Shaping triangular picosecond laser pulses for electron photoinjectors

Abstract

Beam driven wakefield acceleration is one of the most advanced novel accelerator concepts. This process occurs in a plasma or a slow-wave structure in which the drive beam deposits energy in the form of a wake and the main beam is accelerated by this wake. For efficient acceleration, it is important to shape the drive beam current and to increase the so-called transformer ratio: the energy gain by the main beam to the energy loss of the drive beam. Triangular bunch shapes are among the most promising distributions considered for increasing the transformer ratio. In most cases, the drive beam is shaped with the help of electron beam phase space manipulation along with removal of portions of the beam with a scraper. This type of shaping is complicated by itself, causing charge loss and thermal heating, but also leads to beam instabilities due to the altered phase space of the beam. In this paper, we demonstrate experimentally the shaping of picosecond laser pulses in the form of a triangle with the help of a spatial light modulator. Such laser pulses can be used to generate a triangular electron bunch from a photoinjector. Considerations on the required pulse profile, comparison to other shaping methods, experimental results, and numerical analysis of the impact of pulse shaper parameters on the reproducibility of the triangular laser pulse shape are presented.

eXawatt Center for Extreme Light Studies

“…Before amplification, the pulses are profiled to provide the necessary (close to rectangular) shape of the pump pulse of the intermediate, booster and final parametric amplifiers. Since the pulses are chirped, it is convenient to use the spectral approach for their profiling [ 42 , 43 ] . This approach allows tuning the delay in few ps range with approximately kHz resolution to suppress a temporal jitter on ps parametric stages.…”

“…This requires a pump pulse energy of at least 100 J with a quasi-rectangular pulse duration of 3.5 ns, which ensures overlap between the pump pulse and the 3-ns chirped pulse. The quasi-rectangular shape of the pulse is provided by profiling the output pulse [ 42 , 43 ] in the frontend (point 1.2 in Figures 1 and 3). Secondly, the intermediate OPCPA ensures the operation of the XCELS laser with a multipetawatt power and a repetition rate significantly higher than two pulses per day (point 2.2 in Figure 1).…”

Bunch Shaping in Electron Linear Accelerators

“…The first beam physics applications used an SLM system to convert a Gaussian pulse of 9 ps FWHM to a flattop pulse of the same length to drive a photocathode RF gun (Yang et al, 2002). Gaussian pulses were transformed into both triangular and super-triangular laser pulses with an SLM-based system (Kuzmin et al, 2018) as shown in Fig. 14.…”

“…A downstream dispersive section with longitudinal dispersion ξ + and designed to suppress the dispersion introduced by the section upstream of the mask results in the overall shaping function f [x m /(ηh 0 )]. This technique bears similarities with the frequency-domain temporal shaping method commonly encountered in ultrafast laser shaping, where a frequency-chirped laser is dispersed, its spectrum modified with a mask, and subsequently recombined (Kuzmin et al, 2018;Weiner et al, 1988); see Section III.D.3.a.…”

“…(a) unmodified laser pulse shape; (b) spectrum optimized for a right triangle; (c) spectrum optimized for a right triangle with the leading edge replaced by a super-Gaussian section. Courtesy(Kuzmin et al, 2018).…”