A virtual cathode is formed in a cavity, if the injection current from the
diode exceeds the critical current. The propagation properties of the forwa
rd current beyond the virtual cathode are investigated. It is shown that th
e forward current increases significantly as the diode gap distance L-CA in
creases, although the injection current from the diode decreases drasticall
y. Experimental data agree well with theoretical results obtained analytica
lly. (C) 2001 American Institute of Physics.