Abstract
Stable operation of a laser-plasma accelerator near the threshold for electron self-injection in the blowout regime has been demonstrated with 25--60 TW, 30 fs laser pulses focused into a 3--4 millimeter length gas jet. Nearly Gaussian shape and high nanosecond contrast of the focused pulse appear to be critically important for controllable, tunable generation of 250--430 MeV electron bunches with a low-energy spread, $\ensuremath{\sim}10\text{ }\text{ }\mathrm{pC}$ charge, a few-mrad divergence and pointing stability, and a vanishingly small low-energy background. The physical nature of the near-threshold behavior is examined using three-dimensional particle-in-cell simulations. Simulations indicate that properly locating the nonlinear focus of the laser pulse within the plasma suppresses continuous injection, thus reducing the low-energy tail of the electron beam.
Highlights
INTRODUCTIONProgress in the technology of short-pulse laser amplification has made it possible for laser-plasma accelerators (LPAs) to generate quasimonoenergetic (QME) electron beams [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] and to approach the GeV energy range [14,15,16,17,18,19,20,21]
Experiments show a direct correlation [22,23,24] between the generation of collimated electron beams and formation of a unique plasma structure—an electron density ‘‘bubble’’—trailing the relativistically intense laser pulse [25,26,27,28]
The results of 3D PIC simulations indicate that production of the optimal beams in the laboratory may be associated with the precise location of the laser pulse focus and transient dynamics of the laser pulse in plasma, rather than with the stable self-guiding of the pulse until electron dephasing and/or pulse depletion
Summary
Progress in the technology of short-pulse laser amplification has made it possible for laser-plasma accelerators (LPAs) to generate quasimonoenergetic (QME) electron beams [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] and to approach the GeV energy range [14,15,16,17,18,19,20,21]. A 50–60 fs, 60–200 TW laser pulse, propagating through the plasma with the electron density ne $ 5 Â 1018 cmÀ3 [15,18,19,20], rapidly evolves into a relativistic optical shock with a very steep front [44,45] In this situation, a massive polychromatic tail would develop even if self-injection during the early stages of the interaction produces a QME bunch [18,19]. We demonstrate that the near-threshold regime can be accessed in a stable manner, yielding high electron-beam quality over a broad range of laser and plasma parameters. The accelerated electrons exiting the plasma impinge on a fluorescent screen (LANEX) that was imaged with a 12-bit
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