Spectroscopic Study of Anode Plasmas in a Microsecond Electron Beam Diode

Abstract

Experiments have been performed to investigate the visible emissions (370-600 nm) from long-pulse electron-beam-driven carbon anode plasmas, and to correlate the spectroscopic evidence for ions with deviations of the diode current and voltage from Child-Langmuir behavior. Electron beams had peak voltages of -0.8 MV, current densities approaching 103 A/cm2, and pulselengths of about 1 μs. Diode closure resulted in three phases of the beam/plasma evolution. In stage I the deposited electron dose was smaller than that required for anode plasma optical emission (≤250 J/gr). No deviations of the diode operation from conventional Child-Langmuir scaling were observed. In stage II, the electron dose was large enough for anode plasma formation before shorting (≥350J/gr), and low-intensity optical emission was primarily from CII and CIII. During stage II, two types of non-Child-Langmuir diode behavior occurred due to the presence of an anode plasma. An anomalous voltage peaking behavior occurred at an average dose of 740 J/gr, at about 80 percent of the diode shorting time. The diode impedance was constant during this voltage peaking behavior, contrary to the impedance collapse expected from Child-Langmuir behavior in a closing diode. This voltage peaking was accompanied by an increase in continuum emission, particularly at shorter wavelengths. A bipolar impedance level was occasionally observed before voltage peaking, at an average dose of 410 J/gr,at about 66 percent of the predicted shorting time.