Modeling of rod-pinch-diode performance at 2 to 4 MV

Summary form only given. The rod-pinch diode has been scaled up successfully to 2 to 4 MV in experiments on the ASTERIX generator. Well-behaved impedance is achieved for anode-cathode (A-K) gaps /spl ges/ 3.75 mm at 2.7-MV peak voltage and /spl ges/ 6.5 mm at 4-MV peak voltage. Shots with smaller A-K gaps exhibit rapidly decreasing impedance. The electrical behavior of the diode is analyzed with the rod-pinch-diode model. This model generates a current waveform by combining the space-charge-limited (SCL) current and the critical current, evaluated using the measured diode voltage. The model includes several physical parameters to account for anode and cathode plasma expansion, as well as the generation of ion current. The model current is fit to the measured diode current by adjusting these parameters. The model current reproduces the shape and magnitude of the measured current for shots with well-behaved impedance (shots with sufficiently large A-K gap) using physically reasonable values of the model parameters. These parameters are consistent with values determined in previous analyses of rod-pinch shots at 1 to 2 MV on the Gamble II and TriMeV generators. The analysis indicates that the rod-pinch diode on ASTERIX transitions from SCL to critical current early in the pulse. With the large currents in this experiment (55 to 135 kA), the ion current turns on very early in the pulse (3 to 8 ns). The fits are most sensitive to the /spl alpha/-factor in the critical-current formula and to the anode expansion velocity v/sub A/. Values of the /spl alpha/-factor and v/sub A/ are consistent with the values obtained in the previous analyses at 1 to 2 MV.