Simulation and Measurement of Helicon Wave Fields and Plasma Parameters at the Madison AWAKE Prototype

The AWAKE project at CERN opens up the frontier of next generation electron colliders using beam-plasma wakefield acceleration. Acceleration gradients exceeding 1 GV/m have been demonstrated using a laser-ionized plasma. However a full scale accelerator will need a reliable, high density plasma source that scales to kilometer lengths with a high degree of axial density uniformity. The Madison AWAKE Prototype (MAP) is utilizing 30 kW of RF power to generate a helicon plasma with expected densities reaching 10²⁰ m^-3 in a multi-antenna setup. In order to optimize the density profile, an understanding of wave propagation and power deposition is essential. To this end we have developed a finite element model in Comsol that solves for the quasi 3D wavefields and power deposition for a given temperature, density and neutral distribution. We present comparisons of model predictions to experimental measurements for the power deposition. Further we show the helicon density scaling with magnetic field strength, as measured by a newly developed high-speed heterodyne microwave interferometer system. We also show results from the recently established steady-state high-power operation which was made possible by a new antenna cooling system.