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.

http://absimage.aps.org/image/DPP09/MWS_DPP09-2009-000843.pdf

Injection and Trapping of Tunnel Ionized Electrons into Laser Produced Wakes

Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

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Petawatt laser guiding and electron beam acceleration to 8 GeV in laser-heated capillary discharge waveguides

We demonstrate laser wakefield acceleration of quasi-monoenergetic electron bunches up to 15 MeV at 1 kHz repetition rate with 2.5 pC charge per bunch and a core with < 7 mrad beam divergence. Acceleration is driven by 5 fs, < 2.7 mJ laser pulses incident on a thin, near-critical density hydrogen gas jet. Low beam divergence is attributed to reduced sensitivity to laser carrier envelope phase slip, achieved in two ways using laser polarization and gas jet control: (1) electron injection into the wake on the gas jet's plasma density downramp, and (2) use of circularly polarized drive pulses. Under conditions of mild wavebreaking in the downramp, electron beam profiles have a 2D Lorentzian shape consistent with a kappa electron energy distribution. Such distributions had previously been observed only in space or dusty plasmas. We attribute this shape to the strongly correlated collisionless bunch confined by the quadratic wakefield bubble potential, where transverse velocity space diffusion is imparted to the bunch by the red-shifted laser field in the bubble.

Laser-Accelerated, Low-Divergence 15-MeV Quasimonoenergetic Electron Bunches at 1 kHz

Laser-accelerated, low divergence 15 MeV quasi-monoenergetic electron bunches at 1 kHz

Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realisation of an electron source that can operate at kHz-repetition rate while relying on modest laser energy It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam As a result, the beam pointing and electron energy spectrum become CEP-dependent For injection in a density gradient these effects are reduced, as electron injection is mostly longitudinal and mainly determined by the density gradient Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams Mitigation of CEP effects can nevertheless be achieved using density gradient injection

Carrier-envelope phase effects in Laser Wakefield Acceleration with near-single-cycle pulses

Millijoule-energy few-cycle laser pulses excite the plasma wakefield and accelerate electrons at kilohertz repetition rate, generating mega-electron volt (MeV) electron bunches with femtosecond temporal duration for ultrafast electron diffraction applications. By simulating few-cycle laser pulses interacting with the underdense nitrogen plasma, we have studied the mechanism of periodic electron self-injection, which manifests the laser carrier envelop phase (CEP) effect in few-cycle laser wakefield acceleration. A few-cycle laser pulse experiencing a significant wavelength redshift induces the transverse oscillation of the plasma bubble at the same frequency as the laser CEP changes by 2π. The oscillation of the plasma bubble periodically injects free electrons into the bubble at a doubled frequency, broadening the electron energy spread.

Periodic self-injection of electrons in a few-cycle laser driven oscillating plasma wake

Betatron radiation from relativistic electrons accelerated by a laser plasma wakefield is a promising x-ray source. However, the betatron photon number and energy is limited by the MeV electron energy when accelerated by a sub-terawatt laser pulse. Here we have proposed a scheme of enhancing the betatron radiation of electrons accelerated by a millijoule laser pulse. By applying a moderate mid-infrared control laser field, the plasma bubble and electrons injected inside are forced to oscillate transversely at a high, tunable frequency, enhancing the betatron oscillation strength and resulting in soft x-ray radiation.

Ning Tang

Papers

Periodic self-injection of electrons in a few-cycle laser driven oscillating plasma wake

Enhanced soft x-ray betatron radiation from a transversely oscillating laser plasma wake.