Langmuir Probe Method for Precise Evaluation of the Negative‐Ion Density in Electronegative Gas Discharge Magnetized Plasma

Abstract

The paper reports results from Langmuir probe current-voltage (IV) characteristic measurements in argon and oxygen low-gas-pressure magnetized plasma. The plasma was produced in a stainless steel discharge tube with length 1.5 m and diameter 0.17 m with a hot filaments cathode. The wall of the discharge tube was grounded. An axial magnetic field B was created by a solenoid. A platinum cylindrical Langmuir probe with radius R = 5·10 −5 m and length L = 5· 10 −3 m was placed axially and radially at the center of the discharge tube in order to perform measurements along and across the magnetic field.Using the second derivatives of the measured IV probe characteristics, the EEDF in argon and oxygen was evaluated. The derivatives were taken numerically. Measurements in an argon (i.e., in the absence of negative ions) were performed to obtain results for comparison with the measurements in an oxygen gas discharge, where a high density of negative ions is expected.In the measurements of the second derivatives of the IV characteristics with the probe parallel to the magnetic field in oxygen, a peak appears close to the plasma potential due to the registration of negative ions. In the same time, the electron part of the second derivative was substantially suppressed relatively to the case with the probe oriented perpendicular to the magnetic field. Thus using the appropriate orientation of the probe and a low magnetic field, when the electron part of the second derivative is suppressed, since the negative ion part is not affected by the low magnetic field applied, we can evaluate precisely the negative ion density.The results from the plasma parameters evaluation (plasma potential, electron and negative ion densities and electron temperatures) are presented and discussed.

Helicon Volume Production of H− and D− Using a Resonant Birdcage Antenna on RAID

“…Negative ions in plasmas could be theoretically detected by LPs by measuring the shape of the second derivative of the current-voltage probe characteristic. This technique is used when the plasma electronegativity .α = n − /n e is about 1 or more (Popov et al 2013). However, in the case of low-temperature hydrogenic plasmas, the electronegativity is of the order of 0.1, and the negative ion density is too small, resulting in too noisy I-V curves.…”

One-dimensional, multi-fluid model of the plasma-wall transition. II. Negative ions

“…[7][8][9] Direct and indirect measurement methods have been developed, 10,11 and our group was also involved in the development of novel probe methods to measure the energy distribution functions of negative species in low-density plasmas. 12 Different approaches have been used to understand the transition between the plasma and the wall in such complex plasmas. Different fluid models are very popular as they can also be used for magnetized plasmas [13][14][15][16] or in the case when more than just two groups of charged particles (positive ions and electrons) are present or if a cylindrical or spherical geometry is considered.…”

Three-fluid model of the plasma–sheath region for a planar probe immersed in an active oxygen discharge. Validity of the Boltzmann relation

“…In particular, oxygen discharges are used for etching, purifying, oxidation and coating processes [3,5]. All of these applications have aroused the interest of many authors about electronegative plasmas [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], and different models have been developed to describe an oxygen plasma in low-pressure inductive discharges [23][24][25][26][27], parallel-plate capacitive discharges [15], glows [28,29] and RF discharges [30].…”