Unraveling the Link Between Radio-frequency Wave Propagation and High Ionization Efficiency of Helicon Waves

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

The focus of this research project is on the physics of generating very uniform plasmas for future particle accelerator applications. Plasmas, the fourth state of matter, are of importance for many science and industrial applications, one of which is as a medium for next generation particle accelerators. The electrical features of a plasma allow for high accelerating electric field gradients which are up to a factor of a hundred greater than in existing solid-state-based particle accelerators. This can translate into much more compact, energetic, and cheap accelerators. At CERN in Geneva, Switzerland, the AWAKE project aims to use a plasma column as a medium for a next generation electron accelerator based on the wakefield acceleration concept. This grant will support research aimed at improving the understanding of how a plasma column can be formed and tuned for the requirements in the AWAKE accelerator application. The understanding of such plasma sources is also of direct relevance to a variety of other applications, such as plasma processing in industry and life-sciences, plasma generation for fusion-based neutron sources, and development of a wind-tunnel experiment for astrophysical plasma turbulence. The specific method used for plasma generation in this project is the so-called Helicon wave. These are radio-frequency waves in the whistler regime, known broadly for reliable generation of magnetized plasmas at high density with very high ionization fractions. In the magnetic field regimes of 10^3 Gauss, these waves are canonically called Helicons. They are characterized by a transition from an inductively coupled, low density and low ionization regime into a regime of high density and almost complete gas ionization. This transition into the helicon regime and the ionization dynamics in this regime are not yet fully understood. This is linked to the application of a plasma as the medium for particle acceleration. This application requires unprecedented levels of homogeneity in the plasma density in the direction of the accelerated particle beam path. The goal of this research is to resolve the helicon transition and understand the ionization dynamics during the transition and shortly afterwards. Plasma diagnostics based on analyzing the light emitted from the plasma will be used to understand the plasma axial homogeneity and methods of active, local gas injection will be used to gain control of the axial plasma density. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.