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Cryogenic targets

With suitable target configurations, laser-induced plasmas can support field gradients exceeding 1012 V/m, hence the huge interest in PW lasers as compact ion accelerators.

There are many schemes to enhance the spatial confinement of the accelerating field in order to optimize the quality of the accelerated beams like circularly polarized laser light to create a “light sail” [A. P. L. Robinson, P. Gibbon et al., Plasma Phys. & Contr. Fusion, 51, 024004 (2009)], composite targets containing both heavy (Al, Cu) and light (H, C) ion components [Dromey, M. Geissler, A. Karmakar, and P. Gibbon, Phys. Rev. Lett. 105, 155002 (2010)], and so-called mass-limited targets, like frozen pellet targets or cluster-jet targets, which should have sizes roughly equal to that of the diameter of the laser focus.

With frozen hydrogen targets of 10 µm diameter and with a laser intensity of 1020 W/cm2, simulations show that proton energies of a few 10 MeV can be reached. A further advantage of pellet targets is that the target material can be permanently re-supplied with a frequency > 1 kHz.

Cluster-jet targets (see Fig. 3) provide a continuous flux of frozen nm-sized clumps each containing between 10 and 106 hydrogen atoms. According to theoretical calculations for the corresponding plasmas with near-critical densities generated by a PW laser, proton energies up to around 100 MeV may be expected.

It is planned to build a compact laser-induced thermal neutron source using a cluster-jet target from the University of Münster. A converter-target will be bombarded by the laser-accelerated protons and therefore, it will produce a pulsed beam of MeV neutrons. Eventually a tailored moderator will be used to obtain meV neutrons. The test-experiments will be performed at the Arcturus laser facility at the Heinrich-Heine University Düsseldorf.