Investigation of absorption mechanisms in helicon discharges in conducting waveguides

Abstract

This paper investigates the mechanisms by which the helicon and associated Trivelpiece-Gould waves are absorbed in helicon discharges produced in conducting chamber; the experiments were based on a recent theory of damping and absorption of helicon modes in conducting waveguides (Ganguli et al 2007 Phys. Plasmas 14 113503). In particular, it was also planned to investigate the manner in which the absorbed energy is utilized for the production of warm electrons that are needed for ionization because helicon discharges are high density, low T e discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. To this end, two separate regimes were considered. The first was a low pressure (≈0.2-0.3 mTorr), low magnetic field (≈16-20 G) regime where both wave absorption and warm electron production are shown to proceed through Landau damping. The second was a moderate pressure (≈10 mTorr), moderate magnetic field (≈60-65 G) regime, where both power absorption (which is collisional) and warm electron production proceed via high-energy electrons produced by acceleration of bulk electrons (from neighboring regions) across large potential gradients.

Evidence of current free double layer in high density helicon discharge

“…One may improve the situation somewhat by using a magnetic mirror configuration as used in authors' work on electron cyclotron resonance and helicon plasma production. [46][47][48][49][50] Therefore, for the present experiments, use of a magnetic mirror configuration has been necessary for trapping the population of nonthermal electrons.…”

“…For instance, it allows one to determine warm electron density, n w and warm electron temperature, T w (if present) directly along with the other plasma parameters, permitting one to generate profiles for these quantities, which makes a simultaneous comparison of all variables relatively easy. It may be noted that RF LP measurements were quite valuable in authors' previous work 49,50,55 for measuring the plasma parameters. LP analysis procedure is efficient to determine the bulk and warm electron populations.…”

Experimental investigation of current free double layers in helicon plasmas

“…It is these high-energy electrons, which need to be confined magnetically using the designed mirror field so that their presence can be detected in the Langmuir probe measurements. [30][31][32][33] To produce the axial magnetic field (Fig. 1), a set of seven electromagnet coils (M/s Tondon and Co., New Delhi) of bore diameter % 26 cm position coaxially over the plasma chamber.…”

“…The details of LP data acquisition and robustness and accuracy of the LP data measurement; and LP analysis of the acquired data is presented elsewhere. [30][31][32][33] Fig. 2 is one of the representative LP data taken on axis, showing the various electron populations at axial location z % 6.5 cm in the axial profiles to be discussed below in Fig.…”

“…3 presents the axial profiles of various plasma parameters, e.g., bulk plasma density (n 0 ), bulk electron temperature (T e ), warm electron density (n w ), warm electron temperature (T w ), and plasma potential (V p ) at operating pressure % 6 mTorr and RF power % 940 W. At the coil current of 15 A, the average magnetic field in the flat portion of the mirror was B 0 % 66 G. It may be noted that the position z % 0 in the figure represents the center of the second loop antenna, i.e., the RF launching site. [30][31][32] Fig. 3(a) shows the axial magnetic field for ease of reference.…”