Optimization and stabilization of a kilohertz laser-plasma accelerator

Abstract

Laser–plasma acceleration at kilohertz repetition rates has recently been shown to work in two different regimes with pulse lengths of either 30 fs or 3.5 fs. We now report on a systematic study in which a large range of pulse durations and plasma densities were investigated through continuous tuning of the laser spectral bandwidth. Indeed, two laser–plasma accelerator (LPA) processes can be distinguished, where beams of the highest quality, with a charge of 5.4 pC and a spectrum peaked at 2–2.5 MeV, are obtained with short pulses propagating at moderate plasma densities. Through particle-in-cell (PIC) simulations, the two different acceleration processes are thoroughly explained. Finally, we proceed to show the results of a 5-h continuous and stable run of our LPA accelerator accumulating more than 18×106 consecutive shots, with a charge of 2.6 pC and a peaked 2.5 MeV spectrum. A parametric study of the influence of the laser driver energy through PIC simulations underlines that this unprecedented stability was obtained thanks to micro-scale density gradient injection. Together, these results represent an important step toward stable laser–plasma accelerated electron beams at kilohertz repetition rates.

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

“…We point out that these quasimonoenergetic bunches can be suitable for many high repetition applications, although further studies on directional and long-term stability are still needed. 59,60 We have also shown that the processes that give rise to such bunches in TW and sub-TW SM-LWFA do not have the typical DLA energy gain, nor the expected DLA signatures are verified on their electrons. 41 Instead, the bunch characteristics and the acceleration history are compatible with those of the blowout LWFA.…”

“…These sources have also been pointed for the irradiation of biological samples, ultrafast imaging, and femtosecond x-ray generation. 14,59…”

“…In our study, most of the charge is in the range of 1 MeV, with transverse emittances of tens of mm mrad (≲10 mm mrad for 1 TW and sub-TW). However, the fact that the total charge is on the nC scale makes these beams relevant for applications where a high dose of radiation is required, 46,54,59 with the advantage of allowing compact high repetition rate systems.…”

Waveform control of relativistic electron dynamics in an underdense plasma

“…Plasma electrons are then self-injected and accelerated into the wakefield, forming high quality, quasimonoenergetic electron beams [17][18][19] . While most LPAs are driven by laser pulses containing many optical cycles, few-cycle pulses can now be used to excite wakefields and accelerate electrons [20][21][22][23] . In this case, the usual framework of a cycle-averaged ponderomotive force 24 is not sufficient to describe the interaction.…”

“…In our experiments, we are able to detect and control this CEP-dependent pointing of the accelerated electron beam, see illustration in figure 1. Our LPA is driven by laser pulses as short as 1.3 cycles (< 4 fs FWHM in intensity) 16,21,22 . Two active feedback loops allow for stabilization and control of the CEP (see Methods).…”