Two dimensional particle simulations of rf-driven plasma sheaths

Two dimensional PIC simulations are performed to study sheath formation around objects immersed in a magnetized plasma, driven by intense rf waves. The parameters and the configuration are chosen to model the conditions around a Faraday screen bar, during ICRH of tokamaks. The imposed rf field can be obtained from the 3-D antenna simulation in vacuum, using ARGUS. If the magnetic lines intersect the conductor, the sheath is electron depleted. For magnetic lines almost parallel to the conductor surface, ion depletion is dominant, due to larger Larmor radius. The strong field gradient in the sheath accelerates ions to impact energies sufficient to produce sputtering of the screen material. This causes impurity influx to the plasma core, and screen damage. Preliminary results show that 2-D effects are very important when the sheath thickness is comparable to the bar cross-section. Then anomalous ion transport sets in, and the ambipolar diffusion tends to re-establish quasineutrality, depleting the sheath. The possibility of opposite polarity ion and electron sheaths, across the x and y direction respectively, must be examined. In case of intense sheath formation, the authors observe energization of magnetized ions to energies much higher than the time-averaged rectified sheath potential. An explanation through a stochastic acceleration mechanism, triggered by the field rectification, is proposed. Existing 1-D theories are compared with their simulations. 4 refs