  Discharge experiments were carried out at the Eindhoven University of
Technology in 2013. The experimental setup was designed to search for electrons
produced in meter-scale sparks using a 1 MV Marx generator. Negative voltage
was applied to the high voltage (HV) electrode. Five thin (1 mm) plastic
detectors (5 $\rm cm^2$ each) were distributed in various configurations close
to the spark gap. Earlier studies have shown (for HV negative) that X-rays are
produced when a cloud of streamers is developed 30-60 cm from the negative
electrode. This indicates that the electrons producing the X-rays are also
accelerated at this location, that could be in the strong electric field from
counterstreamers of opposite polarity. Comparing our measurements with modeling
results, we find that $\sim$300 keV electrons produced about 30-60 cm from the
negative electrode are the most likely source of our measurements. A
statistical analysis of expected detection of photon bursts by these fiber
detectors indicates that only 20%-45% of the detected bursts could be from soft
($\sim$10 keV) photons, which further supports that the majority of detected
bursts are produced by relativistic electrons.
