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.
Highlights
A number of experiments during the past decade [1,2,3,4,5,6,7] have confirmed laser-wakefield acceleration of charged particles in plasma as a promising technology candidate for driving compact and brilliant x-ray light sources [8,9] and possibly future particle colliders [10,11]
Taking into account current high-end laser technology, acceleration of electron beams to energies beyond the 10 GeV level seems possible only by use of multiple stages in series, i.e. by staging. Pivotal beam parameters such as the transverse emittance must be conserved during the transport of preaccelerated electron bunches into the accelerating phase of a subsequent plasma module to allow for high-energy, highquality beams and applications in photon science and at the particle-beam energy frontier
The assumption if 1⁄4 imþ1 for the derivation of Eq (19) is contributed to the fact that sharp vacuum-plasma interfaces are considered in this study. Another option constitutes the investigation of smooth transitions, much longer than the local betatron frequency in order to match the electron beam adiabatically into the plasma wave, as proposed in earlier works [13,15]. This would imply the beta function of the bunch when emerging from stage i to be smaller than the beta function in the subsequent stage i þ 1 before being adiabatically matched, if < ^ imþ1, reducing emittance growth after a free drift compared to the sharp interface case [cf. derivation of Eq (19)]
Summary
A number of experiments during the past decade [1,2,3,4,5,6,7] have confirmed laser-wakefield acceleration of charged particles in plasma as a promising technology candidate for driving compact and brilliant x-ray light sources [8,9] and possibly future particle colliders [10,11] These plasma waves support extreme field gradients that facilitate GeVenergy gain in centimeter-scale stages [4,5] with the length of a unit and the energy gain in a single stage being fundamentally limited by energy depletion of the driving laser pulse [12]. We derive a general analytic expression for the emittance growth caused from betatron decoherence as expected by insufficient matching of the beta function (bunch size) or of the alpha function (focusing) and compare it to fully three-dimensional particle-in-cell simulations of finite-length electron bunches with low energy spread, externally injected into a laser-driven plasma wave. In addition we discuss quantitatively the fundamental implications of beam-quality degeneration arising from insufficiently controlled electron-beam matching on staged plasma accelerators
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