Control of focusing fields in laser-plasma accelerators using higher-order modes

Abstract

Higher-order laser modes are analyzed as a method to control focusing forces and improve the electron bunch quality in laser-plasma accelerators. In the linear wake regime, the focusing force is proportional to the transverse gradient of the laser intensity, which can be shaped by a superposition of modes. In particular, the transverse wakefield can be arbitrarily small in a region about the axis by adjusting the laser modes. Plasma channel effects, which prohibit the formation of the controlled-focusing region, can be mitigating by introducing a delay between the modes. Modes with parallel polarization produce a beat interference in the laser intensity, which lead to deflecting forces. This can be avoided by using modes with orthogonal polarization, different frequencies, or short pulses that do not overlap. Particle-in-cell simulations are performed of a laser-plasma accelerator in the quasilinear regime driven by high-order modes. Simulations show that, by including the first-order mode, the matched radius of the electron bunch is substantially increased, which for fixed bunch density and emittance implies an increase in the beam charge.

Beamstrahlung considerations in laser-plasma-accelerator-based linear colliders

“…In the present concept, the focusing forces acting on the beams originate from the transverse laser-driven wakefields in the plasma. It should be noted that the beam radius r can be controlled by controlling the transverse wakefields using shaped transverse laser intensity profiles in the quasilinear regime [19]. Independent control of the transverse focusing and longitudinal accelerating forces and, hence, the beam radius, in the LPA is critical for control of emittance [20].…”

Tapered plasma channels to phase-lock accelerating and focusing forces in laser-plasma accelerators

“…(19), f = 1, and for the phase of the transverse field given by Eq. (20), f (z) = 1 − r 2 s (z)/2R 2 ch (z). Equation (27) can be solved for the plasma density variation (taper) to eliminate slippage between the beam and the plasma wave fields.…”

Plasma Undulator Based on Laser Excitation of Wakefields in a Plasma Channel

“…As demonstrated in Ref. [34], using multiple Hermite-Gaussian laser modes can reduce the strong focusing of the wakefields (reduce k β ). Consider the following example of a wakefield excited by two laser modes: n = 0, m = 1, a 0,0 = 0.145, a 0,1 = 0.1, λ 0 = λ 1 = 0.8 µm, and both modes matched to the plasma channel (n 0 = 10 18 cm −3 ) with w 0 = 7 µm.…”

“…Plasma wave excitation using multiple laser modes was considered in Ref. [34]. Using the same formalism as above, an electron in the wakefield driven by two linearly-polarized HermiteGaussian laser modes will produce undulator and betatron radiation with the parameters:…”

“…Note that, in general, the focusing is asymmetric k βx = k βy , however, additional laser pulses, polarized orthogonally (with ∆kL 1) or temporally separated, can be used, following the techniques described in Ref. [34], to control k βy . The undulator strength is given by the parameter…”