Comparing experiments with modeling for light ion helicon plasma sources

Abstract

The ability to obtain high plasma densities with high fractional ionization using readily available, low-cost components makes the helicon a candidate plasma source for many applications, including plasma rocket propulsion, fusion component testing, and materials processing. However, operation of a helicon can be a sensitive function of the magnetic field strength and geometry as well as the driving frequency, especially when using light feedstock gases such as hydrogen or helium. In this paper, results from a coupled rf and transport model are compared with experiments in the axially inhomogeneous Mini-Radio Frequency Test Facility ͓Goulding et al., , p. 107͔ ͑Mini-RFTF͒. Experimental observations of the radial shape of the density profile can be quantitatively reproduced by iteratively converging a high-resolution rf calculation including the rf parallel electric field with a transport model using reasonable choices for the transport parameters. The experimentally observed transition into the high density helicon mode is observed in the model, appearing as a nonlinear synergism between radial diffusion, the rf coupling to parallel electric fields that damp near the plasma edge, and propagation of helicon waves that collisionally damp near the axis of the device. Power deposition from various electric field components indicates that inductive coupling and absorption in the edge region can reduce the efficiency for high-density operation. The effects of absorption near the lower hybrid resonance in the near-field region of the antenna are discussed. Ponderomotive effects are also examined and found to be significant only in very low density and edge regions of the Mini-RFTF discharge.

Superiority of half-wavelength helicon antennae

“…Though the loading resistance can be computed this way, the equilibrium density cannot be found without considering plasma ionization and transport. Only recently have attempts [30][31][32] been made to couple the helicon equations with transport and continuity equations to give the density profiles. For design purposes, the plasma loading was computed with HELIC for uniform n(r).…”

Optical, wave measurements, and modeling of helicon plasmas for a wide range of magnetic fields

“…15 They have also been used for plasma confinement studies, space-plasma simulation experiments, 16 and are of substantial interest as part of the VASIMR thruster engine for Mars space vehicles. [17][18][19] Recent studies of helicon sources have focused on the physics behind their highly efficient ionization and strong wave damping, which is not fully explained by either collisional or Landau damping processes. The significance of the role of a population of fast electrons constituting a nonMaxwellian component of the electron distribution function in the helicon ionization process is a central question.…”

Improved simulation of the ICRF waves in the VASIMR plasma

“…More recently, the implementation of the iterative solver, using the Multigrid-Incomplete Cholesky Preconditioned Conjugate Gradient Squared method (ICCG) demonstrated good convergence with higher computational speed, compared to the direct solver [6]. The recent upgrade of the EMIR code, EMIR3 [7], includes the calculation of the parallel electric field component, so the system of Eqs. (2) is now solved for all three components.…”