Pulsed, intense electron beams for material response studies without the use of external magnetic fields

Direct irradiation of materials by electron beams (e-beams) has been used to study material response.2,3 The desire to utilize high-power (∼ TW) generators to achieve higher specific energy deposition over larger areas has led to several approaches. One approach utilizes a monolithic e-beam diode with an external magnetic field (B field). The external B field allows the diode to operate in the bipolar, space-charge-limited regime without the current being magnetically limited to a lower value. The field also is used to guide the e-beam through the gas-filled region between the vacuum diode and the object to be irradiated. An alternate approach, discussed in this presentation, utilizes multiple diodes electrically in parallel, with each diode running below the critical current to obtain a high current. The e-beams are then scattered in foils and combined in the gas-transport region to achieve the desired irradiation uniformity and area. We report on experiments that have been performed on the Gamble II generator at NRL (∼ 1 MV, ∼ 800 kA, ∼ 60 ns) designed to study this second approach. Diagnostics include diode voltage and current, a net-current monitor, interferometry, spectroscopy, an axial array of Ta-strip x-ray witness plates, and a segmented calorimeter. Experiments have been performed with a single ring diode and two nested ring diodes. Where possible, the measurements are compared with results from ITS and from the recently-developed ABC model4 for the interaction of the e-beam with the gas. Results show that the beam is very nearly charge and current neutralized as it propagates ∼ 20 cm in 1-Torrr N2 gas. Beam scattering from both the anode and a Ti scattering foil in the gas results in a relatively uniform radial beam profile.