Central peaking of magnetized gas discharges

Abstract

Partially ionized gas discharges used in industry are often driven by radiofrequency (rf) power applied at the periphery of a cylinder. It is found that the plasma density n is usually flat or peaked on axis even if the skin depth of the rf field is thin compared with the chamber radius a. Previous attempts at explaining this did not account for the finite length of the discharge and the boundary conditions at the endplates. A simple 1D model is used to focus on the basic mechanism: the short-circuit effect. It is found that a strong electric field (E-field) scaled to electron temperature T e , drives the ions inward. The resulting density profile is peaked on axis and has a shape independent of pressure or discharge radius. This "universal" profile is not affected by a dc magnetic field (B-field) as long as the ion Larmor radius is larger than a.

The Evolution of Discharge Mode Transition in Helicon Plasma Through ICCD Images

“…5 also confirms the simulation speculation [25] that the absorption of RF energy will mainly occur in plasma-vacuum boundary, and the electron density would peak on axis to form big blue column [26].…”

Analysis of Bohm Diffusions Based on the Ion-Neutral Collisions

“…The ion limited current in Bohm's experiment is smaller than the ion gyrocenter shift current and the particle transport is limited by the ion thermal velocity and the ion gyro-radius so that the Bfield dependence of diffusion coefficient in Bohm's experiment is 1/B, while the diffusion coefficient in Simon's experiment is proportional to 1/B 2 . One of the under lying meanings revealed by the validity of short circuit effect (another recent analysis using short circuit effect can be found in [19]) is that the plasma is not self-neutralized by the quasi-neutrality. If the quasi-neutrality is effective then the ions cannot make the charge separation at the beginning and there is no cause for the short circuit effect.…”

Can we estimate plasma density in ICP driver through electrical parameters in RF circuit?

“…It is to be noted that the density profile inside the cylindrical ICP zone (as shown in fig.2) is expected to be parabolic [26]. The condition for parabolic density profile is existing in the present ICP ion sources, ROBIN [14], BATMAN [13], MANITU [27], ELISE [28], TWIN source [29] or in ITER-NBIs [30]; because metallic endplates (faraday shield) ensures the conditions, as mentioned in [26] for parabolic profile. Metallic plate allows short-circuit electron currents [Simon effect] from two different regions of the plasma and influence radial plasma transport from driver axis to the peripheral region.…”

“…Estimated density values are close to the measured data, within the possible error range and lies within few factors. Apart from the accuracy level of probe measurement in a strong RF environment, the other possible reasons for the discrepancies can be: (a) Radial density profile is parabolic [26] in such cylindrical ICPs. The estimated plasma density is an average value near the skin and also the experimental measurements by Langmuir probe are local and near the center.…”

“…Presence of new species (impurities) will affect the collision frequency in conductivity equation (26). However for short RF plasma pulse, impurity density information or prior calibration is required to compute this impact on the conductivity, value i.e.…”