Since 04/2019: AWAKE helicon cell research funded by NSF
Title: “Unraveling the link between radio-frequency wave propagation and high ionization efficiency in helicon plasmas”
Plasmas, the fourth state of matter, are of importance for many science and industrial application. An exciting perspective is the application as a medium for next generation particle accelerators. The electrical features of a plasma allow to drive high electric field gradients used for particle generation, which overcome those in existing solid-state based setups by up to a factor of one hundred. This translates into a significant shortening of the length of next generation linear particle accelerators and therefore savings in costs, construction time and complexity and risk. At CERN in Geneva, Switzerland, the AWAKE project aims on using a plasma column as medium for a next generation Lepton accelerator based on the wake-field acceleration concept. The research in this grant focuses on the plasma physics underlying this promising way of generating plasmas and the understanding on how this plasma is formed and how it can be tuned for the requirements in the accelerator application. The generic understanding of such plasma sources has also a direct relevance to a variety of future applications, for example plasma processing in industry and life-science, plasma generation for fusion based neutron sources and utilization to develop a wind-tunnel experiment for astrophysical plasma science questions.
The specific method used for plasma generation in this grant 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 103Gauss, these waves are canonically called Helicons. They are characterized by a bifurcative transition from a inductively coupled, low density and low ionization regime into a regime of high density and almost complete gas ionization. A direct resolution of this bifurcative transition into the helicon regime and of the ionization dynamics in this regime have not been accomplished. There is still the unresolved question how energy is coupled from the whistler type, right hand RF wave into the neutral gas, such that for a given plasma regime obtained by inductive coupling, the situation transitions into a highly ionized plasma setup. And then, in this setup, a density limit is found which keeps the plasma density from reaching the nominal density as defined by the dispersion relation of the wave at the given field. This is linked to the application of plasmas as a medium in particle accelerators. 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 this bifurcative transition and understand the ionization dynamics at this point 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 necessary axial plasma density.
See more about the project here: https://home.cern/news/news/experiments/awake-more-plasma-more-acceleration
Funding Information: National Science Foundation NSF, PHY1903316