A one-dimensional model, consisting of a hydrodynamic radio-frequency (RF) sheath model for the ion cyclotron range of frequencies heating and an equivalent circuit model, is used to study the structure of the collisionless RF sheath of a fusion plasma containing a beam of energetic electrons. For various energetic electron concentrations and velocities at the plasma-sheath edge, a set of equations describing the model are solved numerically to obtain the potential drop across the RF sheath and the sheath thickness, as well as the spatiotemporal variations of the potential, the ion density, and the background electron and energetic electron densities inside the sheath. Under the current EAST ion cyclotron range of heating conditions, it is observed that even at small beam fluxes, the potential drop across the sheath is enhanced at any time in an RF cycle for different bulk plasma densities, ion temperatures, and frequencies and amplitudes of the disturbance current. When the energetic electron component is included, the physical sputtering yields of the RF sheath wall materials such as titanium and iron become significant as a result of the enhancement of the drop in the sheath potential.

The effect of the non-Maxwellian plasma with enhanced electron tails on the properties of the radio frequency (RF) sheath is studied with a one-dimensional collisionless model, which consists of the sheath model and the equivalent circuit model. In the sheath model, electrons are assumed to obey the Cairns–Tsallis distribution. For various entropic indices q characterizing the degree of electron nonextensivity and parameter α measuring the electron nonthermality state, the electron nonextensivity and nonthermality are found to modify the potential drop across the sheath and the sheath thickness, as well as the spatiotemporal variations of the potential, the ion and electron densities inside the sheath. With the decrease in q and the increase in α, the potential drop across the sheath and the thickness increase at any time in a RF cycle as a result of the increase in superthermal electrons in the non-Maxwellian tail. The dependence of the potential drop across the sheath on q and α is deeply related to the frequency and amplitude of the disturbance current. When the electron nonextensivity and nonthermality are strengthened, the enhancement of the sheath potential drop can cause a significant increase in the ion bombardment energy on the wall, sheath power dissipation, and plasma energy flux to the wall.

Heating with the wave in the ion cyclotron range of frequencies (ICRF) has been used in the development of high-performance H-mode operations in EAST. A different ion cyclotron resonance heating scenario in three-ion component plasma with real experimental parameters on EAST was investigated using a numerical tool. Excellent radio frequency wave absorption was found with an extremely low 3He concentration (0.1%–0.4%) in D-H-(3He) plasma, by adjusting the plasma composition appropriately in our simulation. In this case, the 3He fundamental resonance layer is located between the two ion–ion hybrid resonance-cutoff pairs in close proximity, and therefore E+ of the wave was considerably enhanced near the 3He fundamental resonance layer. The minority 3He tail was estimated to be superenergetic (∼1 MeV) because of the high power carried by each resonant 3He ion. The potential of the three-ion ICRF heating means on EAST was shown, and the scenarios investigated are particularly promising for fast particle generation schemes.

During edge localized modes (ELMs), the sheath evolution in front of the Experimental Advanced Superconducting Tokamak (EAST) upper divertor is studied to estimate the sputtered tungsten (W) atoms from the divertor target. A large potential drop across the sheath is formed during ELMs by compared with inter-ELMs, and the maximum of sheath potential drop can exceed one thousand of eV in current EAST operation. Due to the enhancement of the sheath potential drop during ELMs, the W physical sputtering yield from the deuterium (D) ions and the impurity ions on the upper divertor target is found to be significant. It is established that the sputtered W yield during ELMs is at least higher by an order of magnitude than inter-ELMs, and D ions and carbon (C) ions are the main ions governing the W production for the current H-mode with ELMs discharges. With increase in the pedestal electron temperature, the maximum of the D and C ion impact energy during ELMs shows a nearly linear increase, and the D ions have sufficient impact energy to cause the strong W physical sputtering. As a consequence, the D ions may dominate the sputtered W flux from the divertor target when the C concentration is controlled less than one percent for the higher heating power H-mode with ELM discharges in near future.

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The hotspots on the guard limiters of the lower hybrid wave (LHW) antenna on EAST tokamak not only cause serious damage to the guard limiters, but also strongly degrade the plasma performance due to enhanced impurity productions. In published studies, the heat flux to the limiters is assumed to be carried to limiter walls by electrons which can absorb a small amount of the launched wave energy via interactions with the lower hybrid modes of high parallel refractive index , and the effects of the sheaths formed in front of the limiter surface are ignored. In this work, the heat fluxes to the limiter surfaces are obtained consistently by conducting one-dimension particle-in-cell (PIC) simulations. For the plasma between two guard limiters, while the ions are assumed to be Maxwellian, the electrons are described by using the Fisch model, whose velocity distribution function (VDF) has a resonant plateau in the superthermal region formed by the electron and LHW interaction. Secondary electron emission (SEE) from the limiter surfaces is taken into account by considering two kinds of wall materials, carbon and tungsten. It is found that the sheath potential drop is significantly raised due to the presence of fast electrons. As a result, both ion and electron heat fluxes to the limiters are strongly enhanced compared to those in a Maxwellian plasma, which increases the wall temperature and gives rise to hotspots. The heat flux to the guard limiters increases with the width of the resonant plateau of the electron VDF (i.e. the electron energy). With the enhancement of the sheath potential drop, the physical sputtering yield from the primary and impurity ions on limiter surfaces is found to be significant, which may affect the plasma confinement and discharges.

Hot spots on the EAST tokamak graphite guard limiters of the lower hybrid wave (LHW) antenna may cause a sudden increase of impurity influx and even ending with disruption. A sheath model is developed by taking into account the energetic electron component from the plasma-LHW interaction via electron Landau damping, and then a self-consistent method is used to study the interaction of the plasma across the sheath at the material surface and material thermal response. It is found that the fast electron fluxes driven by both the 2.45 and 4.6 GHz LHWs modify strongly the sheath potential, which then influence on the bombarding ion energy at the material surface and the surface temperature. By calculating the carbon physical sputtering and chemical erosion from guard limiter of the LHW antenna, the carbon production from the guard limiter in two LHW systems is found to have different behavior for the same edge plasma density and electron temperature. Our results show that carbon impurity is mainly from physical sputtering, except for the 4.6 GHz LHW in a narrow range of the high electron temperature where the chemical erosion has a very sharp increase. Due to modification of the sheath potential induced by the fast electrons, the power density is higher with the 4.6 GHz than the 2.45 GHz LHW system. As a consequence the surface temperature is higher at 4.6 GHz and susceptible to reach several hundred of degrees which is prone to chemical erosion.

During ion cyclotron resonance heating, the sheath power dissipation caused by ion acceleration in the radio frequency (RF) sheath is one of the main causes of RF power loss in the tokamak edge region. To estimate the power dissipation of an RF sheath in the ion cyclotron range of frequency (ICRF), a 1D fluid model for the multi-component plasma sheath driven by a sinusoidal disturbance current in the ICRF is presented. By investigation of the sheath potential and ion flux at the wall, it is shown that the larger frequency and lower amplitude of the disturbance current can cause smaller sheath power dissipation. The effect of the energetic ion on the sheath power dissipation depends on the disturbance current. For large amplitude of disturbance current, the increase in the concentration and energy of the energetic ion leads to a decrease in sheath power dissipation. While for a small disturbance current, the sheath power dissipation demonstrates non-monotonic variation with the concentration and energy of the energetic ion. In addition, the sheath power dissipation is found to have a small increase in the presence of light impurity ions with low valence.