IBW generated ponderomotive potential effect on edge plasma in PBX‐M

Edge plasma modifications have been observed during IBW (f=54 MHz) on PBX‐M at very low power (below 100 kW). Reductions of up to a factor of 2 are seen in the density profile over a radial extent of several cm, with little change to the floating potential or electron temperature, as measured by the fast reciprocating probe. At higher powers, the density reduction can be as much as an order of magnitude, with no significant edge electron heating. A 1‐D analytical model and a 2‐D fluid code are used to model these experimental results, which appear to be due to the generation of a ponderomotive potential barrier in the edge plasma. The experimental results are compared with the analytic and numerical model using an RF generated ponderomotive potential. These results are signficant for edge plasma modification concepts such as the rf‐limiter and divertor, and encouraging for IBWH, where power was observed not to be deposited in the plasma edge.