMP current, and increase the density pump out, increasing the\nSOL density, and potentially increasing cross-field transport in this region (hence reducing parallel transport), that less and\nless external D or impurity is needed. This is not shown. The only figure that contains some information in this direction is\nFig. 2, where it would appear that an extremely similar level of D seeding is needed with and without the RMPs to detach the\ndivertor.\nRather, the meaning of “facilitation” used here appears to be a different one. It is used in reference to the value of the\npedestal (or core) density at the time of detachment. Typically, detachment is achieved at higher pedestal Greenwald\ndensities, as noted by the authors. Note that high Greenwald fraction in the plasma core is, in fact, desirable in a reactor,\nmaking pump-out not necessarily a good thing. The use of RMPs to ‘pump-out’ the core (a well known result), means that the\ndetachment obtained here is obtained at lower pedestal Greenwald fraction, hence leading to the statement “RMPs reduce\nthe detachment onset density.” i.e. RMPs break the standard correlation – between SOL and pedestal top density. This\nseems to be the meaning of “facilitation” here. This is factually correct, but misleading, and does not reflect the actual\nphysics at play – or necessarily mean it was ‘easier’ to induce detachment.\nOn page 3, the authors note that even parity is the primary setting. Later in this paper, the authors show strong ELM-\nmitigation combined with a detached divertor obtained with odd parity. None of the profile changes in that case are actually\nshown, and it is not at all clear that the entire discussion of pages 3-5 is relevant for the actual “success case” shown on\npage 6.\nOn page 3-5, even parity discharges are presented. In figure 2: The units of the D ramp-rate are missing. The reader has no\nway of knowing if this is large gas-puff or not. Please include units and information on valve location/calibration. In the EAST\ncase, is it correct that there is ZERO D gas puff – the only fueling in this time window is two neon puffs (how big are these?\nIn elec/s?) Minutia: in the top boxes the DIII-D label reads “no RMP” while the EAST label reads “ W/O RMP.” The authors\nclearly went through some effort to unify the presentation of the data between the two machines (appreciated!) – they can\nthink to unify labels if they wish. And an edge density/Greenwald fraction would be more informative than a core.\nIn the text – f_g – it would be useful if earlier when introducing f_G, the authors also specified that all uses of this term will be\nreferring to pedestal top densities (assuming this is the case). It is worth being very clear, that the main physics message of this first\nsection is not ELM-suppression + detachment. It is, in fact, quite high density, high collisionality. The point is\nsupposed to be that adding RMPs to such a scenario increases the SOL density, and hence makes it easier to detach these\nparticular plasmas. (remains to be shown) But no attempt is made, at this point, to combine detachment with RMP ELM\nsuppression. This distinction should be very clear, because from the abstract and the introduction, this is what the reader is\nexpecting to find.\nComing to figure 3. These are very nice edge profiles, that clearly demonstrate the drop in the core densities and the\nincreasing the SOL densities with increasing RMP current. In some sense, this is a common-sense observation. The density\ngoes down in the core, those particles have to go somewhere. And it was, in principle, already shown in Petrie NF 2011,\nsee Fig. 9. But I don’t believe I’ve ever seen it shown this clearly and for different levels of RMP current. However, a missing\npiece of information that must be provided is the fueling rate (and location) in each case.\nComing to figure 4. The authors highlight this figure as the ‘key physics result’ of this section, and I find this troubling. The\nlabel for this plot is “RMP induced transport correlates with detachment facilitation.” The y-axis here the density (at various\nlocations in the plasma) at which detachment occurs, while the x-axis is the change in the pedestal top density from the RMP\napplication, which one could interpret (from an engineering parameter stand-point) as the applied perturbation field. With\nrespect to detachment, the density on the y-axis that matters most is the SOL-density, which contributes to the power\nexhaust, either by helping exhaust power along field lines (particularly if neutral pressure in the divertor is correlated with the\nincreased density in the SOL (is it?)), or it represents increased cross-field transport. So, focusing on the SOL density as the\nparameter here closest to the relevant physics, we actually see an INCRESASE in the SOL density needed to detach with\nincreasing RMP-current -which would be the opposite of facilitation. But again, we are missing information on the applied D\ngas puff and divertor conditions, such that a proper interpretation of this plot with the available information is not possible.\nBut, noting that the core or pedestal top density at detachment onset goes down with increasing RMP current – is hardly\ngrounds for claiming RMPs facilitate detachment.\nIf at constant D gas puff, detachment were initiated by increasing the RMP current, that would be clear. What needs to be\nshown is that the increased SOL density (due to the RMPs) actually makes it easier to achieve detachment – i.e. at lower D\nor impurity has puff. This should be doable.\nIn principle, I like the addition of the SOL-PS simulations, showing that (assuming purely diffusive transport?) there is an\nincrease in cross-field transport in the presence of RMPs needed to match the profiles. What constraints are used / matched\nwith respect to edge neutral pressures / densities and their distribution? Why is there an “additional power” of 0.5 MW\nincluding in the modeling in the RMP case? The applied powers are the same, and I would assume that the impurity density\n/ lower electron density in the RMP case would lead to lower radiated power, not higher?\nThe SOLPS simulations (with constant transport coefficients?) show that with RMPs the density at the LCFS at which\ndetachment occurs is lower with RMPs, presumably because of enhanced cross-field transport, which is speculated by not\nshown. This is consistent with Fig 3c, explaining why the authors chose to show that plot. But at the same time, if the D gas\npuffing required to achieve detachment was always the same – then this relationship to the ne_SEP is coincidental. I\nstrongly recommend that the authors consider how best to emphasize the local changes in transport in the SOL and divertor,\ndue to the application of RMP – rather than highlighting a correlation with density values in the confined region. One could\ninstead consider to cite the increased SOL density and cross-field transport induced by RMP as facilitating detachment – if\none can show that detachment really did get easier.\nOn page 5, the authors write, “During detachment, the peak impurity emission shifts from the outer divertor target to the x-\npoint. … ” When I stare at Figure 5, in the top right plot, I can clearly see the hot spot along the outer leg – it is small, took me\na bit, but I found it. But to me, it looks like the x-point hot spot is equally bright already in the attached conditions. In the\ndetached conditions, I don’t see a shift, so much as the absence of the previous peak? About the lobes, when I look at the\nleft plots, it looks to me like there are 1-2 pretty clear lobes on the outer leg side. And the red line in the top right box, looks\nlike there are lobes there as well. But the text says, “no evident lobes are observed around the outer diverter leg” - what is\nthe definition of significant? I can’t reconcile / quantify those pictures.\nPage 5, please cite D. Brida as appropriate.\nPage 6: In Fig. 7., for the first case of open circular symbols, could please add “No RMP” to the legend.\nWith odd parity, we see a large increase in ELM frequency, and a corresponding drop in change in stored energy, and a\nsignificantly reduction of the heat flux (I assume this is integrated over the wetted area – and not the peak?). But it is unclear\nif the entire previous discussion regarding changes in plasma profiles, increased SOL density and SOL transport, still apply\nin the odd-parity case? Also, if these experiments are taking place in a low-density, low collisionality pedestal top parameter\nrange? There are no clarifications on experimental conditions. If I were to interpret the “ne” listed quietly in Fig 8 a.as\npedestal top values, then the detached case, is at quite high pedestal top density. Which makes this whole thing look a lot\nlike type II or type III ELMs – as seen in the early days of ELM suppression experiments at higher collisionality (e.g.\nSuttrop NF 2011, Rathgeber NF 2014). Such a scenario would not be hard to detach - previous attempts at ELM mitigation +\ndetachment failed, specifically because they were working in the low collisionality branch. The detachment attempts by\nShin, for example, were in that low collisionality regime when they lost the ELM suppression, as were the experiments\npresented in Kallenbach 2021. Detachment with a variety of small ELM scenarios, on the other hand, has already been\ndemonstrated.\nSo, what does the pedestal do, in these detachment cases? Does any of the previous “Claim 1” discussion still apply? Is this\nstill a low-collisionality scenario? Or, is this an MHD pedestal stability effect from changes in the plasma kinetic profiles,\nopposed to a SOL transport effect? Right hand side of Fig. 7 – what are the D and N fueling rates? What is the edge density\nand fG? Combining a low collisionliaty pedestal with RMP ELM-suppression/mitigation with a high collisionality pedestal\nbottom that is detachment compatible, would indeed be interesting, but this information is lacking. At the moment, this looks\na lot like a high collisionality, small-ELM regime.\nComing to page 7, am I correct that discharge 193108 is a ‘new’ discharge, in the sense that it has not yet been presented to\nthe reader? I presume there is a collisionality scan, maybe induced by changing power? Or by changing fueling? And this is\ndone specifically to give an experiment to the modeling to explore the impact of changing the collisionality on the HFS\nresponse? A few additional sentences here would help the reader follow this section.\nTypo in “coontrol” the top right caption of figure 8 “Magnetic response at the control surface”\nThis discussion on page 7 is helpful. We see clearly under what conditions we expect RMPs to be effective at penetrating\nthe plasma, and align best with the field line structure. With increasing collisionality – odd-parity is better field aligned, and\nso even though the response is low, it is more effective at flattening the profiles? Could we please see those profiles? How do\nthe pedestal characteristics compare to known type II ELM regimes?", "reviewer_3": "The present manuscript addresses a relevant topic in the fusion community, i.e. the relation between plasma detachment,\nmandatory to ensure plasma facing components survivability, and Resonant Magnetic Perturbation, which is the chosen\nstrategy for ITER in order to mitigate the deleterious Edge Localised Modes. In principle the authors are claiming that the use\nof RMPs indeed facilitate the access to detachment, which is achieved at lower density according to their claims.\nFurthermore other advantages arises because of the use of RMP during detached states, as improvement of confinement\nduring detachment, enhanced heat flux mitigation, absence of a second heat flux peak, andsimultaneous achievement of\nsmall ELMs with partial divertor detachment. They claims to have evidences of the aforementioned processes coming from 2\ndifferent devices, DIII-D and EAST.\nI want to clarify that the data presented by the authors are actually relevant and interesting but despite effort I do not think that\nthe conclusion derived are sufficiently robustly supported for being published in such a prestigious journal. In the following\nthe list of my main concerns supporting this conclusion. They are not provided in priority\nordering but just\n1. One the my main concern regards the conclusion derived from Figure 2. According to the authors the use of the RMP\nfacilitate detachment and this in their opinion shown by noticing that detachment is achieved at lower density showing in this\nrespect the central density as a reference. Actually this is to me not appropriate for several reason: first of all RMP\napplication are know to cause a different density peaking and this is associated as well to the process of pump out. A more\nrobust support to the conclusion of the authors would have been derived by showing that detachment is achieved at lower\nseparatrix density. This point is partially answered in Figure 3 where the authors claims that separatrix density at\ndetachment onset is reduced by 10-20%. This is a very delicate question given the uncertainty on the separatrix localisation\nin presence of RMPs (with induced magnetic bulging caused by magnetic stochasticity because of perturbed magnetic field).\nThe authors uses kinetic equilibrium reconstruction to infer separatrix position, but w/o accounting for any 3D effect as\ncaused by RMP. This uncertainity in the separatrix position needs to be considered together the error bars of the density\nestimate shown in Fig 3 (according to which the separatrix density w/o RMPs is within 1 sigma compatible with the value\nwith maximum density pumpout). Therefore I don't think the authors are in the position of claiming that detachment is\noccurring at lower separatrix density with RMPs, which is the relevant figure of merit to be considered as far as exhaust\nphysic is concerned. I would also like to noticed that the different peaking in presence or absence of RMP is seen also from\nFigure 3 given that the ratio ne_core/ne_sep is not constant\n2. Still on Figure 3 I think that the example shown for EAST are rather peculiar: the authors attribute the detachment\nachievements to Ne seeding. Actually detachment is achieved approximately after 0.5s from the second Neon puffing.\nAccording to recent models [Henderson et al IAEA 2023] this time is in the same ball-park of Ne resident time in the diveror\nregion and therefore it would be questionable attributing detachment as caused by impurity injection. Also in this case it is\nnot possible to deduce the separatrix condition (ne_sep, Psep or Te_sep) with or w/o RMPs and therefore we can't state\nunambiguously that detachment is facilitated by RMP applocation.\n3. To support their claim according to which RMP application are facilitating detachment onset, the authors performed a\nseries of SOLPS-ITER simulations with drift-activated. Actually as shown in literature (e.g. Brida NME 2019, Rea NF 2015),\nthe SOL profiles with applied 3D magnetic perturbation are intrinsically 3D and therefore it is not clear which profiles are\ntaken as reference for constraining SOLPS simulations. I do find the use of a 2D code for a clear 3D geometrical problem not\nappropriate and therefore I don't think the results of SOLPS are really promoting the claims of the author: they simply\nconfirm that with increased radial transport easier detachment is achieved (since at similar Psep this will be clearly associated to\nbroader target heat profile thus easier to be detached)\n4. It is actually quite difficult to interpret Fig 6 (a) concerning the DIII-D part. Indeed while it is clearly observed a\nperpendicular heat flux decrease at OSP, there is a simultaneous increase of the flux in a different radial position still of the\norder of 1.5 MW/m^2. It is difficult to intepret this as a signature of partial detachment. I think that the comparison of the radial\nprofiles of main quantities (ne, Te, jsat) might reveal features which are not addressed and considered in the present paper.\nBut this is not clearly a reference case scenario to be compared with RMP cases\n5. Also when comparing even and odd parity the authors does not provide information on the possible difference of\ndetachment onset relevant quantities like ne_sep or Psep between the two different configuration. Any inference of better\nperformances of one configuration w.r.t. to the other would not be possible w/o essential exhaust relevant information\n6. As a minor point, in the introduction the authors made a comment on ITER operation claiming that ITER is expected to\noperate at relatively high density for divertor detachment with [...]. This is actually inaccurate, since ITER is indeed foreseen to\noperate at high gas throughput (i.e. high neutral pressure) but this is not impacting the plasma density achieved since\nplasma fuelling is ensured by Pellet operation. On the other side the chosen strategy for ensuring plasma detachment in\nITER is a combination of high gas throughput and impurity seeding [see Pitts, Nucl. Mat. Energy 2019].\nIn conclusion I don't think the authors provide enough clear clearer evidences of the real benefit in term of detachment accessibility\nfacilitated by application of RMPs. As stated to me detachment onset are occurring within the experimental uncertainty at the\nsame separatrix parameters (ne_sep, te_sep). No information are provided in terms of power crossing the separatrix and\nonly information about core density are claimed as important, which is partially inappropriate. The use of a 2D transport code\nwith respect to an inherent 3D problem is to me of limited use. Therefore I don't think that the paper is adequate for a\npublication in such a prestigious journal"}, "author_response": "Response to Reviewers’ Comments for Manuscript NCOM-24-12833\nTaming Harmful Bursts and Heat Flux in High-Confinement\nTokamak Plasmas\nAddressed Comments for Publication to\nNature Communications\nBy\nQiming Hu, Huiqian Wang, et al.\nWe would like to sincerely thank you once again for your thoughtful and constructive comments\non our manuscript entitled “Taming Harmful Bursts and Heat Flux in High-Confinement Tokamak\nPlasmas” (Manuscript Number: NCOMMS-24-12833).\nIn the previous version of the manuscript, which was originally prepared for submission to another\njournal with a more restrictive length limit, we were unable to provide sufficient details on the\nexperimental data and supporting analysis. In this revised version, we have restructured the\nmanuscript in response to your insightful feedback and incorporated significantly more detail to\nfully address all comments and suggestions.\nPlease find below a point-by-point response to each comment (following the order presented in\nthe decision letter). For clarity, all comments from the editor and reviewers are typeset in boxes,\nand the major revisions in the manuscript have been highlighted in red. With response to each\nreviewer outlined below:\nAuthors’ Response to Reviewer 1\nAuthors’ Response to Reviewer 2\nAuthors’ Response to Reviewer 3\nThank you again for your valuable input.\nSincerely,\nQiming Hu, Huiqian Wang, et al.\nGeneral comments.\nThe authors discussed the integration of the RMP ELM mitigation and divertor\ndetachment in the high collisionality tokamak plasmas. This is a crucial and critical issue\nto the ITER and the realization of the fusion power plant. All results are clearly explained\nand well communicated. However, I am strongly disappointed that the authors conclude\nthe results in this manuscript are cross-machine validations. I believe these results are\njust demonstrations in two devices. I suggest several points that the author should revise\nconsidering these remarks.\nResponse:\nWe sincerely thank the referee for your thoughtful and constructive feedback. We understand\nthe concern regarding the interpretation of results across multiple devices. To address this, we have\nrevised the manuscript to avoid any implication of cross-machine validation. Specifically, we have\nmoved the EAST experimental results to the supplementary materials to focus the main text on the\nDIII-D results. The conclusions have also been modified accordingly to reflect this change. We\nhave carefully revised the wording throughout the manuscript to clarify the scope of our findings,\nimprove the explanation of the data, and eliminate any potentially misleading descriptions. We\nhope these changes address your concerns.\nComment 1\nThe authors discussed the DIII-D results in Figure 2a-d. I do not see these results as\nproof of the facilitation of the divertor detachment by the RMP. For #193022, the\nD_alpha signal obviously dropped, and the ELM was fully suppressed. However, after\nthat, the plasma was again the ELMy, and the ion saturation current J_sat increased for\nboth #193022 and 193106. Also, the J_sat for #193022 at t=4s slightly decreases than\nthe J_sat for #193106. But those are almost comparable. I wonder about the ELMy\nbehavior for #193022. In Figure 2c, the density pump-out was observed for #193022.\nThat suggests the RMP penetrates the pedestal top.\nResponse:\nWe thank the referee for the detailed comments, which helped us improve the clarity of the\npresentation and interpretation of our results. In response:\n1. As clarified in the revised manuscript (page 2 and 5), by “detachment facilitation,” we refer\nto achieving detachment with reduced gas puffing rate and at a lower upstream separatrix\n3\ndensity due to the application of RMP. To strengthen this analysis and remove ambiguity,\nwe have replaced discharge #193022 with #193107, which exhibits more consistent\nbehavior. Compared to #193106 (without RMP), #193107 shows a 17% lower detachment\nonset density and 11% lower D puffing rate with 4 kA RMP applied.\n2\n2. The time trace of D in # 193022 (now 193107) is typical in several tokamak devices and\n\nhighlights the difficulty to achieve ELM suppression when increasing the density via gas\npuffing to pursue divertor detachment, as mentioned in the introduction section. The reason\nis that, with limited RMP strength (here coil current), RMP can only penetrate at low\ndensity to form magnetic island at the pedestal-top to suppress ELM (Q.M. Hu et al 2020\nNucl. Fusion 60 076001, R. Fitzpatrick, Phys. Plasmas 2020 27, 042506. As density\nincreases, the magnetic island may heal, resulting in a return to mitigated ELMy conditions\nrather than complete suppression. Now in the revised manuscript we added description\nabout the change in the ELMy behavior in terms of its frequency and caused energy loss.\n3. Continuous increasing J with increasing density until its roll-over is a typical\nsat\ncharacteristic of divertor detachment (P C Stangeby 2018 PlasmaPhys.Control.Fusion 60\n044022). Since the upstream separatrix electron temperature remains unchanged due to\npower balance, the increasing separatrix density leads to increasing separatrix pressure,\nwhich results in increasing J . The decreased upstream separatrix density in #193022\nsat\ncompared #193106 indicates lower downstream density, which results in the relative\ndecreased J in #193022 compared to #193106. In light of another referee’s suggestion,\nsat\nwe now use the target electron temperature (T ) as a direct indicator of detachment onset.\ne,div\nThe use of T provides clearer and more consistent evidence of the onset of detachment\ne,div\nacross discharges. And the neutral pressure measured by ASDEX-pressure gauge also\nconfirms the detachment onset.\n4. Regarding the density pump-out observed in #193022, we agree this suggests that the RMP\npenetrates the pedestal region. Based on established understanding and modeling (e.g.,\nQ.M. Hu et al. 2020, S.K. Kim et al. 2023), magnetic island formation at the pedestal-foot\nis associated with density pump-out, while island formation at the pedestal-top is\nresponsible for ELM suppression. We interpret that in #193022, during 2.1 s < t < 2.4 s,\nthe RMP penetrated both the pedestal-foot and -top. However, as density increased, the\npedestal-top island healed, terminating ELM suppression. Now for the newly added shot\n193107, 4 kA RMP is not enough to suppress ELMs and still exhibits similar level of\ndensity pump-out.\nWe hope these clarifications, along with the updated discharge selection and revised description,\naddress the referee’s concerns.\nComment 2\nare obvious. For #127746, in the detachment phase, the ELM was mitigated clearly, and\nthe temperature of the divertor was clearly dropped. However, a comparison of #127746\nand 127747 is misleading because, for #127747, the disruption happened by the impurity\naccumulation. In such a sense, unlike the case of DIII-D, this is not a fair comparison.\nResponse:\nThank you for your thoughtful and constructive comment.\nAs stated in the manuscript, the comparison between discharges #127746 and #127747 was\noriginally intended to illustrate the facilitation of detachment by RMP application on EAST.\nSpecifically, #127746, with 3.5 kA n=1 RMP, achieved detachment at a lower upstream density\ncompared to #127747, supporting the concept of RMP-facilitated detachment. #127747 disrupted\nsoon after the detachment by impurity accumulation due to missing impurity removal by either\nELMs or RMP. The condition around detachment onset is comparable that we could make the\ncomparison. Importantly, the impurity-triggered disruption in #127747, much later in #127746,\nfurther underscores a potential benefit of RMP application—not only in facilitating detachment\nbut also in mitigating impurity accumulation, possibly by enhancing impurity exhaust.\nWe acknowledge, however, the referee's concern that the comparison may appear misleading\ndue to the early disruption in #127747 caused by impurity accumulation. To address this, and to\navoid overemphasizing EAST data in the main discussion, we have moved the EAST results from\nthe main text (previously Figure 2e–h) to the supplementary materials. In doing so, we now present\nthe EAST observations as additional supportive evidence, rather than part of the central analysis.\nWe hope these revisions fully address the referee’s concerns.\nComment 3\nThe results in Figure 2 have two serious problems. In the DIII-D, two shots were puffed\ncontinually, and the discharges were transient. However, the results in Figure 2b-d are\nalmost comparable. In the EAST, the discharges were in a steady state. This is good.\nHowever, the divertor detachment needs impurity seeding. Nevertheless, the radiation\nwas not discussed. In other words, one problem is that the DIII-D shot is transient, but\nthe EAST shot is in a steady state. Another problem is that the EAST shot is the impurity\nseeding, but the DIII-D shot is not the impurity seeding. It seems the puzzle. How do we\nconsider that problem?\nResponse:\nraised and clarify that the issues identified do not represent “serious problems,” but rather stem\nfrom a misunderstanding of the scope and intent of the data presentation. To improve clarity and\nrigor, we have made several key revisions to the manuscript, which we summarize below:\n1. Clarification of Discharge Nature (Transient vs. Steady-State): While EAST is capable\nof steady-state operation, the specific EAST discharges presented in this study were\nintentionally transient, selected to match the transient nature of the DIII-D discharges.\nThis allows for a consistent analysis framework across devices, focusing on the detachment\nonset dynamics under evolving plasma conditions. In experiments, we have found that the\ndivertor detachment could happen at a very short time scale as can be seen in the divertor\ntemperature time evolution. The detachment timescale is much shorter than the energy\nconfinement time. We have clarified this point explicitly in the revised manuscript in page\n5 to avoid potential misinterpretation.\n2. Consistency in RMP Application and Purpose: In both devices, RMPs were applied to\nassess their effect on divertor detachment. The primary conclusion for Figure 2 (now\nFigure 4)—that RMPs facilitate detachment at lower upstream densities and lower gas\npuffing rate—holds true across DIII-D (with either D₂ puffing or D +N ) and EAST (with\n2 2\nimpurity seeding). The differing fueling or cooling methods do not compromise the\nintegrity of the observation regarding the role of RMPs. We have revised the text to\nemphasize that this section is not a direct cross-machine validation, but rather a\ndemonstration of a consistent physical trend enabled by RMPs in different experimental\nsetups.\n3. Radiation Analysis: We appreciate the referee’s point regarding the importance of\nradiation in impurity-seeded detachment. In response, we have added radiation data for\nthe DIII-D discharges to the revised manuscript. These additions further support the\nunderstanding of impurity-seeded versus D puffing detachment dynamics and reinforce\n2\nthe interpretation of RMP’s role in facilitating detachment in this context.\n4. Reorganization of EAST Data Presentation: To avoid any confusion and to keep the\ncentral narrative focused on DIII-D results, we have moved the EAST data to the\nsupplementary material. This ensures that the main conclusions of the manuscript remain\nrooted in a single-device analysis while still providing supporting evidence from EAST in\na more appropriate context.\n5. EAST experiments utilized external impurity injection to achieve detachment. While\nin DIIID with graphite wall and divertor, the carbon is dominant impurity source without\nexternal impurity injection. Both machines are trying to increase the radiation dissipation\nto achieve divertor detachment.\nconclusions are sound and that the perceived inconsistencies have been resolved through\nclarification, additional data, and manuscript restructuring. We sincerely thank the referee for\nraising these important points, which have led to a clearer and stronger manuscript.\nComment 4\nIn Figure 3, the authors discussed the RMP-induced transport. From Figures 3a and b,\nI\nunderstood the key to integrate the RMP ELM mitigation and divertor detachment is the\nincreased density of the SOL, not decreased density in the pedestal. In such a sense,\nFigure 3c is not enough because only the onset density of the core and pedestal are shown\nfor the EAST. Since the divertor settings of the DIII-D and EAST are so different, I\nbelieve the authors must discuss the recycling effect in these results. I suspect that the\nbehaviors in the DIII-D and EAST might be different, in other words, the EAST results\nof the separatrix and SOL might deviate significantly from the DIII-D results in Figure\n3c.\nResponse:\nThank you for your thoughtful comments regarding Figure 3 (now Figure 4) and the role of\nRMP-induced transport. We agree that understanding SOL density behavior and edge recycling is\ncritical to interpreting the integration of ELM mitigation and divertor detachment.\nAs you correctly noted, the EAST data in Figure 3c include only core and pedestal onset\ndensities due to the lack of edge profile diagnostics. Given this limitation, and to avoid over-\ninterpretation, we have moved the EAST results to the supplementary material and clarified\nthat they are provided only to support the broader trend of RMP-induced reduction in detachment\nthresholds.\nTo directly address your concern regarding recycling effects, we have added divertor neutral\npressure measurements for the DIII-D discharges in Figure 4 in page 4 of the revised\nmanuscript. These data provide insight into edge recycling behavior and support the interpretation\nthat RMPs facilitate detachment through enhanced particle transport into the SOL.\nAlthough the divertor configurations of EAST and DIII-D differ, both employ closed divertor\ngeometries, which share fundamental characteristics. Our analysis focuses on the common\nobservation that RMPs reduce detachment onset densities and required gas puffing rate, rather than\na detailed comparison of geometry-specific effects, which would require further diagnostics\nbeyond the current scope.\nWe appreciate your feedback, which has helped clarify the scope and strengthen the\npresentation of our results.\nMinor comment: why did the authors show the J_sat in the DIII-D and Te,div in the\nEAST? For the comparison, the same parameter should be discussed. From Figure 7, Te,\ndiv should be shown.\nMinor comment: in Figure 2c and g, the legend should be the ne_bar.\nResponse:\nThank you for your helpful comments and attention to detail. We have revised the manuscript to\nimprove consistency and clarity across figures and parameters:\n• In response to your suggestion, we now show divertor electron temperature (T e,div ) for\nDIII-D in Figure 4, replacing the previously used J in DIII-D. This ensures a consistent\nsat\nand meaningful comparison of divertor conditions across devices, in line with the format\nused in Figure 2.\n• The EAST results have been moved to the supplementary material due to the lack of\nresolved edge profile data, limiting their direct comparability. In the revised main text and\nFigure 4, we focus on DIII-D where full edge profiles are available, including separatrix\nand SOL densities.\n• Additionally, we now present pedestal density instead of core line-averaged density in\nFigure 4 to better represent the edge conditions relevant to detachment onset and to\nmaintain consistency with the local edge measurements.\nWe appreciate your careful review, which has helped improve the clarity and rigor of our\npresentation.\nComment 7\nIn Figure 5, the authors discuss the lobe structure in HFS and conclude that there is no\nlobe structure in LFS. However, if we look at Figure 5 naively, it seems that the\nemissions in LFS reflect the lobe structure. Let's see the following figure.\nIn the region with red circles, the lobe reached the wall from the patterns. What do the\nauthors consider the patterns of the emissions in LHS? The emissions patterns are\ninconsistent with the modeling, but this reflects the lobe.\nResponse:\nThank you for your valuable feedback. We would like to clarify our interpretation:\nOur primary conclusion focused on the high-field side (HFS), where the observed emission\npatterns are consistent with modeled lobe structures. We did not intend to imply that lobe structures\nare entirely absent on the low-field side (LFS). Rather, due to limitations in camera spatial\n8\nresolution and viewing geometry, clear identification of LFS lobes is not feasible in our current\ndiagnostics.\nThe emission features in the red-circled LFS region may suggest lobe-like patterns; however,\nwe interpret these as likely resulting from broadened radiation zones due to the complicated\nLFS wall geometry or projection effects, rather than definitive magnetic lobe structures. The\nmismatch with the modeling further supports that these patterns are not direct evidence of coherent\nLFS lobes.\nTo address this, we have revised the text to clarify that LFS lobe structures cannot be\nconfirmed due to diagnostic constraints in page 8, and that the apparent features may reflect more\ndiffuse or integrated effects.\nWe appreciate your careful observation and have adjusted the discussion to avoid ambiguity\nin our conclusions.\nComment 8\nThe authors discussed the second peak of the heat flux in Figure 6. However, the\nbehaviors of DIII-D and EAST are so different. In DIII-D, the authors pointed out that\nthe second peak disappeared very early before the detachment. And the primary peak\nincreased up to the detachment. On the other hand, in EAST, the second peak\ndisappeared just before the detachment. And the primary peak decreased. I cannot\nconclude that Figures 6b and c are the same. The authors should discuss this in detail.\nResponse:\nThank you for highlighting this important observation. We agree that the heat flux evolution\nin DIII-D and EAST shows different behaviors, and we have revised the manuscript to clarify\nthese differences.\nIn DIII-D, the primary heat flux increases prior to detachment, which we attribute to the\nincreased ion flux, as indicated by the rising J . This reflects the impact of continuous D₂ gas\nsat\npuffing, which raises the SOL density and enhances energy delivery to the divertor. The\nsecondary peak disappears early, due to progressive cooling of outer divertor regions as density\nrises.\nIn contrast, in EAST with pulsed impurity seeding (rather than continuous D₂ puffing), RMPs\ninduces density pump-out, reducing core density below initial levels. This lowers the primary heat\nflux while enabling the secondary peak to persist until detachment (due to delayed SOL cooling).\nThese differences stem from distinct operational approaches—gas puffing in DIII-D versus\npulsed impurity seeding in EAST—and are not contradictory but rather reflective of different\ndetachment dynamics. To avoid overinterpretation, we have moved the EAST results to the\nsupplementary material.\nthe clarity of our discussion.\nComment 9\nRegarding the above comment, in DIII-D, the RMP penetration might be related to the\ndensity or collisionality. The LHD experiment found that the beta (or collisionality)\nsignificantly correlates with the RMP penetration. In DIII-D, the density is ramping up,\nand the collisionality may change according to the density. For the LHD, please see,\nhttps://iopscience.iop.org/article/10.1088/0741-3335/55/1/014014\nResponse:\nThank you for pointing out the potential role of collisionality in RMP penetration and for\nreferencing the valuable LHD study. We agree that parameters such as collisionality and beta—\nwhich are linked to density and temperature—can significantly influence RMP penetration, as\ndemonstrated in LHD [Sakakibara et al., Plasma Phys. Control. Fusion 55, 014014 (2013)].\nIn our DIII-D experiments, the density ramp-up (Fig. 4 now) does lead to increasing pedestal\ncollisionality, which could, in principle, affect RMP field penetration. Based on our previous\nstudies [Q. Hu et al., Nucl. Fusion 60, 076001 (2020); R. Nazikian, Q. Hu et al., Nucl. Fusion 61,\n044001 (2021)], RMP penetration is expected to initiate at low density level near the pedestal\nfoot when RMPs are first applied. The penetration then persists during the subsequent density\nincrease due to the hysteresis behavior and the significantly lower threshold for penetration at\nthe pedestal foot.\nWe have added a discussion in page 7 of the revised manuscript to reference the LHD findings\nand explicitly note the potential influence of density/collisionality on RMP behavior in tokamak\nplasmas. Your comment has helped us better contextualize our results within the broader\nunderstanding of RMP physics.\nComment 10\nI suppose Figure 7 is the highlight of this manuscript. I strongly recommend that the\nauthors add the heating power and the line-averaged density. I am very curious about\nthe discharge in the steady state and how the density pump-out behaves in the even and\nodd parities.\nResponse:\nThank you for recognizing the importance of Figure 7 and for your valuable suggestions. In\nresponse, we have restored the manuscript and revised Figure 7 (now figures 2 and 3) to include\n10\nthe time evolution of heating power, pedestal density, and radiation power, in addition to the\ncore plasma parameters.\nThese additions provide a more comprehensive view of the discharge behavior and clearly\nillustrate the density pump-out effects associated with RMP application in both even and odd\nparities. The evolution of pedestal density highlights the identical impact of RMP parity on\nparticle transport, while the radiation power traces show rise when switch from even to odd parity.\nWe agree that this extended diagnostic context significantly enhances the interpretation of the\nsteady-state phases and the role of RMPs. Thank you again for your insightful feedback, which\nhas helped improve the clarity and depth of the analysis.\nComment 11\nFigure 8c-d are curious. Are those the first time to study the collisionality dependency\nof the plasma response? If not, the authors should refer the papers appropriately.\nResponse:\nThank you for your thoughtful question. While previous studies have explored the impact of\ncollisionality on plasma response—primarily through theoretical modeling and linear MHD\nsimulations—these works do not address the changes in plasma response associated with the\ntransition from attached to detached divertor conditions, and did not show variation in the\nresonance.\nTo our knowledge, this is the first experimental investigation linking plasma response\nvariation with collisionality changes across detachment, as shown in Figures 8a–b. In the\nrevised manuscript in page 8, we have added citations to relevant prior modeling studies for context,\nwhile emphasizing that our results extend beyond those by incorporating direct experimental\nobservations during detachment transitions.\nWe appreciate your comment, which helped us improve the clarity and positioning of our\ncontributions.\nGeneral comments.\nThe reviewer would like to thank the authors for a well written contribution with clearly\npresented information. There are some new additions to our pool of observations\nregarding RMP ELM suppression and connections to SOL transport and detachment.\nHowever, at several points key pieces of information are missing, that are essential to\ninterpret the experimental data and to support the claims that the authors make.\nTo address specifically the points requested by the journal:\n• The quality of the data appears technically sound and appropriately analysed. However,\ninformation is missing. In this sense, it has not been presented in sufficient detail.\n• The potential significance of the results could be high. However, in its present form,\nthis conclusion cannot be drawn.\nKey results: For ITER, having an external “control” knob that would enable us to\nsuppress ELMs while maintaining a stably attached divertor in Q=10 operation with a\nhigh-collisionality pedestal bottom and low-collisionality pedestal top is indeed\nattractive. It is possible, these results contribute to the development of such a tool set,\nbut there is insufficient information to assess if this is indeed the case, or if the\nobservations are a slightly new variation on previously observed phenomena. The\nReviewer suspects that while there is good material here for a Nuclear Fusion paper, the\nresults are not sufficiently novel for a Nature Communication. Detailed comments are\ngiven below.\nResponse:\nWe sincerely thank the reviewer for their thoughtful and constructive feedback. We appreciate\nthe recognition of the technical quality of our data and the potential impact of the work, and we\nfully agree that additional detail and clarity are essential to properly support the claims and\nhighlight the novelty of our findings. In response, we have made substantial revisions to the\nmanuscript to address these points and reinforce the significance of our results.\n1. Addressing Missing Information and Clarifying Methodology\nWe have revised the manuscript to provide the missing contextual and technical information\nthe reviewer requested:\n12\n• Expanded Experimental Detail: We now include additional diagnostics and analysis\nparameters such as time evolution of the separatrix and SOL density, radiation power,\ncalibrated gas puffing rate, required threshold gas puffing rate to trigger detachment versus\nRMP current to clearly demonstrate the evolution of plasma conditions during RMP\napplication and detachment.\n• Simulation Setup Clarified: The SOLPS-ITER modeling section has been expanded to\nintroduce the details in setup.\n• Additional 3D simulations: we add new 3D EMC3-EIRENE simulations to support the\nexperimental observations.\n• Figure Revisions: Based on reviewer comments, EAST results lacking full edge profile\ndiagnostics have been moved to the Supplementary Material to avoid overinterpretation,\nand figures have been updated accordingly for clarity and consistency.\n2. Clarifying Novelty and Contribution to the Field\nWe respectfully emphasize that our study offers several novel experimental insights that go\nbeyond prior work on RMPs:\n• Integrated ELM Mitigation and Detachment Control: This is the first experimental\ndemonstration of simultaneous strong ELM mitigation (or small ELM regime) and\nsustained divertor detachment in a tokamak, resolving a long-standing compatibility issue.\n• Mechanistic Insight into RMP-Enhanced Detachment: We provide evidence that RMP-\ninduced changes in edge/SOL transport reduce the density and gas puffing rate threshold\nfor detachment. This causal relationship has not been previously established in tokamak\nexperiments.\n• Suppression of Secondary Heat Flux Peaks: Contrary to prior experimental observations\nand ITER projections, we show that divertor lobe-related secondary heat flux peaks can be\nsuppressed as detachment progresses—an important result for divertor longevity.\n• Experimental Characterization of Plasma Response vs. Collisionality across\nDetachment: We show, for the first time, how plasma response changes from attached to\ndetached conditions in relation to collisionality—advancing beyond prior modeling-only\nstudies.\nThese findings significantly extend the current understanding of RMP applications and their\ninterplay with boundary plasma conditions.\n3. Relevance to ITER and Reactor-Scale Operation\nOur results offer practical guidance for RMP-based control strategies in ITER and future fusion\ndevices:\n• Reduced Detachment Onset Density: RMPs lower the detachment threshold by 20–30%\nwith less gas injection, allowing detachment to occur at lower pedestal collisionality (ν *\ne\n13\n< 0.1) and without sacrificing core performance (H > 1), which is critical for high-Q\n98\nscenarios.\n• Optimized 3D Magnetic Configuration: Our observation that odd-parity RMPs leads to\nsmall ELMs and stable detachment suggests broader operational flexibility for ITER’s\nRMP coils.\n• Improved Divertor Conditions: The suppression of secondary heat flux peaks and\nenhanced impurity exhaust (especially in EAST metal wall) directly address major divertor\nheat load challenges anticipated in reactor scenarios.\nThese points are now explicitly discussed in the revised Outlook and Abstract.\n4. Suitability for Nature Communications\nWe respectfully believe that the integration of ELM mitigation and divertor detachment via\nRMPs represents a fundamental advancement in boundary plasma control, with implications that\nextend well beyond individual machines:\n• Cross-Device Consistency: Consistent trends across two major tokamaks (DIII-D and\nEAST) strengthen confidence in the generality of our findings.\n• Cross-Disciplinary Relevance: The work intersects plasma physics, 3D magnetic\ntopology, transport theory, and reactor design, making it relevant to both physicists and\nengineers.\n• Transformational Potential: The results provide a framework for resolving one of the\nkey operational trade-offs in magnetic fusion, helping to bridge the gap between current\nexperiments and reactor-scale implementation.\n5. Summary of Revisions\nTo address the reviewer’s concerns and strengthen the manuscript, we have:\n• Included new time traces of heating power, pedestal density, and radiation to support\ninterpretations.\n• Expanded methodological details and added error analysis where appropriate.\n• Reorganized the structure of the draft and figures for clarity, moving EAST results with\nincomplete edge profiles to the Supplementary Material.\n• Added discussion comparing our work to prior modeling studies, clearly identifying novel\ncontributions.\n• Highlighted the implications for ITER and future reactors in both the Abstract and Outlook.\nWe hope these revisions address the reviewer’s concerns and more clearly demonstrate the\nsignificance and novelty of our work. We sincerely appreciate the reviewer's critical insights,\nwhich have greatly improved the manuscript.\nComment 1\nIt would be worthwhile to note here that the Greenwald fraction you are referring to (I\nassume) is an edge Greenwald fraction – pedestal top? When referring to Greenwald\nfraction for which ELM ELM-suppression / mitigation is observed to be effectively,\na good reference would be Fig 4c of Paz-Soldan 2024 (submitted) chrome-\nextension://efaidnbmnnnibpcajpcglclefindmkaj/https://arxiv.org/pdf/2403.03693.pdf\nResponse:\nThank you for this helpful suggestion. In our manuscript, the Greenwald fraction f indeed\nG\nrefers to the pedestal-top electron density normalized to the Greenwald limit. To clarify this point,\nwe have now explicitly defined f as the edge Greenwald fraction on Page 2 to avoid any\nG\nambiguity.\nWe also appreciate the reference to Paz-Soldan et al. (2024, arXiv:2403.03693), which\nprovides valuable insight into the role of pedestal-top Greenwald fraction in ELM suppression.\nWe have cited this work in the revised Discussion section to better contextualize our results within\nthe broader framework of ELM control studies.\nComment 2\nITER is expected to operate at relatively high SOL density, or density at the LCFS.\nRegarding the presence of heat flux lobes in the presence of RMPs and the inability to\nmaintain detachment and RMP-suppression, one can also consider including the\nASDEX Upgrade results:\nD. Brida NF 57 116006 2017\nA. Kallenbach NF 61 016002 2021\nThe former is particularly interesting for the authors with respect to Claim 2, as it\ndemonstrated the removal of the RMP-induced lobe-heat fluxes in detached conditions\nin both L- and H-mode plasmas.\nResponse:\nWe thank the reviewer for the helpful suggestion to contextualize our findings with the\nASDEX Upgrade results. As noted, Brida et al. (2017, Nucl. Fusion 57, 116006) reported the\nsuppression of RMP-induced lobe-related heat fluxes during detachment in both L-mode and H-\nmode plasmas. This finding aligns well with our observation of the disappearance of the second\nheat flux peak under detached conditions (Fig. 6). However, as also noted in that study, no clear\n15\nlobe structures are visible for the strongly detached case. Here, in our experiment, the 2nd peak\nheat flux disappears even before partial detachment.\nThe follow-up study by Kallenbach et al. (2021, Nucl. Fusion 61, 016002) highlights the\nongoing difficulty of simultaneously achieving ELM control and divertor detachment with RMPs,\nfurther underscoring the importance of identifying operational regimes where both can coexist.\nIn our revised manuscript, we now cite both references and incorporate a discussion of how\nour results—achieving partial detachment and ELM suppression through impurity seeding and\noptimized 3D configurations—offer new pathways to resolving this long-standing challenge. We\nthank the reviewer again for pointing out these important studies and for helping us better situate\nour findings within the broader context of RMP physics.\nComment 3\nOn page 2 the manuscript states, “We report that, for the first time, divertor detachment\nfacilitation by RMP is observed in H-mode plasmas in DIII-D and reproduced in EAST.”\nWith this statement, I expect to see in the subsequent pages, a clear experimental\ndemonstration that applying RMPs makes it easier to detach the divertor. And based on\nthe connected statement in the abstract, “The detachment facilitation correlates with the\ndensity pump-out magnitude, consistent with SOL-PS simulations.” I expect to be\nshown that with increasing RMP magnitude (higher and higher SOL density due to\nincreasing pump-out) it gets easier and easier to detach the divertor. Unfortunately, this\nis not shown.\nWe detach tokamak divertors by applying either D or impurity seeding to increase the\nradiation and parallel power loss in the SOL, removing the incident power before it\nreaches the target surfaces. Hence, if applying RMPs ‘facilitates’ detachment, I expect\nto see a figure showing that less D or impurity needed to be puffed to reach a detached\ncondition, compared to the case without RMPs. I expect to see that as I increase the RMP\ncurrent, and increase the density pump out, increasing the SOL density, and potentially\nincreasing cross-field transport in this region (hence reducing parallel transport), that\nless and less external D or impurity is needed. This is not shown. The only figure that\ncontains some information in this direction is Fig. 2, where it would appear that an\nextremely similar level of D seeding is needed with and without the RMPs to detach the\ndivertor.\nRather, the meaning of “facilitation” used here appears to be a different one. It is used\nin reference to the value of the pedestal (or core) density at the time of detachment.\nthe authors. Note that high Greenwald fraction in the plasma core is, in fact, desirable in\na reactor, making pump-out not necessarily a good thing. The use of RMPs to ‘pump-\nout’ the core (a well known result), means that the detachment obtained here is obtained\nat lower pedestal Greenwald fraction, hence leading to the statement “RMPs reduce\nthe detachment onset density.” i.e. RMPs break the standard correlation – between SOL and\npedestal top density. This seems to be the meaning of “facilitation” here. This is factually\ncorrect, but misleading, and does not reflect the actual physics at play – or necessarily\nmean it was ‘easier’ to induce detachment.\nResponse:\nWe thank the reviewer for this insightful and constructive comment, which prompted us to\nclarify the meaning of \"detachment facilitation\" and provide additional evidence supporting our\nconclusions.\nIn the original manuscript, “facilitation” referred specifically to the reduction in the upstream\n(pedestal-top) density required to achieve divertor detachment when RMPs are applied, as\ndemonstrated in Figs. 2 and 3. This occurs because RMP-induced pump-out enhances cross-field\ntransport, raising the SOL density and radiative losses while lowering the core and pedestal density,\nthus reducing the upstream threshold for detachment.\nIn response to the reviewer’s request for a more direct demonstration of how RMPs reduce the\nneed for external fueling (D₂ or impurities), we have now included additional experimental data.\nIn particular, we have added new shot in the new Figure 4 with calibrated gas puffing rate,\nwhich shows a clear reduction (~11%) in the D₂ puffing rate required to trigger detachment in discharges\nwith RMPs (4 kA, n=3) compared to those without. This directly confirms that RMPs lower the\nexternal fueling needed for detachment, consistent with the facilitation concept in a broader\noperational sense.\nIt is important to note that recycling neutrals provide a significantly larger source of particle\nfueling in DIII-D than gas puffing [Leonard, JNM 2009]. Therefore, when considering detachment\nrequirements, both gas puffing and recycling must be taken into account. Meanwhile, the upstream\ndensity at detachment onset, rather than the gas puffing rate alone, serves as a more reliable\nindicator for initiating and assessing detachment.\nWe have revised the manuscript accordingly:\n• Page 2 (Introduction): Clarified that “facilitation” includes both (a) reduced external\nfueling and (b) reduced upstream/pedestal-top density thresholds needs.\n• Page 5 (Results): Added reference to Figure 4 and emphasized the experimental reduction\nin D₂ puffing rate with RMPs and added a subplot (Fig. 4j) to show the required gas puffing\nrate for detachment onset versus the RMP current.\nfractions in reactor cores. Indeed, maintaining high core density is essential for fusion performance.\nHowever, our results show that RMPs enable detachment at lower pedestal densities while still\nachieving good confinement (e.g., H₉₈ ≥ 1.1 in DIII-D), thereby decoupling core and SOL density\nrequirements—an outcome that directly supports ITER’s operational goals. In addition, as\ndiscussed in the outlook, our previous analysis for ITER showed that the magnitude of pump-out\nwill be weaker than recent device either by optimizing the much flexible RMP configuration or\nhigher n (i.e. n=4) RMP.\nWe appreciate the reviewer’s request for clarity, and we believe the additional data and\nrevisions significantly improve the manuscript by refining both the physical interpretation and its\nrelevance to reactor scenarios.\nComment 4\nOn page 3, the authors note that even parity is the primary setting. Later in this paper,\nthe authors show strong ELM-mitigation combined with a detached divertor obtained\nwith odd parity. None of the profile changes in that case are actually shown, and it is not\nat all clear that the entire discussion of pages 3-5 is relevant for the actual “success case”\nshown on page 6.\nResponse:\nWe thank the reviewer for this important observation regarding the parity-specific effects of\nRMPs and the relevance of earlier discussion to the integrated “success case” shown in Fig. 7.\nWhile pages 3–5 primarily focus on even-parity RMPs—used to investigate detachment\nfacilitation—the case shown in Fig. 7 employs odd parity to achieve simultaneous ELM mitigation\nand divertor detachment. We agree that this distinction merits clarification, and we have now made\nthe connection between the two cases more explicit in the revised manuscript.\nKey clarifications and additions include:\n• Restructured the manuscript with more details: We now started with introduction of\nthe results in Figure 7 (now Figure 2), and showed the detailed evolution of the pedestal\ndensity, which clearly shows the same level of density pump-out, consistent with our\nunderstanding that the very edge kink response from even and odd parity produces similar\nmagnetic island at the pedestal-foot and pump-out. We now showed the profiles and\npedestal stability analysis in the supplementary for that discharge under even and odd parity.\nWe have also revised the associated text to clarify how the underlying detachment\nmechanism remains valid in the success case and how parity optimization builds on it to\nachieve integrated control.\n18\n• Parity-dependent response: For the first time, we observe that the RMP-driven plasma\nresponse differs significantly between attached and detached conditions. Specifically,\nunder detached conditions with shifted pedestal profiles, odd parity exhibits stronger\nresonant response at the pedestal-top, enhancing ELM mitigation in high-collisionality\nregimes (as shown in Fig. 8b–e). This highlights the importance of optimizing the 3D\nmagnetic configuration based on plasma conditions. As demonstrated in our earlier work\n(Q. Hu et al., Nucl. Fusion 60, 076001, 2020), RMP-induced magnetic islands at the\npedestal-top are primarily responsible for ELM suppression, while islands at the pedestal-\nfoot drive density pump-out. When switching from even to odd parity, the pedestal-foot\nresonant components—and thus the detachment-relevant physics discussed on pages 3–\n5—remain unchanged. Odd parity, however, offers enhanced pedestal-top response,\nimproving ELM control while retaining the detachment benefits described earlier.\nWe appreciate the reviewer’s comment, which helped us improve the coherence and narrative\nflow of the manuscript by more clearly articulating the connection between parity, pedestal\nresponse, and integrated performance.\nComment 5\nOn page 3-5, even parity discharges are presented. In figure 2: The units of the D ramp-\nrate are missing. The reader has no way of knowing if this is large gas-puff or not. Please\ninclude units and information on valve location/calibration. In the EAST case, is it\ncorrect that there is ZERO D gas puff – the only fueling in this time window is two neon\npuffs (how big are these? In elec/s?) Minutia: in the top boxes the DIII-D label reads “no RMP” while the EAST label reads “ W/O RMP.” The authors clearly went through some effort to unify the presentation of the data between the two machines (appreciated!) – they can\nthink to unify labels if they wish. And an edge density/Greenwald fraction would be more informative than a core.\nResponse:\nWe thank the referee for their careful review and helpful suggestions. To address the concerns\nraised, the following revisions have been made:\n• D₂ injection units and calibration: In Figures 2 and 4, we now explicitly state the units\nof the D₂ gas injection rate for the DIII-D discharges. Additional details regarding the valve\nlocation and calibration have been added to the figure caption and relevant text to provide\nthe reader with a clear understanding of the fueling magnitude.\n• EAST fueling clarification and relocation: We confirm that D₂ puffing was applied with\nfour times of the Neon puffing rate. The Neon injection rate (in electrons/second) is now\nspecified. However, due to the lack of available pedestal profile data in EAST, we have\n19\nmoved this result to the Supplementary Materials, where it still serves to support the\nbroader cross-device context while ensuring the main text remains focused on cases with\ncomplete diagnostic coverage.\n• Label and density normalization consistency: We have unified the labeling across all\nfigures to use “W/O RMP” for consistency. Additionally, we now present pedestal-top\ndensity instead of core density, as this provides a more relevant metric for evaluating\ndetachment physics and better aligns with standard practice.\nThese changes enhance the clarity, consistency, and interpretability of the presented data.\nWe appreciate the referee’s thoughtful comments, which helped improve the rigor and presentation of\nthe manuscript.\nComment 6\nIn the text – f_g – it would be useful if earlier when introducing f_G, the authors also\nspecified that all uses of this term will be referring to pedestal top densities (assuming\nthis is the case). It is worth being very clear, that the main physics message of this first\nsection is not ELM-suppression + detachment. It is, in fact, quite high density, high\ncollisionality. The point is supposed to be that adding RMPs to such a scenario increases\nthe SOL density, and hence makes it easier to detach these particular plasmas.\n(remains to be shown) But no attempt is made, at this point, to combine detachment with RMP\nELM suppression. This distinction should be very clear, because from the abstract and\nthe introduction, this is what the reader is expecting to find.\nResponse:\nWe thank the referee for their insightful comments and the opportunity to clarify the key\nmessages in the manuscript. In response, we have made the following revisions to improve\nclarity and structure:\n1. Clarification of f definition:\nG\nTo avoid ambiguity, we now explicitly define the Greenwald fraction in the Introduction in\npage 2 as follows:“In this paper, f denotes the Greenwald fraction, defined as the ratio of\nG\nthe pedestal-top electron density (n ) to the Greenwald density limit (n ).” This ensures\ne,ped G\nthat all subsequent uses of f are clearly and consistently understood to refer to pedestal-top\nG\nvalues.\n2. Restructuring for narrative clarity:\nTo address the concern about potential confusion between detachment facilitation and ELM\nsuppression with other sections, we have restructured the Results section. Specifically, we have\n20\nmoved the section titled “Integration of RMP ELM control with divertor detachment” to the\nbeginning of the experimental results. This change connects the small ELMs and detachment\nwith the features enabling this integration.\n3. Added transition paragraph:\nTo further guide the reader, we have added a paragraph at the end of the integration section\nthat explicitly states how the integration section connects to subsequent sections (detachment,\nsimulation understanding, heat flux mitigation and plasma response change). This ensures a\nsmoother narrative and aligns with the expectations set in the abstract and introduction.\nWe appreciate the referee’s close reading and helpful suggestions, which have significantly\nimproved the clarity and structure of the manuscript.\nComment 7\nComing to figure 3. These are very nice edge profiles, that clearly demonstrate the drop\nin the core densities and the increasing the SOL densities with increasing RMP current.\nIn some sense, this is a common-sense observation. The density goes down in the core,\nthose particles have to go somewhere. And it was, in principle, already shown in Petrie\nNF 2011, see Fig. 9. But I don’t believe I’ve ever seen it shown this clearly and for\ndifferent levels of RMP current. However, a missing piece of information that must be\nprovided is the fueling rate (and location) in each case.\nResponse:\nWe sincerely thank the referee for their thoughtful evaluation and for drawing a helpful\ncomparison to Petrie et al. (Nucl. Fusion 2011). We are pleased that the referee found the edge\nprofiles in Fig. 3 (now Fig. 4) compelling. In response to the comments, we have made the\nfollowing clarifications and additions:\nWhile increased SOL density under RMPs was previously observed by Petrie et al., our results\naddress two key limitations of that earlier work:\n• Monotonic SOL density trends: In Petrie’s Fig. 9, the SOL density does not increase\nconsistently with fueling rate (e.g., the SOL density at 10 Pa·m³/s is lower than at 6.3\nPa·m³/s), which contradicts expectations for enhanced cross-field transport. In contrast, our\nresults (now further supported by new data in Supplementary Fig. 2) demonstrate a clear\nand monotonic rise in SOL density with increasing D₂ puffing rate at constant RMP current.\nThis trend was reproducibly observed across multiple discharges and RMP amplitudes.\n• Separatrix alignment and T e,sep consistency: The profiles in Petrie et al. lacked precise\nseparatrix alignment, resulting in unphysical variation in T (ranging from 30 to 100 eV)\ne,sep\ndespite similar heating power and plasma current. In contrast, our profiles in Fig. 3 (now\nFig. 4) are anchored to kinetic equilibrium reconstructions constrained by power balance,\n21\nenabling accurate localization of the separatrix (ψ =1) and consistent T values (~90 eV),\nN e,sep\nvalidated by high-resolution Thomson scattering.\nImportantly, as the referee noted, Fig. 3a and 3b clearly show a progressive drop in core density\nand a rise in SOL density with increasing RMP current. While this trend may appear intuitive,\nthis is, to our knowledge, the first time it has been unambiguously demonstrated across a well-\ncontrolled discharge series with sufficient spatial resolution and repeatability to resolve the profile\nevolution at detachment onset.\nRegarding the missing fueling information: we clarify that all discharges in Fig. 3 (now Fig. 4)\nused identical D₂ gas puffing waveforms, calibrated via in-vessel pressure gauges. We have now\nadded the time evolution puffing rate in Fig. 4a, and at the time of detachment onset directly in\nFig. 4j and clarified this detail in the caption.\nWe thank the referee for emphasizing this point, which has helped us enhance the clarity and\nrigor of the manuscript.\nComment 8\nComing to figure 4. The authors highlight this figure as the ‘key physics result’ of this\nsection, and I find this troubling. The label for this plot is “RMP induced transport\ncorrelates with detachment facilitation.” The y-axis here the density (at various\nlocations in the plasma) at which detachment occurs, while the x-axis is the change in the\npedestal top density from the RMP application, which one could interpret (from an\nengineering parameter stand-point) as the applied perturbation field. With respect to\ndetachment, the density on the y-axis that matters most is the SOL-density, which\ncontributes to the power exhaust, either by helping exhaust power along field lines\n(particularly if neutral pressure in the divertor is correlated with the increased density in\nthe SOL (is it?)), or it represents increased cross-field transport. So, focusing on the\nSOL density as the parameter here closest to the relevant physics, we actually see an\nINCRESASE in the SOL density needed to detach with increasing RMP-current -which\nwould be the opposite of facilitation. But again, we are missing information on the\napplied D gas puff and divertor conditions, such that a proper interpretation of this plot\nwith the available information is not possible. But, noting that the core or pedestal top\ndensity at detachment onset goes down with increasing RMP current – is hardly grounds\nfor claiming RMPs facilitate detachment.\nResponse:\nsubstantial clarification and strengthening of our presentation and interpretation. We address the\nconcerns as follows:\n1. Clarification of Figure Reference and Revision\nThe referee’s comment appears to refer to the previous Figure 3 (now updated and relabeled\nin the revised manuscript in Fig. 4), which we originally highlighted as a key physics result.\nWe acknowledge that the previous framing could lead to misinterpretation.\n2. Revised Presentation in Figure 3 (now Fig. 4)\n• The x-axis in the revised figure has been changed from the density pump-out magnitude to\nthe RMP coil current, which more directly reflects the strength of the applied 3D magnetic\nperturbation.\n• The y-axis shows the onset density for detachment at various locations (core, pedestal\ntop, and SOL).\n• The figure now explicitly shows that increasing RMP current leads to lower core and\npedestal-top densities at detachment onset and saturates after 4 kA, highlighting a\ntrend consistent with enhanced edge transport driven by RMPs.\n3. Interpretation of SOL Density Trends\nWe appreciate the referee’s point that the SOL density is a more relevant parameter for\nassessing detachment physics. While Fig. 3c shows that SOL density at detachment onset\nincreases slightly with RMP strength, this does not imply detachment is harder to achieve.\nInstead, the increased SOL density reflects enhanced cross-field transport, which raises the\nupstream density while enabling greater particle and heat spreading to the divertor. The key\nfacilitation mechanism here is not a reduction in required SOL density, but rather increased\nradial transport that elevates SOL density and reduced neutral pressure, thereby\npromoting earlier power dissipation and detachment.\n4. Addition of 3D Edge Plasma Modeling (New Figure 6)\nTo directly address the referee’s request for physical interpretation, we have now included\nEMC3-EIRENE simulations (Fig. 6) that model the effect of 3D RMP fields on the SOL and\ndivertor. These results clearly show:\n• RMP-induced edge stochasticity leads to enhanced radial transport into the SOL.\n• This, in turn, raises local SOL density, supporting more favorable conditions for\ndetachment.\n• The simulations provide strong evidence that the experimentally observed changes in\ndetachment onset are consistent with 3D transport enhancement mechanisms.\nWe have also updated the manuscript to better distinguish between correlation and causation\nwhen referring to detachment “facilitation,” clarifying that our claim is based on reduced gas\n23\npuffing rate, lower pedestal detachment thresholds and enhanced edge transport, supported\nby both measurement and partially by modeling.\nWe are grateful to the referee for encouraging a more rigorous and nuanced analysis, and we\ntrust the revised manuscript now presents a clearer and more robust interpretation of the\nexperimental findings.\nComment 9\nIf at constant D gas puff, detachment were initiated by increasing the RMP current, that\nwould be clear. What needs to be shown is that the increased SOL density (due to the\nRMPs) actually makes it easier to achieve detachment – i.e. at lower D or impurity has\npuff. This should be doable.\nResponse:\nWe thank the referee for this important and insightful comment. As suggested, demonstrating\nthat RMP-induced increases in SOL density leads to detachment at lower D₂ or impurity fueling\nrates would significantly strengthen our conclusions.\nTo address this, we have added a new discharge to the revised manuscript that shows\ndetachment is achieved at a lower D₂ gas puffing rate when RMPs are applied, compared to the\nno-RMP case. This directly supports the referee’s point and reinforces the role of RMP-enhanced\ncross-field transport in reducing the detachment threshold.\nAlthough we had proposed a dedicated experiment to perform a full RMP current scan at\nconstant D₂ fueling, this was unfortunately not feasible due to limited run time and ongoing\nupgrades to the upper divertor hardware. Nevertheless, the newly added example and cross-\nmachine comparison with EAST provide empirical evidence that the application of RMPs can\nlower the fueling requirements for detachment, consistent with the underlying physics discussed.\nWe acknowledge that this does not yet constitute a complete parametric study, but the available\nresults across two devices suggest a robust and reproducible mechanism. We have clarified this in\nthe manuscript and look forward to future experiments to expand on this observation.\nComment 10\nIn principle, I like the addition of the SOL-PS simulations, showing that (assuming\npurely diffusive transport?) there is an increase in cross-field transport in the presence\nof RMPs needed to match the profiles. What constraints are used / matched with respect\nto edge neutral pressures / densities and their distribution? Why is there an “additional\npower” of 0.5 MW including in the modeling in the RMP case? The applied powers are\n24\nthe same, and I would assume that the impurity density / lower electron density in the\nRMP case would lead to lower radiated power, not higher?\nThe SOLPS simulations (with constant transport coefficients?) show that with RMPs the\ndensity at the LCFS at which detachment occurs is lower with RMPs, presumably\nbecause of enhanced cross-field transport, which is speculated by not shown. This is\nconsistent with Fig 3c, explaining why the authors chose to show that plot. But at the\nsame time, if the D gas puffing required to achieve detachment was always the same –\nthen this relationship to the ne_SEP is coincidental. I strongly recommend that the\nauthors consider how best to emphasize the local changes in transport in the SOL and\ndivertor, due to the application of RMP – rather than highlighting a correlation with\ndensity values in the confined region. One could instead consider to cite the increased\nSOL density and cross-field transport induced by RMP as facilitating detachment – if\none can show that detachment really did get easier.\nResponse:\nWe thank the referee for their thoughtful feedback and valuable suggestions regarding the\ninterpretation and presentation of the SOLPS modeling results. We have revised the manuscript to\naddress the raised concerns and clarify the physical insights. The following points summarize our\nrevisions and responses:\n1. Simulation Setup Clarified (Page 6): We now provide a more detailed description of the\nSOLPS simulation setup in the revised manuscript (Page 6), including boundary conditions,\nassumptions, and constraints. Specifically, the modeling is constrained by experimentally\nmeasured upstream density and temperature profiles, as well as divertor neutral pressure\ndata from fast ion gauges. These constraints ensure that the simulation accurately captures\nedge conditions and neutral dynamics.\n2. Interpretation of “Additional Power” in RMP Case: The inclusion of 0.5 MW\nadditional power in the RMP simulation case is based on the experimentally observed\nreduction in stored energy upon application of RMPs. This drop in stored energy implies\nincreased power exhaust into the SOL, which is consistent with the observed rise in inter-\nELM divertor heat flux. Rather than representing a discrepancy in input heating power,\nthis additional term reflects the increased transport of energy across the LCFS under RMPs,\nwhich is required to match the heat flux and profile data in the modeling.\n3. Radiation Power Evolution Now Shown (Figs. 2 and 4): To clarify the role of radiative\nlosses, we have added the time evolution of the core radiation power to Figures 2 and 4.\nThese additions show that the radiation power is increased by applied RMP compared to\nno-RMP cases.\n25\n4. Transport Coefficients in SOLPS Simulations: Contrary to the assumption in the\nreferee’s comment, the SOLPS simulations do not use constant transport coefficients.\nInstead, we use spatially resolved, experimentally derived transport coefficients, which\nbetter reflect the changes in radial transport due to RMPs. These coefficients are adjusted\nto match upstream and divertor diagnostic measurements and reproduce the observed SOL\ndensity profiles with RMPs.\n5. Strengthened Claim for RMP-Enhanced Detachment: The revised manuscript includes\nnew experimental discharges and analysis that directly show lower D₂ puffing rates\nrequired to achieve detachment when RMPs are applied, compared to the no-RMP\ncases. This, along with the observed increase in SOL density and the results from both\nSOLPS and EMC3-EIRENE simulations (now included in Figure 7), provides robust\nevidence that RMP-induced cross-field transport facilitates detachment. We have\nrevised context accordingly to shift emphasis from correlations with core or pedestal-top\ndensity to the more relevant local SOL and divertor conditions impacted by RMPs.\nWe thank the referee once again for these insightful points, which have led to important\nclarifications and improvements throughout the revised manuscript.\nComment 11\nOn page 5, the authors write, “During detachment, the peak impurity emission shifts\nfrom the outer divertor target to the x-point. … ” When I stare at Figure 5, in the top\nright plot, I can clearly see the hot spot along the outer leg – it is small, took me\na bit, but I found it. But to me, it looks like the x-point hot spot is equally bright already in the\nattached conditions. In the detached conditions, I don’t see a shift, so much as the\nabsence of the previous peak? About the lobes, when I look at the left plots, it looks to\nme like there are 1-2 pretty clear lobes on the outer leg side. And the red line in the top\nright box, looks like there are lobes there as well. But the text says, “no evident lobes\nare observed around the outer diverter leg” - what is the definition of significant? I can’t\nreconcile / quantify those pictures.\nPage 5, please cite D. Brida as appropriate.\nResponse:\nWe thank the referee for their careful assessment and insightful comments, which prompted\nimportant clarifications in both the text and interpretation of Figure 5.\n1. Clarification on Emission “Shift” Statement: We acknowledge the referee’s observation\nthat the X-point impurity emission is already present during attached conditions, and that the\ndominant change during detachment is the disappearance of the outer target peak, rather than a\n26\nnew emergence at the X-point. To more accurately reflect the data, we have revised the text as\nfollows:\n“During detachment, the peak impurity emission at the outer divertor target diminishes, while\nthe X-point emission remains prominent. This behavior reflects target cooling and a redistribution\nof radiative losses, with a relatively enhanced role near the X-point.”\n2. Clarification of Lobe Observations: We appreciate the referee’s close reading of the\nimages. The revised manuscript now clarifies the distinction between modeled and observed lobe\nstructures. While the MHD model (left-hand plots in Figure 7 now) predicts lobe-like structures\non both high-field and low-field sides, the experimental CIII emission (right-hand plots) shows no\nstatistically significant lobes on the outer divertor leg. In the revised text, we now state:\n“While the MHD model predicts lobe structures on both the HFS and LFS, experimental C3+\nemission shows pronounced lobes on the HFS only. On the outer divertor leg, lobes are not\nclearly visible due to the limited viewing angle of the TTV camera.”\n3. Citation Added: We now appropriately cite the work of D. Brida et al. on lobes dynamics\non page 7.\nWe are grateful to the referee for prompting these clarifications, which have improved the\nprecision and clarity of our presentation.\nComment 12\nPage 6: In Fig. 7., for the first case of open circular symbols, could please add “No RMP”\nto the legend.\nResponse:\nWe thank the referee for the careful review and helpful suggestion. In the revised manuscript,\nwe have updated the legend of in the original Figure 7 (now Figure 3) to explicitly label the open\ncircular symbols as “No RMP” for improved clarity and consistency.\nComment 13\nWith odd parity, we see a large increase in ELM frequency, and a corresponding drop in\nchange in stored energy, and a significantly reduction of the heat flux (I assume this is\nintegrated over the wetted area – and not the peak?). But it is unclear if the entire\nprevious discussion regarding changes in plasma profiles, increased SOL density and\nSOL transport, still apply in the odd-parity case? Also, if these experiments are taking\nplace in a low-density, low collisionality pedestal top parameter range? There are no\nclarifications on experimental conditions. If I were to interpret the “ne” listed quietly in\nFig 8 a.as pedestal top values, then the detached case, is at quite high pedestal top\n27\ndensity. Which makes this whole thing look a lot like type II or type III ELMs – as seen\nin the early days of ELM suppression experiments at higher collisionality (e.g.\nSuttrop NF 2011, Rathgeber NF 2014). Such a scenario would not be hard to detach - previous\nattempts at ELM mitigation + detachment failed, specifically because they were working\nin the low collisionality branch. The detachment attempts by Shin, for example, were in\nthat low collisionality regime when they lost the ELM suppression, as were the experiments\npresented in Kallenbach 2021. Detachment with a variety of small ELM scenarios, on the\nother hand, has already been demonstrated.\nSo, what does the pedestal do, in these detachment cases? Does any of the previous\n“Claim 1” discussion still apply? Is this still a low-collisionality scenario? Or, is this an\nMHD pedestal stability effect from changes in the plasma kinetic profiles,\nopposed to a SOL transport effect? Right hand side of Fig. 7 – what are the D and N fueling rates? What is the edge density\nand fG? Combining a low collisionliaty pedestal with RMP ELM-suppression/mitigation with a high collisionality pedestal\nbottom that is detachment compatible, would indeed be interesting, but this information is lacking. At the moment, this looks\na lot like a high collisionality, small-ELM regime.\nResponse:\nWe sincerely thank the referee for this thoughtful and detailed critique. We have addressed\neach point below and made corresponding revisions to the manuscript to improve clarity and\ncompleteness.\n1. Applicability of Previous Discussion to Odd-Parity Cases: The effects discussed\npreviously—namely, increased SOL density and enhanced cross-field transport—remain valid in\nthe odd-parity RMP case. As shown in the updated Figure 2 (original Figure 7) and stated in the\nprevious response, the density pump-out magnitude during odd-parity RMP is comparable to that\nin even-parity cases. This suggests that similar SOL transport mechanisms are active in both cases.\nThis parity-insensitive behavior arises because the plasma response near the pedestal foot produces\ncomparable resonant components for both parities, despite the differences in resonant amplitude\nat the pedestal top.\n2. Clarification of Experimental Conditions and Pedestal Collisionality: We have now\nincluded detailed experimental parameters in the revised manuscript:\n• The pedestal top density during detachment with odd-parity RMPs is approximately 4 ×\n10¹⁹ m⁻³, lower than in the D₂ puffing-only discharges as shown in Fig. 2.\n• The pedestal collisionality (ν e *) in these cases remains below 1.5 (Figure 2d), which are\nmoderate compared to usual high collisionality (above 5) during detachment collisionality\nregimes. In contrast, the D₂ puffing-only cases exhibit higher pedestal collisionality (ν *>\ne\n1.5). These clarifications are now stated explicitly in the manuscript.\n28\n3. Distinction from Type-II/III ELMs: We appreciate the referee’s comparison to\npreviously observed high-collisionality small-ELM regimes and have revised the text to\ndifferentiate our scenario:\n• Our discharges operate with heating power >4× the L-H threshold, which is well above\nthe typical range for Type-III ELMs.\n• The pedestal collisionality remains ν e *< 1.5, far below the ν e * ~5-10 regime typical of Type-\nII ELMs.\n• The transition from mitigated to small ELMs is directly linked to RMP parity change,\nwith no adjustments in power or fueling, indicating a magnetic topology-driven mechanism\nrather than one reliant on collisionality changes.\n• The pedestal profiles and ELITE calculations in the supplementary (Figure 1) show\nidentical characteristics for even and odd parity RMP.\n4. Fueling and Edge Conditions: In response to the request for additional fueling and edge\nparameters:\n• The time evolution of D₂ and N₂ fueling rates, has been added to the revised figures.\n• The time evolution of the pedestal density (n ₑ,ped, and also separatrix density) is added,\nwhich is approximately 30% lower than in D₂-only cases—again confirming the low-\ncollisionality regime.\n5. Pedestal Behavior and Role of SOL Transport: We have conducted ELITE calculations\nfrom low to high density during gas puffing, which shows that the pedestal remains near the\nballooning boundary of peeling-ballooning mode throughout the discharge, suggesting MHD\npedestal stability does not deteriorate during detachment. Instead, the observed detachment\nappears to be driven by RMP-enhanced cross-field and SOL transport, consistent with the\nbroader conclusions of the paper.\nWhile we agree that detachment in high-collisionality, small-ELM regimes has been\ndemonstrated previously, our results reveal a novel operating point: successful detachment with\nRMP-mitigated small ELMs in a moderate collisionality regime. This overcomes long-standing\nchallenges in achieving simultaneous ELM mitigation and divertor detachment at reactor-relevant\nparameters. These findings are now more clearly articulated in the revised manuscript, with all\nrelevant conditions and analysis provided to support our claims.\nComment 14\nComing to page 7, am I correct that discharge 193108 is a ‘new’ discharge, in the sense\nthat it has not yet been presented to the reader? I presume there is a collisionality scan,\nmaybe induced by changing power? Or by changing fueling? And this is done\nspecifically to give an experiment to the modeling to explore the impact of changing the\n29\ncollisionality on the HFS response? A few additional sentences here would help the\nreader follow this section.\nResponse:\nWe thank the referee for the thoughtful observation and for highlighting the need for greater\nclarity in this section.\nTo clarify: Discharge 193108 was previously shown in Figure 3 (now Figure 4) for\ncomparison of edge profiles under different RMP currents and is discussed alongside discharge\n193022, with which it shares a similar setup. The key distinction is that discharge 193108 was\nconducted with a higher RMP coil current (5 kA), allowing us to isolate the effects of increased\nRMP amplitude on detachment onset and SOL transport.\nWhile this discharge was not originally emphasized, we now clearly state its purpose and role\nin the RMP scan. This scan, conducted under fixed D₂ puffing conditions, helps disentangle the\ninfluence of RMP-induced transport changes from those driven by fueling or input power. It\nprovides empirical context for the modeling work by illustrating how increased RMP strength\ncorrelates with enhanced SOL transport and earlier detachment, consistent with the\nsimulation results.\nRevisions to the manuscript: We have added the following clarification in the relevant\nsection on Page 8:\n“Discharge 193108, as shown in Fig. 4g and 4h, was performed with increased RMP current\n(5 kA) under otherwise similar conditions to discharge 193106.”\nThese revisions ensure that readers can follow the experimental logic and understand the\nrelevance of this discharge in supporting the modeling analysis.\nComment 15\nThis discussion on page 7 is helpful. We see clearly under what conditions we expect\nRMPs to be effective at penetrating the plasma, and align best with the field line\nstructure. With increasing collisionality – odd-parity is better field aligned, and so even\nthough the response is low, it is more effective at flattening the profiles? Could we please\nsee those profiles? How do the pedestal characteristics compare to known type II ELM\nregimes?\nResponse:\nWe thank the referee for the insightful comments and helpful suggestions.\nTo address the request, the relevant pedestal profiles under even and odd-parity RMP\nconditions have been added to the supplementary materials. These provide a clearer picture of\nthe profile modifications.\n• Collisionality: In our experiments, the pedestal collisionality (ν e *) remains below 1.5 (see\nFigure 2d). This is substantially lower than typical Type-II ELM regimes, where ν *≥ 5\ne\n(e.g., Suttrop et al., NF 2011; Beurskens et al., PPCF 2013).\n• Pedestal Structure: Unlike the broad, shallow pedestal typically observed in Type-II ELM\nconditions, our discharges maintain a steep pressure gradient near the ballooning\nboundary of peeling-ballooning stability limit (Figure 1 in supplement), which is consistent\nwith low-collisionality, small-ELM regimes.\n• ELM Dynamics and Origin: While the small ELMs observed here are similar in\namplitude to Type-II ELMs, their onset and structure are directly correlated with RMP-\ninduced magnetic topology changes, rather than increased collisionality. This supports\nthe interpretation that the ELM behavior arises from MHD-driven field structure\nmodifications, not classical Type-II mechanisms.\nThese points clarify that while the profile flattening becomes more pronounced at higher\ncollisionality (as seen in the odd-parity case), the underlying physics remain distinct from the\nType-II ELM regime.\nGeneral comments.\nThe present manuscript addresses a relevant topic in the fusion community, i.e. the\nrelation between plasma detachment, mandatory to ensure plasma facing components\nsurvivability, and Resonant Magnetic Perturbation, which is the chosen strategy for\nITER in order to mitigate the deleterious Edge Localised Modes. In principle the authors\nare claiming that the use of RMPs indeed facilitate the access to detachment, which is\nachieved at lower density according to their claims.\nFurthermore other advantages arises because of the use of RMP during detached states, as improvement of confinement\nduring detachment, enhanced heat flux mitigation, absence of a second heat flux peak, andsimultaneous achievement of\nsmall ELMs with partial divertor detachment. They claims to have evidences of the aforementioned processes coming from 2\ndifferent devices, DIII-D and EAST.\nI want to clarify that the data presented by the authors are actually relevant and interesting but despite effort I do not think that\nthe conclusion derived are sufficiently robustly supported for being published in such a prestigious journal. In the following\nthe list of my main concerns supporting this conclusion. They are not provided in priority\nordering but just\nResponse:\nWe sincerely thank the referee for their thoughtful and constructive review. We appreciate\nyour recognition that the topic is both relevant and timely for the fusion community, particularly\nregarding the integration of detachment and RMP-based ELM control—both critical to ITER and\nfuture reactor scenarios.\nWe acknowledge your concern that the conclusions presented in the manuscript must be more\nrobustly supported. In response, we have undertaken substantial revisions to improve the clarity,\nrigor, and evidential basis of our findings. Specifically:\n1. Expanded Data and Analysis: We have added new experimental results, including\nadditional discharges in DIII-D, to better illustrate trends in detachment onset and SOL\ntransport under varying RMP conditions. For EAST, we have moved the results to the\nsupplementary material due to limited edge profile data, while preserving their relevance\nto cross-device consistency.\n2. Improved Discussion of Physical Mechanisms: The revised manuscript clarifies the\ninterpretation of RMP-induced facilitation of detachment, disappearance of the second\nheat flux peak, explicitly distinguishing pedestal and SOL behavior, and introducing\n32\nadditional diagnostics (e.g., T , radiation power evolution, and neutral pressure trends)\ne,div\nto support our claims.\n3. New Modeling Support: We have incorporated new 3D EMC3-EIRENE simulations to\nbetter demonstrate how RMP-modified magnetic topology contributes to increased cross-\nfield transport and reduced detachment thresholds.\n4. Clarified and Structured Presentation: We have restructured the Results section to\nseparate the mechanisms behind detachment facilitation from the integrated scenario\nachieving simultaneous small ELMs and partial detachment. This improves the logical\nflow and avoids conflating distinct findings.\nWe trust that these revisions address the concerns raised and provide a clearer, more\ncompelling foundation for the conclusions drawn. We thank the referee again for their valuable\nfeedback, which has significantly improved the quality and clarity of our manuscript.\nComment 1\nOne the my main concern regards the conclusion derived from Figure 2. According to\nthe authors the use of the RMP\nfacilitate detachment and this in their opinion shown by noticing that detachment is achieved at lower density showing in this\nrespect the central density as a reference. Actually this is to me not appropriate for several reason: first of all RMP\napplication are know to cause a different density peaking and this is associated as well to the process of pump out. A more\nrobust support to the conclusion of the authors would have been derived by showing that detachment is achieved at lower\nseparatrix density. This point is partially answered in Figure 3 where the authors claims that separatrix density at\ndetachment onset is reduced by 10-20%. This is a very delicate question given the uncertainty on the separatrix localisation\nin presence of RMPs (with induced magnetic bulging caused by magnetic stochasticity because of perturbed magnetic field).\nThe authors uses kinetic equilibrium reconstruction to infer separatrix position, but w/o accounting for any 3D effect as\ncaused by RMP. This uncertainity in the separatrix position needs to be considered together the error bars of the density\nestimate shown in Fig 3 (according to which the separatrix density w/o RMPs is within 1 sigma compatible with the value\nwith maximum density pumpout). Therefore I don't think the authors are in the position of claiming that detachment is\noccurring at lower separatrix density with RMPs, which is the relevant figure of merit to be considered as far as exhaust\nphysic is concerned. I would also like to noticed that the different peaking in presence or absence of RMP is seen also from\nFigure 3 given that the ratio ne_core/ne_sep is not constant\nResponse:\nWe thank the referee for the careful and insightful evaluation of our interpretation of Figure 2\nand for highlighting the importance of separatrix density as a more appropriate metric for assessing\ndetachment facilitation.\n1. Use of Separatrix Density as a Diagnostic: We fully agree that the separatrix density\nprovides a more relevant figure of merit for detachment studies than core or line-averaged\ndensity, especially in the context of edge plasma and exhaust physics. While Figure 2\noriginally referenced central density, this was supplemented in Figure 3 with an analysis\nof separatrix density inferred from kinetic EFIT reconstructions. Now we provided the time\nevolution of the separatrix and SOL density in Figure 4, which more consistently shows\nthe lower separatrix density with RMP regardless of the uncertainty.\n2. Uncertainty in Separatrix Localization: We acknowledge the limitations associated with\nusing axisymmetric equilibrium reconstructions (i.e., kinetic EFIT) in the presence of\nRMPs, where 3D magnetic perturbations can distort flux surfaces and introduce\nuncertainties in the precise localization of the separatrix. The separatrix location is\ndetermined by the power balance technique (Leonard NF 2017), and with the profile\nevolutions (no big change in the heat flux width from Te profile and no big change on the\nseparatrix power) , the power balance technique could provide a more accurate separatrix\nlocations. While this caveat is now more explicitly discussed in the revised manuscript, we\nalso emphasize that the trend of decreasing separatrix density with increasing RMP current\nis observed systematically across multiple discharges and remains consistent within the\nbounds of experimental uncertainty. This trend, while modest (~10–20%), supports the\ninterpretation that RMPs lower the density threshold for detachment onset.\n3. Complementary Metric: Gas Puffing Requirement: To strengthen our argument beyond\nthe separatrix density, we have now included data showing that the required D₂ gas puffing\nrate to achieve detachment is reduced when RMPs are applied. This metric is less sensitive\nto equilibrium reconstruction assumptions and provides additional evidence for\ndetachment facilitation due to RMPs.\n4. Density Peaking Consideration: We agree with the referee that RMPs alter the density\npeaking factor due to core density pump-out. As observed in Figure 3 (now Figure 4), the\nn /n ratio varies with RMP amplitude. We now explicitly acknowledge this in the\ne,core e,sep\nrevised text and emphasize that our conclusions are based on multiple consistent indicators\n(separatrix density trend, gas puffing requirement, and divertor parameters), not solely on\nn .\ne,core\nIn summary, while we recognize the limitations in separatrix position accuracy under 3D\neffects, we believe the convergence of several independent indicators—separatrix density, gas\nfueling threshold, and divertor behavior—provides a robust basis for our conclusion that RMPs\n34\nfacilitate detachment. We thank the referee again for highlighting these important considerations\nand have revised the manuscript accordingly to reflect this more nuanced interpretation.\nComment 2\n2. Still on Figure 3 I think that the example shown for EAST are rather peculiar: the authors attribute the detachment\nachievements to Ne seeding. Actually detachment is achieved approximately after 0.5s from the second Neon puffing.\nAccording to recent models [Henderson et al IAEA 2023] this time is in the same ball-park of Ne resident time in the diveror\nregion and therefore it would be questionable attributing detachment as caused by impurity injection. Also in this case it is\nnot possible to deduce the separatrix condition (ne_sep, Psep or Te_sep) with or w/o RMPs and therefore we can't state\nunambiguously that detachment is facilitated by RMP applocation.\nResponse:\nWe thank the referee for the thoughtful comments and clarification regarding the EAST\nexample, and we agree that accurate interpretation of the timing and cause of detachment is\nessential.\nFirst, we believe the referee intended to reference the original Figure 2 rather than Figure 3 in\nthis comment. We apologize for any confusion caused by this overlap.\nWe fully acknowledge that the detachment onset in EAST (e.g., shot 127747) occurs\napproximately 0.5-0.8 s after the second Neon (Ne) puff. As the referee correctly points out, this\ntimescale is comparable to the expected Ne residence time in the divertor region, as suggested in\nrecent modeling work (e.g., Henderson et al., IAEA 2023). This implies that the delayed radiative\neffects of impurity injection must be considered when attributing causality. However, we note that\nthis timing remains consistent with impurity-driven radiative cooling processes that can trigger\ndetachment, particularly in regimes where Ne remains confined to the divertor volume.\nThat said, we also recognize the limitations in our ability to quantitatively assess separatrix\nconditions (e.g., ne_sep, P_sep, Te_sep) in EAST due to diagnostic constraints. Consequently, we\nagree with the referee that it is not possible to make a definitive, unambiguous statement about\ndetachment facilitation by RMPs in the EAST case alone.\nIn response to this concern, we have moved the EAST data and analysis to the\nsupplementary materials, and now focus the main text of the manuscript on the DIII-D dataset,\nwhere separatrix conditions are more reliably diagnosed. The EAST results are retained as\nsupporting evidence, demonstrating similar trends but not forming the primary basis of our\nconclusions.\ninterpretation and the presentation of our results.\nComment 3\n3. To support their claim according to which RMP application are facilitating detachment onset, the authors performed a\nseries of SOLPS-ITER simulations with drift-activated. Actually as shown in literature (e.g. Brida NME 2019, Rea NF 2015),\nthe SOL profiles with applied 3D magnetic perturbation are intrinsically 3D and therefore it is not clear which profiles are\ntaken as reference for constraining SOLPS simulations. I do find the use of a 2D code for a clear 3D geometrical problem not\nappropriate and therefore I don't think the results of SOLPS are really promoting the claims of the author: they simply\nconfirm that with increased radial transport easier detachment is achieved (since at similar Psep this will be clearly associated to\nbroader target heat profile thus easier to be detached)\nResponse:\nWe thank the referee for the thoughtful comments regarding the use of SOLPS-ITER in\nmodeling RMP-induced detachment and acknowledge the concern that a 2D simulation tool may\nnot fully capture the intrinsically 3D nature of the edge plasma under magnetic perturbations.\n1. On the 3D Nature of the Problem: We fully agree that the application of RMPs leads to\n3D modifications of the SOL and divertor conditions, as highlighted in previous studies (R.\nWilcox, et al, PRL 117. 135001 (2016)). In our work, the upstream profiles used to\nconstrain the SOLPS-ITER simulations are based on experimental measurements, with the\nseparatrix temperature fixed at 80 eV following prior characterization. This choice helps\nmitigate the sensitivity of the simulations to localized 3D displacements, and was made\nwith the intent of focusing on the role of increased radial transport, not reproducing the\ndetailed 3D structure of the plasma.\n2. On the Purpose of the SOLPS-ITER Simulations: We do not claim that the 2D\nsimulations capture the full complexity of RMP-induced 3D effects. Instead, our use of\nSOLPS-ITER was intended to isolate and examine the general influence of enhanced radial\ntransport—commonly associated with RMP application—on detachment onset. As\ncorrectly noted by the referee, and as acknowledged in our manuscript, these simulations\ndemonstrate the expected trend: that enhanced radial transport at fixed P leads to target\nsep\nflux broadening and easier access to detachment. These results qualitatively support our\nexperimental findings but are not presented as a comprehensive model of the 3D plasma\nbehavior.\n36\n3. Addition of 3D Modeling: In response to the referee’s valid concern, we have now\nincorporated 3D EMC3-EIRENE simulations into the revised manuscript (now Fig. 6).\nThese simulations include the full 3D magnetic field geometry and reveal that RMPs\nreduce the separatrix density while increasing the SOL density—directly reflecting the\nexperimental trends observed in our study. The simulations also show solid detachment\nwith RMP while marginal to detachment if using the same D2 gas puffing and Carbon\nerosion but without RMP. This addition significantly strengthens the modeling support for\nour conclusions.\n4. Clarification of EAST Data Usage: To further ensure clarity and focus, we have moved\nthe EAST experimental results (previously presented in the main text) to the supplementary\nmaterials. While they support the general trends observed on DIII-D, they were limited by\ndiagnostic access and are no longer part of the primary evidence used to support our central\nclaims.\nWe appreciate the referee’s critical input, which has prompted us to improve both the rigor and\nclarity of our analysis.\nComment 4\n4. It is actually quite difficult to interpret Fig 6 (a) concerning the DIII-D part. Indeed while it is clearly observed a\nperpendicular heat flux decrease at OSP, there is a simultaneous increase of the flux in a different radial position still of the\norder of 1.5 MW/m^2. It is difficult to intepret this as a signature of partial detachment. I think that the comparison of the radial\nprofiles of main quantities (ne, Te, jsat) might reveal features which are not addressed and considered in the present paper.\nBut this is not clearly a reference case scenario to be compared with RMP cases\nResponse:\nWe thank the referee for their detailed observations and the opportunity to clarify our\ninterpretation of Figure 6(a) and its relevance as a reference case.\n1. Context of Shot 193106: Figure 6(a) corresponds to DIII-D shot #193106, the details of\nwhich are also presented in Figure 4, where the transition to partial detachment is\ncharacterized by a sharp drop in divertor electron temperature (Tₑ) to approximately 5 eV,\nproviding an independent indication of detachment onset. This reinforces the interpretation\nof this discharge as a valid reference case for partial detachment.\n2. Heat Flux Redistribution: We emphasize that the outward shift and reduction of the\nprimary heat flux is consistent with partial detachment physics (A. Kallenbach etal 2015\nNucl.Fusion 55 053026), where a drop in upstream power reaching the outer strike point\nleads to redistribution toward more peripheral divertor regions. This effect, together with\n37\nreduced Jₛₐₜ and Tₑ at the outer strike point and increased J at the SOL, supports the\nsat\nconclusion that a partial detachment state is achieved in this shot.\n3. Justification as a Reference Case: The main purpose of Figure 6 is to contrast divertor\nheat flux behavior in similar discharges with and without RMP application. In the RMP\ncase, the second heat flux peak is suppressed prior to the onset of detachment, and overall\nheat flux mitigation is more pronounced. The inclusion of 193106, as a case without RMP\nbut showing partial detachment, provides a necessary baseline to highlight these RMP-\ninduced changes. Therefore, we respectfully disagree with the referee’s assertion that it is\nnot a suitable reference scenario.\nWe hope these clarifications, along with the additional divertor Tₑ data now shown in Figure\n4, resolve the referee’s concerns and strengthen the case for our interpretation.\nComment 5\n5. Also when comparing even and odd parity the authors does not provide information on the possible difference of\ndetachment onset relevant quantities like ne_sep or Psep between the two different configuration. Any inference of better\nperformances of one configuration w.r.t. to the other would not be possible w/o essential exhaust relevant information\nResponse:\nWe thank the referee for raising this important point regarding the comparison between even\nand odd parity RMP configurations, and the need to examine detachment-relevant parameters such\nas separatrix density (nₑ,sep) and power across the separatrix (Pₛₑₚ).\nIn response to this comment, we have made several revisions to strengthen the analysis:\n1. Expanded Diagnostic Coverage: We now provide the pedestal-top, separatrix, and\nSOL density evolution for the discharge in question, along with the total and divertor\nradiated power and stored energy, to ensure a comprehensive picture of the plasma\nconditions across the RMP parity transition. These additions are included in the revised\nmanuscript in Figure 2.\n2. Clarification of Plasma Conditions: Additional detail has been added to original Figure\n7 (now Figure 2), explicitly showing that the key global parameters—such as fueling rate,\nand heating power—remain effectively unchanged when switching from even to odd parity.\nThis establishes a controlled experimental context, allowing us to isolate the observed\neffects to the change in magnetic topology.\n3. Observed Differences: Under otherwise similar plasma conditions, switching to odd\nparity results in a clear transition to smaller ELMs, accompanied by a recovery in stored\nenergy and an increase in βₙ. These changes are consistent with enhanced confinement and\n38\nimproved divertor performance, forming the basis for our inference that the odd parity\nconfiguration is more effective in this regime.\nWe appreciate the referee’s constructive suggestion, which has led us to significantly enhance\nthe clarity and completeness of our analysis.\nComment 6\n6. As a minor point, in the introduction the authors made a comment on ITER operation claiming that ITER is expected to\noperate at relatively high density for divertor detachment with [...]. This is actually inaccurate, since ITER is indeed foreseen to\noperate at high gas throughput (i.e. high neutral pressure) but this is not impacting the plasma density achieved since\nplasma fuelling is ensured by Pellet operation. On the other side the chosen strategy for ensuring plasma detachment in\nITER is a combination of high gas throughput and impurity seeding [see Pitts, Nucl. Mat. Energy 2019].\nResponse:\nWe thank the referee for their helpful clarification regarding ITER’s operational strategy. In\nresponse, we have revised the relevant section of the introduction to more accurately describe the\nexpected approach to divertor detachment in ITER.\nSpecifically, we now clarify in both the introduction and outlook sections that ITER is designed\nto operate with high gas throughput, which increases neutral pressure in the divertor region,\nwhile plasma fueling will primarily be provided by pellet injection, rather than by the gas\nthroughput itself. Furthermore, we explicitly note that ITER's strategy for achieving divertor\ndetachment involves a combination of high gas throughput and impurity seeding, as discussed\nin Pitts, Nucl. Mater. Energy 2019.\nWe appreciate the referee for drawing attention to this point, and we have corrected the\nmanuscript accordingly to reflect a more accurate representation of ITER’s operational plans.\nComment 7\nIn conclusion I don't think the authors provide enough clear evidences of the real benefit\nin term of detachment accessibility facilitated by application of RMPs. As stated to me\ndetachment onset are occurring within the experimental uncertainty at the same\nseparatrix parameters (ne_sep, te_sep). No information are provided in terms of power\ncrossing the separatrix and only information about core density are claimed as important,\nwhich is partially inappropriate. The use of a 2D transport code with respect to an\n39\ninherent 3D problem is to me of limited use. Therefore I don't think that the paper is\nadequate for a publication in such a prestigious journal\nResponse:\nWe thank the referee for the detailed review and critical assessment. We understand the\nimportance of providing robust and unambiguous evidence for the role of RMPs in facilitating\ndivertor detachment, and we have made substantial revisions to the manuscript to address the\nconcerns raised. Specifically, we have taken the following steps:\n1. Detachment Accessibility and Separatrix Conditions: While we acknowledge the\nchallenges in precisely determining separatrix parameters such as n , T , and P\ne,sep e,sep sep\n(particularly in EAST due to diagnostic limitations), we emphasize that our comparative\nanalysis is based on discharges with and without RMPs under otherwise similar conditions.\nTo strengthen this point, we have now included additional measurements and time traces\nof pedestal-top, separatrix, and SOL densities, along with radiation power and total stored\nenergy. These additions help to more clearly illustrate the differences in edge behavior and\ndetachment onset with RMP application.\n2. Power Crossing the Separatrix: We recognize the referee’s concern regarding the lack\nof direct P measurements. Although this remains experimentally challenging, we have\nsep\nadded a more thorough discussion of inferred power crossing the separatrix based on\navailable measurements and profiles, to better contextualize the observed detachment\nbehavior.\n3. Core Density and Detachment Onset: We agree that core density alone cannot fully\ncharacterize detachment accessibility. Our emphasis was intended to reflect its correlation\nwith edge cooling and density buildup. We now clarify this point and expand the discussion\nto emphasize the broader set of edge parameters relevant to detachment, including SOL\nand pedestal conditions, supported by both experimental observations and modeling.\n4. Use of 2D vs. 3D Modeling: We fully agree with the referee that RMP-induced effects are\ninherent 3D. Our initial use of SOLPS-ITER was intended to isolate the role of increased\nradial transport in detachment facilitation, not to capture the full 3D complexity. To address\nthis limitation, we have now included results from 3D EMC3-Eirene simulations. These\nsimulations explicitly account for the magnetic topology imposed by RMPs and show that\nRMP application leads to reduced separatrix density and enhanced SOL density—\nconsistent with our experimental findings and reinforcing our interpretation of RMP-\nenhanced edge transport promoting detachment.\nIn light of these extensive revisions—both in terms of additional experimental analysis and\nincorporation of 3D modeling—we believe our manuscript now presents a significantly more\nrobust case for the role of RMPs in improving detachment accessibility. We sincerely thank the\n41\nreferee for their constructive feedback, which has contributed to a clearer and stronger presentation\nof our results. We hope that the revised version now meets the expectations for publication in this\njournal.\n41"}]

Decision letter