Coherent-Radiation Spectroscopy of Few-Femtosecond Electron Bunches Using a Middle-Infrared Prism Spectrometer

Modern, high-brightness electron beams such as those from plasma wakefield accelerators and free-electron laser linacs continue the drive to ever-shorter bunch durations. In low-charge operation ($\sim 20\mathrm{pC}$), bunches shorter than 10 fs are reported at the Linac Coherent Light Source (LCLS). Though suffering from a loss of phase information, spectral diagnostics remain appealing as compact, low-cost bunch duration monitors suitable for deployment in beam dynamics studies and operations instrumentation. Progress in middle-infrared (MIR) imaging has led to the development of a single-shot, MIR prism spectrometer to characterize the corresponding LCLS coherent beam radiation power spectrum for few-femtosecond scale bunch length monitoring. In this Letter, we report on the spectrometer installation as well as the temporal reconstruction of 3 to 60 fs-long LCLS electron bunch profiles using single-shot coherent transition radiation spectra.

Figure 1

Layout of the LCLS beam line with locations of relevant diagnostics along the LCLS linac (see text).

Figure 2

Schematic for MIR spectroscopy of CTR with CTR envelope shown as dashed lines. CTR from a thin foil inserted into the electron beam ($A$) is imaged with reflective optics ($B–E$) to the spectrometer entrance slit ($F$), chromatically dispersed by a custom KRS-5 prism ($H$), then spectrally resolved on a linear detector array ($J$).

Figure 3

Fit of computed detector position $x$ as a function of spatial frequency $\kappa$ to a sample of spectral lines (left) with detail view over sampled points (left, inset) and spectral resolution $\Delta \kappa$ (right).

Figure 4

$T(\kappa )$ reconstruction results for an $L2$ chirp scan of a 40 pC bunch with (a) raw data $I_{\mathrm{meas}}(\kappa ;{\varphi }_{L2})$, best fit solutions for (b) $|f_{\mathrm{sim}}(\kappa ;\varphi ){|}^2$ and (c) transmission function $T(\kappa )$, and (d) final corrected spectra $|f_{\mathrm{meas}}(\kappa ;{\varphi }_{L2}){|}^2=I_{\mathrm{meas}}(\kappa ;{\varphi }_{L2})/T(\kappa )$.

Figure 5

Example beam profile reconstruction for 13.4 GeV, 150 pC electron bunches at four different $L2$ rf phase settings around full compression showing (a) data and interpolated curves up to $\kappa =5000{\mathrm{cm}}^{-1}$ with (b) resulting current profiles and true FWHM bunch lengths $\Delta z$.

Figure 6

Comparisons for the beam current profile dependence on $L2$ rf phase for a 13.4 GeV, 150 pC electron bunch with (a) simulated profiles as a function of ${\varphi }_{L2}$, (b) profiles reconstructed from measured MIR spectra, and (c) FWHM bunch lengths $\Delta z$ from square current profile fitting including LCLS BLM2 measurement.