Exploration of Helicon Plasmas for Next-Generation Particle Accelerators at the Madison AWAKE Prototype (MAP)*

Plasma wakefield accelerators, such as the Advanced Proton Driven Wakefield Accelerator (AWAKE) project, require a highly uniform, scalable, reproducible and high-density plasma source. One of the most promising solutions uses helicon waves for plasma breakdown and sustainment. Helicons are routinely used to create high-density plasmas but are still an active area of research. We present work in support of AWAKE that has solved some of the fundamental questions in helicon research. Our experimental studies were performed at the Madison AWAKE Prototype (MAP), a new and versatile platform for high-power, high-density helicon experiments. Densities were measured with LIF and a new high-speed, high-precision heterodyne microwave interferometer. In order to optimize power coupling into MAP we have developed a helicon antenna optimization strategy based on a global particle and power balance model combined with an antenna power spectrum analysis. We found that even at relatively low RF power of 1.3 kW, we could reach core densities up to 2.3x10¹⁹ m^-3 in good agreement with our global power balance model. This result indicates that future operation at 30 kW is likely to reach the 10²⁰ m^-3 density regime needed for AWAKE. We have shown that different antenna lengths led to vastly different plasma performance, thus validating our antenna design strategy. In addition, we have studied helicon wave propagation inside MAP using a newly developed, quasi-3D, finite element model, that massively reduces computational costs compared to existing tools. A comparison between computational modeling and experimental results on MAP allowed us to resolve long-standing questions regarding the directionality of helicon plasmas and the preference of so-called right-handed whistler modes. Specifically, we have shown that both effects are linked and arise from the interaction of helicon wavefields with radial plasma density gradients, a discovery that allows us to direct the plasma discharge.