Long pulse measurements from a cold field emission cathode *

A test bed for high perveance field-emission cathodes and photocathodes has been developed.  The diode has been designed for voltages up to 500kV, has the ability to test cathode diameters < 2.5 cm, and an adjustable A-K gap from 2-8 cm.  The cathode is driven by 20-Ohm, 400 kV, PFN Marx capable of providing a 2.6 μs pulse.  We also have the ability to crowbar the pulse down to ~300 ns.  Preliminary measurements of cathode emission at V > 200 kV and E > 60 kV/cm are presented.  These consist of cathode induced light emission, extracted currents > 100 A (J > 40 A/cm²), and corresponding current density distributions downstream.  Transient field emission effects are observed for t > 150 ns.  A preliminary analysis of field emission effects over the 0.3-2 μs pulse length range is also presented.