High Time Resolution Axial Particle Balance in AWAKE Helicon Plasmas

A high density plasma (n_e ≈ 10²¹ m^-3) is needed to achieve Wakefield acceleration of electrons from an axial electric field in the GV/m range. It has been shown that sufficient densities are achievable in helicon plasmas during 5 ms pulses. It is still necessary to determine the axial density homogeneity, which is required to stay within 0.25% for Wakefield acceleration. An argon laser induced florescence technique to measure the axial density homogeneity and derive a particle balance on sub-millisecond timescales is being developed. Acousto-optic modulators have been successfully tested for frequency shifting to avoid the need for the typical frequency scan. Utilizing these frequency shifters, the new diagnostic will be capable of acquiring 12-point density data after only one 5ms pulse, and ion velocity data and an axial particle balance after only two 5ms pulses. The particle balance will indicate if additional fueling is necessary to correct the axial density homogeneity.