Evidence of current free double layer in high density helicon discharge

Abstract

This paper investigates the formation of double layer (DL) in helicon plasmas. In the experiment, argon plasma production is using the excitation of m ¼ À1 helicon mode with magnetic mirror field with high mirror ratio of $1:1.7. We have specifically used the radio frequency compensated Langmuir probe (LP) to measure the relevant plasma parameters simultaneously so as to investigate the details about the plasma production. The DL, which consists of both warm and bulk populations towards higher potential region and only dense bulk plasmas towards the lower potential region downstream the antenna, is present in the transition region. LP measurements also show an abrupt fall of density along with a potential drop of about 20 V and e DV p k T e % 12 within a few cm. The potential drop is equal to the difference of the electron temperatures between the two plasma regions forming the DL, which is present in the plateau region of mirror, unlike in several prior studies on the DL formation in the region of strong gradients in the magnetic field. The DL is strong, current-free, electric double-layer with estimated thickness of about 10 Debye lengths.

Study of axial double layer in helicon plasma by optical emission spectroscopy and simple probe

“…the strength of the magnetic field, the filling gas pressure and the excitation frequency of RF power [1,2]. To find a mechanism which makes the helicon plasma source highly efficient, research has been done over the last several decades to understand helicon wave energy absorption [3][4][5][6][7][8]. Landau damping of the helicon wave is one such mechanism, observed in several experiments [2][3][4].…”

“…Among them, a Langmuir probe based on the saturation ion current, which is a new RFcompensated structure [17,18], is the most commonly used device to ascertain the plasma density and evaluate the electron temperature. Sahu et al [5][6][7] used an RF-compensated Langmuir probe combined with a B-dot to measure the DL in the plateau regions of the magnetic field under mirror-like magnetic field topology in a conducting cylindrical chamber. Charles [13] used retarding field energy analyzers (RFEAs) to discover the DL in a helicon plasma and the ion beam downstream of the DL.…”

Experimental investigation of current free double layers in helicon plasmas

“…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.…”

“…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.…”

“…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.…”