Stability of ionisation-injection-based laser-plasma accelerators

Abstract

Laser-plasma acceleration (LPA) is a compact technique to accelerate electron bunches to highly relativistic energies, making it a promising candidate to power radiation sources for industrial or medical applications. We report on the generation of electron beams from an 80 MeV-level LPA setup based on ionisation injection (II) over a duration of 8 hours at a repetition rate of 2.5 Hz, resulting in 72000 consecutive shots with charge injection and acceleration. Over the full operation time the moving averages of the total beam charge of 14.5 pC and the charge between 70-80 MeV did not drift on a detectable level. The largest source of shotto-shot jitter was in the beam charge (26% standard deviation), which was most strongly correlated with fluctuations in the plasma density (3.6% standard deviation). Particle-in-cell simulations demonstrate that this was chiefly caused by stronger laser self-focusing in higher density plasmas, which significantly increased the ionised charge along with the emittance of the beam. The nonlinearity of this process imposes tight constraints on the reproducibility of the laser-plasma conditions required for a low jitter II-LPA output if self-focusing plays a role in the laser evolution.

Polarized Electron Beams from Laser Plasma Acceleration and Their Polarimetry

“…A robust and stable LPA source employing a 10 Hz, 25 TW Ti:Sapphire laser system has already been commissioned and has been used as the electron source for the novel PLASMED X all-optical inverse Compton source for medical applications. Stable acceleration of electron bunches over several hours was demonstrated [9]. For the LEAP project, the existing setup will be modified to enable the demonstration of polarized electron beams from a LPA.…”

“…Based on previous measurements [9] and particle-in-cell simulations using FBPIC [10], the following beam parameters are expected: A tunable central energy from 30 to 70 MeV, a FWHM energy spread of about 10 %, an electron spot size of 1 µm, a divergence of 3 mrad, and an electron beam charge of 10 pC. The choice of target gas is an important consideration for any polarized LPA source employing a prepolarized plasma source as it defines the achievable fraction of polarized electrons and the wavelength required for the dissociation of the molecules.…”