Nonuniform discharge currents in active plasma lenses

Active plasma lenses have attracted interest in novel accelerator applications due to their ability to provide large-field-gradient (short focal length), tunable, and radially symmetric focusing for charged particle beams. However, if the discharge current is not flowing uniformly as a function of radius, one can expect a radially varying field gradient as well as potential emittance degradation. We have investigated this experimentally for a 1-mm-diameter active plasma lens. The measured near-axis field gradient is approximately 35% larger than expected for a uniform current distribution, and at overfocusing currents ring-shaped electron beams are observed. These observations are explained by simulations.

Figure 1

MHD simulations of a 1-mm-diameter plasma lens yield the radial distribution of (a) the electron temperature and plasma density, (b) the transverse current density, and (c) the magnetic field profile. A simplified model was proposed (plotted as solid blue curves), based on $J(r)\propto T_e(r{)}^{3/2}$, with $T_e(r)\propto u(r{)}^{2/7}$ as described in Ref. [34]. One can observe an approximately 35% enhancement in near-axis current density and B-field gradients.

Figure 2

Schematic concept of the experimental setup. Electron beams produced by a jet-blade assembly [35] were propagated through the plasma lens to either a transverse charge diagnostic (phosphor screen) or energy distribution diagnostic (magnetic spectrometer). The bottom left inset shows the (simplified) plasma profile at the LPA, while the bottom right inset displays the measured discharge current profile (red curve) and profile used for the MHD simulations (black curve).

Figure 3

Electron beam size (red circles) and peak signal level (blue circles) on the phosphor screen in a current timing scan at $8\times 10^{16}{\mathrm{cm}}^{-3}$ He atomic density. The dashed black line displays the current at which one would expect focusing in case of uniform radial current distribution.

Figure 4

Charge distribution on the phosphor screen at various discharge timings (and thus discharge currents). Experimental data in (a)–(e), simulations based on a plasma lens with nonuniform current in (f)–(j), and uniform current in (k)–(o). For overfocusing currents ($>60\mathrm{A}$), the nonuniform current simulations (h)–(j) match the observed ring-shape beam profiles, while for the underfocused current of 27 A the beam profile is filled in for both experiment (a) and simulation (f).