Time-resolved proton focus of a high-power ion diode

An improved understanding of the factors that control the axial focus of applied-B ion diodes was obtained from time-resolved diagnostics of ion-beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3-mm diameter from a 4.5-cm-radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2 on the 1-TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time-resolved information about virtual cathode evolution, the self- and applied-magnetic-field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.