Understanding helicon plasmas

Abstract

This paper presents a comprehensive overview of work on the helicon plasmas and also discusses various aspects of RF power deposition in such plasmas. Some of the work presented here is a review of earlier work on theoretical [A. Ganguli et al., Phys. Plasmas 14, 113503 (2007)] and experimental [A. Ganguli et al., Plasma Sources Sci. Technol. 20(1), 015021 (2011)] investigations on helicon plasmas in a conducting cylindrical waveguide for m = −1 mode. This work also presents an approach to investigate the mechanisms by which the helicon and associated Trivelpiece-Gould (TG) waves are responsible for RF power deposition in Helicon discharges. Experiment design adopts the recent theory of damping and absorption of Helicon modes in conducting waveguides [A. Ganguli et al., Phys. Plasmas 14, 113503 (2007)]. The effort has also been made to detect the warm electrons, which are necessary for ionization, because Helicon discharges are of high density, low Te discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. Experimental set up also comprises of the mirror magnetic field. Measurements using RF compensated Langmuir probes [A. Ganguli et al., Plasma Sources Sci. Technol. 17, 015003 (2008)], B-dot probe and computations based on the theory shows that the warm electrons at low pressure (0.2–0.3 mTorr) Helicon discharges, are because of the Landau damping of TG waves. In collisional environment, at a pressure ≈10 mTorr, these high-energy electrons are due to the acceleration of bulk electrons from the neighboring regions across steep potential gradients possibly by the formation of double layers.

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.…”

Whistler waves with angular momentum in space and laboratory plasmas and their counterparts in free space

“…As in unbounded plasmas helicons have for a fixed frequency two spatial modes with different radial wavelengths, a helicon wave, and a shorter wavelength Trivelpiece-Gould mode (TG) [122][123][124]. These two modes also exist in unbounded plasmas where the difference lies in the angle of wave propagation.…”

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.…”