Concurrence of monoenergetic electron beams and bright X-rays from an evolving laser-plasma bubble

Abstract

Desktop laser plasma acceleration has proven to be able to generate gigaelectronvolt-level quasi-monoenergetic electron beams. Moreover, such electron beams can oscillate transversely (wiggling motion) in the laser-produced plasma bubble/channel and emit collimated ultrashort X-ray flashes known as betatron radiation with photon energy ranging from kiloelectronvolts to megaelectronvolts. This implies that usually one cannot obtain bright betatron X-rays and high-quality electron beams with low emittance and small energy spread simultaneously in the same accelerating wave bucket. Here, we report the first (to our knowledge) experimental observation of two distinct electron bunches in a single laser shot, one featured with quasi-monoenergetic spectrum and another with continuous spectrum along with large emittance. The latter is able to generate high-flux betatron X-rays. Such is observed only when the laser self-guiding is extended over 4 mm at a fixed plasma density (4 × 10 18 cm −3 ). Numerical simulation reveals that two bunches of electrons are injected at different stages due to the bubble evolution. The first bunch is injected at the beginning to form a stable quasi-monoenergetic electron beam, whereas the second one is injected later due to the oscillation of the bubble size as a result of the change of the laser spot size during the propagation. Due to the inherent temporal synchronization, this unique electron-photon source can be ideal for pump-probe applications with femtosecond time resolution.S ynchrotron light sources are powerful in generating bright X-rays for a wide range of applications in basic science, medicine, and industry (1). However, these machines are usually large in size and expensive for construction and maintenance and are thus unaffordable to many would-be users. With the advent of tabletop ultrashort and ultraintense lasers, laser plasma acceleration (LPA) proposed by Tajima and Dawson (2) has demonstrated its great potential as a compact accelerator and X-ray source. Significant progress in LPA was made in the last decade (3-11): Well-collimated (approximately millirad) quasi-monoenergetic electron beams were first observed in 2004, and the electron energy above gigaelectronvolts over centimeter-scale acceleration lengths were demonstrated in several laboratories in the last few years.While accelerating longitudinally in the laser wakefield, the electron beams also oscillate transversally (wiggling motion) due to the transverse structure of the wakefield, which emits wellcollimated betatron X-rays (12-14). Among several mechanisms to generate X-ray radiation from laser-plasma interactions (15-20), betatron radiation is straightforward and able to deliver larger X-ray photon fluxes per shot [∼10 8 phs/shot (21)] and higher photon energies [up to gamma rays (22)]. The betatron oscillation frequency is given by ω β = ω p (2γ) −1/2 , where ω p is the plasma frequency and γ is the Lorentz factor of the accelerated electron beam. For large-amplitude betatron oscillations (i.e., a few mi...

Resonantly Enhanced Betatron Hard X-rays from Ionization Injected Electrons in a Laser Plasma Accelerator

“…The gas jet can generate well-defined uniform gas density profiles in the range of 1 × 10 18  cm −3 ~ 3 × 10 19  cm −3 by changing the gas stagnation pressure34, for which the density information is based on the hydrodynamic calculations reported by Hosokai37.…”

Enhanced betatron radiation by steering a laser-driven plasma wakefield with a tilted shock front

“…However, in order to obtain a high-quality highenergy e beam, electron injection and acceleration should be carefully manipulated via a well-performed LWFA, where a large oscillation amplitude r b has to be avoided. Some ideas of manipulating r b to enhance betatron radiation have been demonstrated; [18][19][20][21][22] however, the produced x-rays had continuum spectra because the controllability was at the cost of sacrificing the e-beam quality. Recently, an idea of a helical plasma undulator has been proposed to produce controllable synchrotron-like radiation by inducing centroid oscillations of the laser pulse 23,24 in a plasma channel.…”

Diagnosis of bubble evolution in laser-wakefield acceleration via angular distributions of betatron x-rays

“…While, the departure from the matching condition usually causes instability of the bubble structure. The evolution of the bubble structure can induce multi-injection, [5][6][7][8][9] leading to large-energy spread beams. Meanwhile, the unstable evolution may cause transverse oscillations of the bubble structure, leading to collective transverse oscillations of the electron beam, 10,11 which will enlarge the divergence and the transverse emittance of the electron beam, and even shorten the dephasing length which eventually reduces the maximum energy gain of the beam.…”

“…Betatron radiations are generated when the relativistic electrons perform transverse oscillations, referred to "betatron oscillations," due to the transverse focusing forces of the wakefiled, 12 resonant oscillations in the laser field, 13,14 and the unstable evolution of the bubble structure which is mentioned above. 6 The characteristics of the betatron radiation, such as photon flux, photon energy spectrum, angular distribution and the brilliance, are determined by the characteristics of the accelerated electron beam, such as beam charge, energy spectrum, amplitude of transverse oscillations, and beam duration, [15][16][17] and hence depend on the dynamic evolution of the wakefield. Therefore, by studying the characteristics of the betatron radiation, we can retrieve the behaviors of the electron beams, and, in principle, we can reconstruct the process of the bubble dynamic evolution.…”

“…6 This laser system delivers up to 200 TW in power with a FWHM pulse duration of 60 fs at the central wavelength of 806 nm. The laser pulse was focused by an f/12 off-axis parabola to a focal spot of 18 lm with 35% energy concentration.…”