Fast Faraday cup to measure neutralized drift compression in intense ion charge bunches

Abstract

Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-chargedominated 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.

Source-to-target simulation of simultaneous longitudinal and transverse focusing of heavy ion beams

“…This beam frame temperature translates into the laboratory frame coordinates as energy variation E 2E i T k 1=2 . Previous measurements of t p min have been 2 -4 ns on NDCX with E i 300 keV K ions and L 130 cm [14,15]. This pulse width implies a total T k 1 eV or E 1 keV and energy variation E=E i 0:003 for NDCX.…”

“…II neglects electron inertia and assumes a constant density of 10 12 -cm ÿ3 . In this section, we utilize a more realistic kinetic electron-ion PIC model for the plasma and assess the impact of the plasma density ranging from 10 9 -10 11 cm ÿ3 (roughly the range seen in experiments [14,15]) on the beam focal spot. As above, we reinject beam particles at the z 275 cm plane from the integrated simulation.…”

“…In the presence of a perfectly neutralizing plasma, the compression is only limited by the accuracy of the applied velocity tilt and the ion longitudinal temperature [13]. Longitudinal compression factors of 70 have been demonstrated on the neutralized drift compression experiment (NDCX) [14,15].…”

Neutralized drift compression experiments with a high-intensity ion beam

“…A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe. The characteristics of the cup have been published elsewhere [21].…”

Overview of theory and simulations in the Heavy Ion Fusion Science Virtual National Laboratory

“…In recent experiments [5,6] on the NDCX facility at LBNL, a carefully developed tilt waveform yielded sixty-fold longitudinal compression. Figure 1 shows the evolution of the longitudinal compression factor, as measured using the fast Faraday cup [7] and as simulated using the LSP code, a kinetic model, and a hybrid model [8]. The modeling shows that the compression achieved to date has been limited by known tilt waveform errors that can be further reduced; an improved pulser is planned.…”