Derivation of the Helicon Particle Balance by Laser Induced Fluorescence in MAP*

A moderately high plasma density (10²¹ m^-3) with very high axial uniformity is needed to achieve wakefield acceleration of electrons in the GV/m range in AWAKE plasmas. While pulsed helicon plasmas are capable of reaching sufficient densities transiently, it is not known if steady state helicon plasmas can sustain these densities. Using laser induced fluorescence (LIF) it is possible to infer plasma density in steady state and pulsed helicon plasmas. Initial results in a steady state plasma in the Madison AWAKE Prototype (MAP) indicate a peaked axial and hollow radial density profile at low power of 1.3 kW in a 5.2 cm diameter chamber. Radial and axial flow velocities of ions and neutrals are also measured by LIF, and these velocities are used to derive the particle source rates and particle balance in the plasma. Previous work on helicons indicated a dominant axial flux, but we measure a radial flux of the same order of magnitude as the axial flux, much stronger than merely Bohm. The radial flux suggests a different fueling mechanism for the plasma compared to devices with larger diameter. The 2D particle balance indicates how helicon plasmas are formed and sustained. This information about helicons can provide insight into how to adapt these plasmas for more uniform and higher densities on axis, so that they may approach the AWAKE parameter requirements.