Measurement of ion acceleration and diffusion in a laser-driven magnetized plasma

Abstract

Here we present results from an experiment performed at the GSI Helmholtz Centre for Heavy Ion Research. A mono-energetic beam of chromium ions with initial energies of $\sim 450$ MeV was fired through a magnetized interaction region formed by the collision of two counter-propagating laser-ablated plasma jets. While laser interferometry revealed the absence of strong fluid-scale turbulence, acceleration and diffusion of the beam ions was driven by wave-particle interactions. A possible mechanism is particle acceleration by electrostatic, short scale length kinetic turbulence, such as the lower-hybrid drift instability.

Figures (6)

• Figure 1: Schematic of the experimental setup fielded at the GSI facility. The target foils are separated by 1.95 mm and the dimensions of their mesh structure are shown. The nhelix (nanosecond high-energy laser for heavy ion experiments) laser operating at $\lambda_\mathrm{L}=1053$ nm with a pulse duration of $\tau_\mathrm{L}=10$ ns (approximately linear 3 ns rising and falling edges) is split into two beams of approximately equal energy (25-35J), each illuminating one of the two opposing polypropylene (2:1 ratio of hydrogen atom number to carbon) target foils. The laser spot size is 200 µ m on each foil, larger than indicated by the scale in the figure. The UNILAC ion beam direction is into the page and its approximate size is indicated in magenta. The interferometry laser (green) probes along the axis orthogonal to both the ion beam and target normals but its size is not shown to scale -- the actual field of view spans an area several times larger than the foil separation.
• Figure 2: Example of interferometry data at $t=10$ ns after the start of the laser drive. (A) and (B) show interferograms recorded before and during the shot, respectively; the images have been cropped, and their contrast adjusted for clarity. In addition, a rotation is applied to correct for a small random tilt in the interferometry imaging of $\lesssim 5\degree$, such that the target foils are parallel to the x-axis of the images. The propagation direction of the ion beam is shown in magenta in (A). Processing the interferograms results in (C), a map of line-integrated electron density. The projection of the target is filled in with gray; white indicates regions where the density is above the maximum value measurable or where high density gradients cause self-interference effects that distort the fringe pattern. (D) shows a close-up of the interaction region bounded by the yellow box in (B), overlaid with fringes traced in red.
• Figure 3: Interferometry data taken at $t=5$ ns after the start of laser drive, used to estimate the temperature of each plasma jet prior to collision. (A) shows the line-integrated electron density map, similar to Fig. 2C but only driven from one side. A lineout is taken along the dashed red line in the y direction as displayed in (B). The dashed black line shows a least squares fit parameterized by the plasma sound speed, assuming a planar, isothermal expansion.
• Figure 4: CR-39 data and analysis. (A) shows a background shot with no plasma, cropped to a small region at the periphery of the ion beam. Note the sharp 'knife edge' positioned at y=0 mm which is exploited in this analysis. (B) shows a shot where the ion beam traverses the magnetized plasma (double-sided drive) at $t=10$ ns, spatially aligned with the background shot. There is a clear increase in pit density away from the knife edge in comparison with the background as demonstrated in (C). (D) shows the radial distribution of pits moving away from the beam center, with the background subtracted. A Gaussian least-squares fit is shown as the red dashed curve and is used to quantify the magnitude of deflection.
• Figure 5: Examples of ion energy spectra extracted from Time-of-Flight data, probing the interaction region at $t=10$ ns. The background is obtained by averaging the 6 pulses before the pulse of interest (PoI), with standard deviation shaded in red. Normalization is performed such that the area below the background and PoI profiles equate to 1. (A) shows spectra with a small change in shape between the PoI and background pulses. (B) shows a much larger change, with the PoI exhibiting two distinct peaks. There is noticeable variance between the averaged background pulses in (A) and (B), which precludes direct comparison of spectra between shots.