Hydrodynamic Analysis Of The Radio Frequency Sheath In The Ion Cyclotron Range Of Frequency In Fusion Plasma

Ion cyclotron resonance heating(ICRH)is one of the main auxiliary heating methods in tokamaks and has been succeeded in many experiments.However,the radio frequency(RF)sheaths driven by the Ion cyclotron range of frequency(ICRF)wave may cause strong interactions between plasma and boundary materials,leading to a series of unwanted problems such as impurity sputtering,power losses,hot spots,etc.,which will not only change the plasma parameters,and affect the plasma performance,but also damage the boundary materials and shorten materials’ life.Therefore,the study of the characteristics of the RF sheath driven by ICRF wave and its effect on power dissipation and impurity sputtering is of great significance for improving the efficiency of ICRH,extending the materials’life,and controlling the impurity level in core plasma.In this work,based on the fluid model,together with the equivalent circuit model,a one-dimensional collisionless RF sheaths model for fusion plasma in ICRF is developed.In the fluid model,the effects of energetic ion components and impurity ion components on the sheath structure are considered,and ion temperature is considered.The relationship between the instantaneous potential at the wall and the sheath thickness are determined self-consistently by the equivalent circuit model.Our numerical results show that the amplitude and frequency of disturbance current caused by ICRH and the plasma parameters have great effects on the sheath structures.The sheath potential increases with the increase of disturbance current and decreases with the increase of frequency.The increase in background ion temperature,plasma density,and the energy or concentration of energetic ion will cause decrease in the sheath potential.For the light impurity ions,their concentration is usually less than a few percent in tokamak boundaries,under this condition,the changing in concentration of impurity ions has little effect on the sheath characteristics.Sheath potential is an important factor affecting sheath power dissipation.The decrease in amplitude of disturbance current and the increase in frequency lead to the decrease in sheath potential,thus the sheath power dissipation decreases due to the decrease of acceleration of sheath on ions.When the energy or concentration of energetic ion increase,the sheath potential decreases,while the ion flux to the material surface increases.For the small disturbance current,the sheath power dissipation decreases first and then increases with the increase in energy or concentration of energetic ion;for the large disturbance current,with the increase in energy or concentration of energetic ion,the decrement of sheath potential is far larger than the increment in ion flux,therefore,the sheath power dissipation decreases.For the carbon,titanium,iron and molybdenum materials,with the increase of the sheath potential,ions such as hydrogen,deuterium,lithium,carbon,oxygen,etc.will obtain much more energy through the acceleration of the large sheath potential,therefore,the sputtering yield of impurities increases evidently.When the sheath potential is small,the yield of impurities is mainly caused by energetic ion;with the increase of sheath potential,for ions will obtain much more energy through the acceleration of the sheath,the sputtering yield caused by other ions is significantly increased,and will higher than that of energetic ion.The increase of sheath potential is an important reason leading to the increase of impurity sputtering.