Microclump effects in magnetically-immersed electron diodes

Summary form only given. Magnetically-immersed electron diodes are being developed to produce needle-like, high-current, electron beams for radiography applications. An immersed diode consists of a needle cathode and a planar anode/bremmstrahlung converter which are both immersed in a strong solenoidal magnetic field (12-50 T); nominal parameters are 10 MV, 40 kA, 0.5 mm radius cathode, and 5-35 cm anode-cathode gaps. A physical picture of normal and abnormal diode behavior is emerging. Normal diode behavior occurs for times 0/spl les/t/spl les//spl tau/, where the transition time /spl tau/ is typically 30 ns; during this time, bipolar space-charge limited flow occurs, which scales well to desired radiography parameters of high dose and small spot size. Abnormal diode behavior occurs for t/spl ges//spl tau/, which results in substantial increases in spot size and current (impedance reduction). This abnormal behavior appears to be caused by an increase in ion charge in the gap, which may result from poor vacuum, impurity ions undergoing ion-ion stripping collisions during transit, or microclumps undergoing stripping collisions during transit. The potential effects of microclumps on diode behavior are reported here.