Fast PhotoDiode (FPD) Non-Perturbative Diagnostic System for Reconnection Measurement in PHASMA*

Magnetic reconnection causes bulk plasma heating and particle energization through the conversion of stored magnetic energy to particle kinetic and thermal energy. In the PHAse Space MApping (PHASMA) experiment, electron-only magnetic reconnection occurs when two kink-free flux ropes merge without significant participation from ions. Light emitted by flux ropes contains information about flux rope mergers and reconnection dynamics. The Fast PhotoDiode (FPD) system, a non-perturbative diagnostic, is designed to optically resolve the evolution of flux ropes and locate reconnection in PHASMA. The FPD system uses axial and radial arrays of photodiodes paired with fast electronics to produce high signal-to-noise ratio (>50dB) and high bandwidth (>100 MHz) measurements of light emission. Both the push and pull phases of reconnection can be identified using light emission and these measurements are well correlated with magnetic probe results. Both push and pull type reconnection occur within a single discharge. Optically derived measurements of electron heating are compared with incoherent Thomson scattering and triple Langmuir probe measurements. Results indicate that FPD arrays could serve as non-perturbative diagnostics for monitoring flux rope and reconnection dynamics in lab plasmas.