Use of an electrostatic barrier scrape-off layer as a technique to reduce impurity sources from bulk plasma bombardment of radiofrequency antenna structures

A new technique is proposed that may be used to reduce the bombarding flux of plasma onto critical RF antenna components (Faraday shield) yet maintain a high plasma density relatively near the antenna to maximize coupling efficiency. Limiters, which are normally employed to protect the Faraday shield from direct plasma contact, are modified and biased electrically positive, forming a narrow boundary layer plasma with a significantly reduced density e-folding length. A simplified two-fluid transport model which includes phenomenological cross-field diffusion and mobility is used to estimate the effect of the induced electrostatic barrier scrape-off layer (EBSOL) on the local density e-folding length. The model predicts that the influence of an applied bias on the local density gradient depends only on the relative magnitudes of cross-field ion and electron mobilities. Characteristic density e-folding lengths in front of the Faraday shield may be reduced by up to a factor of five with positive bias potentials of the order of three times the local electron temperature. The result is insensitive to the magnetic field strength and suggests that an order of magnitude or more reduction in the ion sputtering rate on the Faraday shield may be achievable. The result also suggests that cross-field mobility may play an important role in scrape-off layer transport, even in the absence of an externally imposed bias – an effect that is typically overlooked in tokamak scrape-off layer transport analysis. As a specific example of the EBSOL technique, an arrangement that might be employed to protect the Faraday shield of an Alcator C-MOD ICRF antenna structure is considered. Bias potentials, currents, heat fluxes and reduced densities at the Faraday shield are estimated for anticipated plasma conditions. The dimensionless phenomenological transport parameter Λ (= Tμ⊥/D⊥) is shown to be important for assessing the practical viability of the EBSOL technique, with Λ ≪ 1 implying that multiple biased limiter electrodes in minor radius may be required.