TEMPORAL CONTROL OF RADIATION PULSE USING GAS FILLIED DRIFT CELLS ON HERMES III

Significant erosion of the front of the HERMES-III electron beam in drift cells filled with N{sub 2} gas is measured for pressures greater than 100 Torr (as much as 10 ns [3 m] at 630 Torr for 11-m length cells). Little rise time (RT) sharpening or pulse width reduction of the subsequent radiation pulse generated at bremsstrahlung targets terminating the cells, however, is measured. Three-dimensional numerical simulations show that the simultaneous virulence of the resistive-hose instability with increased pressure degrades the spatial coherence of the leading edge of the beam, degrading in turn the RT of the radiation pulse. Transport in the lower-pressure ion-focused regime, on the other hand, does permit intense radiation fields to be produced near the beam axis at the target, with RTs and pulse widths that can be controlled with pressure. Over the range 5 to 50 mTorr, the RT and full-width half-maximum (FWHM) of the on-axis radiation pulse are inversely related to pressure and can be varied from {approximately}5 to {approximately}2 ns and {approximately}8 to {approximately}3 ns, respectively. At 10 mTorr, for example, the RT and FWHM are 4.2 {plus_minus} 1.7 ns and 6.1 {plus_minus} 1.3 ns, and the corresponding peak dose, peak-dose rate, andmore » useful area of exposure are {approximately}25 krad (CaF{sub 2}), {approximately}4 Trad (CaF{sub 2})/s, and {approximately}300 cm{sup 2}, respectively.« less