A megavolt test stand for measuring cathode and anode emissions with nanosecond pulses

Summary form only given. A new test stand has been constructed for measuring emissions from cathode and anode surfaces under high electrical stress in vacuum. An L-3 PulseRad is used as the generator to provide ~50-ns electrical pulses of up to 1 MV. The usual sealed glass X-ray tube has been replaced by a stacked ring insulator so that custom load hardware can be used with the generator. The initial configuration of the load hardware was designed to measure electron emission from cathode surfaces. The cathodes are flat disks with a three inch OD. Using a flat cathode surface eliminates a geometric component of field enhancement. The anode sits below the cathode surface with an adjustable AK gap. With this geometry we can test areas of several square centimeters with variable peak electric fields up into the MV/cm range. A vacuum voltmeter is used in this configuration, both to carefully measure the applied voltage and to act as a dummy load. This eliminates concerns about damaging the generator in case the test load stays at high impedance. Bdot probes on the cathode holder measure the current from the sample. An X-ray camera placed above the system looks through the cathode at the high Z anode to give a time integrated view of X-ray emission from the anode, helping to diagnose uniformity of emission. Optical access to the cathode surface is provided by vacuum windows, allowing for the use of cameras, spectrometers and interferometers. This new test stand will first be used to investigate the role of surface material, machining, cleaning and coatings on the temporal development of electron emission from cathode surfaces. Later, the test stand will be modified to examine ion emission from the cathode and anode surfaces.