A proposed near-term experiment for pulse length minimization on NDCX-II

The recently completed second-generation Neutralized Drift Compression eXperiment (NDCX-II)¹ provides a very tool for the understanding of “warm dense matter” (WDM) as well as for heavy ion fusion (HIF). The latest status of NDCX-II is to accelerate 50 nC Li⁺ to 130 keV and compresses the beam with 7 induction cells (accelerating elements). The duration of the ion beam will be shortened from 500 ns to less than 70 ns down the machine. The pulse length of the beam will be largely affected by the compression schedule of each cell in the acceleration section. If the beam is over-compressed, particle overtaking can take place, leading to irreversible growth of the longitudinal emittance, which is a beam parameter that directly relates to the pulse length. Using WARP simulation, the compression schedule has been re-optimized, by maximally compressing the beam without particle overtaking and associated irreversible longitudinal emittance growth. In the present configuration, the beam will drift down to the target after leaving all the acceleration elements. Further reduction of the longitudinal emittance of the beam can lead to additional reduction of the final pulse length on the target. We propose to apply a generalization of a recent technique² developed by Jack Woo to reduce the longitudinal emittance of the NDCX-II beam. First, two adjacent current monitors will be used to measure the beam energy profile. Second, with this energy profile, we can apply a correction using one or two voltage correction cells. In the case of the two-step correction, the first correction cell will correct the density fluctuation and the second will correct the velocity fluctuation. We will show the net effect of these correction techniques on the final pulse length of the NDCX-II beam. Experimental demonstration of these pulse minimization techniques will have significant implications for future target experiments.