# Impurity convection reversal caused by edge localized turbulence for generating a stationary edge radiative layer in the HL-2A tokamak

G.Q. Xue, W.L. Zhong, Z.X. Wang, X.L. Zou, J.M. Gao, R. Ke, S. Zheng, X.R. Zhang, D.M. Fan, W.P. Guo, et al.

► To cite this version:

G.Q. Xue, W.L. Zhong, Z.X. Wang, X.L. Zou, J.M. Gao, et al. Impurity convection reversal caused by edge localized turbulence for generating a stationary edge radiative layer in the HL-2A tokamak. Nuclear Fusion, 2024, 64 (12), pp.126042. 10.1088/1741-4326/ad7ed3 . cea-04788843

HAL Id: cea-04788843

https://cea.hal.science/cea-04788843v1

Submitted on 18 Nov 2024

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Impurity convection reversal caused by edge localized turbulence for generating a stationary edge radiative layer in the HL-2A tokamak

G.Q. Xue<sup>1,2</sup>, W.L. Zhong<sup>2,*</sup>, Z.X. Wang<sup>1,*</sup>, X.L. Zou<sup>3</sup>, J.M. Gao<sup>2</sup>, R. Ke<sup>2</sup>, S. Zheng<sup>1</sup>, X.R. Zhang<sup>1,2</sup>, D.M. Fan<sup>2</sup>, W.P. Guo<sup>2</sup>, M.K. Han<sup>2</sup>, X.X. He<sup>2</sup>, M. Jiang<sup>2</sup>, J.C. Li<sup>4</sup>, Y.G. Li<sup>2</sup>, A.S. Liang<sup>2</sup>, L. Liu<sup>2</sup>, S.Q. Wang<sup>2</sup>, T.B. Wang<sup>2,5</sup>, J. Wen<sup>2</sup>, G.L. Xiao<sup>2</sup>, J.Q. Xu<sup>2</sup>, Z.C. Yang<sup>2</sup>, J. Yin<sup>2</sup>, X. Yu<sup>2</sup>, Y.R. Zhu<sup>2</sup>, Z.B. Shi<sup>2</sup>, M. Xu<sup>2</sup> and HL-2A team<sup>2</sup>

E-mail: zhongwl@swip.ac.cn and zxwang@dlut.edu.cn

Received 29 March 2024, revised 27 August 2024

Accepted for publication 24 September 2024

Published 15 October 2024

<figure><img src="image_2.png" /></figure>

CrossMark

## Abstract

The effect of impurity on edge turbulence was investigated on the HL-2A tokamak using neon supersonic molecular beam injection (SMBI). For the first time, a stationary radiative layer accompanied by edge localized turbulence (ELT) lasting more than 100 ms is observed at the edge region $ρ = 0.7 \sim 0.9$. This duration is much longer than the SMBI pulse length (1.2 ms), indicating that the impurity diffusion is on the whole canceled out by the impurity convection without spreading of the localized turbulence during this period. This implies that the impurity convection is reversed from the positive to the negative impurity density gradient region. This reversal could be explained by the parallel impurity compression mechanism. The observed turbulence is a broadband electrostatic turbulence (25–70 kHz) propagating in the electron diamagnetic drift direction with a normalized poloidal wavenumber of $0.1 \le k_θρ_s \le 0.4$. It has been found that the excitation of the ELT strongly depends on the local collisionality $ν^*$. This feature is qualitatively predicted by the spectral multiscale gyrokinetic turbulence simulation, which suggests that the turbulence could be a dissipative trapped electron mode. This experiment demonstrates that mutual interaction between externally injected impurity ions and edge turbulence can generate a stationary edge radiative layer and avoid impurity core accumulation. These results could provide a potential approach for the formation of a stable impurity radiative layer by impurity injection in tokamaks.

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ School of Physics, Dalian University of Technology, Dalian, China</footnote>

<footnote n="2">² Southwestern Institute of Physics, Chengdu, China</footnote>

<footnote n="3">³ CEA, IRFM, Saint-Paul-Lez-Durance, France</footnote>

<footnote n="4">⁴ College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, China</footnote>

<footnote n="5">⁵ ITER Organization, Saint Paul-lez-Durance, France</footnote>

<footnote>* Authors to whom any correspondence should be addressed.</footnote>
Keywords: impurity injection, edge turbulence, impurity radiative layer

(Some figures may appear in colour only in the online journal)

# 1. Introduction

In magnetic fusion devices, external impurity injection and impurity transport are of great importance for plasma confinement and control optimization of future magnetic fusion reactors. Recently, low or medium Z impurities seeding as an optimized technique has been widely applied on the tokamaks and stellarators for heat flux mitigation, plasma turbulence control and plasma confinement enhancement. Firstly, it is worth pointing out that the high radiation plasma scenarios with impurity seeding are necessary for upcoming ITER operations [1, 2]. In particular, the edge steady-state impurity radiation layer distributes most of the power evenly over the first wall, which could effectively mitigate heat flux on plasma-facing components (PFCs) [3, 4]. Nitrogen and neon are the preferred choices for the impurity gas to be seeded into the divertor, for favoring the edge and SOL radiation, and thus to achieve divertor detachment, such as in JET [5, 6], DIII-D [7, 8], ASDEX-Upgrade [9, 10], EAST [11] and HL-2A [12] tokamaks. Secondly, H-mode plasma is usually accompanied by edge-localized modes (ELMs) crash, which releases a large number of particles and plasma energy to PFCs. Experimental studies have demonstrated that seeded impurity can control the behavior of ELMs [13–16], and it has been evidenced that the ELMs behavior is very sensitive to the impurity ratio in HL-2A tokamak [17].

Additionally, it has been observed that plasma microinstabilities can be changed with the impurity seeding, resulting in influencing the plasma total confinement. The reduction of ion and electron thermal diffusivities induced by impurity injection has been investigated, which could be related to stabilization of ion temperature gradient (ITG) or electron temperature gradient driven turbulence as observed in NSTX [18, 19], HL-2A [20], JET [21, 22] and LHD [23, 24] H-mode plasma discharges. In experiments, the transport of particles is reduced by impurity seeding, leading to an enhancement in the pedestal pressure and global energy confinement. For instance, it is claimed that the edge coherent mode generated by lithium and boron powder injection in EAST could provide continuous particle exhaust for maintaining small ELM plasma regimes and avoiding impurity accumulation [25, 26]. Similar results have been obtained in DIII-D with edge bursty chirping mode driven by Li injection [15]. Turbulence switches from ITG to resistive interchange mode turbulence when the density exceeds a certain density threshold owing to a magnetic hill configuration in LHD [27, 28], which is rarely observed on tokamaks. Multiple coupling effects from impurity injection have also been observed. In DIII-D L-mode and TEXTOR radiation improvement mode (RI-mode) plasma discharges, the energy confinement improvement is due to the combination of two effects, including the impurity directly stabilizing turbulence and $E \times B$ velocity shear [29–31]. Similar to the

RI-mode, core density peaking and confinement improvement has been observed when B powders are injected into WEST L-mode plasmas. These phenomena are strongly correlated with higher collisionality, lower Te/Ti and higher $Z_{eff}$, which can substantially reduce turbulence [32, 33]. Pulsed injections of boron carbide granules into Wendelstein 7-X stellarator (W7-X) plasmas increases the ion temperature and stored energy transiently at the same time, turbulence intensity is reduced and the radial electric field becomes more negative [34]. Theoretical works show that the impurity ions influence on the plasma turbulence and heat transport [35] mainly depend on the plasma dilution (increasing $Z_{eff}$) effect [31, 36], peaking of the impurity density profile [37, 38], electron and ion collision frequencies [39] and the isotope effect [40, 41] and so on.

Although impurity injection has been shown to improve confinement to some extent, large amounts of impurities can lead to impurity core accumulation, resulting in degrading the plasma confinement or even plasma disruption [42]. Furthermore, impurity seeding can also dilute the plasma in the core, which would ultimately reduce the number of useful fusion reactions in a future fusion reactor. Thus, how to achieve enhanced radiative capacity by external impurity seeding while avoiding impurity core accumulation is a key issue in fusion reactors. In this paper, the co-existence of a stationary radiation layer and edge localized turbulence (ELT) without continuous impurity injection is observed for the first time in the HL-2A tokamak. As a new regime, it is desirable to efficiently avoid impurity core accumulation and to mitigate the heat load on the PFCs.

The paper is organized as follows. Section 2 introduces the experimental setup, key diagnostics and impurity injection systems. In section 3, the experimental observation of the edge turbulence excited by the supersonic molecular beam injection (SMBI) Ne seeding is presented. The characteristics of the localized turbulence are shown in section 4. Section 5 mainly describes the basic physical mechanism of turbulence excitation, and turbulence simulation results. Finally, the conclusions are summarized in the last section, followed by discussions.

# 2. Experimental setup

Experiments with impurity seeding are carried out in the L-mode plasma in the HL-2A tokamak, whose major and minor radii are $R = 1.65$ m and $a = 0.36$ m, respectively. The plasma current $I_p = 160$ kA and toroidal magnetic field $B_t = 1.33$ T. The auxiliary heating includes neutral beam injection (NBI), electron cyclotron resonance heating (ECRH) and lower hybrid current drive (LHCD). A typical plasma configuration is fixed as shown in figure 1. The
<figure><img src="image_4.png" /><figcaption>**Figure 1.** The cross section of HL-2A shows a typical plasma shape and the arrangement of main diagnostics.</figcaption></figure>

plasma line averaged electron density is obtained using a far-infrared laser interferometer [43]. Radial profiles of the electron density $n_e$ is measured with frequency modulated continuous microwave reflectometers [44] and the electron temperature $T_e$ is measured with the electron cyclotron emission radiometer [45]. Both the ion temperature and the toroidal velocity are measured by the charge exchange recombination spectroscopy system [46]. The effective charge number $Z_{eff}$ is estimated using the visible spectroscopy diagnostic [47]. Additionally, the total plasma radiated power is recorded by two poloidal arrays of fast absolute extreme ultraviolet photodiodes with multiple chordal views of the main plasma and lower outer divertor [12]. Density fluctuations are measured by beam emission spectroscopy (BES) [48] and Doppler reflectometry (DR) [49]. The 32 channels of BES are deployed as a 16 (radial) × 2 (poloidal) 2D array at the outer mid-plane as shown in figure 1, dedicated to the measurement of the edge density fluctuations in our experiments.

Specifically, for the external impurity seeding experiments, the pure neon (Ne) gases are seeded into plasma by using SMBI system, which was first proposed on the HL-1 tokamak [50]. It was then widely implemented in many fusion devices, such as JT-60U [51], LHD [52], EAST [53], etc. Recently, four SMBI systems were installed in the HL-2A tokamak,

which are equipped on the mid-plane and divertor. It has a higher fueling efficiency and deeper particle deposition than gas puffing [54]. The particle number of the injected fueling gas or impurity gas can be easily determined by controlling the injected pulse width and pressure. Additionally, SMBI is also used to study the density feedback control [55], plasma transport [56] and ITB formation [57]. In particular, it has been observed that the SMBI can trigger the L-H transition by lowering the power threshold [58]. ELMs could be successfully mitigated and suppressed by fueling gas or impurity gas seeding, such as $D_2$, Ne, Ar, etc [59].

### 3. Excitation of an ELT by impurity seeding

Figure 2 displays the time evolution of key plasma parameters with pure Ne impurity seeding in L-mode plasma. The plasma current maintains 160 kA. The auxiliary heating power of NBI, ECRH and LHCD is up to 1.9 MW as shown in figure 2(a). The Ne SMBI is seeded at 1095 ms on the outer mid-plane, as shown from the monitor of the SMBI in figure 2(b). The pulse length is 1.2 ms with the pressure of 5.85 bar. At the same time, the line-averaged electron density (figure 2(b) blue line) increases gradually. It clearly shows that the electron density rises over the entire range of minor radius by comparing electron density profiles before and after the SMBI in figure 3(a). After impurity injection, the averaged effective charge number $Z_{eff}$ increases from 3 to 4 (figure 2(c)), which could be considered to be mainly the contribution of externally impurity ions. With neon seeding, there is a pronounced enhancement in the plasma total radiation as seen from figure 2(d), which is increased by a factor of about 10% than that before the SMBI injection. Figure 2(e) is the time evolution of the Ne III line emission integral intensity measured by visible spectroscopy diagnostics, which increases after the SMBI. Due to the radiative cooling effect caused by impurity injection, both the ion temperature (figure 3(b)) and the electron temperature (figure 3(c)) in the edge region decrease slightly, and the electron temperature in the core decreases substantially. However, there is a moderate increase in the core ion temperature. It could also be found that the plasma stored energy remains almost constant and the energy confinement time maintains 15 ms after impurity injection as shown in figure 2(f). The energy confinement factor $H_{89}$ maintains at about 1. This means that the impurity injection does not degrade the plasma global confinement.

Externally injected impurities typically function as particle sources, resulting in plasma perturbations to some extent. This differs from previous observations in H-mode plasma with a higher pedestal density in HL-2A tokamak, where Ne injection was found to reduce the growth rate of ITG, thereby decreasing the ion heat flux and increasing the Ti [25]. Figure 4(a) shows the spatiotemporal evolution of the plasma total radiation power increment after SMBI compared with the reference time $t = 1090$ ms (before SMBI). It is interesting to note that the radiation power sharply increases at the localized normalized radius $\rho = 0.7 \sim 0.9$ after impurity seeding.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Main plasma parameters for the pure neon seeded by SMBI in HL-2A tokamak: (a) LHCD, NBI and ECRH heating power, (b) monitor of the SMBI pulse, line-averaged electron density, (c) effective charge number, (d) total plasma radiation power, (e) Ne III line emission intensity, (f) plasma stored energy and energy confinement time.</figcaption></figure>

We can qualitatively characterize that the Ne impurities are concentrated in this edge localized region. More importantly, the impurity radiative layer persists for 100 ms in this region, much longer than the SMBI neon pulse length (1.2 ms) and the particle confinement time (~20 ms). This indicates that the Ne impurity particle flux may be close to zero under the free source of impurity, and the impurity diffusion is on the whole canceled out by the impurity convection. This also suggests that impurity core accumulation is prevented by forming a stationary radiative layer at the edge region.

Based on the BES diagnostic with high temporal resolution, an ELT with a frequency of 25–70 kHz after Ne seeding is observed from the spectrogram of density fluctuations in figure 4(b), where located at the region of $\rho = 0.83$. It is important to emphasize that the ELT and impurity radiative layers were maintained for the same duration. For a more intuitive observation of turbulence changes, the integrated intensity of the ELT is shown in figure 4(d). It is worth noting that the excitation and maintenance of the ELT are consistent with the radiation enhancement (figure 4(a)) after Ne impurity seeding. This implies a strong correlation between the ELT

excitation and impurity injection. However, the ELT is not observed in the magnetic fluctuations measured by the mid-plane Mirnov coils in both the high-field and low-field sides (figure 4(c)), suggesting that this mode may be an electrostatic mode. These phenomena of ELT excitation were reproducible when the plasma discharges under similar heating power and seeding conditions.

Specifically, figure 5(a) shows power spectra of density fluctuations at times before and after Ne injection, and the peak around 50 kHz represents the ELT. However, figure 5(b) displays the spectra of magnetic fluctuations, and no ELT is observed. The low-frequency coherent mode that appears on the magnetic perturbation spectrum may be a magnetohydrodynamic instability, which is not our focus in this research as it is independent of impurity injection. Thus, this confirms that the ELT is an electrostatic mode. Furthermore, due to the high perpendicular wavenumber measured by the DR diagnostic [48], the ELT is not being captured on both the density perturbations and the phase perturbations by the DR.

Impurities are injected into the plasma as neutrals and undergo an ionization process that is dependent on the
<figure><img src="image_6.png" /><figcaption>Figure 3. The (a) density, (b) ion and (c) electron temperature profiles before (blue line) and after (red line) SMBI.</figcaption></figure>

plasma temperature, thereby raising radiation levels. The radiation power of impurity in plasmas includes contributions from bremsstrahlung, line radiation, and recombination radiation [60]. Consequently, the radiation power induced by a particular impurity species could be written as

$$P_q = \sum_q P_{q1} + P_{qrec} + P_{qbr}, \quad (1)$$

where $P_{q1}$ is the line radiation from the impurity in the $q$th state, $P_{qrec}$ is the recombination radiation, $P_{qbr}$ is the bremsstrahlung. For low and medium Z impurities, such as nitrogen or neon, impurity radiation is concentrated in the edge region [61]. In contrast, higher Z impurities, such as tungsten, can continue to radiate as they penetrate deeper into the plasma core [62]. The radiative power for an impurity species can be calculated from the cooling factor using a collisional-radiative model. Figures 6(a) and (b) show the cooling factor Lz (Te) as a function of electron temperature for hydrogen, neon and carbon, respectively, from the Atomic Data and Structure Analysis database under coronal equilibrium [63]. This clearly illustrates that the cooling factor Lz is sensitive to the plasma electron temperature. Therefore, figures 6(c) and (d) give

the radial profiles of the cooling factor corresponding to the experimental temperature profiles after impurity injection, as shown in figure 3(c). It is observed that there is a very slight increase in the cooling factor of carbon after Ne injection due to decreased Te, which remains below the radiation efficiency of neon. Meanwhile, the edge carbon emission intensity, as measured by VUV, did not exhibit a pronounced increase following the Ne injection. The contribution of carbon impurities to the increase in radiation after SMBI is not significant. This suggests that the main increase in the radiated power at the edge comes from Ne.

Assuming local ionization balance and thus ignoring the transport effect, the local radiation power density can be written as $P_{rad} = n_e^2 \cdot L_H(T_e) + n_e \cdot n_z \cdot L_z(T_e)$. The local Ne impurity density profile at 1120 ms is shown in figure 7(a). This indicates that there is still a peaking distribution of Ne impurity at the edge region, even when considering the temperature dependence of the cooling factor. Figure 7(b) displays the radial profile of the electron density during the short duration of the SMBI pulse. This increment is attributed solely to the ionization of externally injected Ne impurity ions. It can be observed that the peak of density increments is mainly concentrated in the edge region of $\rho = 0.7 \sim 0.9$. Based on the array BES data, the relative intensity of the ELT is estimated by $\tilde{n}/n$, where $\tilde{n}$ is filtered within a frequency bandpass of 25–70 kHz. As can be seen from the orange square in figure 7(c), the relative intensity of the turbulence increases significantly after the SMBI. The ELT is localized at about $\rho = 0.7 \sim 0.9$, in the same region as the impurity radiative layer. It is clearly observed that the background turbulence is essentially unchanged in the $\rho < 0.7$, but there is a decrease in background turbulence at the $\rho > 0.9$ after the impurity injection. It is suggested that turbulent spreading may not be present beyond the impurity distribution interval. These results demonstrate that deposition and distribution of impurities excites and maintains the ELT at the edge region. In turn, the ELT may lead to balanced impurity transport, generating a stationary peaked impurity distribution at the edge region.

## 4. Characteristics of the ELT

The BES 2D array can be used for correlation analysis in both poloidal and radial directions. The two-point cross correlation method is used to calculate the wavenumber spectrum $S(k,f)$ with different spatial channels, which is given as follows [64, 65]:

$$S(k,f) = \frac{1}{M} \sum_{i=1}^{M} I_{\Delta k} [k - k_i(f)] |P_{X_1 X_2}^{j}(f)|, \quad (2)$$

where M is the number of the realization divided from the fluctuations, $P_{X_1 X_2}^j(f)$ is the cross-power of the density fluctuations which have the poloidal separation of $\Delta x$, and the local wavenumber is given by $k_i(f) = \Delta \theta_{1,2}^i(f) / \Delta x$, $\theta_{1,2}^i(f)$ is the phase shift between two fluctuations in the frequency domain. The indicator function $I_{\Delta k}(x)$ is defined as

$$I_{\Delta k}(x) = \begin{cases} 1 & -\Delta k/2 \le x \le \Delta k/2 \\ 0 & \text{elsewhere} \end{cases}. \quad (3)$$
<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Monitor of the SMBI pulse; and spatiotemporal evolution of the increment of plasma radiation power density after Ne impurity injection, (b) amplitude spectrogram of density fluctuation at $\rho = 0.83$, (c) amplitude spectrogram of magnetic fluctuation, (d) integrated ELT intensity in the frequency range of 25–70 kHz.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5.** The time-averaged spectra of (a) density fluctuations measured by the BES and (b) magnetic fluctuations measured by the Mirnov coils at times before and after impurity injection.</figcaption></figure>

Figure 8(a) shows the poloidal wavenumber spectrogram $S(k_\theta, f)$ obtained from two poloidal BES channels, separated by 1.35 cm, which are located at $\rho = 0.83$ in the mid-plane. The ELT has a short poloidal normalized wavenumber range

of $0.1 \le k_\theta \rho_s \le 0.4$, where, $\rho_s = c_s/\Omega_i$; $c_s = \sqrt{T_e/m_i}$ is the ion sound speed, and $\Omega_i = eB/m_i$ is the ion gyrofrequency. It is observed that the mode propagates in the electron diamagnetic drift direction. More intuitively, the cross-power spectra of the
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Cooling factor Lz as a function of electron temperature for (a) hydrogen, (b) neon and carbon, as the sum of line radiation, recombination radiation and bremsstrahlung in coronal equilibrium, the radial profile of cooling factor for (c) hydrogen at time 1120 ms after neon injection, and (d) neon and carbon before (dashed line) and after (solid line) SMBI.</figcaption></figure>

two separate BES signals are plotted in figure 8(b), at time before and after Ne injection. The inferred mode numbers are $m \sim 40$ and $n \sim 15$, which are estimated by $m = k_\theta r$ and $n = m/q$ respectively.

Figure 9 displays the poloidal wavenumber spectra at different radial positions from core to edge before and after the impurity injection. It is observed that the background turbulence with low poloidal wavenumber is almost unchanged at $\rho = 0.42$ as shown in figures 9(a1) and (b1). The ELT is clearly observed at $\rho = 0.72$ and $\rho = 0.87$ in figures 9(b2) and (b3), which differs from the k-spectrum before impurity injection (figures 9(a2) and (a3)). This suggests that the ELT does exhibit strong radial localization without turbulence spreading into the plasma core. Furthermore, the reduction in low-frequency background turbulence propagating in the ion diamagnetic drift direction at the edge region ($\rho = 0.99$) after impurity injection is shown in figures 9(a4) and (b4). Figure 10(a) shows the turbulence perpendicular rotation velocity $u_\perp$ measured by multi-channel Doppler reflectometry [66] before and after SMBI. In Doppler reflectometry, a microwave beam is launched into the plasma at an angle that is oblique to the normal cutoff surface, and the backscattered microwave

beam is measured [67, 68]. The $u_\perp$ can be given by $u_\perp = \frac{2\pi f_D}{k_\perp}$, where $f_D$ is the Doppler shift of the spectrum of the backscattered signal, and $k_\perp$ is the perpendicular wavenumber of the measured turbulence. The $u_\perp$ obtained from the Doppler shift contains two components: $u_\perp = v_{E \times B} + v_{\text{phase}}$, where $v_{E \times B}$ is the $E \times B$ drift velocity and $v_{\text{phase}}$ is the phase velocity of turbulence. It has been shown that $v_{E \times B} \gg v_{\text{phase}}$ at the plasma edge, thereby approximating $u_\perp \approx v_{E \times B}$ [69, 70]. It can be observed in figure 10(a) that the $u_\perp$ profile is changed after SMBI. Here, the positive represents the ion diamagnetic drift direction, while the negative represents the electron diamagnetic direction. The corresponding velocity shear $du_\perp/dr$ profiles are shown in figure 10(b). After Ne injection, the velocity shear increases near the LCFS ($\rho = 0.95 \sim 1$), as highlighted by the shaded region. This enhanced velocity shear may contribute to the suppression of the background turbulence.

## 5. Driving mechanism of the ELT

To understand the underlying physical mechanism of excitation of the ELT, we analyzed the dependence of the ELT on
<figure><img src="image_10.png" /><figcaption>**Figure 7.** The profile of (a) Ne density, (b) plasma electron density and (c) ELT relative intensity.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 8.** (a) Poloidal wavenumber spectrum of the density fluctuations, (b) turbulence PSD before (blue) and after Ne seeding (pink), time averaged over a window of 30 ms.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 9. Poloidal wavenumber spectra of the density fluctuations at different radial positions before (a1)–(a4) and after (b1)–(b4) SMBI.</figcaption></figure>

collisionality, density and temperature gradients. The results are given in figure 11. Relative density fluctuations by the BES versus the normalized collisionality are shown in figure 11(a), which defined [71, 72] here as

$$v^* = (a/c_s) \left( \sqrt{2\pi} e^4 n_e / \left( m_e^{1/2} T_e^{3/2} \right) \right) \ln \Lambda, \quad (4)$$

$$\ln \Lambda = 24 - \ln \left( \sqrt{n_e}/T_e \right), \quad (5)$$

where $\ln \Lambda$ is the Coulomb Logarithm. From figure 11(a), it is observed that the ELT is triggered when the normalized collisionality exceeds a certain threshold. Additionally, we compared other possible turbulence drive terms, results from figures 11(b) and (c) show that the excitation of the ELT is insensitive to the normalized electron density gradient and temperature gradient. After SMBI, the normalized density

gradient and temperature gradient are, essentially, the same as in the presence of background turbulence, and both are in a similar parameter interval. These results suggest that normalized collisionality plays a key role in exciting the ELT.

In order to better understand the ELT excitation mechanism, theoretical simulation was performed with the spectral multiscale gyrokinetic code CGYRO, which is used to calculate theoretical micro-instabilities and transport in the edge plasma [73]. The measured plasma parameters at $\rho = 0.85$ are used as input, where the ELT peaks. Figure 12 shows the normalized real frequency $\omega/(C_s/a)$ and linear growth rate $\gamma/(C_s/a)$ as a function of collisionality at $k_\theta \rho_s = 0.2$. It is found that there is a mode transition in the real frequency with the collisionality exceeding a threshold of about 0.3 in figure 12(a). Negative values of the real frequencies represent the modes propagating in the electron diamagnetic drift direction. When the normalized collisionality is
<figure><img src="image_13.png" /><figcaption>**Figure 10.** The profile of (a) $u_{\perp}$ velocity and (b) velocity shear before and after SMBI.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 11.** The relative density fluctuations versus the (a) normalized collisionality, (b) electron density gradient and (c) electron temperature gradient.</figcaption></figure>

below 0.3, the growth rate of the collisionless electron trapped mode decreases significantly with increasing collisionality. However, once the collisionality exceeds the threshold, the corresponding growth rate of the dominated mode rises with increasing collisionality, as shown in figure 12(b), where

the mode is a dissipative trapped electron mode (DTEM). Experimentally, the turbulence is excited as the collisionality exceeds a threshold as shown in figure 11(a). Thus, the dependence on collisionality suggests that the observed ELT could be a DTEM. It is known that the linear growth rate
<figure><img src="image_15.png" /><figcaption>**Figure 12.** Dependence of (a) real frequency and (b) growth rate on the collisionality at times before and after SMBI.</figcaption></figure>

of the DTEM increases with the collisional frequency [74]. The turbulence simulation shows that the direction of particle flux induced by DTEM can be reversed by collisions. A radially inward particle flux is obtained and the diffusivity is inversely proportional to the collisional frequency by gyrokinetic simulation [75].

**6. Conclusions and discussions**

Excitation of an ELT due to impurity injection is observed in the HL-2A tokamak. After the Ne impurity seeding by SMBI, it is interesting to note that the increase in the radiation power is mainly localized at $\rho = 0.7 \sim 0.9$. The impurity radiative layer persists for 100 ms, much longer than the SMBI pulse length (1.2 ms) and the energy confinement time (15 ms). This indicates that the Ne impurity is mainly concentrated in a localized region, with negligible radial transport under the free source of impurity. Broadband turbulence with frequency in the range of 25–70 kHz, namely ELT, is observed after impurity seeding from BES measurements in the region where the impurity ions are concentrated. Poloidal correlation analysis shows that the normalized poloidal wavenumber of ELT is $0.1 \le k_\theta \rho_s \le 0.4$ and ELT propagates in the electron diamagnetic drift direction, indicating that the ELT is an electrostatic turbulence. Turbulence simulations with CGYRO suggest that there is a threshold value in the collisionality for the excitation of the ELT, which is in qualitative agreement with the

experimental observations. This prediction suggests that the observed ELT could be a DTEM. In turn, the excited turbulence may play a role in maintaining the edge peaked impurity profile. Our findings may offer a promising approach for the formation of a stable impurity radiative layer through impurity injection in tokamaks.

In addition, figure 13 shows the heat flux and ion saturation current on the outer divertor targets before and after Ne injection measured by Langmuir probes [76]. Both the peak heat flux and ion saturation current do not change much after Ne injection. The results demonstrate that the plasma remains attached to the divertor targets after Ne injection.

The key point in this experiment is how to maintain a stable impurity radiative layer at the edge region without continuously impurity injection. In general, impurity diffusion occurs in the opposite direction of the impurity density gradient. Therefore, impurity diffusion is inward on the impurity positive density gradient and outward on the impurity negative density gradient side. Combining the experimental observations, the impurity convection should be balanced by diffusion. Thus, it can be inferred that the direction of impurity convection is outward in the impurity positive gradient and inward in the impurity negative gradient region as shown by schematic diagram in figure 14. Convective velocity mechanisms in the theory of impurity turbulent transport have been extensively investigated [77–79]. Impurity convection consists of three terms, including curvature pinch, thermodiffusion pinch,
<figure><img src="image_16.png" /><figcaption>**Figure 13.** (a) Heat flux and (b) ion saturation current on the outer divertor targets before and after Ne injection measured by Langmuir probes.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 14.** Schematic diagram of impurity transport; table: type of convective pinch and its direction on TEM.</figcaption></figure>

and parallel impurity compression [80]. First, the curvature pinch arises from the perpendicular compression of $E \times B$ drift velocity. Its direction is determined by the sign of the normalized precession frequency, which depends on the magnetic shear. In general, for positive magnetic shear, the curvature pinch is inward, regardless of plasma turbulence [81]. In contrast, strong negative magnetic shear can reverse the sign of the convection, driving the impurity particle toward the edge. Second, the thermodiffusion pinch term originates from the compression of the diamagnetic drift velocity, and is proportional to the temperature gradient [82, 83]. The direction of thermodiffusion is inward when the TEM turbulence is dominated and outward when the ITG turbulence is dominated. In addition, this term is dependent on $1/Z$, decreasing with higher $Z$ impurities. The third one is the parallel compression pinch term from parallel velocity fluctuations produced

along the field line by the electrostatic turbulence [84]. The parallel compression pinch is directed outwards for electron turbulence, and inwards for ion turbulence. It increases with increasing $Z/A$ and decreasing $q$. The direction of impurity convection due to TEM is summarized in the table of figure 14. The final direction of total convection is determined by the contributions of the main components. Experimental observations suggest that the parallel impurity compression pinch dominates in the impurity positive density gradient region, while the thermodiffusion pinch or curvature pinch dominate in the impurity negative gradient region. However, the magnetic shear and the temperature gradient do not undergo significant changes in the impurity concentration region, and the reversal of impurity convection is probably due to the polarity of the impurity density gradient. The theoretical work investigating the dependence of the parallel impurity compression on
impurity density gradient is beyond the scope of this paper and the comparison of the experiments will be explored in future work.

## Acknowledgment

This work is supported by the National Natural Science Foundation of China under Grant Nos. U21A20440, 11925501 and 12205083, and National Key R&D Program of China with No. 2022YFE03180200.

## ORCID iDs

G.Q. Xue https://orcid.org/0000-0002-8583-9918

J.M. Gao https://orcid.org/0000-0002-9546-5139

R. Ke https://orcid.org/0000-0003-0117-0098

X.R. Zhang https://orcid.org/0000-0002-1943-8913

M.K. Han https://orcid.org/0000-0001-8735-8440

X.X. He https://orcid.org/0000-0002-1088-4845

T.B. Wang https://orcid.org/0000-0003-1759-3853

J. Wen https://orcid.org/0000-0001-6645-3641

J.Q. Xu https://orcid.org/0000-0003-4834-3005

M. Xu https://orcid.org/0009-0001-3059-7026

## References

[1] Asakura N. et al 2010 *J. Plasma Fusion Res.* **9** 136–41
(available at: www.jspf.or.jp/JPFRS)

[2] Huber A. et al 2014 *41st EPS Conf. on Controlled Fusion and Plasma Physics (Berlin, Germany)* (available at: https://info.fusion.ciemat.es/OCS/EPS2014PAP)

[3] Messiaen A.M. et al 1994 *Nucl. Fusion* **34** 825

[4] Zhang K. et al 2016 *Chin. Phys. B* **25** 065202

[5] Field A.R. et al 2017 *Plasma Phys. Control. Fusion* **59** 095003

[6] Loarte A. et al 1998 *Nucl. Fusion* **38** 331

[7] Wang L. et al 2021 *Nat. Commun.* **12** 1365

[8] Eldon D. et al 2019 *Nucl. Mater. Energy* **18** 285

[9] Reinold F. et al 2015 *Nucl. Fusion* **55** 033004

[10] Kallenbach A. et al 2015 *Nucl. Fusion* **55** 053026

[11] Xu G.S. et al 2020 *Nucl. Fusion* **60** 086001

[12] Gao J.M. et al 2023 *Nucl. Fusion* **63** 036006

[13] Beurskens M.N.A. et al 2008 *Nucl. Fusion* **48** 095004

[14] Maingi R. et al 2011 *Phys. Rev. Lett.* **107** 145004

[15] Osborne T.H. et al 2015 *Nucl. Fusion* **55** 063018

[16] Urano H., Nakata M., Aiba N., Kubo H., Honda M., Hayashi N., Yoshida M. and Kamada Y. 2015 *Nucl. Fusion* **55** 033010

[17] Zhong W.L. et al 2019 *Nucl. Fusion* **59** 076033

[18] Maingi R. et al 2009 *Phys. Rev. Lett.* **103** 075001

[19] Canik J.M. et al 2011 *Phys. Plasmas* **18** 056118

[20] Xue G.Q. et al 2021 *Nucl. Fusion* **61** 116048

[21] Glöggler S. et al 2019 *Nucl. Fusion* **59** 126031

[22] Gabriellini S. et al 2023 *Nucl. Fusion* **63** 086025

[23] Nespoli F. et al 2023 *Nucl. Fusion* **63** 076001

[24] Nespoli F. et al 2022 *Nat. Phys.* **18** 350–6

[25] Hu J.S. et al 2015 *Phys. Rev. Lett.* **114** 055001

[26] Sun Z. et al 2021 *Nucl. Fusion* **61** 014002

[27] Kinoshita T. et al 2024 *Phys. Rev. Lett.* **132** 235101

[28] Toi K. et al 2004 *Nucl. Fusion* **44** 217

[29] Staebler G.M., Jackson G.L., West W.P., Allen S.L., Groebner R.J., Schaffer M.J. and Whyte D.G. 1999 *Phys. Rev. Lett.* **82** 1692

[30] McKee G., Burrell K., Fonck R., Jackson G., Murakami M., Staebler G., Thomas D. and West P. 2000 *Phys. Rev. Lett.* **84** 9

[31] Tokar M.Z., Ongena J., Unterberg B. and Weynants R.R. 2000 *Phys. Rev. Lett.* **84** 5

[32] Bodner G. et al 2024 *Plasma Phys. Control. Fusion* **66** 045022

[33] Bodner G. et al 2022 *Nucl. Fusion* **62** 086020

[34] Lunsford R. et al 2021 *Phys. Plasmas* **28** 082506

[35] Li J. et al 2021 *Nucl. Fusion* **61** 126008

[36] Dominguez R.R. and Staebler G.M. 1993 *Nucl. Fusion* **33** 51

[37] Moradi S., Tokar M.Z. and Weyssow B. 2010 *Phys. Plasmas* **17** 012101

[38] Du H., Wang Z.-X. and Dong J.Q. 2016 *Phys. Plasmas* **23** 072106

[39] Tsang K.T. et al 1977 *Nucl. Fusion* **17** 261

[40] Zurro B. et al 2014 *Plasma Phys. Control. Fusion* **56** 124007

[41] Guo W., Wang L. and Zhuang G. 2016 *Phys. Plasmas* **23** 112301

[42] Isler R.C. et al 1984 *Nucl. Fusion* **24** 1599

[43] Li Y.G., Zhou Y., Li Y., Deng Z.C., Wang H.X. and Yi J. 2017 *Rev. Sci. Instrum.* **88** 083508

[44] Zhong W.L. et al 2014 *Rev. Sci. Instrum.* **85** 013507

[45] Shi Z.B. et al 2014 *Rev. Sci. Instrum.* **85** 023510

[46] Yu D.L. et al 2014 *Rev. Sci. Instrum.* **85** 11E402

[47] Liu L. et al 2020 *IEEE Trans. Plasma Sci.* **48** 8

[48] Ke R. et al 2018 *Rev. Sci. Instrum.* **89** 10D122

[49] Zhong W.L. et al 2015 *Nucl. Fusion* **55** 113005

[50] Yao L. et al 2001 *Nucl. Fusion* **41** 817

[51] Takenaga H. et al 2009 *J. Nucl. Mater.* **390–1** 869–75

[52] Murakami A., Miyazawa J., Suzuki C., Yamada I., Morisaki T., Sakamoto R. and Yamada H. 2012 *Plasma Phys. Control. Fusion* **54** 055006

[53] Zheng X., Li J., Hu J., Li J., Ding R., Cao B. and Wu J. 2013 *Plasma Phys. Control. Fusion* **55** 115010

[54] Yao L.H. et al 2004 *Nucl. Fusion* **44** 420

[55] Chen C.Y. et al 2016 *Rev. Sci. Instrum.* **87** 093503

[56] Xiao W.W. et al 2010 *Rev. Sci. Instrum.* **81** 013506

[57] Xiao W.W. et al 2010 *Phys. Rev. Lett.* **104** 215001

[58] Zhong W.L. et al 2020 *Nucl. Fusion* **60** 082002

[59] Xiao G.L. et al 2023 *Rev. Mod. Plasma Phys.* **7** 2

[60] Hulse R.A. et al 1983 *Nucl. Technol. Fusion* **3** 259–72

[61] Casali L. 2014 et al *EPJ Web Conf. vol* 79 p 01007

[62] Wang T.B. et al 2022 *Plasma Phys. Control. Fusion* **64** 084003

[63] Summers H.P. 1994 Atomic data and analysis structure user manual

[64] Beall J.M., Kim Y.C. and Powers E.J. 1982 *J. Appl. Phys.* **53** 3933

[65] Zhong W.L. et al 2010 *Phys. Plasmas* **17** 112307

[66] Shi Z.B. et al 2016 *Rev. Sci. Instrum.* **87** 113501

[67] Zou X.L. et al 1999 *26th EPS Conf. On Controlled Fusion and Plasma Physics (Maastricht, The Netherlands)* pp 1041–4 (available at: https://infoscience.epfl.ch/entities/publication/b65490e3-9881-4a35-9b28-a0ee922272e6)

[68] Conway G.D. et al 2004 *Plasma Phys. Control. Fusion* **46** 951

[69] Hirsch M., Holzhauer E., Baldzuhn J., Kurzan B. and Scott B. 2001 *Plasma Phys. Control. Fusion* **43** 1641

[70] McKee G.R. et al 2000 *Phys. Plasmas* **7** 1870

[71] Sauter O., Angioni C. and Lin-Liu Y.R. 1999 *Phys. Plasmas* **6** 2834

[72] Conway G.D. et al 2006 *Nucl. Fusion* **46** S799

[73] Candy J., Belli E.A. and Bravenec R.V. 2016 *J. Comput. Phys.* **324** 73

[74] Xu X.Q. and Rosenbluth M.N. 1991 *Phys. Fluids B* **3** 627
[75] Zhao C. and Xiao Y. 2018 *Phys. Plasmas* **25** 102303

[76] Huang Z.H. et al 2022 *Plasma Sci. Technol.* **24** 054002

[77] Gravier E. et al 2019 *Phys. Plasmas* **26** 082306

[78] Garbet X., Dubuit N., Asp E., Sarazin Y., Bourdelle C., Ghendrih P. and Hoang G.T. 2005 *Phys. Plasmas* **12** 082511

[79] Futatani S., Garbet X., Benkadda S. and Dubuit N. 2010 *Phys. Rev. Lett.* **104** 015003

[80] Weiland J., Jarmén A.B. and Nordman H. 1989 *Nucl. Fusion* **29** 70

[81] Isichenko M.B., Gruzinov A.V. and Diamond P.H. 1995 *Phys. Rev. Lett.* **74** 22

[82] Garbet X., Garzotti L., Mantica P., Nordman H., Valovic M., Weisen H. and Angioni C. 2003 *Phys. Rev. Lett.* **91** 03500

[83] Lim K., Gravier E., Lesur M., Garbet X., Sarazin Y. and Médina J. 2020 *Plasma Phys. Control. Fusion* **62** 095018

[84] Angioni C. and Peeters A.G. 2006 *Phys. Rev. Lett.* **96** 095003