Temporal evolution of anode and cathode plasmas in a vircator diode

Summary form only given, as follows. The electron beam current in the diode region of a vircator at Himeji Institute of Technology is well characterized by the electron space-charge-limited current in bipolar flow. The beam current is well below the critical current defined for diode pinching and the observed microwave emission is accompanied by the strongly pinched electron beam. These results suggest the presence of anode and cathode plasmas expanding in the anode-cathode (A-K) gap. In this paper we report the experimental observation of the evolution of anode and cathode plasmas in time and space. The experiments were performed with the pulsed power generator "HARIMA-II" characterized by the maximum output voltage of 400 kV, output impedance of 3 /spl Omega/ and pulse duration of 50 nsec. The electron beam diode consists of an annular cathode of 3.0 cm in diameter and 1 mm in thickness, and an aluminum foil anode of 15 /spl mu/m in thickness. An anode plasma appears on the anode surface immediately after the rise of a beam current and at about 10 nsec later a cathode plasma on the cathode surface is formed. Both plasmas expand with approximately the same speed of 2/spl times/10/sup 4/ m/sec. The A-K gap is closed by plasmas after about 100 nsec from the onset of anode plasma. The microwave emission occurs just before the gap closure. The anode plasma in the initial phase was identified as a hydrogen plasma with an aid of the optical filter for H/sub /spl alpha// and H/sub /spl beta// lines.