High Time Resolution Axial Particle Balance in AWAKE Helicon Plasmas

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 helicon plasmas are capable of reaching sufficient densities at the beginning of a 5 ms pulse, it is not known if they meet the strict uniformity requirement of 0.25% for use in AWAKE. Using laser induced fluorescence, it is possible to derive a particle balance in helicon plasmas. A new high speed laser induced fluorescence technique is being developed to measure densities and flow velocities with time resolution as short as at least 1 ms. This new diagnostic will be capable of providing density and flow velocity measurements for both argon ions and neutrals, and from this data, the particle balance is derived. This particle balance not only provides the necessary details about the axial density profile, but it also gives insight into the physics of the ion and neutral dynamics that establish this profile. In particular, the plasma pressure gradient that could cause a depletion of neutral particles on axis could have a significant impact on the axial density profile especially in limiting the achievable density in a helicon plasma.