Particle Balance Measurements by Laser Induced Fluorescence in a Two Antenna Helicon for Plasma Wakefield Acceleration*

A high plasma density (10²⁰ - 10²¹ m^-3) with very high axial uniformity of under 0.25% is needed to achieve beam-driven wakefield acceleration of electrons in the GV/m range in AWAKE plasmas. Helicon plasmas have been known for several years to be capable of reaching these high densities. To determine whether helicon plasmas can achieve the necessary uniformity, it is helpful to understand the helicon particle balance. Using laser induced fluorescence, it is possible to measure the 2D particle balance in a helicon plasma. We present these measurements for various relevant RF configurations. We utilize two identical antennas, each capable of delivering up to 10 kW of RF power to the plasma. We investigate the effects of adding the second antenna, and the effects of increasing RF power. Understanding how these two inputs influence the particle balance is key to understanding how helicon plasmas can be used to sustain uniform, high, on-axis plasma densities for use in AWAKE.