Active Plasma Lensing for Relativistic Laser-Plasma-Accelerated Electron Beams

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of $3000\mathrm{T}/\mathrm{m}$, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

Figure 1

(a) Schematic concept of the focusing force in an active plasma lens. (b) The focal length $F_0$ for 300-MeV electrons and chromatic dependency $|\mathrm{\Delta }F/F_0|$ is displayed for a state-of-the-art solenoid (black curve), quadrupole triplet (blue curve), and active plasma lens (red curve), illustrating the advantage of the active plasma lens (cm-scale focal length with reduced chromatic dependence).

Figure 2

Schematic of the jet-based LPA with active plasma lens. A thin tape is spooled every laser shot in order to protect the capillary from remnant laser light. Energy-integrated (from the phosphor screen) and energy-dispersed (from the magnetic spectrometer) beam-size measurements are recorded.

Figure 3

(a) Current trace for the sapphire-based capillary discharge. (b) Scaling of the magnetic field with radius as obtained from MHD simulations at $I_0=330\mathrm{A}$ (solid curve). Linear field gradients (dashed curve) are observed to be linear up to $R/2$, with $R$ the capillary radius ($125\mu \mathrm{m}$ in this case).

Figure 4

Single-shot images of the transverse charge distribution as obtained by the energy-integrated phosphor screen. Relative to the lens-off case in (a), the electron beam is (b) focused (converging) to a smaller size at $I_0=45$ A and (c) overfocused (diverging) to a larger size at 74 A.

Figure 5

Comparison between single-shot experimental (a) and simulated (b) magnetic spectrometer images during a scan of the timing of the electron beam with respect to the discharge pulse. The yellow insets in (a) display the timing of the electron beam as vertical lines.

Figure 6

Data (red circles) on the transverse electron-beam size $\sigma$ are compared to simulation (${\sigma }_{\text{sim},\text{final}}$, solid blue curve) in case of a weak focusing current in (a), where the electron beam is well focused by the active plasma lens, and in case of a stronger focusing current in (b), where an extra oscillation within the lens yields a much stronger chromatic dependency. The black dashed curves display simulation results assuming uniform current density ($B_{\varphi }\propto r$) and perfect resolution.