Optical plasma torch electron bunch generation in plasma wakefield accelerators

A novel, flexible method of witness electron bunch generation in plasma wakefield accelerators is described. A quasistationary plasma region is ignited by a focused laser pulse prior to the arrival of the plasma wave. This localized, shapeable optical plasma torch causes a strong distortion of the plasma blowout during passage of the electron driver bunch, leading to collective alteration of plasma electron trajectories and to controlled injection. This optically steered injection is more flexible and faster when compared to hydrodynamically controlled gas density transition injection methods.

Figure 1

Experimental setup: An electron beam driver and one or two moderately synchronized laser pulses interact in an underdense medium with (at least) two species with different ionization thresholds such as hydrogen and helium. One Ti:sapphire laser pulse is focused to intensities of the $I\approx 10^{14–15}\mathrm{W}/{\mathrm{cm}}^2$ level in order to generate the localized hydrogen/helium plasma torch in the path of the electron beam driven blowout. In contrast to the self-ionized cases (i) and (ii), for case (iii) another, high-energy fraction of the laser pulse is used to preionize the hydrogen in order to allow for a stronger blowout.

Figure 2

On-axis density lineouts for cases (i) and (ii), where hydrogen and helium are neutral (a), and (iii), where hydrogen is preionized (b). The electron bunch driver (green) propagates to the right, and the torch is produced by a laser pulse with $a_0=0.015$ in case (i) and $a_0=0.025$ in case (ii) and (iii). The zoomed insets show the plasma torch profile obtained via numero-analytical ADK calculations (brown) compared to densities obtained in the simulation (blue/red).

Figure 3

Optical plasma torch injection for case (i). In (a) the plasma torch is generated in the path of the drive beam, and in (b) the torch is crossed, leading to blowout amplification and injection. The injected hydrogen electrons are shown in (c) and (d) after $z\approx 10.4\mathrm{mm}$ of acceleration, where maximum energies of $E\approx 440\mathrm{MeV}$ are reached.

Figure 4

Plasma torch injection in case (iii). In preionized hydrogen, at gas densities of $n_{\mathrm{He}}=n_{\mathrm{H}}=1\times 10^{16}{\mathrm{cm}}^{-3}$ a pronounced blowout is generated (a) and the tunable helium torch (b) downramp leads to dark-current free witness bunch formation (c) and allows for massive charge at substantially beam-loaded levels (d).