Thomson-Backscattered X Rays From Laser-Accelerated Electrons

We present the first observation of Thomson-backscattered light from laser-accelerated electrons. In a compact, all-optical setup, the “photon collider,” a high-intensity laser pulse is focused into a pulsed He gas jet and accelerates electrons to relativistic energies. A counterpropagating laser probe pulse is scattered from these high-energy electrons, and the backscattered x-ray photons are spectrally analyzed. This experiment demonstrates a novel source of directed ultrashort x-ray pulses and additionally allows for time-resolved spectroscopy of the laser acceleration of electrons.

Figure 1

(a) Setup of the Thomson-backscatter experiment: The main laser pulse is divided into two pulses by a $90/10$ beam splitter both of which are focused by off-axis parabolic mirrors into a He gas jet. The backscattered radiation is observed with an x-ray CCD camera. A frequency-doubled laser pulse is used for shadowgraphy of the interaction region. (b) Shadowgram of the interaction region. The pump pulse is incident from the left generating the plasma channel, the probe pulse from the right. The bright area in the center of the image is due to the $2\omega$ plasma self-emission within the relativistic channel, visible since the camera integrates over the entire interaction period. (c) Interaction scheme: $\vartheta$ is the angle of observation with respect to the electron propagation, and $d\Omega$ is the solid angle of detection.

Figure 2

(a) Typical Thomson-backscattered photon spectrum recorded with the x-ray CCD camera at a delay between pump and probe pulse of 200 fs averaged over 10 laser shots. The error bars indicate the standard error for a single channel derived from the fluctuations of 10 shots. The spectrum is corrected for background and filter transmission. (b) Solid line: electron spectrum obtained from photon spectrum in (a) according to Eq. (3). For comparison, an electron spectrum recorded with a conventional magnet spectrometer under similar conditions is shown as a gray line (rescaled for visibility) [24].

Figure 3

Varying the delay between pump and probe pulse: The integral number of photons in the interval $430$ to $2030\mathrm{eV}$ averaged over 10 laser shots is plotted over the delay between the laser pulses. The number of photons was normalized to the peak of the respective curves to facilitate comparison between data recorded under varying circumstances. A delay of 0 fs corresponds to both pulses interacting in their common focus. Inset: Sketches of three scenarios of different pump-probe delay where the pump pulse is incident from the left generating a relativistic channel. The region where pump and probe pulse overlap is indicated by a hatched box.