Helicon experiments and simulations in nonuniform magnetic field configurations

Abstract

Wave, antenna impedance, plasma density, and temperature anisotropy measurements are carried out for a helicon plasma source in nonuniform and uniform static magnetic fields. Strong axial density gradients associated with the nonuniform magnetic fields are observed to affect wave fields, absorption, and source efficiency. The wave field and antenna input impedance measurements are compared with a new simulation code which also calculates Poynting power flow and wave absorption profiles. Wave amplitude measurements are shown to decay more rapidly and the phase velocity varies over a wide range for the nonuniform static magnetic field case.

Measurements of helicon wave propagation and Ar II emission

“…By moving the Langmuir probe along the z-direction at r ϭ5 cm, we find that the density during the flattop peaks at 1.1ϫ10 12 /cm 3 about 12 cm from the double-half-turn coil ͑located at zϭ7.5 cm͒. It then decreases to a lower density of 2ϫ10 11 /cm 3 at zϭ40 cm and slowly decreases at further axial positions as shown in Fig. 4.…”

“…In this paper we report on experiments performed on the WOMBAT 7 helicon experimental facility in which were created argon plasmas with densities in the range 10 11 18 The upper Ar II 443 nm emission state can be excited directly from the atomic ground state or from the ionic state by electron collisions. Thus, electrons of fairly high energy are required to directly populate these states, and emission at 443 nm is indicative of their presence.…”

“…Collisional processes, 1,2 Landau damping, 2,3 antenna localized acceleration, 4,5 mode conversion near the lower hybrid frequency 6 and nonlinear trapping [7][8][9][10] of fast electrons have been examined for different operating regimes. The effects of an axial variation in the source static magnetic field and electron temperature anisotropy has been measured and modeled by Guo et al 11 These efficient sources are considered for argon lasers, 12 silicon wafer etching, 13 materials processing, optical integrated circuit thin film creation, 14 space plasma simulation 15 and plasma thrusters for space vehicles. 16 The central question in the physics of helicon sources is the cause of their highly efficient ionization and strong wave damping, which is not well explained by either collisional or Landau damping processes.…”

Laser ionization and radio frequency sustainment of high-pressure seeded plasmas

“…Figure 6 shows the power absorbed as a function of the radius. Our studies show 33 that a five-turn helical antenna ionizes and absorbs much more power for the same plasma than a Nagoya type-III coil 34 for the same plasma conditions.…”

“…The vacuum impedance of the antenna was measured with the network analyzer to be Z ant ϭ0.02ϩi 72 ⍀, indicating that the vacuum reactance of this antenna is approximately five times higher than a comparable type-III antenna. 34 The experiment is carried out to determine at what maximum pressure the discharge can be initiated with the capaci- tive rf fields alone. The argon pressure is varied from 200 mTorr to 80 Torr.…”