Abstract
The generation of high intensity, ultrashort x-ray pulses enables exciting new experimental capabilities, such as femtosecond pump-probe experiments used to temporally resolve material structural dynamics on atomic time scales. Thomson backscattering of a high intensity laser pulse with a bright relativistic electron bunch is a promising method for producing such high-brightness x-ray pulses in the 10--100 keV range within a compact facility. While a variety of methods for producing subpicosecond x-ray bursts by Thomson scattering exist, including compression of the electron bunch to subpicosecond bunch lengths and/or colliding a subpicosecond laser pulse in a side-on geometry to minimize the interaction time, a promising alternative approach to achieving this goal while maintaining ultrahigh brightness is the production of a time-correlated (or chirped) x-ray pulse in conjunction with pulse slicing or compression. We present the results of a complete analysis of this process using a recently developed 3D time and frequency-domain code for analyzing the spatial, temporal, and spectral properties an x-ray beam produced by relativistic Thomson scattering. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, laser focus, and the transverse and longitudinal phase space of the electron beam were examined. Simulations of chirped x-ray pulse production using both a chirped electron beam and a chirped laser pulse are presented. Required electron beam and laser parameters are summarized by investigating the effects of beam emittance, energy spread, and laser bandwidth on the scattered x-ray spectrum. It is shown that sufficient temporal correlation in the scattered x-ray spectrum to produce sub-100 fs temporal slice resolution can be produced from state-of-the-art, high-brightness electron beams without the need to perform longitudinal compression on the electron bunch.
Highlights
The use of short laser pulses to generate high intensity, ultrashort x-ray pulses enables exciting new experimental capabilities, such as femtosecond pump-probe experiments used to temporally resolve the structural dynamics of high-Z materials on atomic time scales [1,2]
The development of a femtosecond, hard x-ray source capable of probing inner-shell electron properties on atomic time scales would open up regions of currently underexplored science, such as phase transitions in materials under shock loading and chemical reaction dynamics
Thomson backscattering of an intense laser pulse with a high-brightness electron beam is a promising means of meeting the demanding specifications of such an x-ray source
Summary
Thomson backscattered photons are produced when an electron beam collides with a photon beam (i.e., laser). Where c is the speed of light, is the total Thomson cross section, and n0 r; t and n0e r; t are the photon and electron density in the electron beam rest frame, respectively To generalize this for a relativistic electron beam, it can be noted that the total number of scattered photons is Lorentz invariant, and that the above expression can be expressed covariantly as the integration of the product of the electron four current, J ecne 1; e , and the photon four flux, cn 1; ck=! The validity of Eq (5) requires the scattering remain linear, which means the normalized vector potential within the laser pulse, given by a0 eA=mc, where m is the rest mass of the electron, is much less than unity It is assumed there is no recoil of the electron, implying the incident photon energy in the electron’s rest frame is much less than the electron rest mass. It is noted that the calculations presented in this paper assume the background motion of each electron through the incident laser pulse is ballistic, which is a good approximation provided the two above conditions are met, and the plasma oscillation period (1=!p ) of the electron beam is much longer than the interaction time, which implies that space-charge effects can be neglected
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