Fast (10 ns) x-ray pulse production from a 60 ns generator using a plasma opening switch

摘要

Summary form only given. There is interest in developing a capability for generating a bremsstrahlung pulse with approximately 10 ns full width at half maximum (FWHM) using several-MeV electrons at a few-hundred kA. One approach to do this is to use a plasma opening switch (POS) to reduce the output pulse width of a standard pulsed power generator. Similar output pulses have been achieved in the past using a POS on the Gamble II generator (2 MV, 1 MA, 60 ns).[l] This paper reports new experiments on Gamble II using cable-gun and flashboard plasma sources and several configuration variations to establish promising techniques to generate the desired output pulse from conventional, longer-pulse generators. Gamble II has a coaxial, 3-ohm water-filled output line connected through an insulator to an inductive vacuum section, initially short-circuited by the POS plasma. During the short circuit phase of the POS, energy increases in the vacuum inductance. At peak current, the inductively-stored energy is about 80% of the energy in the forward-going power pulse. The POS opens quickly, switching the current to a downstream electron-beam diode. Bremsstrahlung is generated when electrons impact a tantalum anode. The resulting x-ray pulse width depends on when the POS opens. The time of opening is controlled by adjusting the initial density and electrode dimensions. Fast switching on Gamble II occurs when the center conductor diameter of the coaxial POS is 5 cm, the plasma length is 8 cm, and when opening starts prior to the end of the forward-going power pulse. The diode load is located 25 cm downstream from the load-end of the POS. This separation is necessary to avoid interference of the POS plasma with the electron-beam diode, which consists of a 6-cm diameter hollow cylindrical cathode, spaced 1-cm from a planar tantalum anode. If the POS conducts for about 80 ns, the current reaches 700 kA prior to switching. The forward-going power pulse is decreasing from its maximum at- this time, and the voltage at the insulator crosses zero about 10 ns later. The combination of high conducted current and a diminishing power pulse result in a 9-ns x-ray FWHM, with 500 kA and 3 MV at the diode. Increasing the conduction time results in reduced diode power, while reducing the conduction time results in increased x-ray pulse width.