Computational and Analytical Optimization of Helicon Antennas with a Fast Full Wave Solver Exploiting Azimuthal Fourier Decomposition

Abstract: Plasma wakefield accelerators (PWA), such as AWAKE, require homogenous high-density plasmas. The Madison AWAKE Prototype (MAP) has been built to create a uniform argon plasma in the $10^{20}\,\mathrm{m^{-3}}$ density range using helicon waves. Computational optimization of MAP plasmas requires calculating the helicon wavefields and power deposition. This task is computationally expensive due to the geometry of high-performance half-helical antennas and the small wavelengths involved. We show here for the first time how the 3D wavefields can be accurately calculated from a small number of 2D-axisymmetric simulations. Our approach exploits an azimuthal Fourier decomposition of the non-axisymmetric antenna currents to massively reduce computational cost and is implemented in the Comsol finite-element framework. This new tool allows us to calculate the power deposition profiles for 800 combinations of plasma density, antenna length, and radial density profile shape. The results show the existence of an optimally coupling antenna length in dependence on the plasma density. This finding is independent of the exact radial profile shape. We are able to explain this relationship physically through a comparison of the antenna power spectrum with the helicon dispersion relation. The result is a simple analytical expression that enables power coupling and density optimization in any linear helicon device by means of antenna length shaping.