Simulations of the pinch dynamics in a planar self-magnetically pinched diode

Summary form only given. When used for radiography applications, the planar self-magnetically-pinched diode (SMPD) consists of an /spl sim/8-mm-diameter hollow cylindrical cathode and a planar high-atomic-number anode separated by an anode-cathode gap spacing of /spl sim/10 mm. For these applications, it is important to understand how the charged-particle dynamics in the diode influence the electron energy and angular distributions on the anode. The particle-in-cell code LSP is used to simulate the charged-particle flow in the diode and to characterize these electron distributions. SMPD operation consists of three phases: a low-voltage, low-current phase characterized by space-charge-limited flow (SCL), a high-voltage, high-current phase characterized by strongly-pinched magnetically-limited (ML) flow, and an intermediate phase where the beam is weakly pinched (WP). During the SCL phase, the electrons are spread over a relatively large area of the anode. The electron flow is stable and laminar with a mono-energetic energy distribution on the anode and an average incidence angle that decreases from /spl sim/30/spl deg/ to the normal to /spl sim/5/spl deg/ as the self-magnetic field increases. During the WP phase, the electrons are pinched into a radius roughly equal to the cathode radius with few electrons crossing the diode axis. The electron flow is stable with an energy distribution that remains mono-energetic and an average incidence angle that increases from /spl sim/5/spl deg/ to /spl sim/45/spl deg/ as electron orbits begin to cross and the flow becomes non-laminar. During the ML phase, the electrons are strongly pinched within the central 1-3 mm diameter portion of the anode with many electrons crossing the diode axis. The flow is turbulent and unstable with an energy distribution that has significant components at energies higher and lower than the applied voltage. The onset of the instability is very rapid and results in a precipitous drop in the average i- cidence angle from /spl sim/45/spl deg/ to /spl sim/25/spl deg/.