Injection and Trapping of Tunnel-Ionized Electrons into Laser-Produced Wakes

A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the $K$ shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the $L$ shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.

Figure 1

OSIRIS simulation of the injection of tunnel-ionized electrons ($n_e=7\times {10}^{18}{\mathrm{cm}}^{-3}$; $a_0=2$). As the laser pulse propagates to the right it ionizes a $9:1$ mix of He and ${\mathrm{N}}_2$ and drives a wake. (a) The envelope of the normalized vector potential $a_o$ of the laser (dashed line) and the ionization state of nitrogen atoms (solid line) on axis. The superimposed trajectories (1) and (2) in frame (b) represent simulation electrons ionized into the wake from the $K$ shell of nitrogen. Electron (1) is ionized close to the axis and is trapped by the wakefield, while electron (2), ionized earlier and off axis, slips over the potential well and is not trapped. The solid line labeled $E_z$ refers on axis longitudinal electric field of the wake. (c) The normalized wake potential $\overline{\Psi }$ on axis, with particular points relevant to the physics of the trapping mechanism depicted.

Figure 2

A comparison between the observed (a) and (b) and the simulated (c) and (d) electron spectra created at a $n_e\sim 1.4\times {10}^{19}{\mathrm{cm}}^{-3}$ from a $9:1$ $\mathrm{He}:{\mathrm{N}}_2$ gas mix.

Figure 3

Measured (diamonds) and simulated (circles) integrated charge of electrons with energy $>25\mathrm{MeV}$ vs laser $a_o$ and normalized power $P/P_c$. Solid lines correspond to the $a_o$ required to ionize the 6th and 7th electron of nitrogen in one cycle of the laser pulse using the ADK model. Dashed line is the experimental signal detection limit. $n_e\sim 1.4\times {10}^{19}{\mathrm{cm}}^{-3}$; $9:1$ $\mathrm{He}:{\mathrm{N}}_2$ gas mix.

Figure 4

The measured spectral intensity of the laser pulse (solid line) observed after propagating through a plasma created from pure helium gas (a) and from a $9:1$ $\mathrm{He}:{\mathrm{N}}_2$ gas mix (b). For both spectra $n_e$ was $\sim 1.5\times {10}^{19}{\mathrm{cm}}^{-3}$ and $a_o\approx 1.65$. The dashed line is the normalized spectral intensity of the laser.

Figure 5

Measured (diamonds) and simulated (circles) electron beam divergence ($e^{-1/2}$) transverse to the laser polarization vs laser $a_o$. An undispersed electron beam ($a_o\approx 2.1$) is shown inset, here the arrow indicates direction of laser polarization.