Since the first observation of Mega-electronvolt (MeV) proton beams produced during the interaction of high-intensity laser pulses with thin foils1,2, the field of laser-ion acceleration has seen rapid progress. Quasi-static, Teravolt-per-meter (TV/m) electric fields are generated during these interactions, orders of magnitude higher than those achievable with conventional accelerators. Due to the short acceleration length (∼10 μm) required for protons to reach energies of several tens of MeV in these fields, various potential applications such as the production of medically relevant radio-isotopes3, hadron therapy4,5, or the realization of an ultra-short frontend for conventional accelerators6,7,8 are under discussion. These applications would all benefit from the availability of an intense and compact high-energy proton source, preferably with a high repetition rate. However, broad, quasi-thermal energy distributions of the protons and a low energy-conversion efficiency from the laser to the proton beam have been major issues in the past, which need to be solved before the envisaged applications can be realized.

MeV-protons can be generated via the process of Target Normal Sheath Acceleration (TNSA)9. Here, a high-intensity laser pulse ionizes the front side of a solid target, e.g. a thin foil, generating hot electrons. These electrons propagate through the target to form a sheath at its back side. Rear-surface atoms are first ionized and then accelerated away from the target by the associated TV/m-electric fields. While μm thick metal targets are commonly used, protons stemming from surface contaminations are favored due to their highest charge-to-mass ratio, q/m, hence dominating the acceleration. Nevertheless, a significant fraction of the laser energy is distributed among heavier ions10 reducing the energy conversion from laser to protons. For high-repetition rate lasers with sub-100 fs pulse duration and 10–200 TW peak power, this efficiency has been reported to be 1% or less11. So far, only single-shot, PW-class lasers with pulse energies of a few 100 J have reached conversion efficiencies of 6% with a proton-beam half-angle of ∼30°12. Furthermore, using the PW-laser system VULCAN, conversion efficiencies as high as 15% from laser energy to protons could be achieved when using thin Au-foil targets irradiated by a double-pulse structure13. Finally, TNSA protons typically exhibit a Boltzmann-like spectrum14, but can be modified by specially prepared targets15,16.

Another process producing MeV protons is Radiation Pressure Acceleration (RPA)17,18. Here, the laser pulse is reflected at the target’s front side pushing the electrons in the forward direction, i.e. into the target. The negatively charged electrons in turn pull along the positive ions. RPA requires balancing the pressure from laser radiation with the pressure from charge separation in the target. This is primarily achievable with nm-thin solid targets19,20. Yet another possible mechanism is the acceleration of ions by collisionless electrostatic shocks (Collisionless Shock Acceleration, CSA). Such shocks can, e.g., be generated at a sharp transition from a hot, dense plasma to a cooler plasma of lower density21. A strong electric field spike is formed at the shock front, accelerating ions from the less dense plasma that the shock propagates into.

The problem that a considerable fraction of the energy is imparted to heavier ions occurs in all of these mechanisms. Hence, there is good reason to consider pure hydrogen targets, which can readily be produced e.g. with gas jets, ensuring that protons are the only accelerated ion species. However, both RPA and shock acceleration demand that an over-critical plasma be generated. For regular-pressure gas jets, long-wavelength lasers such as CO 2 -lasers are required. Here, narrow-band proton beams have been observed22,23 but with low conversion efficiencies (4 × 10−4). If near-IR, high-power lasers are to be used, they require either the use of ultra-high pressure gas jets24,25 or hydrogen targets at near-solid density26,27,28. Furthermore, a self-replenishing target, which is well-suited for high-repetition rate operation would be beneficial. Over the last few years, there has been considerable research on the application of solid-hydrogen as the target material. The results from these measurements show a high conversion efficiency from laser energy to protons when using 300 ps-long26 or sub-ps pulses28,29. In most of these experiments, temperature-like proton spectra following a Boltzmann distribution were detected with cutoff-energies in the range of 1 MeV26 or up to 20 MeV28. In the results presented by Gauthier et al.27, a quasi-monoenergetic feature around 1 MeV was observed but the exact origin of this feature has not yet been identified. Furthermore, Göde et al. found that due to the presence of a preplasma on the target rear surface, Weibel-type instabilities affecting the formation of the hot-electron sheath on the target rear surface can strongly modulate the generated beam profile in the transverse direction30, rendering such beams problematic for applications, which require a smooth proton beam.

In this paper, we report the successful application of a cryogenic solid hydrogen target31,32 for laser-driven proton acceleration. Using a Joule-class, 1/40-Hz laser system, we achieved both a high energy conversion efficiency from the laser pulse to the accelerated proton beam and a cutoff-energy in excess of 20 MeV while still exhibiting a rather smooth beam profile. Furthermore, we observed clear non-thermal features in the proton spectra, which can be explained to be the result of an electro-static, collisionless shock occurring in the low-density corona surrounding the solid-hydrogen filaments, therefore offering a new explanation for our – and potentially also for other – experimental results.