Temporal Evolution of Ion Velocity Distribution Function (IVDF) in a Pulsed, Current-Free, Helicon Generated, Expanding Magnetoplasma

摘要

Summary form only given. Current-free plasma expansion in a divergent magnetic field is surprisingly common and is found on a variety of spatial scales and in a variety of applications. Plasma expansion is essentially equivalent to a pressure gradient arising from a change in the plasma density. The density gradient can give rise to a potential gradient that retards motion of the lighter plasma electrons but accelerates the more massive ions downstream. The solar wind expansion and corresponding creation of the interplanetary electric field is a classic example of this process. Also, there is strong experimental evidence in support of Alfven''s hypothesis that the aurora results from energetic electrons precipitating onto the upper atmosphere and that the electrons in space could be accelerated by double layer (DL) electric fields with components parallel to the terrestrial magnetic field. Under certain external conditions, theoretical simulations and experimental observations showed that in a helicon plasma expanding into a weaker magnetic field, a DL with a width of a few tens of Debye lengths can form at the end of helicon plasma. The DL accelerates ions to Mach numbers of order 2. In this work, we present the temporal evolution of both parallel and perpendicular to the magnetic field argon ion velocity distribution functions (IVDFs) in pulsed helicon plasmas obtained by using a time resolved laser induced fluorescence technique with 1 ms resolution. At 600 W of RF power and 1 mtorr pressure, for pulses of 200 ms at a duty cycle of 0.5, the DL forms in the first 40 ms of the discharge. The parallel argon ion flow speed rises from 400-500 m/s at the beginning of the pulse to 3000-3500 m/s at the end of the pulse. Perpendicular measurements showed no change in the perpendicular flow speed, consistent with a DL electric field parallel to the background magnetic field