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Preference of Right-Handed Whistler Modes and Helicon Discharge Directionality due to Plasma Density Gradients

Marcel Granetzny, Oliver Schmitz, Michael Zepp

Abstract

Whistlers are magnetized plasma waves in planetary magnetospheres. Bounded whistlers, known as helicons, can create high-density laboratory plasmas. We demonstrate reversal of the plasma discharge direction by changing either antenna helicity or magnetic field direction. Simulations reproduce these findings only in the presence of a radial density gradient. Inclusion of such a gradient in the wave equation gives rise to azimuthal shear currents which for the first time consistently explains the preference of right- over left-handed whistlers and the discharge directionality in helicon plasmas.

Preference of Right-Handed Whistler Modes and Helicon Discharge Directionality due to Plasma Density Gradients

Abstract

Whistlers are magnetized plasma waves in planetary magnetospheres. Bounded whistlers, known as helicons, can create high-density laboratory plasmas. We demonstrate reversal of the plasma discharge direction by changing either antenna helicity or magnetic field direction. Simulations reproduce these findings only in the presence of a radial density gradient. Inclusion of such a gradient in the wave equation gives rise to azimuthal shear currents which for the first time consistently explains the preference of right- over left-handed whistlers and the discharge directionality in helicon plasmas.
Paper Structure (2 sections, 24 equations, 7 figures, 1 table)

This paper contains 2 sections, 24 equations, 7 figures, 1 table.

Table of Contents

  1. Derivation of the Wave Equation in a Plasma with Radial Density Gradient
  2. Symbols used in this Work

Figures (7)

  • Figure 1: Dependence of the helicon plasma density and light emission on the antenna helicity - left-helical (LH) or right-helical (RH) - and background magnetic field direction - leftward or rightward in this figure.
  • Figure 2: Top: CAD model of the core components of the MAP experiment. Bottom: Right-half-helical antenna with currents during one half of the RF cycle in blue and axial launch and azimuthal rotation directions for the different modes in green.
  • Figure 3: Simulation results for an RH antenna with magnetic field in the negative $\bm{\hat{z}}$ direction, representing the green case in Fig. \ref{['fig:directionalityExp']}. Top: Measured plasma density and simulated wave magnetic field strength. Bottom: Axial power deposition by azimuthal modes.
  • Figure 4: Comparison of simulation results for the four combinations of antenna helicity and magnetic background field direction, representing the green, orange, red and blue cases in Fig. \ref{['fig:directionalityExp']}. White contour lines show plasma density in units of $10^{19}\mathrm{\; m^{-3}}$.
  • Figure 5: Simulated power deposition profiles for helicon discharges with different field directions and antenna helicities in plasmas with radial density gradient (top) and uniform density (bottom). Green and orange as well as blue and red curves are overlapping, respectively.
  • ...and 2 more figures