Plasma Physics and Controlled Fusion

The use of the lower (left-hand) X-mode cutoff for measuring core to edge radial profiles of the perpendicular plasma velocity $u_\perp$ and radial electric field E_r via microwave Doppler reflectometry/backscattering (DR) is demonstrated in the ASDEX Upgrade tokamak (AUG). With high toroidal magnetic fields $B_{\mathrm{T}} = -2.8$ T and core plasma densities $\overline{n}_{\mathrm{oe}} \sim 1 \times 10^{20}$ m⁻³ the lower X-mode cutoff becomes accessible to the AUG low-field-side launch, V-band Doppler reflectometers. Example $u_\perp$ velocity profiles are presented for high-confinement H-mode plasmas, with and without edge-localized-modes, and are found to compare well with velocity measurements from Charge exchange recombination spectroscopy. With the aid of 2D full-wave numerical simulations of the actual experimental conditions, E_r profile measurement errors, due to uncertainties in the DR probing wavenumber $k_\perp$ and backscatter location r, plus estimates of the minimum wavenumber $\Delta k_\perp$ and radial Δr error-bars, are obtained from the instrument response function. Some practical limitations and issues involving the lower X-mode cutoff for core tokamak/stellarator DR probing are also investigated using 2D full-wave and 3D ray/beam tracing codes.

The behavior of runaway electrons of Frascati Tokamak Upgrade (FTU) discharges is investigated through the comparison of experimental synchrotron emission spectra and visible images with their synthetic counterparts. Synchrotron spectra are measured in an unprecedented wide wavelength range (450–4000 nm) while synchrotron images are collected by a visible CCD camera. The simulated spectra and images are calculated with the synthetic synchrotron radiation diagnostic SOFT (Synchrotron-detecting Orbit Following Toolkit) code. The aim of this work is to extend the study of runaway electrons dynamics in FTU also to post-disruption phases. The runaway number, radial profile, energy and pitch angle have been evaluated during their whole time evolution, from the start-up to the post-disruption phase, assuming a given runaway electrons (RE) distribution function. The runaway number is found to increase by two orders of magnitude after the disruption, while the energy and pitch angle maintain similar values before and after the disruption. The runaway electrons are mostly distributed in the core of the plasma. The inferred maximum RE energy and pitch angle are in agreement with the results of simulations based on a runaway electron test particle model.

The propagation of high-frequency (HF) waves through inhomogeneous magnetized plasmas, to investigate energy absorption mechanisms, mode conversion, and the resulting stimulated electromagnetic emissions (SEEs) is studied, aiming to illuminate the underlying physical mechanisms. A Fully electromagnetic (EM) particle-in-cell method is employed to simulate the interaction between a high-power left-hand circularly polarized HF wave and a magnetized plasma with a linearly increasing density gradient. Two conditions are considered: magnetic field aligned with the HF wave propagation direction and perpendicular to the HF wave propagation direction. The dynamics of HF wave–plasma interactions, the nature of mode conversion, excited stimulated modes, and the conditions that enhance or inhibit SEEs are studied. Parallel propagation to the magnetic field induces small amplitude plasma modes near the left-hand cutoff frequency. In perpendicular propagation into the magnetic field, the incident circularly polarized HF wave decomposes into two distinct polarization modes: the ordinary mode (O-mode) and the extraordinary mode (X-mode). The O-mode behaves as a linearly polarized EM wave and reflects at the ordinary wave cutoff frequency which leads to excitation of plasma modes. However, the X-mode propagating partly transverse and partly longitudinal through the plasma stops propagating at the right-hand cutoff frequency. At this point, it is absorbed and converted into strong EM and electrostatic modes, resulting in a substantial energy loss. The results indicate that considering an elliptically polarized HF wave instead of a circular one can lead to more efficient heating of plasmas. The generated linear and nonlinear plasma modes are investigated using fast Fourier transform analysis. The suppression of SEEs at integer multiples of electron gyroharmonic frequencies observed in experiments is also investigated. It is shown that the resonance of the transmitted HF wave at the upper-hybrid frequency causes this suppression. These results have important implications for plasma diagnostics and heating experiments.

The radio frequency (RF) negative hydrogen ion source is employed in neutral beam injection (NBI) system for magnetic confined fusion devices. To satisfy the required beam current of negative hydrogen ions in NBI for fusion, the surface production on the plasma grid (PG) surface is introduced to increase the amount of negative hydrogen ions. In this paper, a 2D fluid model is established, including volume production and surface production of negative hydrogen ions. This work focuses on the spatial distributions of negative hydrogen ions with different gas pressures, PG bias potentials, magnetic filter filed (MFF) positions and RF powers. The results show that the density of negative hydrogen ions is enhanced by the surface production. As the gas pressure increases, the increase in the negative hydrogen ion density is dominated by the increase in the conversion of neutral hydrogen atoms at the PG surface. As the bias potential increases, the distribution of negative hydrogen ions is slightly shifted towards the PG due to the reduction of electric field near the PG. As the MFF position shifts towards the PG, the increase in the negative hydrogen ion density is dominated by the increase in the conversion of positive hydrogen ions at the PG surface. Moreover, the negative hydrogen ion density increases with the increased RF power, which is dominated by the increase in the conversion of positive hydrogen ions at the PG surface. The model facilitates the understanding of the negative hydrogen ion distribution and the optimization of negative hydrogen ion production.

In the past decades, observations from magnetically confined fusion experiments have revealed instances of two-plasmon decay instabilities between injected X-mode waves for second harmonic electron cyclotron heating and trapped upper hybrid (UH) waves near half frequency of the injected wave. In this study, we demonstrate that developed models used to assess the two-plasmon decay instability in fusion plasmas are also applicable to general low-temperature laboratory plasmas. We carry out a parameter scan where a reduced analytical model is used to find optimal plasma conditions for the growth rates of the instability for an injected X-mode wave with frequency of $5.8\,\mathrm{GHz}$. To verify the behaviour of the trapped UH waves and the estimated growth rates, we conduct 1D particle-in-cell simulations in the case of low-temperature plasmas. Lastly, a lower limit to the magnetic field strength is found, where the growth rate of the instability significantly declines.

In tokamak plasmas, non-linear interplay between transport and sources/sinks takes place for all transported quantities (current, heat, particles and momentum). Thanks to integrated modelling frameworks, we can iterate physics-based quasilinear turbulent transport models over multiple confinement times. Such modelling allows us to predict current, temperature, density and rotation profiles, and to disentangle the causality at play behind the modelled time evolution. An intense validation effort of such modelling against experimental measurements has been ongoing and has progressed our understanding. In dynamical phases, the so-called 'cold pulse' physics have been explained in the AUG tokamak, the isotope impact in plasma current ramp-up is understood in the JET tokamak, and the impact of the particle source (from neutral beam injection) on tungsten core accumulation has been clarified in JET and AUG. In stationary phases, the saturation of the ion temperature in electron-heated WEST plasmas has been clarified, and the energy content has been predicted with higher accuracy than empirical scaling laws with respect to the plasma current, magnetic field, plasma size and gas fueling, both in L and H modes on AUG. The validation of physics-based integrated modelling allows control optimisation in preparation for ITER operation as well as risk reduction for the design of future reactors. However, despite the reported progress, physics gaps remain on this path. For example, unlike today's devices, ITER-class devices will be opaque to neutrals and fuelled by pellets. In the absence of a physical understanding of the transport in the pedestal, extrapolation is uncertain. Moreover, in burning plasmas, the non-linear coupling between the central core profiles and the fusion power is very strong. The uncertainties in profile predictions due to unverified and unvalidated reduced transport models in such high-pressure plasmas lead to uncertain fusion power predictions. Solutions on how to address these challenges within integrated modelling will be proposed.

During its 40 years of operations, the Joint European Torus (JET) tokamak has consistently pushed the physics and engineering boundaries of fusion research, providing the scientific community with a unique testing ground for theories and innovative ideas. This paper covers a selection of remarkable contributions of JET to various fields of tokamak science, from transport and plasma heating studies to plasma-wall interaction and D-T experiments, and their impact on the fusion research progress.

Turbulence, a fascinating and intricate phenomenon, has captivated scientists over different domains, mainly for its complex cross-scale nature spanning a wide range of temporal and spatial scales. Despite significant advances in theories and observations in the last decades, some aspects of turbulence still remain unsolved, motivating new efforts to understand its underlying physical mechanisms and refine mathematical theories along with numerical models. This topical review explores recent findings from the Parker Solar Probe mission, providing a distinctive opportunity to characterize solar wind features at varying heliocentric distances. Analyzing the radial evolution of magnetic and velocity field fluctuations across the inertial range, a transition has been evidenced from local to global self-similarity as proximity to the Sun increases. This behavior has been reconciled with magnetohydrodynamic theory revising an old concept by emphasizing the evolving nature of the coupling between fields. This offers inspiration for novel modeling approaches to understand open challenges in interplanetary plasma physics as the heating and acceleration of the solar wind, as well as, its evolution within the inner Heliosphere.

This paper reviews current understanding of key physics elements that control the H-mode pedestal structure, which exists at the boundary of magnetically confined plasmas. The structure of interest is the width, height and gradient of temperature, density and pressure profiles in the pedestal. Emphasis is placed on understanding obtained from combined experimental, theoretical and simulation work and on results observed on multiple machines. Pedestal profiles are determined by the self-consistent interaction of sources, transport and magnetohydrodynamic limits. The heat source is primarily from heat deposited in the core and flowing to the pedestal. This source is computed from modeling of experimental data and is generally well understood. Neutrals at the periphery of the plasma provide the dominant particle source in current machines. This source has a complex spatial structure, is very difficult to measure and is poorly understood. For typical H-mode operation, the achievable pedestal pressure is limited by repetitive, transient magnetohydrodynamic instabilities. First principles models of peeling–ballooning modes are generally able to explain the observed limits. In some regimes, instability occurs below the predicted limits and these remain unexplained. Several mechanisms have been identified as plausible sources of heat transport. These include neoclassical processes for ion heat transport and several turbulent processes, driven by the steep pedestal gradients, as sources of electron and ion heat transport. Reduced models have successfully predicted the pedestal or density at the pedestal top. Firming up understanding of heat and particle transport remains a primary challenge for developing more complete predictive pedestal models.

This article reviews applications of Bayesian inference and machine learning (ML) in nuclear fusion research. Current and next-generation nuclear fusion experiments require analysis and modelling efforts that integrate different models consistently and exploit information found across heterogeneous data sources in an efficient manner. Model-based Bayesian inference provides a framework well suited for the interpretation of observed data given physics and probabilistic assumptions, also for very complex systems, thanks to its rigorous and straightforward treatment of uncertainties and modelling hypothesis. On the other hand, ML, in particular neural networks and deep learning models, are based on black-box statistical models and allow the handling of large volumes of data and computation very efficiently. For this reason, approaches which make use of ML and Bayesian inference separately and also in conjunction are of particular interest for today's experiments and are the main topic of this review. This article also presents an approach where physics-based Bayesian inference and black-box ML play along, mitigating each other's drawbacks: the former is made more efficient, the latter more interpretable.

Characteristics and operational regimes of electron density, electron temperature, and energy confinement have been investigated in the JT-60SA first plasma operation phase. Working gases for these plasma discharges are hydrogen (H₂) and helium (He). A plasma current I_p of 1.2 MA has been achieved at a toroidal magnetic field B_T of 2 T. In I_p = 1 MA, the operational regimes of a line-averaged electron density ¯n_e are 2.7 × 10¹⁸ – 5.2 × 10¹⁸ m⁻³ for the He-gas injected plasma and 2.5 × 10¹⁸ – 6.5 × 10¹⁸ m⁻³ for the H₂-gas injected plasma. A maximum electron temperature in this operation phase is 2.0 keV in the core region in the case of He-gas-injection and 1.13-MW electron cyclotron resonant heating (ECH). A global energy confinement time τ_E of L-mode divertor plasmas for both the He-gas-injected and H₂-gas-injected plasmas strongly depends on the total input power. In the similar total input power regimes, τ_E increases with an increase in ¯n_e, leading to the possibility of these plasmas in the linear ohmic confinement regime. The global energy confinement time of the H₂-gas injected plasmas, which is in the range of 100 - 200 ms, is compared with the ITER89-P scaling law, resulting in τ_E being 0.6 to 1.2 times the scaling, mostly around or slightly below it. The dependence of τ_E on ¯n_e matches the neo-Alcator scaling law and reaches the ITER89-P scaling law in higher-density plasma in the case of the similar total input power regimes.

We demonstrate symplecticity of the flow map generated by the guiding-center tracer GORILLA. Since the underlying algorithm relies on a piecewise linear interpolation of the Hamiltonian on a tetrahedral grid, usual proofs based on a twice continuously differentiable Hamiltonian are not applicable. The analysis is pinned down to the critical section near the boundaries of tetrahedral elements, where the Hamiltonian vector field is non-smooth. We show that it is possible to retain symplecticity of the piecewise linear system as a limiting case of a parametrised family of smooth Hamiltonian systems nearly everywhere in phase-space. Limitations are discussed for the case of the X- and O-point in the magnetic field topology. The connection to Hdiv-conforming finite element discretisations and their interface conditions is pointed out. Finally, the practical implications for edge transport modelling are elaborated. For this purpose, we analyse the footprints of magnetic field lines intersecting the divertor plates of a tokamak with resonant magnetic perturbations. These footprints are shown to be in line with the expected behaviour of invariant manifolds of the underlying Hamiltonian system. This demonstration of physical consistency at low-order discretisation lays the basis for further developments of highly efficient edge transport solvers.

A Doppler backscattering (DBS) diagnostic has recently been installed on the Tokamak à Configuration Variable (TCV) to facilitate the study of edge turbulence and flow shear in a versatile experimental environment. The dual channel V-band DBS system is coupled to TCV's quasi-optical diagnostic launcher, providing access to the upper low-field side region of the plasma cross-section. Verifications of the DBS measurements are presented. The DBS equilibrium v⊥ profiles are found to compare favorably with gas puff imaging (GPI) measurements and to the Er inferred from the radial force balance of the carbon impurity. The radial structure of the edge Er × B equilibrium flow and its dependencies are investigated across a representative set of L-mode TCV discharges, by varying density, auxiliary heating and magnetic configuration.

Plasma wakefield acceleration holds remarkable promise for future advanced accelerators. The design and optimization of plasma-based accelerators typically require particle-in-cell simulations, which can be computationally intensive and time consuming. In this study, we train a neural network model to obtain the on-axis longitudinal electric field distribution directly without conducting particle-in-cell simulations for designing a two-bunch plasma wakefield acceleration stage. By combining the neural network model with an advanced algorithm for achieving the minimal energy spread, the optimal normalized charge per unit length of a trailing beam leading to the optimal beam-loading can be quickly identified. This approach can reduce computation time from around 7.6 minutes in the case of using particle-in-cell simulations to under 0.1 seconds. Moreover, the longitudinal electric field distribution under the optimal beam-loading can be visually observed. Utilizing this model with the beam current profile also enables the direct extraction of design parameters under the optimal beam-loading, including the maximum decelerating electric field within the drive beam, the average accelerating electric field within the trailing beam and the transformer ratio. This model has the potential to significantly improve the efficiency of designing and optimizing the beam-driven plasma wakefield accelerators.

We have developed TorbeamNN: a machine learning surrogate model for the TORBEAM ray tracing code to predict electron cyclotron heating and current drive locations in tokamak plasmas. TorbeamNN provides more than a 100 times speed-up compared to the highly optimized and simplified real-time implementation of TORBEAM without any reduction in accuracy compared to the offline, full fidelity TORBEAM code. The model was trained using KSTAR electron cyclotron heating (ECH) mirror geometries and works for both O-mode and X-mode absorption. The TorbeamNN predictions have been validated both offline and real-time in experiment. TorbeamNN has been utilized to track an ECH absorption vertical position target in dynamic KSTAR plasmas as well as under varying toroidal mirror angles and with a minimal average tracking error of 0.5cm.

The use of the lower (left-hand) X-mode cutoff for measuring core to edge radial profiles of the perpendicular plasma velocity $u_\perp$ and radial electric field E_r via microwave Doppler reflectometry/backscattering (DR) is demonstrated in the ASDEX Upgrade tokamak (AUG). With high toroidal magnetic fields $B_{\mathrm{T}} = -2.8$ T and core plasma densities $\overline{n}_{\mathrm{oe}} \sim 1 \times 10^{20}$ m⁻³ the lower X-mode cutoff becomes accessible to the AUG low-field-side launch, V-band Doppler reflectometers. Example $u_\perp$ velocity profiles are presented for high-confinement H-mode plasmas, with and without edge-localized-modes, and are found to compare well with velocity measurements from Charge exchange recombination spectroscopy. With the aid of 2D full-wave numerical simulations of the actual experimental conditions, E_r profile measurement errors, due to uncertainties in the DR probing wavenumber $k_\perp$ and backscatter location r, plus estimates of the minimum wavenumber $\Delta k_\perp$ and radial Δr error-bars, are obtained from the instrument response function. Some practical limitations and issues involving the lower X-mode cutoff for core tokamak/stellarator DR probing are also investigated using 2D full-wave and 3D ray/beam tracing codes.

The behavior of runaway electrons of Frascati Tokamak Upgrade (FTU) discharges is investigated through the comparison of experimental synchrotron emission spectra and visible images with their synthetic counterparts. Synchrotron spectra are measured in an unprecedented wide wavelength range (450–4000 nm) while synchrotron images are collected by a visible CCD camera. The simulated spectra and images are calculated with the synthetic synchrotron radiation diagnostic SOFT (Synchrotron-detecting Orbit Following Toolkit) code. The aim of this work is to extend the study of runaway electrons dynamics in FTU also to post-disruption phases. The runaway number, radial profile, energy and pitch angle have been evaluated during their whole time evolution, from the start-up to the post-disruption phase, assuming a given runaway electrons (RE) distribution function. The runaway number is found to increase by two orders of magnitude after the disruption, while the energy and pitch angle maintain similar values before and after the disruption. The runaway electrons are mostly distributed in the core of the plasma. The inferred maximum RE energy and pitch angle are in agreement with the results of simulations based on a runaway electron test particle model.

In the past decades, observations from magnetically confined fusion experiments have revealed instances of two-plasmon decay instabilities between injected X-mode waves for second harmonic electron cyclotron heating and trapped upper hybrid (UH) waves near half frequency of the injected wave. In this study, we demonstrate that developed models used to assess the two-plasmon decay instability in fusion plasmas are also applicable to general low-temperature laboratory plasmas. We carry out a parameter scan where a reduced analytical model is used to find optimal plasma conditions for the growth rates of the instability for an injected X-mode wave with frequency of $5.8\,\mathrm{GHz}$. To verify the behaviour of the trapped UH waves and the estimated growth rates, we conduct 1D particle-in-cell simulations in the case of low-temperature plasmas. Lastly, a lower limit to the magnetic field strength is found, where the growth rate of the instability significantly declines.

The next generation of Petawatt-class lasers presents the opportunity to study positron production and acceleration experimentally, in an all-optical setting. Several configurations were proposed to produce and accelerate positrons in a single laser stage. However, these configurations have yielded limited positron beam quality and low particle count. This paper presents methods for improving the injection and retention of positrons obtained via Bethe–Heitler pair production and accelerated using direct laser acceleration in a plasma channel. The work first introduces a semi-analytical model which predicts laser energy depletion in this highly nonlinear regime. We demonstrate through PIC simulations that accelerated electrons can induce charge inversion within the channel, leading to positron trapping and acceleration. We investigate how laser focusing position, channel wall density, target foil position and target thickness influence positron creation and retention. Our configuration can achieve an 8-fold increase in positron retention compared to previous studies and a higher number of positrons produced overall. This work establishes a robust, single-stage approach for obtaining positron beams, opening new avenues for experiments with Petawatt-class lasers and potential applications in electron–positron collisions and QED cascades.

The estimation of the poloidal velocity of the turbulence and the poloidal mean flow velocity are important quantities for the study of sheared flows on turbulence and transport. The estimation depends on the underlying model of the turbulence. Beside the propagation time of the turbulence, its decay with the fading time must be considered. For the description of the propagation, the elliptical approach (EA) is applied, which takes into account the propagation and fading time of the turbulence. The model has been applied successfully in experimental fluid dynamics and is confirmed by direct numerical simulations, also. In this paper, the EA is applied in the analysis of density fluctuations, measured by poloidal correlation reflectometry at two different fusion devices, TEXTOR and W7-X. For the latter, it is demonstrated that the EA is necessary for a correct description of the turbulence propagation. In addition, the velocity modulations are investigated, which in principle can be either generated by an oscillation of the propagation time of density fluctuations and/or an oscillation of the fading of the turbulence. An example for low frequency velocity oscillations in W7-X will be given in the paper, showing a relation between turbulence properties and small oscillations on the measured diamagnetic plasma energy.

We demonstrate symplecticity of the flow map generated by the guiding-center tracer GORILLA. Since the underlying algorithm relies on a piecewise linear interpolation of the Hamiltonian on a tetrahedral grid, usual proofs based on a twice continuously differentiable Hamiltonian are not applicable. The analysis is pinned down to the critical section near the boundaries of tetrahedral elements, where the Hamiltonian vector field is non-smooth. We show that it is possible to retain symplecticity of the piecewise linear system as a limiting case of a parametrised family of smooth Hamiltonian systems nearly everywhere in phase-space. Limitations are discussed for the case of the X- and O-point in the magnetic field topology. The connection to Hdiv-conforming finite element discretisations and their interface conditions is pointed out. Finally, the practical implications for edge transport modelling are elaborated. For this purpose, we analyse the footprints of magnetic field lines intersecting the divertor plates of a tokamak with resonant magnetic perturbations. These footprints are shown to be in line with the expected behaviour of invariant manifolds of the underlying Hamiltonian system. This demonstration of physical consistency at low-order discretisation lays the basis for further developments of highly efficient edge transport solvers.

A Doppler backscattering (DBS) diagnostic has recently been installed on the Tokamak à Configuration Variable (TCV) to facilitate the study of edge turbulence and flow shear in a versatile experimental environment. The dual channel V-band DBS system is coupled to TCV's quasi-optical diagnostic launcher, providing access to the upper low-field side region of the plasma cross-section. Verifications of the DBS measurements are presented. The DBS equilibrium v⊥ profiles are found to compare favorably with gas puff imaging (GPI) measurements and to the Er inferred from the radial force balance of the carbon impurity. The radial structure of the edge Er × B equilibrium flow and its dependencies are investigated across a representative set of L-mode TCV discharges, by varying density, auxiliary heating and magnetic configuration.

This computational study delves into the intricate interplay of alloying elements on the generation, recombination, and evolution of irradiation-induced defects. Molecular dynamics simulations were conducted for collision cascades at room temperature, spanning a range of primary knock-on atom energies from 1 to 10 keV. The investigation encompasses a series of model crystals, progressing from pure Ni to binary concentrated solid solution alloys (CSAs) such as NiFe₂₀, NiFe, NiCr₂₀, and NiFeCr₂₀ CSA. We observe that materials rich in Cr actively facilitate dislocation emissions and induce the nucleation of stacking fault tetrahedra in the proximity of nanovoids, due to Shockley partial interactions. This result is validated by molecular static simulations, which calculate the surface, vacancy, and defect formation energies. Among the various shapes considered, the spherical void proves to be the most stable, followed by the truncated octahedron and octahedron shapes. On the other hand, the tetrahedron cubic shape is identified as the most unstable, and stacking fault tetrahedra exhibit the highest formation energy. Notably, among the materials studied, NiCr₂₀ and NiFeCr₂₀ CSAs stood out as the sole alloys capable of manifesting this mechanism, mainly observed at high impact energies.

We have developed TorbeamNN: a machine learning surrogate model for the TORBEAM ray tracing code to predict electron cyclotron heating and current drive locations in tokamak plasmas. TorbeamNN provides more than a 100 times speed-up compared to the highly optimized and simplified real-time implementation of TORBEAM without any reduction in accuracy compared to the offline, full fidelity TORBEAM code. The model was trained using KSTAR electron cyclotron heating (ECH) mirror geometries and works for both O-mode and X-mode absorption. The TorbeamNN predictions have been validated both offline and real-time in experiment. TorbeamNN has been utilized to track an ECH absorption vertical position target in dynamic KSTAR plasmas as well as under varying toroidal mirror angles and with a minimal average tracking error of 0.5cm.

One of the problems of the liquid metal tokamak divertor technology is the redeposition of the released metal on the tokamak vessel walls. In this work an investigation of the removal of the redeposited metals from grounded conducting walls of tokamak was done by a low-temperature Ar plasma generated by electron cyclotron waves, RF-ECWR, at 13.56 MHz in a new/modified system with a tunable plasma potential. The plasma potential was controlled in this system in the range from 0-270 V. The plasma potential control was achieved by adding an additional DC voltage supply on ECWR electrode via a suitable inductor having high impedance at the 13.56 MHz frequency. Metallic Sn thin films were deposited in the first step on the conducting doped Si wafer by means of medium frequency pulsed magnetron sputtering from pure Sn target. In the next step, the Sn films were removed from the Si substrate by ion etching (ion sputtering) by accelerated Ar ion fluxes. The etching efficiency was investigated in dependence on different values of the applied RF power and different values of the adjusted plasma potential. The ion density, electron temperature and the ion flux density impinging on the sample were determined by an RF probe system at different ECWR plasma conditions. The etching efficiency of Sn coatings was evaluated by profilometry measurement, SEM photography and electron microprobe analysis. It was found that this new/modified configuration of the ECWR plasma source can prospectively be used for cleaning of the grounded walls of tokamak from deposited Sn layers.