Gas density structure of supersonic flows impinged on by thin blades for laser–plasma accelerator targets

Abstract

Density transition injection is an effective technique for controllably loading electrons into a trapped phase for laser plasma accelerators. One common technique to achieve the required fluid structure is to impinge a thin blade on the plume of a supersonic nozzle. Density transitions induced in this way are often assumed to be bow shocks and therefore sharp, but two-dimensional simulations and fluorescence measurements presented in this work show that in many cases of interest the density transition accessible to a laser propagating transverse to the shock is an intercepting shock, and therefore shock thickness and density vary with pressure, laser height and blade position. The fluid dynamics of a supersonic nozzle impinged on by a thin, flat object are explored through simulations and relevant features verified via planar laser-induced fluorescence measurements. Implications of results for tuning electron beam injectors in laser plasma accelerators are discussed.

Stable and high quality electron beams from staged laser and plasma wakefield accelerators

“…This added flexibility decouples injection and acceleration in the PWFA stage. In particular, by setting the correct orientation of the astigmatic focus, the shock can be created perpendicular to the beam axis, crucial for the generation of high quality witness beams [53,54] and hard to achieve with supersonic shock formation. In addition, the position jitter of the HOFI injector is only a few µm and smaller than typically achieved with wire-generated shocks [43].…”

Effect of nozzle curvature on supersonic gas jets used in laser–plasma acceleration

“…31 For d à ¼ 0.6 mm, isentropic calculations yield M e ¼ 7.1 whereas for d à ¼ 0.8 mm, M e ¼ 5.7. The higher exit Mach numbers were chosen as a sequel to previously lower M e nozzles used [35][36][37] as high exit Mach numbers correspond to more flat-top density profiles. 32 Both d à were also chosen so that, with a backing pressure of 500 psi, isentropic exit density q e would be on the order of 10 19 cm À3 , the optimal LPA density range.…”

“…[29][30][31][32] On the other hand, a sharp high-density profile is useful for high repetition rate LPA driven by mJ-level laser pulses 33,34 and electron injection. [35][36][37][38][39][40][41][42][43][44] One common method of creating desired density profiles is by producing a supersonic gas jet through converging-diverging (C-D) nozzles.…”

“…Numerous studies have been conducted in the context of LPA on nozzle manufacturing techniques 45 and nozzle design. [27][28][29]31,32,35,[46][47][48][49][50][51][52][53] While the diverging section curvature of C-D nozzles has been examined and optimized for various applications in past studies, 48,[54][55][56][57][58] more investigation of the diverging section curvature's effect in an LPA context is needed.…”

Symmetric and asymmetric shocked gas jets for laser-plasma experiments

“…As mentioned earlier, most of the experiments relying on the gradient injection method use a blade inserted in the flow after the nozzle. The physics of supersonic gas jets impinged by a blade has been recently thoroughly described 23 and such design works well with millimetricscale targets used in experiments with high-power lasers where the Rayleigh length is relatively long, and thus where distance and positioning constraints are not too stringent. But in high-repetition rate laser-plasma accelerators with an energy of only a few millijoules per pulse, it is necessary to focus the laser tightly in order to achieve relativistic intensities.…”