Breakdown characteristics of plane electrodes having spacers in cryogenic gaseous helium

The vapor phase of saturated superfluid helium suffers from electrical breakdown easily near the Paschen minimum condition of gaseous helium. It is very important from the magnet protection viewpoint to know the surface breakdown voltage in that condition. We measured breakdown voltage of surface or gap breakdown in gaseous helium somewhat higher than the saturation temperature for various pressures (1.33 to 101 kPa) using parallel plane electrodes with a GFRP spacer. In the tested regime the breakdown voltage obeys Paschen´s law and changes only with the product of density and thickness. In the lower density-thickness product range surface breakdown can hardly occur before the gap breakdown develops. Whereas in the higher density-thickness product range surface breakdown voltage is lower than the gap breakdown voltage. Within the scatter of the data the breakdown voltage is independent of spacer diameter and thickness