Undulator-Based Laser Wakefield Accelerator Electron Beam Energy Spread and Emittance Diagnostic

Abstract

Abstract. The design and current status of experiments to couple the Tapered Hybrid Undulator (THUNDER) to the Lawrence Berkeley National Laboratory (LBNL) laser plasma accelerator (LPA) to measure electron beam energy spread and emittance are presented

Undulator radiation from laser-plasma-accelerated electron beams

“…Since the energy spread contains electrons above and below the fundamental wavelength, the broadening of the emission spectrum happens on both the positive and negative sides of the fundamental orders. The uniform broadening of the emission spectrum is directly correlated to the energy spread within the electron beam [7].…”

“…The emittance of the electron beam has two separate effects on the XUV spectrum [7]. First, the emittance of the electron beam causes some electrons to travel at an angle with respect to the axis of propagation.…”

Ultrafast Diagnostics for Electron Beams from Laser Plasma Accelerators

“…where L u is the undulator periodicity, K = eB §L u j2%mc is the strength parameter, y is the Lorentz factoi for the oscillating electrons, and 6 is the angle of the electron trajectory with respect to the axis of the undulator Thus, for a given set of undulator parameters L" and K, the on axis contribution to the bandwidth of the n,i, harmonic peak will be lineatly dependent on the energy spread da,, = (n(Oo/y)dy In addition, the angular distribution of odd harmonics has a maximum at 9 = 0, while even harmonics have an emission pattern with a zero at 9 = 0 Electrons propagating along the undulator axis do not contribute to the even harmonics while electrons propagating off-axis do The ratio of even to odd harmonic intensities therefoi e gives us a measure of the angular spread of the electron bunch, which, coupled with knowledge of the bunch source size, provides a measuie of the emittance Simulations done using the code SPECTRA [16] for electron bunches with varying energy spread and emittance confirm the dependence of undulator spectral features on energy and emittance (Fig 2) Experiments are cunently under way to test this concept [14,17] BETATRON X-RAY EMISSION Synchrotron emission is also emitted dunng interaction of the electrons with the wakefield and can be used to gain information about the acceleration process itself Electrons injected off axis into the wakefield experience an ion channel which provides radial forces causing betatton oscillations The oscillations occur with amplitudes on the scale of few to hundreds of microns, determining the size of the electron bunch, and resulting in emission of soft X rays in the few keV range, known as betatron ladiation The amplitude and periodicity of the betatron oscillations depend sensitively on the shape of wakefield, the physics of the injection, and the presence of strong E fields from the drive laser, thus the betatron x-rays can provide valuable insight into the nature of the interaction between the laser, the electrons and the plasma In addition, as the betatron oscillation period is much shorter than that which can be achieved using external undulators, betatron x-rays from LPAs hold promise as a compact source of radiation in the keV energy range…”