Transverse emittance growth in staged laser-wakefield acceleration

Abstract

We present a study on the emittance evolution of electron bunches, externally injected into laser-driven plasma waves using the three-dimensional particle-in-cell (PIC) code OSIRIS. Results show order-ofmagnitude transverse emittance growth during the injection process, if the electron bunch is not matched to its intrinsic betatron motion inside the wakefield. This behavior is supported by analytic theory reproducing the simulation data to a percent level. The length over which the full emittance growth develops is found to be less than or comparable to the typical dimension of a single plasma module in current multistage designs. In addition, the analytic theory enables the quantitative prediction of emittance degradation in two consecutive accelerators coupled by free-drift sections, excluding this as a scheme for effective emittance-growth suppression, and thus suggests the necessity of beam-matching sections between acceleration stages with fundamental implications on the overall design of staged laser-wakefield accelerators.

Efficient numerical modelling of the emittance evolution of beams with finite energy spread in plasma wakefield accelerators

“…In addition, the phase space emittance of beams with finite energy spread and significant divergence may increase dramatically during the expansion in the vacuum downstream the plasma target [14,15]. The challenges to match beams into the plasma as well as to extract them can be substantially mitigated and the quality deterioration suppressed by the use of tapered vacuum-to-plasma and plasma-to-vacuum transitions, respectively, as indicated by several studies [13,[16][17][18][19]. The rigorous investigation of the beam injection and extraction processes in PWFA typically involves the use of particle-in-cell (PIC) simulations.…”

“…The emittance evolution, depicted in Fig. 6, shows the growth during the injection owing to the betatron decoherence (compare [12,13]) and the growth after the beam exits the plasma targets during the expansion in vacuum (compare [14,15]). It can be seen that the emittance growth during the injection is reduced in the tailored plasma profile and the growth rate during the vacuum drift is significantly mitigated in this profile.…”

“…For the plasma, 9 macro-particles per cell were used in the middle of the simulation domain down to 1 macro-particle per cell at the simulation domain boundaries. (6) if not matched (compare [13]). The various subsets oscillate at slightly different frequencies according to (15) due to the varying energy.…”

“…As shown in previous works [12,13], the transverse phase space emittance, which is a figure of merit for the transverse beam quality, grows during the injection and propagation in a plasma target if beams are not matched appropriately. In addition, the phase space emittance of beams with finite energy spread and significant divergence may increase dramatically during the expansion in the vacuum downstream the plasma target [14,15].…”

Conceptual and Technical Design Aspects of Accelerators for External Injection in LWFA

“…This means that while the beam is accelerated, its spot size must decrease according to σ 0 ∝ E −1/4 . A mismatch of the injected beam will cause an increase of the beam normalized emittance according to [28]:…”