Laser plasma acceleration with a negatively chirped pulse: all-optical control over dark current in the blowout regime

Abstract

Recent experiments with 100 terawatt-class, sub-50 femtosecond laser pulses show that electrons self-injected into a laser-driven electron density bubble can be accelerated above 0.5 gigaelectronvolt energy in a sub-centimetrelength rarefied plasma. To reach this energy range, electrons must ultimately outrun the bubble and exit the accelerating phase; this, however, does not ensure high beam quality. Wake excitation increases the laser pulse bandwidth by red-shifting its head, keeping the tail unshifted. Anomalous group velocity dispersion of radiation in plasma slows down the red-shifted head, compressing the pulse into a few-cycle-long piston of relativistic intensity. Pulse transformation into a piston causes continuous expansion of the bubble, trapping copious numbers of unwanted electrons (dark current) and producing a poorly collimated, polychromatic energy tail, completely dominating the electron spectrum at the dephasing limit. The process of piston formation can be mitigated by using a broad-bandwidth (corresponding to a few-cycle transform-limited duration), negatively chirped pulse. Initial blue-shift of the pulse leading edge compensates for the nonlinear frequency red-shift and delays the piston formation, thus significantly suppressing the dark current, making 3 the electron rest mass, n 0 is the background electron density and e is the electron charge. Even with the Lorentz factor γ g approaching 100, the bubble is a 'slow' structure capable of capturing and accelerating initially quiescent electrons of the ambient plasma [22,23,[45][46][47]. Optical diagnostics directly correlate the generation of a collimated electron beam with bubble formation [48][49][50][51][52]. While other (e.g. all-optical) injection schemes are currently being explored [53][54][55][56][57], electron self-injection has its own advantages: it greatly reduces the technical complexity of the experiment, preserving flexibility in parameters and enabling a single-stage acceleration of nano-Coulomb (nC) charge [23,47].Accelerated electrons eventually outrun the slow bubble. They exit the accelerating phase within a time intervalis the bubble radius and k p = ω pe /c. In strongly rarefied plasmas, where γ g k p R b , dephasing takes many Rayleigh lengths 5 . Propagation of the pulse over this distance relies on a combination of relativistic and ponderomotive self-guiding [58][59][60]. Upon entering the plasma, the pulse, with P P cr and duration τ L < 2π/ω pe , self-focuses until full electron cavitation is achieved, and the charge-separation force balances the radial ponderomotive force; the pulse is then guided until depletion (here, P cr = 16.2γ 2 g GW is the critical power for relativistic self-focusing [61]). As this force balance is approached, the bubble size oscillates, causing electron injection during a brief time interval. A QME electron bunch thus forms early [45,62,63]. However, transient dynamics before the onset of self-guiding [23], as well as laser evolution during self-guiding [5,7,22,23,[63][64][65], may cause ...

Sub-millimeter-scale, 100-MeV-class quasi-monoenergetic laser plasma accelerator based on all-optical control of dark current in the blowout regime

“…The radial grid is 3 times coarser, with 30 particles per radial cell; the time step ω 0 Δt ≈ 0.67. Under these conditions, WAKE correctly captures all relevant physics of pulse propagation and evolution of the bubble [7,8]. CALDER-Circ uses the grid Δz ≈ λ 0 /50 ≈ 16 nm and Δr = 15.6Δz (where r = x 2 + y 2 ); 45 particles per cell; and ω 0 Δt = 0.1244.…”

“…The bubble readily traps relativistic electrons from its sheath [1,5,6,7], reducing the technical complexity of the experiment while preserving flexibility in parameters [2,5,7]. Robustness of self-injection is rooted in the adiabatically slow evolution of the bubble, which, in turn, is tied to the nonlinear optical evolution of the driver [5,6,7,8]. Therefore, controlling relativistic optical phenomena, such as self-focusing [9], phase self-modulation and pulse self-compression [10], provides an opportunity to manage the fully kinetic self-injection process and optimize electron beam parameters [8,11].…”

“…First, short dephasing length reduces the energy gain in a dense plasma. Secondly, massive continuous self-injection (dark current), destroys electron beam quality long before dephasing [7,8]. Both difficulties have the same underlying cause.…”

“…Not only does formation of the ROS slow down the pulse, reducing the dephasing length, decreasing the energy gain [1]; it also causes constant expansion of the bubble, trapping copious amounts of unwanted electrons, and producing a poorly collimated, polychromatic energy tail completely dominating the spectrum [7,14,15]. We proposed to suppress formation of the ROS by using a broad bandwidth [corresponding to a few-cycle transformlimited duration (TLD)] negatively chirped pulse [8]. The initial blue-shift of the leading edge compensates for the nonlinear frequency red-shift, thus keeping the pulse uncompressed, and the dark current low through dephasing.…”

“…As a bonus, the higher average frequency of the pulse, together with slower etching of its front, effectively increases the dephasing length, further boosting electron energy. Using this approach, low-dark-current acceleration of a 180 pC bunch to 660 MeV is possible over a merely 1.7 mm distance [8].…”