# Influence of 3 T magnetic field on RF plasma self-bias and erosion pattern for the ITER wide angle viewing system

Tomás C Sousa, Artem Dmitriev, Paul Hiret, Youpeng Wang, Laurent Marot, Ernst Meyer, Jérôme Moritz, Oliver Bieri, Sébastien Dine, Frederic Le Guern, et al.

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## To cite this version:

Tomás C Sousa, Artem Dmitriev, Paul Hiret, Youpeng Wang, Laurent Marot, et al. Influence of 3 T magnetic field on RF plasma self-bias and erosion pattern for the ITER wide angle viewing system. Nuclear Fusion, 2026, 66, pp.026042. 10.1088/1741-4326/ae358d. hal-05472254

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https://hal.science/hal-05472254v1

Submitted on 22 Jan 2026

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# Influence of 3 T magnetic field on RF plasma self-bias and erosion pattern for the ITER wide angle viewing system

To cite this article: Tomás C. Sousa et al 2026 *Nucl. Fusion* **66** 026042

View the [article online](#) for updates and enhancements.

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# Influence of 3T magnetic field on RF plasma self-bias and erosion pattern for the ITER wide angle viewing system

Tomás C. Sousa<fnref n="1" />,*, Artem Dmitriev<fnref n="1" />,*, Paul Hiret<fnref n="1" />,*, Youpeng Wang<fnref n="1" />,*, Laurent Marot<fnref n="1" />,*, Ernst Meyer<fnref n="1" />,*, Jérôme Moritz<fnref n="2" />,*, Oliver Bieri<fnref n="3" />,<fnref n="4" />,*, Sébastien Dine<fnref n="5" />,*, Frederic Le Guern<fnref n="6" />, Juan Piqueras<fnref n="6" /> and Sebastien Vives<fnref n="7" />

E-mail: tomas.sousa@unibas.ch

Received 7 October 2025, revised 15 December 2025

Accepted for publication 8 January 2026

Published 21 January 2026

<figure><img src="image_9.png" /></figure>

CrossMark

## Abstract

The ITER Wide Angle Viewing System (WAVS) relies on in-vessel metallic first mirrors (FM) to transport optical signals from the plasma to out-of-vessel diagnostics. Under the conditions of ITER, energetic plasma particles induce sputtering of plasma-facing components, and the resulting sputtered material can re-deposit on these mirrors, resulting in reflectivity degradation. In-situ plasma removal using radio-frequency (RF) capacitively coupled plasma discharges is one of the proposed methods to mitigate this issue. Experiments were conducted in a 3 T homogeneous magnetic field using a medical magnetic resonance imaging (MRI) system at the University Hospital of Basel. The WAVS First Mirror Unit consists of two mirror electrodes (M1 and M2), one or both of which can be powered with RF, while the remaining surfaces act as grounded areas. Systematic measurements of the self-bias voltage were performed for varying discharge conditions, including gas species (argon and helium), pressure (0.1–10 Pa) and mirror capacitance, as well as the angle $\theta$ between the mirror surface and magnetic field. The results revealed a strong dependence of the self-bias on the grounded-to-powered area ratio $A_g/A_p$, which is affected by the magnetic confinement. Erosion and deposition patterns were examined in cases where one or both mirrors were powered. Erosion occurred primarily where a powered mirror faces a grounded surface, whereas overlapping discharges in the dual-powered case resulted in low ion kinetic energy and preferential net deposition at the overlapping area. These

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<footnote n="1">¹ Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland</footnote>

<footnote n="2">² Institut Jean Lamour, Université de Lorraine, 2 allée André Guinier, BP 50840, 54011, Nancy, France</footnote>

<footnote n="3">³ Division of Radiological Physics, Department of Radiology, University Hospital Basel, Petersgraben 4, 4031 Basel, Switzerland</footnote>

<footnote n="4">⁴ Department of Biomedical Engineering, University of Basel, Gewerbestrasse 14, 4123 Allschwil, Switzerland</footnote>

<footnote n="5">⁵ SOLAYL SAS, 4 Impasse Agrippa d'Aubigné, 28000 Chartres, France</footnote>

<footnote n="6">⁶ F4E, c/Josep Pla 2, E-08019 Barcelona, Spain</footnote>

<footnote n="7">⁷ IRFM, CEA, F-13108 Saint Paul lez Durance, France</footnote>

<footnote>* Author to whom any correspondence should be addressed.</footnote>
observations were supported by one-dimensional simulations of plasma potential evolution under representative conditions. These results provide critical insight into RF plasma behaviour in fully magnetised, geometrically asymmetric discharges and support the development of robust in-situ cleaning strategies for ITER optical diagnostics.

Supplementary material for this article is available online

Keywords: ITER, first mirror, plasma cleaning, wide angle viewing system, magnetised plasma, low-temperature plasma physics

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

# 1. Introduction

Optical diagnostic systems in ITER rely on First Mirror Units (FMUs) to reflect optical signals from the fusion plasma to out-of-vessel diagnostics. These metallic mirrors are exposed to the full range of plasma-facing conditions, including irradiation by neutrons and gammas, high heat loads, erosion from fast neutrals originating from charge-exchange reactions, and deposition of eroded materials from Plasma Facing Components (PFCs) [1, 2].

The revised ITER baseline [3, 4] introduces two major modifications relevant to the erosion and deposition dynamics on first mirrors. The full replacement of beryllium (Be) with tungsten (W) as the primary first wall material alters the nature of the eroded species transported through the plasma [5]. Secondly, to recover the oxygen gettering capacity lost with beryllium, a 50–100 nm boron coating will be periodically deposited via glow discharge boronisation [3, 6]. These changes will likely shift the impurity transport and deposition profile near FMUs, especially in regions of reduced plasma exposure, where boron redeposition may become non-negligible [1]. The implications of these changes on the optical properties of the metallic first mirrors must therefore be re-evaluated under these new ITER-relevant conditions.

To restore optical reflectivity and maintain diagnostic performance, various mirror recovery techniques have been investigated. Among them, radio-frequency (RF) low-temperature capacitively coupled plasma (CCP) discharges have emerged as the most promising candidate, owing to their compatibility with remote activation and potential to homogeneously sputter deposited contaminants using energetic ion bombardment [7–13]. These discharges have been studied across a wide range of configurations, including different driving frequencies, pressures, gas compositions, mirror geometries, and electrical circuit designs. While most experimental campaigns were performed under simplified geometries and low or even no magnetic field, the new updated baseline introduces a new level of complexity due to operational constraints.

The Wide Angle Viewing System (WAVS) is one of many diagnostics in ITER situated in four equatorial ports (3, 9, 12 and 17). They measure electromagnetic radiation originating from the tokamak [14–16]. The light collection is done through FMUs that face a toroidal magnetic field of up to 3.5 T with

viewing angles between magnetic field lines and the first mirrors varying with port geometry [17]. Any successful in-situ cleaning method must therefore operate reliably under such high-field conditions and with different angles between the magnetic field and the face of the mirrors [18, 19].

The operation of CCP plasma in the presence of strong magnetic fields introduces a set of physical phenomena fundamentally distinct from unmagnetised discharges [20]. Charged particles are confined along magnetic field lines due to the Lorentz force, leading to the formation of elongated plasma columns with strongly suppressed cross-field transport [21]. This confinement occurs when the cyclotron frequency of species $x$, $\omega_{cx} = q_x|\vec{B}|/m_x$, where $q_x$ is the charge, $|\vec{B}|$ is the magnitude of the magnetic field and $m_x$ is the mass, exceeds the particle collision frequency $\nu_x$, a regime in which particles are designated as magnetised, both electrons and ions reach this regime at 3 T, 1 Pa, which are typical values in the first mirror environments in a tokamak [22]. As a consequence, the plasma potential ($V_p$) increases significantly compared to the unmagnetised case [20]. This occurs due to the reduced electron loss across the field, requiring a steeper potential gradient to maintain ambipolarity, i.e. the net flux of ions and electrons should be zero [20]. The elevated $V_p$ enhances ion energies at surfaces, even non-powered ones, potentially leading to erosion where deposition would otherwise occur.

Other relevant experimental parameters were investigated, including the gas species, argon (Ar) or helium (He), as well as operating pressures of 0.1, 1, and 10 Pa. Both the gas species and the operating pressure significantly influence the plasma characteristics. Given the notable mass disparity ($m_{Ar}/m_{He} \approx 10$), the plasma sheath dynamics and consequently the ion energy distribution functions vary substantially. As pressure increases, the sheath transitions from collisionless to a more collisional regime, significantly altering the sheath behaviour and ion dynamics [23]. Additionally, the mirror capacitance was modified by incorporating a ceramic plate. This ceramic provides electrical isolation between the optical mirror surface and the actively water-cooled substrate, thus ensuring that RF-driven plasma formation occurs exclusively at the mirror surface [24]. To better understand the plasma parameters under our experimental conditions, one spatial dimension and three velocity component (1d/3v) Particle-in-Cell (PIC)
simulations [25, 26] were performed for three distinct representative experimental configurations.

These coupled field-induced confinement effects on ion dynamics, elevated plasma potentials, and evolving first wall materials highlight the necessity of evaluating FMU cleaning strategies in ITER. This study aims to characterise the interplay between magnetic field strength, experimental parameters, and erosion/deposition behaviour in a configuration replicating ITER's diagnostic geometry, using a 3T homogeneous magnetic field and compare these results with similar experimental conditions from our previous paper at 0T [24].

## 2. Experimental

### 2.1. WAVS in MRI of university hospital of Basel

The WAVS mock-up comprises two mirrors, M1 and M2 (figure 1), the path followed by light from the fusion plasma onto the optical diagnostics sensors is schematised by the yellow arrows. More information on the WAVS mock-up is available in our previous contribution [24]. In this configuration, each mirror contains non-magnetic metallic stainless steel insets that can be used for erosion/deposition studies (more details are given in section 4). The mirror areas are similar with M1 at $1.52 \times 10^{-3} \text{ m}^2$ and M2 at $1.73 \times 10^{-3} \text{ m}^2$. The angle between the mirrors is 19.6°. The dimensions of the FMU assembly are $0.3 \text{ m} \times 0.31 \text{ m} \times 0.24 \text{ m}$. The mock-up was placed inside a cylindrical aluminium vacuum chamber with a length of 1.20 m and a diameter of 0.52 m fixed on a support base on-rails (figure 2). The vacuum chamber was then placed in the magnetic isocenter of the bore of a magnetic resonance imaging (MRI) machine (MAGNETOM Prisma, Siemens Healthineers, Forchheim, Germany) providing a 3T static and homogeneous (1 part per million over an empty cylindrical volume with a 0.5 m diameter that covers the WAVS) magnetic field, achieved by superconducting coils cooled with liquid helium. The vacuum chamber was evacuated to a base pressure of $10^{-4} \text{ Pa}$ using a turbo-molecular pump outside the MRI room. The vacuum and gas inlet connections were made with long (>6 m) lines, and the pressure was controlled using a Penning gauge placed close to the turbo pump. To assess the pressure in a magnetic environment, the pressure inside the chamber was calibrated beforehand (without magnetic field) for different Ar and He flows using a Baratron mounted directly on the chamber and a Penning gauge close to the turbo pump. This system was mounted at the University Hospital of Basel and a photo of the MRI machine and of the vacuum chamber as well as the Computer-Aided Design (CAD) of the chamber are provided in figure 2.

The RF CCPs discharges were generated using a 13.56 MHz RF generator (Hüttinger Elektronik, model PFG 300 RF) coupled to a matching network (model PFM 1500). The output of the matchbox was connected either through a T-connector (in the dual-powered configuration) or directly to the powered mirror (in single-powered configuration). The connections were made using S_10 172_B-11 low loss coaxial cable in air and RG393 in vacuum. During the experiment it

<figure><img src="image_11.png" /><figcaption>**Figure 1.** The Wide Angle Viewing System (WAVS) mock-up features a First Mirror Unit (FMU) composed of two opposing mirrors. (a) Front view: M1 is positioned on the left and M2 on the right; yellow arrows illustrate the path of incident light as it enters the system and undergoes successive reflection by both mirrors. (b) Top view: M1 is on the right and M2 on the left; the angle $\alpha$ denotes the angle between the magnetic field and the mirror surface. Each mirror contains embedded sample insets for material characterisation.</figcaption></figure>

was possible to either power both mirrors (dual powered configuration) or power only one (single powered M1 or M2 configurations), the schematic can be consulted in the supplementary information (figure S1). The capacitance of M1 was also varied by adding a ceramic plate, changing between 113 pF and 486 pF while the capacitance of M2 was fixed at 795 pF [24]. The angle of the mirrors' surface in relation to the magnetic field $\alpha$ was also varied according to two possible equatorial port views, the divertor and left view [14]. Therefore, $\alpha$ for the pair (M1, M2) was (11°, 9°) for the divertor view and (55°, 35°) for left view, replicating the ITER configuration and shown in table 1.

### 2.2. Mirror insets

The insets of mirrors M1 and M2 were characterised using x-ray photoelectron spectroscopy (XPS) for surface chemical analysis, energy-dispersive x-ray spectroscopy (EDX)
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 2.** (a) MRI system located at the University Hospital of Basel, with the cylindrical vacuum chamber inserted into the bore of the scanner on the right. (b) CAD schematic of the WAVS mock-up inside the vacuum chamber. The FMU was mounted on a base allowing precise rotation to predefined orientations, enabling control over the angle between the mirrors surface and the external magnetic field.</figcaption></figure>

<table><caption>**Table 1.** Angles of the surface of M1 and M2 to the magnetic field for different ITER views based on the location of the WAVS in the reactor [14, 15].</caption><thead><tr><th>Views</th><th>M1 (°)</th><th>M2 (°)</th></tr></thead><tbody><tr><td>Right</td><td>67</td><td>43</td></tr><tr><td>Left</td><td>58</td><td>38</td></tr><tr><td>Divertor</td><td>11</td><td>9</td></tr></tbody></table>

and focused ion beam (FIB) for thickness determination of the deposited layers. For XPS analysis, the electron spectrometer was equipped with a hemispherical analyser (Leybold EA10/100 MCD) and a non-monochromatic Mg Kα x-ray source (hν = 1253.6 eV) to perform atomic core-level spectroscopy. The EDX measurements were conducted using a Philips XL30 Scanning Electron Microscope (SEM). Five spectra were acquired at positions near the sample centre, each at different electron beam energies (5, 10, 15, 20, and 25 kV). These spectra were subsequently fitted with STRATAGem software [27–29]. The thickness determination using EDX procedure follows the methodology detailed in chapter 3.2 of [30, 31]. The Zeiss FIB-SEM Crossbeam 540

was used for cross-section analysis and direct thickness measurement. Some sample insets were characterised before and after plasma exposure using either XPS, SEM, EDX, or a combination of these techniques; in contrast, FIB was only done after plasma exposure.

### 2.3. SimSmith RF circuit simulations

A full RF model at 13.56 MHz of the setup was developed with the RF circuit analysis software SimSmith 18.4 [32]. The same modelling tool was used by the authors in a previous study [24]. The formalism in SimSmith allows for a block element approach and it is well suited for this work as it also allows to compare directly experimental measurements like self-bias as a function of generator power and study the influence of the power coupling efficiency (PCE), defined as the power provided to the discharge divided by the generator power for different cable lengths and cable types in our experiment. The circuit has the RF power generator and the CCP discharge at both ends and the matching network in the middle; mirror M1 is biased and mirror M2 is grounded. The impedance of the M1 mirror is represented as a simple capacitor, $C_{MIRROR}$, shunting the CCP discharge impedance.

## 3. Results and discussion

### 3.1. Influence of magnetic field on self-bias as a function of experimental conditions (0 vs. 3 T)

The following section studies the self-bias as a function of applied power compared with the similar case at 0 T [24]. A further analysis is provided to differentiate the influence of the field and of the varied experimental parameters such as gas, pressure, mirror capacitance and angle of the mirror in relation to the magnetic field. Assuming a primarily capacitive sheath at 1 Pa, 13.56 MHz, the electrode(s) self-bias $V_{DC}$ can be calculated as [19, 33]:

$$V_{\text{DC}} = V_{\text{RF}} \frac{(L_g/L_p) - (A_g/A_p)}{(L_g/L_p) + (A_g/A_p)} \quad (1)$$

where $V_{RF}$ is the amplitude of the applied RF potential, $A_g$ is the plasma-wetted grounded area, i.e. the portion of the grounded surface directly in contact with the plasma, $A_p$ is the powered area, and $L_g, L_p$ are respectively the sheath thicknesses at these grounded plasma-wetted and powered areas. The results in the following subsections have direct impact on the applied RF potential, sheath thickness and the area ratio $A_g/A_p$. The latter being the most dominant effect for the difference between the 0 and 3 T case since, as it was mentioned in the introduction, by applying a magnetic field the radial diffusion of the electrons sharply decreases and the plasma is confined along the magnetic field lines, as a consequence the plasma wetted grounded area decreases and the magnitude of the self-bias decreases [11, 18, 19].

To achieve impedance matching, different cable lengths were used (from 5.5 to 7 m), therefore the PCE may be
affected. To ensure that the results from different discharges, in the following subsections, can be compared independently from the cable lengths one has to prove that the PCE does not varies significantly in range of interest. For that a full RF model at 13.56 MHz of the setup was simulated with SimSmith where PCE was calculated as a function of the cable lengths. This results showed the maximum PCE was 1.5% for single and 1.6% for dual powered mirrors (figure S2). It has to be noted that for this configuration the PCE range was low, of 1 to 7% [24]. Therefore, given that the discharge power is not changing significantly with the provided power, it can be concluded that the range of cable length used in this work does not change significantly the results and therefore it can be discarded when comparing the results.

### 3.1.1. Gas.
Measurements were done at 0 and 3 T at 1 Pa for mirror capacitances of 113 pF (M1) and 795 pF (M2), and $\alpha$ was 55° for M1, 35° for M2. Figure 3 illustrates this behaviour for both single- and dual-mirror powered configurations (figures 3(a) and (b) respectively). Although the gas-dependent difference is small for the single-powered mirror, both configurations show the same trend: achieving a given self-bias requires more power in He than in Ar. This difference can be attributed to different ion mobility and ionisation energy. As mentioned in section 1, $m_{Ar}/m_{He} \approx 10$, so the ion Bohm speed $c_s = \sqrt{kT_e/m_i}$ is around 3.2 times lower for Ar than for He for the same $T_e$. The time-averaged ion current density $J_i \approx en_s c_s$ is therefore smaller for Ar for the same density in front of the sheath $n_s$. To maintain zero net current over each RF cycle, the powered electrode must therefore develop a more negative bias in Ar than in He to compensate. Concerning the ionisation energy, He has a significantly higher ionisation energy ($E_{ion,He} = 24.6$ eV) compared to Ar ($E_{ion,Ar} = 15.8$ eV) and a lower ionisation cross-section, resulting in a lower ionisation rate and consequently lower plasma density $n_s$ compared to Ar under identical RF discharge conditions [34, 35]. To achieve a comparable ion flux and sheath potential, the electron temperature must therefore increase, necessitating higher input power in He discharges.

The quantities $\Delta P_g$ and $\Delta P_B$ represent the power difference required to maintain a fixed self-bias when changing gas species (He versus Ar) and magnetic field conditions (0 versus 3 T), respectively. For both single and dual powered configurations, the gas-dependent power difference $\Delta P_g$ remains relatively small and nearly constant, with a maximum difference around 50 W observed for the dual-powered configuration at 3 T. In contrast, the field-dependent power difference $\Delta P_B$ significantly increases with more negative self-bias voltages for both gases, reaching up to around 200 W for He at the dual-powered configuration. The increase of $\Delta P_B$ can be explained by different rate of change of the self-bias with power. These different slopes arise because magnetic confinement at 3 T reduces the plasma-wetted grounded area, altering the grounded-to-powered area ratio and thus changing the rate at which the self-bias decreases with power, as described by

<figure><img src="image_14.png" /><figcaption>**Figure 3.** Self-bias voltage as a function of input RF power for He and Ar plasmas under magnetic fields of 0 and 3 T. (a) Single powered cleaning configuration. (b) Dual mirror powered configuration (figure S1). In both cases, the applied pressure was fixed at 1 Pa and the mirror capacitances were M1 - 113 pF and M2 - 795 pF. The variables $\Delta P_B$ and $\Delta P_g$ are the power differences by changing field and changing gas, respectively.</figcaption></figure>

equation (1). Another way of interpreting it is that by fixing an Ag/Ag, equation (1) becomes a function of $V_{RF}$ (which is a function of power) and can be though out as a line equation starting at the origin. With magnetic field, Ag becomes smaller, therefore the slope becomes smaller in relation to the non-magnetic case. This yields two lines with different slope which naturally have an increase of $\Delta P_B$.

The effect of magnetic field strength on required RF power is comparable for both single and dual powered electrode configurations and shows minimal dependency on the gas species.

### 3.1.2. Pressure.
As the pressure increased, the magnitude of the self-bias voltage decreased significantly (figure 4). This behaviour was primarily attributed to the increased collisionality of the plasma species with the background gas, which affected sheath dynamics and voltage distribution. Indeed, at
<figure><img src="image_15.png" /><figcaption>**Figure 4.** Self-bias voltage as a function of input RF power in Ar plasmas at three different pressures: 0.1, 1, and 10 Pa. (a) Discharges at 0 T; (b) Discharges at 3 T.</figcaption></figure>

higher pressures, the density of neutral atoms ($n_n$) increased, leading to a decrease in the mean free path ($\lambda$) of electrons and ions and an increase in the number of collisions.

The increased frequency of ion-neutral collisions alters the electrical character of the sheath, making it more resistive. Consequently, the applied RF voltage is distributed more symmetrically between the grounded and powered electrode sheaths, even in a geometrically asymmetric configuration. This reduction in sheath voltage asymmetry results in a smaller self-bias, meaning that the powered electrode becomes less negatively biased [23]. However, at 3 T a non-monotonic trend is observed between 0.1 and 1 Pa (figure 4(b)): the self-bias voltage becomes more negative with increasing pressure. This unexpected increase in self-bias may be attributed to differences in electron and ion magnetisation. When the mean free path $\lambda$ becomes comparable to the ion Larmor radius $R_L$, ions become effectively more mobile due to increased cross-field transport, whereas electrons—more strongly magnetised—experience reduced mobility. Without magnetic field ions and

electrons outside sheath regions move without a preferential direction, while under the present conditions, the cross-field electron mobility is reduced by ~ $10^9$ and the ions by ~ $10^4$ in relation to the parallel-field mobility. Both species are magnetised, but ions remain effectively much more mobile across the field than electrons. This differential behaviour can influence the sheath dynamics and alter the self-bias voltage, as discussed in [36]. Further measurements are required to confirm and quantify this effect.

A pronounced difference was observed between the 0 T and 3 T conditions in figure 4. At 50 W, the self-bias reached approximately –500 V at 0 T for both 0.1 and 1 Pa, whereas at 3 T it was significantly reduced to around –200 V, corresponding to a reduction of about 60%. At 10 Pa, the discrepancy became even more pronounced: at 100 W, the self-bias was approximately –500 V at 0 T, but dropped to roughly –150 V at 3 T, indicating a reduction of nearly 70%. This behaviour resulted from the combined effects of collisional sheath dynamics and magnetic confinement, which restricts cross-field transport and reduces the grounded surface area accessible to the plasma.

### 3.1.3. Mirror capacitance.
The change in capacitance did not significantly affect the transmitted power to the load (mirror + plasma). An increase in mirror capacitance, decreases the imaginary part of the load, therefore reducing the load impedance, allowing a higher current to flow. As a result, the RF voltage decreased to maintain nearly constant power delivery. Similar observations have been reported in [24]. In this work, the same trend is reproduced in figure 5, both with and without an applied magnetic field, where the self-bias of single powered mirrors with varying capacitances was analysed. SimSmith simulations were carried out for our configuration by varying the mirror capacitance and calculating the self-bias as a function of power, as shown in figure 5(a). Consequently, the effective RF voltage $V_{RF}$ decreases. At 13.56 MHz and $B=0$, since $A_g \gg A_p$, and from equation (1), when $A_g/A_p \gg L_g/L_p$, it follows that $|V_{DC}| \approx V_{RF}$. This establishes a direct proportionality between the PCE and the self-bias voltage.

## 3.2. Angle in relation to B-field

Understanding the isolated influence of electrode orientation relative to the magnetic field on RF plasma properties—particularly the self-bias voltage—is challenging due to the interplay between sheath dynamics, field-line confinement, and geometric asymmetries. To clarify this analysis and gain insights relevant for the different views of the WAVS, we first examined a simplified model case in the MRI system. This setup (illustrated in figure S4) consisted of two electrodes:
<figure><img src="image_16.png" /><figcaption>**Figure 5.** Self-bias voltage as a function of RF power for three different mirror capacitances (113, 486, and 795 pF) at (a) 0 T and (b) 3 T. In (a) the dashed lines are the simulated data in SimSmith.</figcaption></figure>

a fixed, grounded electrode and a powered electrode whose angle $\theta$ relative to the grounded electrode could be varied.

The measured self-bias voltage increases as a function of the electrode rotation angle $\theta$, reflecting the influence of geometrical asymmetry on plasma behaviour. When the electrodes were nearly perpendicular ($\theta \approx 0^{\circ}$), the plasma generation was weak, resulting in a low self-bias voltage of approximately -15 V. As $\theta$ increased toward 90°, corresponding to a parallel orientation, the self-bias magnitude rose steadily, reaching around -220 V. This trend is shown in figure 6 and highlights the dependence of the self-bias voltage on the grounded-to-powered area ratio ($A_g/A_p$), consistent with equation (1).

To qualitatively describe this dependence, a simplified sinusoidal model ($\sin\theta$) was applied as a first approximation, where $\sin\theta \approx A_g/A_p$. The intuition for this first approach originated from a previous work from Soni et al [19], where the whole chamber was rotated in relation to the magnetic field lines in the MRI chamber and a dependence of

<figure><img src="image_17.png" /><figcaption>**Figure 6.** Effect of electrode orientation on the self-bias voltage $V_{DC}$ at 1 Pa and 50 W in a magnetised RF plasma (3 T). (a) and (b): schematic of the experimental setup with a fixed grounded electrode and a powered electrode rotating from 0° (parallel to the field) to 90° (perpendicular). (c): self-bias voltage as a function of tilt angle $\theta$, alongside a simple model assuming $\sin\theta \approx A_g/A_p$.</figcaption></figure>

$A_g/A_p$ was observed. Using this assumption and starting from equation (1), the self-bias voltage can be approximated as:

$$V_{DC} = V_{RF} \frac{(L_g/L_p) - \sin\theta}{(L_g/L_p) + \sin\theta} \qquad (2)$$

During the experiment the input RF power was set to remain constant, therefore the applied $V_{RF}$ was not fixed but rather adjusted self-consistently with the plasma impedance. As a result, discrepancies between this simple model and experimental data at intermediate angles (as evident from the $\sin\theta$ function in figure 6) might arise from two, not mutually exclusive, main factors:

(i) The simplified assumption $\sin\theta \approx A_g/A_p$ might inadequately represent intermediate angle conditions. Specifically, plasma generated beyond the main column—albeit at lower densities—could still contribute significantly to the current balance. This would effectively enlarge the effective grounded area $A_g$, leading to increased sheath asymmetry and a more negative self-bias voltage.

(ii) In typical non-magnetised discharges, the sheath thickness ratio $L_g/L_p$ is often negligible compared to $A_g/A_p$ due to pronounced geometric asymmetry. However, under magnetised conditions as in this experiment, the effective grounded electrode area $A_g$ is reduced and becomes comparable to $A_p$. Consequently, the sheath thickness ratio $L_g/L_p$ significantly impacts the self-bias as it can become comparable to $A_g/A_p$. Furthermore, the shape and thickness of the sheath strongly depend on the angle between the electrode surface and magnetic field lines, characterised as the Chodura sheath effect [18, 37]. Although direct sheath thickness measurements were not performed, numerical estimates indicate substantial sensitivity: figure S5 illustrates that even a moderate change in $L_g/L_p$ by approximately 0.06 at fixed $A_g/A_p = 0.4$ can alter the self-bias voltage by roughly 65 V. Therefore,
<figure><img src="image_18.png" /><figcaption>**Figure 7.** Schematic representation of the WAVS chamber showing the plasma column orientation and mirror configurations for two ITER-relevant views. Left column: high-angle setup corresponding to the left view with M1 and M2 oriented at 55 and 35°, respectively. Right column: low-angle setup corresponding to the divertor view with M1 and M2 at 11 and 9°. (a), (d) single-powered M1 configuration; (b), (e) single-powered M2 configuration; (c), (f) dual-powered configuration. The horizontal magnetic field $\vec{B}$ confines the plasma along field lines.</figcaption></figure>

slight sheath thickness variations at intermediate electrode angles may explain significant deviations from the simplified sinusoidal model predictions.

Having established the basic understanding from the simplified single-electrode experiment, we now address the more complex case relevant for the WAVS setup, featuring two electrodes within the MRI chamber. Specifically, angular configurations representative of two distinct WAVS orientations, namely, the left and divertor views (table 1). Figure 7 illustrates these two fixed electrode orientations as separate columns, while the rows depict different powering scenarios: single-powered (M1), single-powered (M2) and dual-powered (both M1 and M2).

A precise calculation of the area ratio $A_g/A_p$ for the WAVS setup was difficult to perform due to several reasons:

(i) In the dual-powered case (c), the plasma-wetted column was on both mirrors. This created a unique situation where the ions in the common plasma volume did not clearly

experience a defined grounded area, making it ambiguous how to assign the grounded surface. In figure 8, the powered area for case (c) was considered as the combined area of M1 and M2, and the grounded area was taken as the total surface where plasma contacted grounded elements. This is discussed in more detail in section 4.2.

(ii) In cases (d) and (f), part of the plasma column exited the WAVS chamber and came into contact with the chamber quartz window, as shown in supplementary information figure S3. Since the window was made of a non-conductive material, it could not be considered a grounded surface, and in fact may have charged during operation.

(iii) For all configurations, the complex geometry of the WAVS made it challenging to determine the true grounded area. It was not straightforward to identify which surfaces were actually being wetted by the plasma, and which should be included in the calculation of $A_g$.

Figure 8 displays the self-bias as function of power for several $\alpha$ angles in the cases of single or dual powered electrodes. The grounded to powered area direction, as the single powered M1 case has the highest $A_g/A_p$ and more negative self-bias for a fixed power. As expected from equation (2), the self-bias becomes more negative with increasing $A_g/A_p$. On the right of figure 8 are the schematics of setups, with colour coded borders, for single (b) and (d) or dual powered (a) and (c) cases. The higher $A_g/A_p$, indicated on top of each schematic, the more negative the self-bias is for the same power.

## 4. Erosion dynamics at 0/3 T in the WAVS

In the following section we present our findings regarding the erosion of insets placed in WAVS mirrors and consequential plasma sputtering of those under 3 T magnetic field. We present for both argon single and dual powered discharges and draw comparisons from the results obtained at 0 T. In this section the capacitance of the powered mirrors is 486 pF for M1 and 795 pF for M2.

### 4.1. Single powered

Single mirror powered experiments were performed with the M1 electrode powered under the conditions listed in table 2. At both 0 and 3 T (at 55°), M1 upon visible inspection, was not deposited. This was consistent with the sheath potential drops in both cases, which exceeded 200 V, leading to ion energies high enough to sputter most relevant materials that were deposited in the insets (see figure S6). The estimated eroded thickness $T_{eroded}$ was calculated based on the sputtering yield $Y(V_{DC})$, the ion flux $\Gamma_i$, the ion charge $e$, the atomic density of the target material $n_{at}$, and the plasma exposure time $t$:

$$T_{eroded} \approx \frac{Y_{Fe}(V_{DC}) \Gamma_i t}{n_{at}} \quad (3)$$
<figure><img src="image_19.png" /><figcaption>**Figure 8.** Self-bias voltage as a function of applied RF power for various mirror configurations in the WAVS setup under a 3 T magnetic field. The four curves correspond to dual- and single-powered configurations at two angular orientations relative to the magnetic field: 55, 35° (left view) and 11, 9° (divertor view). Mirror capacitance was held constant across all displayed configurations. The schematics (a) to (d) on the right illustrate the respective mirror geometries and plasma column positions, with the calculated grounded-to-powered area ratios ($A_g/A_p$) indicated above each configuration. These ratios strongly influence the resulting self-bias, with larger $A_g/A_p$ values (e.g. single 55°, $A_g/A_p = 0.62$) producing more negative biases.</figcaption></figure>

<table><caption>**Table 2.** Experimental parameters and calculated eroded thickness for single mirror cleaning experiments with M1 powered and M2 grounded in an Ar discharge at 1 Pa. The magnetic field magnitude $B$, self-bias voltage $V_{DC}$, and plasma exposure time $t$ are experimental conditions, while the sputtering yield $Y$ and eroded thickness are calculated values. The eroded thickness was estimated from the sputtering yield and exposure time according to equation (3).</caption><thead><tr><th>B (T)</th><th>V<sub>DC</sub> (V)</th><th>t (h)</th><th>Y (at/ion)</th><th>Thick. (nm)</th></tr></thead><tbody><tr><td>0</td><td>-300</td><td>2</td><td>0.40</td><td>~344</td></tr><tr><td>3</td><td>-197</td><td>1</td><td>0.77</td><td>~660</td></tr></tbody></table>

where:

* $Y_{Fe}(V_{DC})$ is the sputtering yield (atoms sputtered per incoming ion) dependent on the self-bias voltage,

* $\Gamma_i$ is the ion flux to the electrode surface (ions $cm^{-2}s^{-1}$),

* $t$ is the plasma exposure time (s),

* $n_{at}$ is the atomic density of the target material (atoms $cm^{-3}$).

Table 2 includes the estimated sputtered thicknesses for both magnetic field conditions. A constant ion flux of $\Gamma_i = 1 \times 10^{15}$ ions $cm^{-2}s^{-1}$ was assumed for the unmagnetised case [23]. Based on literature, the ion flux can increase by a factor of up to 3 under strong magnetic confinement due to improved plasma confinement and enhanced parallel transport along field lines [18, 19]. Applying this scaling, the calculated erosion was approximately 344 nm after 1 h at 3 T, and 660 nm after 2 h at 0 T. Despite the comparable erosion rate of M1 under both

field conditions, the deposition observed on M2 differed substantially (figure 9). At 0 T, M2 was visibly darkened, and a significant stainless steel layer, sputtered from M1, was redeposited on its surface (figure 9(a)). In contrast, at 3 T, no visible changes were observed on M2 (figure 9(b)), although XPS revealed traces of stainless steel on its insets (i, ii and iii). These insets were composed of stainless steel substrates coated with thin rhodium (i) or molybdenum films (ii and iii), and stainless steel was detected atop these coatings, indicating that deposition was still present (from material of the eroded M1 mirror onto M2) but greatly reduced in the magnetised case (see figure S7 of the supplementary information).

The reduced deposition observed on M2 at 3 T was unexpected, given that the estimated erosion of M1 was still substantial. Since sputtered particles are neutral, their trajectories are not directly affected by the magnetic field. Assuming identical neutral gas conditions (1 Pa) and negligible changes in neutral transport, the large reduction in deposition at 3 T suggested that conditions at the M2 surface were altered. One plausible explanation lies on the suppression of electron losses to the walls given the limitation of cross-field transport of these particles, this lead to an increase of plasma potential to maintain current balance. This effect has been shown to increase $V_p$ substantially with increasing magnetic field strength, although it saturates at high fields, as reported in our previous contribution [20]. In this study no direct measurements of the plasma potential were performed in these configurations.

In conclusion, the reduction in net deposition on M2 at 3 T may be attributed to the increased plasma potential and ion
<figure><img src="image_20.png" /><figcaption>**Figure 9.** Post-exposure visual comparison of M2 after single mirror cleaning with M1 powered at 0 (a) and 3 T (b). Prior to the experiment, both M2 surfaces and insets appeared shiny and metallic. After exposure at 0 T, M2 developed a visible brownish layer, indicative of deposition, whereas at 3 T the surface remained visually metallic, suggesting significantly reduced deposition. The schematic (c) shows the insets analysed by XPS in figure S7. Insets (i), (ii), and (iii) revealed only low levels of stainless steel (less than 5 nm for iii) as we detect molybdenum by XPS. This is consistent with reduced material transfer under magnetic confinement.</figcaption></figure>

energy at its surface, allowing for local sputtering of the depos-
ited material, and reducing the net accumulation despite com-
parable neutral transport conditions. In figure 11(a), the PIC
simulation demonstrates that ions can have significant energy
and sputter the grounded mirror M2.

4.2. Dual powered

A dual mirror cleaning discharge using Ar was carried out with
both M1 and M2 electrodes powered simultaneously at 55 and
35°, respectively, with a self-bias of -70 V for a total dur-
ation of two hours. The geometry of the plasma exposure is
illustrated in at the top of figure 10, where the *double-wetted
plasma areas*, the areas where both plasma columns intersect
in the mirrors surface, are highlighted in brown, in green are
the *single-wetted plasma areas*, where there's no intersection
between the plasma columns. In this configuration, the plasma
column originating from M1 extended to mostly cover the
entire surface of M2, including its insets. However, the reverse
was not true, the plasma column originating from M2 did not
completely reach all insets on M1.

Post-exposure visual inspection revealed signs of material
deposition on the mirror surfaces, as illustrated in figures 10(c)

<figure><img src="image_21.png" /><figcaption>**Figure 10.** Top: Schematic of the plasma exposure geometry showing M1 and M2 in the dual cleaning configuration. Double-wetted plasma area is shown in brown and single-wetted plasma area in green. Bottom: Schematic of electrode M1 with insets 1–4 (b) and M2 with insets 5–6 (a). Coloured circles around insets indicate which techniques were used for inset characterisation i.e. EDX, XPS, and FIB. Using all these characterisations techniques, it was determined if the inset was eroded or deposited. To visualised these results, a colour coding reflects whether the region exhibited net deposition or erosion, brown and green inset colour, respectively. The photos in (c) and (d) show the brown region of deposition. XPS results are shown in figure S8, and EDX data in table S1 and figure S9.</figcaption></figure>

and (d). To investigate these observations in detail, sur-
face characterisation was performed on insets 1 to 6 (shown
in figures 10(a) and (b)) using XPS and EDX techniques.
Coloured circles around each inset indicate which diagnostic
technique was applied. Based on the combined results of these
analyses, each inset was classified as either eroded or depos-
ited. A colour scheme was adopted to represent this classifica-
tion: insets shown in green correspond to erosion, while brown
indicates net deposition.

The characterisation results demonstrated that samples
located within the plasma double-wetted area exhibited net
deposition, whereas those situated outside this region, partic-
ularly where the plasma column interacted with a grounded
surface, underwent net erosion. Detailed results are presented
in the supplementary information figures S8 (XPS), S9 (EDX),
and table S2. Specifically, XPS spectra shown in figure S8 for
insets 2, 4, 5, and 6 revealed the presence of stainless steel deposits (which is the material of the mirrors' surface that does not include insets), as evidenced by the detection of Fe 2p and Cr 2p peaks. In figure S9, EDX measurements for insets 1 and 3 before and after plasma exposure indicated a decrease in the intensity of the deposited film and a corresponding increase in the substrate signal (notably iron), confirming the occurrence of erosion.

This is a striking result, as it was not observed in the absence of magnetic field (0 T) [24]. To understand the physical mechanisms underlying this behaviour, one-dimensional PIC simulations were performed to replicate key aspects of the experimental configuration, particularly the interception of plasma columns. Three configurations were simulated: a powered electrode facing a grounded electrode, two powered mirrors with a 180° phase difference, and two powered mirrors operated in phase. These simulations yielded estimates of the plasma potential and ion impinging energy, providing valuable insight into sheath dynamics and ion acceleration under different phase conditions. The resulting plasma conditions allowed conclusions to be drawn on the observed erosion and deposition patterns in the presence of a magnetic field.

In the single-powered configuration, figure 11(a), the asymmetry in driving conditions results in moderate ion energies (~100 eV). In figure 11(b), the in-phase configuration ($\Delta\phi = 0°$) causes the average plasma potential in the overlap region between the two electrodes to remain close to zero, suppressing ion acceleration ($E_i \approx 0$ eV). In contrast, in figure 11(c), when electrodes 1 and 2 are driven out of phase ($\Delta\phi = 180°$), the increased swing in plasma potential leads to higher ion energies and more uniform erosion across both mirrors. In the PIC simulations, the RF amplitude was fixed at $V_{RF} = 400$ V, and the DC self-bias ($V_{DC}$) was set to zero for simplicity. While in experiments the self-bias is typically negative due to discharge asymmetry, this simplification was justified, as the goal was to isolate the role of the phase difference between the electrodes on ion energy. Since ion acceleration is mainly governed by the instantaneous potential difference between plasma and electrode, neglecting $V_{DC}$ in this context does not significantly affect the interpretation of the ion dynamics.

The observed experimental results of erosion and deposition patterns on the WAVS mirrors could be extrapolated from the results of the PIC simulations. Mirrors M1 and M2 were connected via cables of identical length and material to the same RF generator. However, both mirrors do not have the same capacitance, i.e. 486 and 795 pF for M1 and M2, respectively. To verify the influence of capacitance on the phase, a SimSmith simulation was conducted, which showed that the mirrors are in phase (figure S10). As a result, ions in this region experienced little acceleration and thus did not erode the mirror surfaces. In contrast, ions in single-column regions faced grounded surfaces, traversed sheath potentials, and arrived with higher energies, causing erosion. Neutral material sputtered in these zones could then be redeposited in the double-wetted plasma region.

<figure><img src="image_22.png" /><figcaption>**Figure 11.** One-dimensional PIC simulation results and schematics of three discharge configurations with an applied $V_{RF} = 400$ V and no self-bias. Left column: electrode geometry and magnetic field $\vec{B}$ direction. Right column: temporal evolution of plasma potential and applied RF voltages. Cases shown: (a) single-sided powered electrode, (b) dual powered electrodes in phase ($\Delta\phi = 0°$), and (c) dual powered electrodes out of phase ($\Delta\phi = 180°$). In (a), ions are accelerated toward the grounded surface due to a potential drop between plasma and ground. In (b), the plasma potential remains near zero, resulting in negligible ion acceleration ($E_i \approx 0$ eV). In (c), the out-of-phase driving increases the plasma potential swing and leads to enhanced ion energies on both electrodes.</figcaption></figure>

From the PIC simulation, one can extrapolate to a configuration similar to the WAVS with M1 at 55° and M2 at 35°. Figure 12 illustrates the spatial profile of the plasma potential along the x-direction and the corresponding ion energy in the configuration where both electrodes were powered in phase, (figure 11(b)) as implemented in the experiment (figure 10). On the right, a schematic of the plasma discharge cross-section depicts how the magnetic field $\vec{B}$ confined the plasma along field lines, resulting in the formation of two adjacent plasma columns.

In the central region represented where the two columns overlap—the double-column plasma region—the plasma potential remained approximately constant and close to the time-averaged RF voltage $V_{RF}$, here assumed negative since in reality there is a negative self-bias that increases the potential drop (not the case in the simulation). The left-hand graph of figure 12 shows that, in this region, ions experienced negligible electric fields and therefore gained no energy before reaching the electrode surfaces. In contrast, ions in the single-sided regions, where powered electrodes faced grounded surfaces,
<figure><img src="image_23.png" /><figcaption>**Figure 12.** Schematic representation of plasma potential and ion energy in an RF plasma along the x direction. Mirrors (or electrodes) M1 and M2 are powered in-phase ($\Delta\phi = 0^\circ$). Right: Cross-sectional view of the discharge under a horizontal magnetic field $\vec{B}$, showing both powered electrodes. Left: Corresponding profile along the x-axis of the plasma potential and ion energies. In the central double-wetted region, the average plasma potential remains near $\overline{V_{RF}}$, resulting in negligible electric field and low ion impinging energy on M1 and M2 ($E_{ion} \approx 0$ eV). In contrast, ions reaching the mirrors outside this region gain significant energy from the sheath potential. In region A, marked with a green circle the line were expanded for the sake of interpretation.</figcaption></figure>

traversed a significant sheath potential and reached the surfaces with higher kinetic energy. This explained the observed pattern: net erosion occurred where ion energies were higher, and preferential deposition was found in the central region where ion bombardment was lower, as seen in figure 10. In the photos c) and d) of this figure the deposition pattern observed did not match exactly the double-column plasma region, this could be due to edge effects that created an enhanced electric field on the edges of the electrode allowing for some ions to acquire enough kinetic energy and impact the mirror (more extensive discussion on the edge effect can be found in [18]).

**5. Conclusion**

In this study, we investigated the influence of strong magnetic fields on the self-bias voltage and erosion dynamics of RF capacitively coupled plasmas discharges applied to an optical diagnostics for ITER. The experiments were performed inside a 3 T MRI system using a full-scale mock-up of the WAVS, reproducing the geometry expected for in-vessel equatorial port diagnostics in ITER. These discharges were characterised under varying pressure, gas species, electrode capacitance, and orientation of the mirrors relative to the magnetic field. The main findings are summarised below.

**1. Magnetic confinement effects:** a strong magnetic field leads to radial confinement of the plasma column along magnetic field lines, significantly reducing the plasma-wetted

grounded area. This, in turn, reduced the magnitude of the negative self-bias voltage. However, magnetic confinement simultaneously increased the plasma potential due to reduced electron losses to the wall, potentially enhancing ion energies at both powered and grounded surfaces.

**2. Dependence on grounded-to-powered area ratio ($A_g/A_p$):** systematic variation of the mirror orientation $\theta$ with respect to the magnetic field demonstrated that increasing the effective grounded area wetted by the plasma resulted in more negative self-bias voltages. In these magnetised conditions, the self-bias is described by $V_{DC} \sim \sin(\theta)$, the deviations from this curve were attributed to sheath effects and plasma expansion beyond the visible column, as well as limitations in defining grounded surfaces in complex geometries [20].

**3. Erosion behaviour under single- and dual-powered configurations:** In single-powered configuration (M1 powered), strong erosion was observed on the powered mirror at both 0 T and 3 T, with deposition on the grounded mirror at 0 T and strongly reduced deposition at 3 T. This was attributed to increased plasma potential at 3 T enabling sputtering on the grounded surface as demonstrated by the PIC simulation. In dual-mirror cleaning with in-phase ($\Delta\phi = 0^\circ$) RF signals, a well-defined central region of net deposition—corresponding to the double-wetted plasma area—surrounded by regions of clear erosion was observed. This pattern was confirmed by XPS and EDX measurements and supported by PIC simulations of plasma potential.
**4. Role of RF phase:** PIC simulations showed that when both mirrors were powered in-phase, the plasma potential in the overlap region remained close to $V_{RF}$, resulting in minimal sheath drop and low impinging energy of ions onto the mirror surface ($E_{ion} \approx 0$ eV). In contrast, $\Delta\phi = 180^\circ$ induced a high average plasma potential, leading to uniform erosion on both mirrors. This highlights the critical role of the relative RF phase in governing the spatial evolution of the plasma potential, thereby directly influencing the ion energy and the resulting erosion uniformity across magnetically aligned plasma columns.

The results presented here provide key guidance for the deployment of reliable in-situ cleaning methods for optical diagnostics. However, in ITER, the RF line transmission will be longer and will utilise different cable types, necessitating adequate RF matching to achieve a low voltage standing wave ratio (VSWR). In our experiment, the VSWR was not measured, which influences the power coupling efficiency, as demonstrated using SimSmith RF engineering model. Additionally, it is now foreseen that the ITER magnetic field will be off every two weeks since boronisation requires a non-magnetised glow discharge [3], therefore one may not need to remove impurities in magnetic field, given that during the two-week experiment period where the reactor is running, the mirrors' reflectivity does not drop significantly. Nevertheless, for the purpose of a continuous magnetically confined fusion reactor, if diagnostics that necessitate mirrors are included, the removal of impurities has to be done in magnetic field.

On a broader scale this result contributes to an understanding of plasma-surface interaction in fully magnetised, homogeneous field-aligned CCP discharges.

**6. Outlook**

These results demonstrated the complex interplay between magnetic confinement, RF phase, geometry, and material properties in determining erosion and deposition behaviour in ITER-relevant CCP cleaning systems. In particular, the identification of a central deposition zone in dual in-phase cleaning poses challenges for homogeneous impurity removal from ITER first mirrors. In the specific case of the WAVS geometry, where the plasma columns intersect, the absence of a significant sheath potential in the double-wetted region suppressed ion acceleration, favouring net deposition. To overcome this, alternative strategies such as applying a controlled RF phase shift between electrodes may enable the manipulation of ion impinging energy on the mirrors in the overlap region. Alternatively, a single-mirror cleaning scheme remains a viable option under magnetised conditions, as the non-powered mirror exhibits minimal net deposition due to elevated plasma potentials and the possibility of sputtering.

Future directions will focus on phase-shifted dual-electrode driving experiments where the self-bias at a fixed power is measured for different phase shifts. The study of boron-based coatings and their removal from first mirrors must be further addressed to follow recent ITER first-wall changes.

**Acknowledgment**

The authors would like to thank the Swiss Nanoscience Institute for their financial support. This work was supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under Contract Number 22.00424 and 24.00510. The work leading to this publication has been funded partially by Fusion for Energy under contracts F4E-OPE-1098. This publication reflects the views only of the author, and Fusion for Energy cannot be held responsible for any use which may be made of the information contained therein.

**ORCID iDs**

Tomás C. Sousa ID 0009-0007-0513-9983

Artem Dmitriev ID 0000-0001-6790-167X

Paul Hiret ID 0000-0002-2119-8648

Youpeng Wang ID 0000-0002-7112-5199

Laurent Marot ID 0000-0002-1529-9362

Ernst Meyer ID 0000-0001-6385-3412

Jérôme Moritz ID 0000-0002-4593-6270

Oliver Bieri ID 0000-0002-6755-5495

Sébastien Dine ID 0000-0003-0405-7568

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