Demonstration of the synchrotron-type spectrum of laser-produced Betatron radiation

Abstract

Betatron X-ray radiation in laser-plasma accelerators is produced when electrons are accelerated and wiggled in the laser-wakefield cavity. This femtosecond source, producing intense X-ray beams in the multi kiloelectronvolt range has been observed at different interaction regime using high power laser from 10 to 100 TW. However, none of the spectral measurement performed were at sufficient resolution, bandwidth and signal to noise ratio to precisely determine the shape of spectra with a single laser shot in order to avoid shot to shot fluctuations. In this letter, the Betatron radiation produced using a 80 TW laser is characterized by using a single photon counting method. We measure in single shot spectra from 8 to 21 keV with a resolution better than 350 eV. The results obtained are in excellent agreement with theoretical predictions and demonstrate the synchrotron type nature of this radiation mechanism. The critical energy is found to be Ec = 5.6 ± 1 keV for our experimental conditions. In addition, the features of the source at this energy range open novel perspectives for applications in time-resolved X-ray science.A femtosecond X-ray beam, called Betatron, can be produced by focusing an intense femtosecond laser pulse at relativistic intensities, on the order of 10 18 − 10 19 W.cm −2 , onto a gas jet target. Interacting with the quasi-instantaneously created under-dense plasma, the laser pulse excites a wakefield in which electrons can be trapped and accelerated to high energies in short distances [1][2][3][4][5]. These electrons perform Betatron oscillations across the propagation axis, and emit Xray photons [6-10] (radiation from accelerating chargedparticles). The Betatron radiation consists on a broadband X-ray beam, collimated within 10's mrad, with a femtosecond duration [11].During the past few years, several experiments have been dedicated, at different laser facilities, to the characterization of Betatron radiation. Even if the origin of the radiation was clearly identified, its spectrum has never been precisely determined. This information is however crucial to improve our knowledge of the physical mechanisms driving the source, identify the electrons participating to the emission, and determine the most appropriate routes for its development. In addition, for any potential application the precise shape of the spectrum must be known.So far, spectra estimations were either based on the measurement of the transmission through an array of filters or by using the diffraction from crystals. The use of filters is the most elementary method and it allows a single shot measurement. The results obtained using this method are generally fitted with the synchrotron distribution theoretically predicted [12][13][14][15]. However, this rely on the assumption that the spectrum is synchrotron-like and can not give any deviation from such distribution, or details in the structure of the spectrum. When the Bragg diffraction from a crystal is used, the resolution is important but the characterization range is limit...

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

“…Two techniques were employed to measure the betatron radiation spectra in a single-shot. First, in the case of no TSF when the photon yield of betatron radiation was low, a backilluminated x-ray CCD camera operated in a single-photoncounting (SPC) mode 38,39 was used to measure the spectra below 20 keV, which could be used to measure the e-beam transverse emittance with a high resolution as well. [40][41][42] The piling events 43 could be avoided by satisfying the low photon flux requirement.…”

High flux femtosecond x-ray emission from the electron-hose instability in laser wakefield accelerators

“…X-ray flux was measured using an Andor iKon-M BR-DD camera, with x-ray sensitivity up to 30 keV and a 13 × 13 mm 2 chip size with 1M pixels, which was placed 2.5 m downstream from the interaction region and shielded from the laser light with a 20 μm Be window. The single hit spectrum was measured using standard single photon counting techniques [29] and adjusted for the quantum efficiency (QE) of the camera and any filters the x-ray beam passes through [30], as in Fig. 2(b).…”

Tunable monoenergetic electron beams from independently controllable laser-wakefield acceleration and injection

“…Among other applications, LWFA has been integral to the development of compact x-ray sources based on various mechanisms, such as betatron radiation [6,7], conventional undulator-based synchrotron radiation [8,9], and inverse Compton scattering [10][11][12]. Independent control of the electron beam parameters is one of the most significant advances still needed in order for these applications to become practical.There has been recent progress towards achieving this goal using the following approaches: (i) optical injection, involving two independent laser pulses, one to drive the wake and the other to inject electrons [13][14][15][16][17]; (ii) plasmaprofile tailoring, either by means of a machining laser pulse [18][19][20] or by the introduction of obstacles into the gas flow [21-23]; or (iii) the use of distinct media for injection and acceleration [24][25][26][27][28]. While these methods have resulted in tunable quasimonoenergetic electron beams, none of them were able to control the energy spread and charge as the beam was tuned in energy.…”

“…There has been recent progress towards achieving this goal using the following approaches: (i) optical injection, involving two independent laser pulses, one to drive the wake and the other to inject electrons [13][14][15][16][17]; (ii) plasmaprofile tailoring, either by means of a machining laser pulse [18][19][20] or by the introduction of obstacles into the gas flow [21][22][23]; or (iii) the use of distinct media for injection and acceleration [24][25][26][27][28]. While these methods have resulted in tunable quasimonoenergetic electron beams, none of them were able to control the energy spread and charge as the beam was tuned in energy.…”