Observation of Synchrotron Radiation from Electrons Accelerated in a Petawatt-Laser-Generated Plasma Cavity

Abstract

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x-rays extending to 50 keV is observed. The measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy turned into x-rays. PACS numbers: Valid PACS appear hereThe advent of high power lasers has led to rapid progress in the field of plasma based particle acceleration [1]. In particular, the measurement of monoenergetic electron beams from wakefields generated by short lasers [2] has stimulated great interest in producing such beams and understanding their dynamics. One potential use for these compact sources of energetic particles is as a driver for novel light sources. Laser-accelerated electrons could be injected into a magnetic undulator realizing a compact tunable-energy femtosecond x-ray source synchronized to the laser. A laser-based x-ray source could be downsized further, using the self-generated magnetic and electrostatic fields of the plasma channel as a miniature undulator [3]. For electron beams of sufficiently high quality, an ion channel laser analogous to conventional free electron lasers may be feasible [4]. X-rays can also be produced in intense laser-plasma interactions by nonlinear Thomson scattering [5].Relativistic electron beams have also been measured from interactions at very high laser intensities, where electrons gain energy directly from the laser [6]. At high intensity, the ponderomotive force of the laser can expel plasma electrons leaving a positively charged ion channel. Electrons inside the channel experience a net focusing force due to the space charge and undergo oscillation at the betatron frequency ω β = ω p / √ 2γ z0 , where ω p is the plasma frequency and γ z0 is the Lorentz factor associated with the electrons motion along the plasma channel. Electrons resonant with the laser frequency can gain energy from the transverse electric field of the laser, which can be directed into longitudinal momentum through the v × B force [7]. Accelerating charges radiate electromagnetic radiation. For small betatron strength parameters a β = γ z0 r β ω β /c 1 (undulator limit), the spectrum of the radiation will be narrowly peaked about the resonant fre-is the Doppler factor and α is the angle between the direction of observation and the direction of γ z0 [8]. This highlights the interdependency of spectral and angular distributions. As a β → 1, emitted radiation also appears at harmonics of the resonant...

Bright spatially coherent synchrotron X-rays from a table-top source

“…peak brightness than previous laser-driven betatron sources 6,7 . Furthermore, we demonstrate that the radiation has an appreciable degree of spatial coherence.…”

“…3a) with an X-ray CCD (charge-coupled device) detector 7 . Assuming the spectrum is synchrotron-like, the measured E crit is (29 ± 13) keV for n e = (1.0 ± 0.4) × 10 19 cm −3 , and the 5 mm nozzle (see Fig.…”

Spontaneous emergence of non-planar electron orbits during direct laser acceleration by a linearly polarized laser pulse

“…The fields E and B are superpositions of the fields of the wave [Eqs. (2) and (3)] and the fields of the channel [Eqs. (4) and (5)].…”

Laser-plasma electron acceleration in dielectric capillary tubes

“…Figure 5(d) shows the X-ray beam distribution produced in a capillary tube of length 20 mm and diameter 200 µm, n e = 7 × 10 18 cm −3 , and a different set of filters (a base of 6 µm mylar and 3 µm Al, with a 3 µm thick vertical stripe of Al and a 3 µm thick horizontal stripe of Sn). Assuming a synchrotron-like spectrum, using the tabulated transmission data of the filters and known CCD sensitivity, an estimate of the critical energy E c = 1.3 keV was deduced using a leastsquares method [15].…”