Abstract
Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-charge-dominated ion beams which must undergo longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the effective limits of neutralized drift compression, which occurs due to an imposed longitudinal velocity tilt on the drifting beam and subsequent neutralization of the beam's space charge with background plasma. The accurate and temporally resolved measurement of the ion beam's current and pulse length, which has been longitudinally compressed to a few nanoseconds duration at its focal plane, is a critical diagnostic. This paper describes the design and experimental results for a fast and accurate ion beam probe, which reliably measures the absolute beam current in the presence of high density plasma at the focal plane as a function of time. A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe.
Highlights
One of the more significant challenges in developing heavy ion drivers is found in the final transport section leading to the target, where ion beam compression in space and time is required in order to achieve conditions necessary for heavy ion fusion and high energy density physics applications
In a heavy ion driver, intense beams of ions [1,2,3] are transported through a final-focus magnet system to the target chamber, where they must transversely focus onto the target with a final diameter of a few millimeters or less [4,5]
A high density plasma, positioned between the final-focus magnets and the target chamber, provides an electron population which neutralizes the beam’s space charge and allows the intense beams to be focused beyond the regular space-charge limit [8]
Summary
One of the more significant challenges in developing heavy ion drivers is found in the final transport section leading to the target, where ion beam compression in space and time is required in order to achieve conditions necessary for heavy ion fusion and high energy density physics applications. In order to meet the requisite current densities on target, there is a need to determine the physical and technological limits of such longitudinal focusing, achieved by applying a time-dependent velocity tilt to the beam and subsequently allowing it to drift through a background plasma, thereby neutralizing the beam’s space charge as the beam’s pulse length compresses longitudinally [13]. This concept is referred to as ‘‘neutralized drift compression,’’ and the upgrade of the NTX facility is called the Neutralized Drift Compression Experiment-1A (NDCX-1A). The modified Faraday cup could be described as a type of gridded-energy
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