Wave propagation and absorption simulations for helicon sources

A two-dimensional (r,z) computer code is developed to examine antenna coupling, wave propagation, and power absorption in a helicon plasma source. The code calculates the electromagnetic wave fields and power absorption in an inhomogeneous, warm plasma. An iterative solution which incorporates warm plasma thermal effects has been added to the code to examine collisionless Landau and collisional heating mechanisms. This is the first and currently the only available code to include the axial and radial variations in the plasma density and applied magnetic field profiles as well as the corresponding local temperature effects to the wave field and electron heating calculation. The effects of the applied magnetic field (B0 = 10--1200 G), 2-D (r,z) density ( ne0 = 1011--1013 cm-3) profiles, residual neutral gas pressures of 0.5--10 mTorr and the antenna spectrum on collisional and collisionless wave field solutions are investigated. The primarily electrostatic TG mode dominates the heating at low magnetic fields and deposits its wave energy near the edge region. At higher magnetic fields (B0 > 80 G), the propagating helicon mode transports and deposits its energy in the core plasma region away from the antenna. Collisional damping of the helicon waves is shown to be the dominant heating mechanism for moderate pressures (p > 2 mTorr) and higher densities (ne > 1012 cm -3) . However, at low pressures and densities (p < 2 mTorr, ne < 1012 cm -3) Landau damping becomes important and heats the electrons mainly near the antenna region where the resonant electrons have velocities near the wave phase velocity. Comparisons between the code results and selected experimental data are carried out for uniform and nonuniform applied magnetic field profiles. In particular, the experimentally measured plasma density profile is used as input to our linear code to compute the axial wave magnetic field Bz, antenna impedance, and electron heating profiles. It is shown that the strong axial inhomogeneities in the plasma density profile cause distributed reflections and prevent the wave from propagating to the far end of the source. Wave reflections are also caused by the gradients in the applied magnetic field profiles. The effect of an Electron Cyclotron Resonance (ECR) zone on wave absorption in a helicon operation is also investigated.