A laser-driven ion beamline for generating well-defined ultra-short ion bunches at highest intensities

The LIGHT collaboration [1] has installed a laser-driven ion beamline at GSI Helmholtz Center for Heavy Ion Research. For the first time it is now possible to study the feasibility and potential of shaping laser-driven ion beams for future applications. We report on the temporal recompression of a laser-accelerated ion bunch.
In the presented experiment (c.f. Fig. 1), a dedicated arm of the high-power laser PHELIX was used to drive a TNSA proton source using gold and titanium foils. The 650 fs short, 20 J laser pulse produces the typical exponentially decaying energy spectrum with about 10^10 particles at an energy of 10±0.5 MeV and energy cut-off at 28.4 MeV. The protons are captured by a pulsed high-field solenoid, energy selected and modulated in a conventional radiofrequency cavity and transported along a drift line to the end station by means of permanent magnetic quadrupoles. However, the long drift between the laser target and the cavity introduces a temporal spread-out of the polychromatic beam.
Most recently, we accomplished a recompression of the ion bunch by a well-chosen acceleration voltage of the rf cavity achieving phase-focusing in the following 3.5 meter long drift behind the cavity. At the end station we measured a central energy of 7.8 MeV; up to 5×10^8 protons could be temporally compressed to a bunch with duration of 462±40 ps (FWHM). The bunches show a moderate energy spread between 10 % and 15 % and are available at 6 m distance to the source, thus well separated from the harsh laser-acceleration environment. Such well-defined sub-nanosecond intense ion bunches are ideal for the generation and study of warm dense matter and can probe transient phenomena with unprecedented time resolution.
Fig. 1. LIGHT beamline experiment setup: a) TNSA proton source driven by the PHELIX laser and captured by the high-field solenoid b). The transported particles are rotated in longitudinal phase space by the cavity c). The permanent magnetic quadrupole doublets d), e) and optionally f) transport the beam towards the end station g) where the beam was diagnosed.
References
[1] S. Busold et al., Nucl. Instr. Meth. Phys. Res. A 740, 94 (2014).