Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator

Abstract

Due to their ultra-short duration and peak currents in the kA range 1,2 , laserwakefield accelerated electron bunches are promising drivers for ultrafast X-ray generation in compact free-electron-lasers (FELs), Thomson-scattering or betatron sources [3][4][5] . Here we present the first single-shot, high-resolution measurements of the longitudinal bunch profile obtained without prior assumptions about the bunch shape. Our method allows complex features, such as multi-bunch structures, to be detected. Varying the length of the gas target, and thus the acceleration length, enables an assessment of the bunch profile evolution during the acceleration process. We find a minimum bunch duration of 4.2 fs (full width at half maximum) with shot-to-shot fluctuation of 11% rms. Our results suggest that after depletion of the laser energy, a transition from a laser-driven to a particle-driven wakefield occurs, associated with the injection of a secondary bunch. The resulting double-bunch structure might act as an elegant approach for driver-witness type experiments, i.e. allowing a non-dephasinglimited acceleration of the secondary bunch in a plasma-afterburner stage 6,7 .Since the first demonstration of high-quality, quasi-monochromatic electron beams in 2004, laser wakefield acceleration (LWFA) has become a reliable scheme to accelerate electrons bunches to energies in the GeV range in plasma accelerator stages a few cm long 8-12 . The small scale of the acceleration structure, confining the bunch to a fraction of the plasma wavelength, implies bunch durations in the femtosecond range.Determining the detailed longitudinal profile of the generated bunches is important for understanding the accelerator dynamics, enabling accelerator control, and for determining their potential applications, such as driving compact FELs 13,14 . However, the limited temporal resolution of traditional methods, such as electro-optic sampling 15 , prevents their application to measuring the ultra-short bunches produced by an LWFA.Although recent experiments confirmed the ultra-short nature of LWFA electron beams 1,2 these relied on the assumption of a Gaussian longitudinal profile when determining the electron bunch duration. As in earlier work 2 , we determine the bunch profile from measurements of the spectrum of coherent transition radiation (CTR).However, our experiments advance prior work in several key aspects: (i) the bandwidth of the recorded spectrum covers a spectral range of more than 4 octaves at high resolution; (ii) the spectrum was recorded in a single-shot, preventing shot-to shot fluctuations in the electron bunch parameters distorting the measured spectrum; (iii) the

Generation of ultrafast electron bunch trains via trapping into multiple periods of plasma wakefields

“…The most reliable way to find which injection scenario happened in the experiment would be to directly measure time profile of the electron beams, for example via Coherent Transition Radiation (CTR) technique 23,25 . However, one can get an insight into the electron beam time profile indirectly via the analysis of beam loading in the experimental data.…”

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

“…Few-femtosecond duration is achievable by radio-frequency compression [20][21][22][23] or all-optical terahertz techniques [24]. At relativistic energies, few-femtosecond and attosecond durations have been achieved by inverse free-electron laser processes [25] and laser-wakefield accelerators [26][27][28][29]. Sub-relativistic electron pulses of attosecond durations can be made by ponderomotive forces in standing laser waves [30,31] or by time-varying electric fields [32][33][34]; see figure 1 for an overview.…”

“…However, in practical experiments, the emitted spectrum is often substantially restricted in the short-wavelength region by the influence of the lateral size of the electron beam at the interface [51], which causes destructive optical interference. This high-frequency cutoff restricts the applicability of CTR pulse-length characterizations to electron beams with very small lateral dimensions [48] or beams at highly relativistic energies [25][26][27][28][29]. Unfortunately, the ultrashort electron pulses in ultrafast atomic-scale diffraction need sub-relativistic energies (tens to hundreds of keV) in order to provide a suitable de Broglie wavelength for atomic diffraction [52].…”

“…(c) Similarly, a well-focused electron beam passing close to a laser-excited nanostructure can also experience timedependent forces and temporal compression [58]. (d) Inverse free-electron laser process or laser-wakefield accelerators can also produce few-femtosecond or attosecond electron pulses [25][26][27][28][29]. that the detector can see depends on the projected bunch size into detection direction.…”

Carrier-envelope phase controlled isolated attosecond pulses in the nm wavelength range, based on coherent nonlinear Thomson-backscattering

“…MeV energy range [21], recently with already sub-10 fs pulse length [28,29], and a work on bunch compressing [30] predicting electron bunches of 2 as duration and 5.2 MeV energy. Several research groups on laserwakefield acceleration reported about quasi-mono-energetic, fs or sub-fs electron bunch trains [31][32][33], and one group reported about a single (isolated) bunch [34], having 10-100 pC charge (i.e. 10 7 -10 8 electrons) and an energy in the few to few hundred MeV range.…”