Spectroscopy of betatron radiation emitted from laser-produced wakefield accelerated electrons

Abstract

X-ray betatron radiation is produced by oscillations of electrons in the intense focusing field of a laser-plasma accelerator. These hard x-rays show promise for use in femtosecond-scale time-resolved radiography of ultrafast processes. However, the spectral characteristics of betatron radiation have only been inferred from filter pack measurements. In order to achieve higher resolution spectral information about the betatron emission, we used an x-ray charge-coupled device to record the spectrum of betatron radiation, with a full width at half maximum resolution of 225 eV. In addition, we have recorded simultaneous electron and x-ray spectra along with x-ray images that allow for a determination of the betatron emission source size, as well as differences in the x-ray spectra as a function of the energy spectrum of accelerated electrons.

A plasma wiggler beamline for 100 TW to 10 PW lasers

“…The concept of a plasma wiggler was discussed theoretically [26,31], with the first experiments subsequently being carried out to characterize the beam divergence, keV energy range, broad spectral width [27], source size [32], detailed spectral shape [33,34], betatron oscillation amplitude [35,36] and pulse duration [37,38]. Methods to influence the betatron spectrum by tuning the oscillation amplitude, at ultra-relativistic laser intensities (directlaser-acceleration) [28], with density-tailored plasmas [39] and with laser profile shaping [40] have been discussed and experimentally implemented, demonstrating good control over the betatron mechanism.…”

R&D paths of pixel detectors for vertex tracking and radiation imaging

“…Pixellated semiconductor sensors may offer energy resolution comparable to that of commercial spectroscopy detectors with the added benefits of spatial resolution. Fully depleted CCD on high resistivity substrate [51], developed mostly for astronomical applications, have an energy resolution of ∼150 eV FWHM at 5.9 keV and are successfully applied in imaging and XRF analyses at light sources and for plasma diagnostics [52]. Similarly, pnCCDs have demonstrated excellent energy resolution and are used at LCLS [53].…”

Ultrafast Diagnostics for Electron Beams from Laser Plasma Accelerators

“…The betatron radiation can be characterized by a strength parameter analogous to the "K" of an undulator ap = yipCOp/c, where 2rp is the oscillation amplitude, cop = w p /^/2yis the betatron oscillation frequency, (O v is the plasma frequency and c is the speed of light Various groups have demonstrated the detection of betatron radiation from LPAs, and have shown correlations of the spectra with parameters of the accelerator eg [18,19,20] An important result that has come from these measurements is the determination of the source size of the x-rays, which was accomplished by placing knife-edges or wire meshes m the x-ray beams and observing the sharpness of the shadow cast on an imaging detector Source sizes varying from ~2 ^tm to seveial hundred micions have been measured in different conditions As the source size is correlated to the oscillation amplitude, ip, which is correlated to the transverse size of the electron bunch, tins technique provides valuable infoiraation about the physics of the acceleration and the [10] quality of the accelerator. These measurements also provide precise information about the shottoshot variation in the emission point of the electrons, which can be used to determine the stability of the accelerator.…”

“…In a recent leport [18] a technique was implemented to recoid measuiements of single-shot, spatially lesolved spectra of betatron x-rays with high spectral resolution, by the use of an x-ray CCD Images analyzed by pei forming a histogram of single-pixel absorption events (SPAE) [21] produced x-ray spectra with an unprecedented lesolution of 225 eV, FWHM, and a range of over 10 keV A source emitting iron k shell x-ray lines was used to provide a calibration of the energy per pixel count Preliminary data show two important features not previously lesolved The first is iron and chromium fluorescence lines associated with the interaction of electrons and x-rays with the stainless steel of the vacuum chamber, and the second is the betation continuum The ratio of the amplitude of the fluoiescence lines to that of the betatron continuum was found to vaiy significantly with changes in the accelerator parameters, calling into question the validity of previous spectral analyses, based on filter packs, which cannot distinguish between these two components ELECTRO-OPTIC DIAGNOSTICS Electro-optic (EO) sampling has become a widely-used technique for the measurement of electron-bunch durations and temporal structure In this technique, either the relativistic Coulomb fields of the electron bunch or coheient transition radiation (CTR) in the THz frequency band emitted by the electron bunch traversing a dielectric boundary is used to induce birefringence in an elcctro-optically active crystal, such as gallium phosphide (GaP) or zinc tellunde (ZnTe) An optical probe, timed to overlap with these strong electric fields, is used to sample the temporal profile of the fields, from which the duration of the electron bunch can be deduced This technique is very powerful because it can be used in configurations that are non-or weakly-interacting with the electron bunch, allowing it to be used in conjunction with other diagnostics In addition, it provides the high tempoial resolutions required for measuring the sub picosecond electron bunches produced m LPAs The EO sampling process can be split into two conceptual parts generation of the temporally-varying birefringence, and sampling of the birefringence Three prominent methods for each will be discussed Generating the birefringence. In the first method (Direct Coulomb Sampling), the EO crystal is placed near to the path of the accelerated electrons, so that the Coulomb fields penetrate it, resulting m a transient birefringence An optical probe pulse, propagating parallel to the beam line overlaps the induced fields in the crystal, and is imprinted with the temporal profile of these fields Due to lelativistic contraction, the Coulomb field profiles will be longitudinally compressed by an amount dependent on the electron energy The field temporal profile will thus be a convolution of the charge profile with the longitudinal extent, x e = y/cy, of the electron Coulomb fields, where y is the transverse distance from the beam axis Provided the electrons are sufficiently energetic (y large) and the crystal is sufficiently close (y small), the field profile will represent the bunch profile well In general, however, a polychromatic electron bunch will have a field profile (at the crystal) significantly different than its charge profile, with the lower energy electrons having a longer longitudinal field-extent than the high energy components At a distance of 1 mm, foi example, the convolution factor will be approximately 1 7 ps for a 1 MeV component while it will be about 1 7 fs for a 1 GeV component In addition, since the field-strength scales as y, the EO signal will be strongly biased towards the high energy component of the electron bunch In scenarios where it is the behavior of the high-energy component that is of interest, such as in the FEL application, this bias can be advantageous, whereas if it is the actual longitudinal charge distribution that is ...…”