Optimization of radiographic spot for the TriMeV accelerator
The authors are examining diode configurations for the 3-MeV, 30-kA TriMeV electron-beam accelerator at Bechtel Nevada. These include a flat-cathode diode with focusing into a gas cell and a magnetically-immersed diode similar to that used on HERMES III at Sandia National Laboratories. They are in the process of optimizing the diode designs using the hybrid kinetic-fluid simulation code IPROP which models both vacuum flow and gas breakdown. These configurations will be fielded on TriMeV this year. The TriMeV voltage pulse is somewhat unique in that it has a 3-ns rise and fall time with a 14-ns flat top. This sharp rise and fall in voltage make it ideal for studying beam focusing. The flat-cathode diode uses a 3.5-cm radius velvet electron emitter and a 4-cm anode-cathode gap that has produced a 23 kA beam self-focused in vacuum into a gas cell of 2.8-cm radius. The time-integrated beam spot has been optimized experimentally with 10 Torr nitrogen in the ballistic focusing regime where self fields are minimal. New currents < 5 kA were measured at the gas cell wall for 5--15 Torr nitrogen. Below 2 Torr, the net current increases rapidly to the full beam current. IPROP has simulated this behavior in reasonable agreement with experiment. The code predicts that, due to finite plasma current decay, the time-integrated spot is increased as the beam focus moves upstream of the target. If the diode voltage is sufficiently stable, IPROP predicts a root-mean-square (RMS) radius <2 mm in the runaway regime (1 Torr). In an immersed TriMeV diode, a 10--15 kA electron beam is emitted from a sub-millimeter needle cathode into a strong 12--18 Tesia solenoidal field. The remaining current is lost to the outer wall. The large fields are necessary to limit growth of the magnetized ion-hose instability. With careful design of the cathode structure, the authors can minimize the current flowing into the diode from large radius. IPROP calculates an RMS radius <1 mm. With close collaboration of theory and experiment, the authors hope to reduce of the beam spot size and better understand the impedance behavior of these diodes.
- Research Organization:
- Mission Research Corp., Albuquerque, NM (US)
- Sponsoring Organization:
- Sandia National Laboratories
- OSTI ID:
- 20067599
- Country of Publication:
- United States
- Language:
- English
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