Self-magnetically-pinched radiography diode with laser-generated plasma assist

Summary form only given. The self-magnetic-pinch (SMP) electron beam diode is being developed for 4 to 10 M V, 30 to 50 ohms, 50 ns, flash radiography of explosively driven objects by AWE, SNL, and NRL. The goal is a reproducible <; 2-mm FWHM diameter radiographic spot at several hundred Rads (Si) at 1 m. In this talk a new approach is proposed where the outer diameter of an ~50-ohm vacuum impedance MITL is tapered down to ~7 cm diameter to drive an ~3 cm diameter hollow cathode that will be adjusted to operate at <;40 ohm. Undermatching the diode impedance to the MITL helps reduce the MITL vacuum electron flow with only a small reduction of voltage. Recent 3-D PIC simulations have shown that a finite length small outer diameter MITL helps symmetrize and center the electron beam pinch location. A flat carbon anode with a centered <;2-mm diameter W rod extending a few mm from the surface will be placed 2-3 cm from the cathode. This geometry is similar to the negative-polarity non-reentrant rod pinch diode which PIC simulations suggested would operate well at 10 MV. However, they also show that the electron beam initially spreads out to a large diameter potentially leading to ion production from undesirable locations, and the electrons do not pinch to the end of the rod until the diode is close to 10 MV. To mitigate these problems it is suggested that a pulsed laser illuminate the end of the W rod through the center of the hollow cathode to generate low-density plasma before the power pulse arrives. The proper laser energy, timing, and rod tip material and shape must be chosen so that the resultant laser-ablated plasma will touch the tip of the hollow cathode just as the main power pulse arrives at the diode. This plasma density should be just high enough to initially short the gap and conduct the initial electron beam to the tip of the rod. It is speculated that, as the voltage increases and the low density plasma erodes from the cathode tip, the diode w- ll reach critical current with most of the electrons still focused on the tip of the rod. Numerical simulations will be presented along with preliminary experimental results.