Hard X-Ray Spectrum Measured in a Solar-Relevant Lab Experiment That Undergoes Fast Magnetic Reconnection*

The Caltech MHD jet experiment creates a collimated jet that kinks causing lateral acceleration which then instigates a fast secondary Rayleigh-Taylor instability. This breaks the jet and a hard X-ray burst is detected.  A CMOS camera modified to function as an X-ray spectrometer reveals that the pulse consists of a non-uniform distribution of X-ray energies centered around 8 keV. The camera has an aluminum foil sheet blocking visible light and no lens. X-ray photons transiting the foil deposit energy into random camera pixels so that a histogram of the pixels gives the energy spectrum. These measurements complement a plastic scintillator having nanosecond time resolution but poor energy resolution.  It is proposed that despite the short collision mean free path, an inductive electric field associated with jet breaking accelerates a subgroup of electrons to keV energies without any of these electrons undergoing collisions. It is further proposed that the fast electrons suddenly decelerate via collisions and radiate X-rays. Extrapolation to the solar corona and/or chromosphere predicts the acceleration of a small subset of electrons to very large super-thermal energies by sub-Dreicer electric fields.