Emittance Preservation in an Aberration-Free Active Plasma Lens

Active plasma lensing is a compact technology for strong focusing of charged particle beams, which has gained considerable interest for use in novel accelerator schemes. While providing $\mathrm{kT}/\mathrm{m}$ focusing gradients, active plasma lenses can have aberrations caused by a radially nonuniform plasma temperature profile, leading to degradation of the beam quality. We present the first direct measurement of this aberration, consistent with theory, and show that it can be fully suppressed by changing from a light gas species (helium) to a heavier gas species (argon). Based on this result, we demonstrate emittance preservation for an electron beam focused by an argon-filled active plasma lens.

Figure 1

(a) Experimental setup: an electron bunch was tightly focused by a quadrupole triplet into an APL after passing a thin polymer window. The lens consisted of a gas-filled sapphire capillary with internal gas inlets (b) connected to an external gas flow regulator, and was discharged using two copper electrodes connected to a Marx generator producing high-voltage pulses with 410–450 A peak current (c). A two-axis mover scanned the beam transversely across the capillary aperture, deflecting the beam onto an OTR screen immediately downstream (d). With this screen retracted, the beam was instead focused by a quadrupole doublet onto another OTR screen, allowing measurement of emittance using a quadrupole scan (e). Additionally, a dipole spectrometer with a Chromox screen was used to measure the beam energy and energy spread.

Figure 2

Measurement of the magnetic field per discharge current for a scan of beam-to-lens offsets in (a) helium and (b) argon, where the uncertainty (blue error bars) represents the standard deviation of the mean. A strong nonlinearity is observed in helium, consistent with the JT model (gray line), whereas in argon the measurement is consistent with the expectation from a uniform current density (orange lines).

Figure 3

Quadrupole scan emittance measurements for both (a) a helium and (b) an argon APL, performed multiple times at peak current timing (blue error bars). Additionally, the background emittance was measured in the absence of current (black error bars) before and after discharges to estimate any emittance drift (gray lines). The predicted emittance growth in helium (orange rectangles) based on the measured nonlinear field [see Fig. 2] is in good agreement with the measured values. In argon, all measurements are consistent with emittance preservation. Emittance drift is modeled with a linear fit in all measurements except one (argon #3), where a quadratic fit produces a tighter bound.