Plasma-filled Ion Diode Experiments On PBFA-II

An Applied-B diode is used to generate a focussing light-ion beam on the PBFA-II accelerator at Sandia National Laboratories. This diode has an initial high-impedance phase, which can last up to 20 ns, during which the ions that form the beam are produced, and electron flow builds an electron sheath that allows for enhanced ion emission. Because of this high impedance phase, current lags voltage in the diode. This impedance history also complicates the coupling of a Plasma Opening Switch (POS) to the diode, since the preferred load for a POS is one that reaches the desired impedance in less than 5 ns. One method of reducing the initial diode impedance is use of a plasma-filled load. A low density plasma prefill is supplied to the A-K gap prior to the power pulse arrival. Electrons from this plasma help to populate the electron sheath so that enhanced current flow can begin sooner. In addition, current flowing through this prefill can allow for magnetic insulation of the electron flow downstream of the opening POS to begin immediately, thus helping the POS to open more completely. MAGIC Applied-B diode simulations predict earlier ion current enhancement, and bigger dl/dt. Experiments on the LION accelerator at Cornell University using a prefilled Applied-B diode and a POS resulted in up to 80% more energy coupled to the load.