Coupling of laser energy into plasma channels

Abstract

Diffractive spreading of a laser pulse imposes severe limitations on the acceleration length and maximum electron energy in the laser wake field accelerator (LWFA). Optical guiding of a laser pulse via plasma channels can extend the laser-plasma interaction distance over many Rayleigh lengths. Energy efficient coupling of laser pulses into and through plasma channels is very important for optimal LWFA performance. Results from simulation parameter studies on channel guiding using the particle-in-cell (PIC) code VORPAL [C. Nieter and J. R. Cary, J. Comput. Phys. 196, 448 (2004)] are presented and discussed. The effects that density ramp length and the position of the laser pulse focus have on coupling into channels are considered. Moreover, the effect of laser energy leakage out of the channel domain and the effects of tunneling ionization of a neutral gas on the guided laser pulse are also investigated. Power spectral diagnostics were developed and used to separate pump depletion from energy leakage. The results of these simulations show that increasing the density ramp length decreases the efficiency of coupling a laser pulse to a channel and increases the energy loss when the pulse is vacuum focused at the channel entrance. Then, large spot size oscillations result in increased energy leakage. To further analyze the coupling, a differential equation is derived for the laser spot size evolution in the plasma density ramp and channel profiles are simulated. From the numerical solution of this equation, the optimal spot size and location for coupling into a plasma channel with a density ramp are determined. This result is confirmed by the PIC simulations. They show that specifying a vacuum focus location of the pulse in front of the top of the density ramp leads to an actual focus at the top of the ramp due to plasma focusing, resulting in reduced spot size oscillations. In this case, the leakage is significantly reduced and is negligibly affected by ramp length, allowing for efficient use of channels with long ramps.

Particle-in-cell simulations of quasi-phase matched direct laser electron acceleration in density-modulated plasma waveguides

“…32 Next, a neutral gas region of length λ H,1 is defined with a uniformly distributed hydrogen atom density n a (x, y, z) = n a0 = 1.25 × 10 19 cm −3 . As shown in Fig.…”

Guiding of high-intensity laser pulses in 100-mm-long hydrodynamic optical-field-ionized plasma channels

“…The plumes created by gas flow through the pinholes would have had a longitudinal scale length comparable to the pinhole diameter, which in turn ≈2z R . As such the plume regions could in principle have played a role in coupling the guided beam into the main channel [45,46]. The plasma channel would have extended into the entrance plume since the axicon focus extended into this region.…”

Beam emittance preservation using Gaussian density ramps in a beam-driven plasma wakefield accelerator

“…It is, therefore, necessary to use numerical methods to study the evolution of the beam through a given plasma density profile. This exercise bears some resemblance to the matching of a high intensity laser pulse into a laser wakefield accelerator plasma source [33].…”