Ion Doppler Tomography for Ion Temperature and Velocity Measurements During High Guide Field Magnetic Reconnection

Abstract

A novel ion Doppler diagnostic is developed to measure ion temperature and out-of-plane velocity during magnetic reconnection on the Magnetic Reconnection Experiment (MRX) and the Facility for Laboratory Reconnection (FLARE). The diagnostic utilizes tomographic inversion of line integrated measurements and is capable of resolving ion temperatures of 5 eV and toroidal velocities of 4 km/s of Helium plasmas. A custom spectrometer, utilizing a high-throughput volume phase holographic (VPH) grating was constructed to achieve maximum fidelity measurements across a range of operation conditions in MRX. A novel tomographic inversion technique was also developed to address the low photon counts measured on MRX. The technique utilizes a least-squares minimization technique based on the trust region reflective algorithm to discover the most appropriate emissivity, out-of-plane velocity, and ion temperature profiles corresponding to the line-integrated measurements taken by the diagnostic. The algorithm utilizes a collisionless emission model for an axisymmetric plasma including radial velocity. The diagnostic performance is demonstrated during the study of ion energization during high guide field magnetic reconnection experiments on MRX. Multiple guide field strengths greater than the value of the reconnecting field are studied in the collisionless regime. Mostly flat, elevated ion temperatures near 10 eV are measured with no discernible trend in the structure or magnitude of heating across various guide field values. Slight peaks in out-of-plane velocity are measured at both conditions, with slightly greater magnitudes measured at higher guide fields. The electric field's role in ion energization and heating is discussed at all conditions and the influence of heating due to polarization drift is highlighted for a single condition.