Collisionless Energy Absorption in Plasmas: A Theoretical and Experimental Investigation in Spherical Geometry

摘要

We are investigating the RF impedance characteristics of a small spherical probe immersed in a laboratory plasma. The experimental facility is the large Space Physics Simulation Chamber at the Naval Research Laboratory. The data daken are from network analyzer measurements of the reflection coefficient obtained when applying a low-level RF signal to the probe, which is either near-floating potential or negatively DC-biased in a low-pressure plasma. As is well known, sheaths form around objects placed inside plasmas. The electron density is smaller inside the sheath, and the reduction in density alters the plasma impedance. Surprisingly, the impedance becomes 'resistive,' even though the plasma is effectively collisionless, at frequencies below the bulk-plasma frequency, thus leading to collisionless energy absorption.