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

Abstract

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 (∼20 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. 41.60.Dk, 41.75.Ht Generation and characterization of femtosecond duration electron bunches are of paramount importance in high-energy collider, plasma wakefield accelerator (PWFA), and x-ray free-electron laser (FELs) applications. In a PWFA, driven by either a laser or an electron bunch, the duration of the accelerated bunch is intrinsically shorter than the plasma oscillation period which is typically on the order of 100 fs [1]. In x-ray FELs, it is understood that reducing the charge per bunch alleviates internal space charge and other nonlinear, beam-induced forces allowing for a corresponding reduction of the minimum possible duration. For example, when operating with tens of picocoloumb bunch charges the Linac Coherent Light Source (LCLS) [2] is capable of generating electron and x-ray pulses with durations of just a few femtoseconds [3]. Diagnosis of the longitudinal electron distribution is therefore highly desirable for FEL optimization studies and presents a unique challenge to the resolution of existing diagnostics, reaching into the few-fs scale. In this letter we present first results using a newly developed frequency-domain diagnostic with advantages of economy and robustness over direct timedomain diagnostics [4][5][6] for the LCLS. The prism-based spectrometer demonstrated yields novel, single-shot measurements over a wide, middle-infrared (MIR) spectral range previously unexplored in longitudinal beam diagnostics.Frequency-domain bunch length measurements are based on observing the power spectrum of light from an electron bunch undergoing a radiative process, occurring when the beam experiences a change in trajectory or medium [7,8]. In the low-frequency range, where κ ∼ < (2πσ z ) −1 with σ z the electron bunch length and κ the spatial frequency κ ≡ 1/λ, the radiated power has a well-known coherent enhancement that scales with the number of electrons N over the incoherent beam radiation background. In the one-dimensional bunch limit where the transverse electron beam size is much smaller than the transverse coherence length, the coherent spectral density profile c...

Ultrashort Free-Electron Laser X-ray Pulses

“…Another essential requirement for every ultrafast experiment that relies on a synchronized optical laser for pump or probe is precise information about the relative arrival time between the two lasers. Steady progress in this regard has been made [51,52,58,69,101,103,[117][118][119][120], as was demonstrated throughout this review, and electron acceleration designs relying on superconducting linear accelerators [19,20] open up even better prospects for temporal synchronization [121]. Nevertheless, a complete and direct single-shot measurement in the time domain that reports both the exact XFEL pulse temporal structure and arrival time simultaneously and which is applicable over a broad range of X-ray pulse durations remains elusive.…”

Non-destructive measurement and monitoring of separation of charged particle micro-bunches

“…On the other hand, to enable the THz radiation source tunability and its stable operation, 1-5 the accurate knowledge of the distance between bunches is also required, as the spacing between micro-bunches defines the spectrum of the radiation. [18][19][20][21][22][23][24] In both examples and in other applications driven by such beams, the development of a non-invasive, a single-shot system capable of monitoring the distance between micro-bunches is important, and in many cases, non-destructive, single-shot evaluation of the bunch-to-bunch distance is still an unresolved challenge. At present, the most popular techniques used to study micro-bunch separations are via measurements of autocorrelation functions 2,18,19 and use of a transverse deflection cavity.…”

“…The use of cSPr was due to cSPr monitor compactness and convenience. 21,22 The coherent radiation frequency spectrum from a single femtosecond bunch is broadband (up to tens of THz), [21][22][23][24] and for a single electron bunch consisting of N e electrons, the energy generated at frequency x into a solid angle dX is given by [21][22][23][24] d 2 I=ðdxdXÞ / ðd 2 I e =ðdxdXÞÞ ðN e À 1ÞN e jFðxÞj 2 ; (1) where I is the energy emitted by the bunch, I e is the energy emitted by a single electron, and F(x) is the normalized "form factor," i.e., Fourier transform of the bunch temporal profile. [21][22][23][24] If a pre-bunched beam is used, i.e., the beam consisting of M micro-bunches separated by interval Dt, the…”

Characterization of non-relativistic attosecond electron pulses by transition radiation from tilted surfaces

“…A prerequisite for applying few-femtosecond or attosecond electron pulses to studies of light-matter interaction is a proper characterization of temporal shape and duration. A widespread method in electron-pulse metrology is a spectrally resolved measurement of coherent transition radiation (CTR) [39][40][41][42][43][44][45][46][47][48][49], i.e. recording and analyzing the electromagnetic fields that are emitted by a multi-electron bunch that hits an interface between media of different refractive index [40,50].…”