Experimental study of the pinch-beam diode with thin, unbacked foil anodes

Pinch-beam diodes are used extensively for intense electron and ion beam production. The large aspect ratio (diode radius/anode-cathode gap, R/D) diodes utilize the large electron path length (-R) relative to the ion path length (-D) to enhance ion production, over the bipolar limit, by R/D. Pinch-reflex diodes further enhance the electron path length and thus the ion production efficiency by incorporating thinner-than-electron-range anode foils attached to a flat solid anode (a few mm behind the anode foils) by a small, on-axis conducting stalk. Electron reflexing is induced by the magnetic field generated by the return current through the conducting stalk. In both cases, large R/D\´s are needed for high ion production efficiencies thus necessitating low impedance (Z ∝ D/R) diode operation. In this work, a new "backless" anode is studied in which the thin foil is held to the anode potential only along its circumference with the region behind it totally void. In this case, the self-pinching electrons are allowed to reflex between the real cathode and a self-formed virtual cathode behind the anode foil until either the virtual cathode dissipates or the electrons lose their energy in the foil. This diode has two potential advantages over conventional pinch-beam or pinch-reflex diodes. The ion production efficiency can be high even at high impedances (smaller R/D\´s) and the anode debris caused by the dissipating electron beam is substantially reduced. The experiments are carried out on the Gamble II generator (13 MV, 0.85 MA, 60 ns). When thin plastic anodes are used, efficient (−60%) positive ion production is observed. Thinner foils result in earlier ion production and larger time-integrated total ion energies. Furthermore, totally unbacked foils are significantly more ion-efficient at early times than standard reflexing (backed) foils of the same thickness. When high-Z metallic foils are used as anodes, the bremsstrahlung radiation - rovides an additional diagnostic for gaining insight into the dynamics of the electron orbits. Thin, unbacked foils produce more than twice the radiation compared with the same thickness foils backed with a low-Z material to prevent electron reflexing. The experimental results and their implications will be discussed.