Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator

Abstract

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma 1 can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread 2-5 , low emittance 6 and up to 1 GeV peak energy 7,8 . Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection 9 , contains a temporal feature that can be identified as a 15 pC, 1.4-1.8 fs electron bunch (root mean square) leading to a peak current of 3-4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources 10,11 , and will benefit the realization of laboratory-scale free-electron lasers 12-14 .The ponderomotive force generated by the focused laser pulse is proportional to the gradient of the laser intensity. It pushes electrons out of regions of high intensity and separates them from the ions, thus creating a plasma wave that propagates in the wake of the laser pulse with a phase velocity close to c, the speed of light in vacuum. The characteristic length of the accelerating cavity that forms behind the driving laser pulse is the plasma wavelength λ p . In a typical laser wakefield acceleration experiment, λ p = 10-30 µm for plasma densities n e = 10 18 -10 19 cm −3 . The electric field changes along the length of the plasma wave, therefore, to generate an electron beam with low energy spread and low divergence, the electron bunch should reside within the focusing and accelerating phase of the wave, which has a length λ p /4. These heuristic arguments indicate that one would expect the bunch duration to be ultrashort, τ < λ p /4c ≈ 10 fs (refs 9,15). However, to the best of our knowledge, such short durations have not previously been directly measured.Traditional techniques to measure the electron bunch duration, such as streak cameras and radio-frequency sweeping cavities, do not have the temporal resolution required for femtosecond bunches. Therefore, we have employed a method in the frequency domain and measured the coherent transition radiation (CTR) that is emitted by the electron bunch as it passes through a thin metallic foil. CTR is an established particle beam diagnostic and has been used to diagnose micro-structures of picosecond-bunches 16,17 and for benchmarking simulations of femtosecond-bunch dynamics

Shock-Front Injector for High-Quality Laser-Plasma Acceleration

“…One of the important applications of the inherently fewfs long electron bunches [3,4] is currently the generation of ultrashort XUV/x-ray bursts with narrow or broad bandwidth via undulator radiation [5], Thomson backscattering [6], or betatron radiation [7,8]. Especially for narrowband x rays, a precise control of the electrons' peak energy E peak and a low full width at half maximum (FWHM) energy spread ÁE are important.…”

Excitation and Control of Plasma Wakefields by Multiple Laser Pulses

“…Laser-driven plasma accelerators have made impressive progress [7] in recent years. They can now generate electron beams with energies comparable to those used in synchrotrons and FELs (a few GeV), but in accelerator stages only a few centimetres long [8][9][10], with bunch durations in the femtosecond range [11][12][13], and with properties ideal for generating femtosecond duration visible to X-ray pulses [14][15][16][17][18][19][20].…”

Narrowband inverse Compton scattering x-ray sources at high laser intensities

“…its transverse radius is σ r,FWHM = 1.8 µm, its duration is 10 fs and the angular divergence is σ θ,FWHM = 0.2 mrad. These parameters are realistic for the current laser plasma accelerated electrons [82][83][84][85], which show great promise towards compact photon sources. The laser beam has a Gaussian shape in every direction, normalized peak amplitude a 0 = 2.83, and r.m.s.…”