Changes in diode operating characteristics as a function of applied magnetic field geometry

Abstract

Summary form only. In order to propagate a lithium ion beam to the axis at PBFA II (Particle Beam Fusion Accelerator II), a specific magnetic field geometry is required. If the ions lose no energy in the gas cell, and if lithium ions originate at the anode in the singly charged state and are stripped to the triply charged state in the gas cell membrane, conservation of canonical angular momentum requires that Psi /sub a/=-2 Psi /sub g/, where Psi is the magnetic field stream function and the subscripts a and g refer to its values at the anode and the gas cell, respectively. When Psi not=0, it has proved difficult experimentally to operate the diode efficiently and reliably. It has been hypothesized that the difficulty is associated with the dynamics of virtual cathode formation. Experiments have been carried out, along with analytical and simulation studies, to elucidate this problem. Factors that appear to be important are the anode surface area, the magnitude and uniformity of the magnetic insulation between the cathode tip and the active anode surface, the amount of radial magnetic flux in the diode feeds, and the magnetic field topology in the diode region. These factors may alter the dynamics of charge accumulation in the diode and thereby affect the timing of ion current.<>