SNL/NRL Experiments on Gamble II
Under SNL sponsorship, NRL is conducting ion beam experiments at 1.2 MV with a new extraction diode on
Gamble II. The 2D magnetic field code DATHETA was used to predict the proper anode shape to produce the
desired ion beam profile. The diode will generate and focus protons for self-pinched transport experiments. (An
ion transport scheme will be required for high-yield defense and energy applications to protect diode hardware
from intense bursts of x rays and gamma rays.) With self-pinched transport, the azimuthal magnetic field
produced by the net current in a low-pressure gas pinches the beam and allows it to propagate within a narrow
channel. The self-pinched transport approach is considered the most promising for both light and heavy ion
approaches to ICF because it removes the need for focusing hardware within the target chamber. An added
benefit for the light ion approach is that self-pinched transport may reduce requirements on ion beam
divergence by a factor of 2. At half the predicted focal length of the diode (36 cm from the anode) the
experimental proton beam, injected through a foil into 1-Torr air, is recorded on a witness plate and imaged
through multiple pinholes onto radiochromic film. Preliminary analysis suggests that the beam focusing is similar
to the DATHETA predictions. Variations in beam current density as a function of beam radius are controlled by
adjusting the magnetic field shape. A counterbalancing magnetic field pulse has been designed to remove
undesired angular momentum in the beam to maximize the amount of beam that focuses on axis. After the focus
is optimized, the protons will be used in self-pinched transport experiments. Simulations with the 3D, hybrid
(fluid electrons, particle ions) IPROP code (developed by MRC) will guide the transport experiments. The goal
of the Gamble-II experiments is to demonstrate efficient self-pinched transport of protons before conducting
follow-on experiments relevant to high yield with lithium ions on the SABRE accelerator at SNL.
Other Reports on High Energy Density and Inertial Confinement Fusion
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