Laser-driven proton acceleration has emerged as a promising compact and cost-effective alternative to conventional accelerators. Over the past decade, significant advances have been made, with proton beams reaching near 100 MeV in experiments [1] and novel acceleration mechanisms have been developed. A recent record of 160 MeV was achieved at HZDR using the DRACO laser [2], and stable high-repetition-rate (HRR) operations have been demonstrated on various laser platforms [3]. We will present recent experimental and numerical results obtained using innovative target designs like micro-coils and cryogenic targets that have a strong potential to make these sources more suitable for applications.

Helical coil targets are widely used in experiments [4,5] to focus, collimate, bunch, and accelerate protons generated via the Target Normal Sheath Acceleration process. These targets enable the production of highly focused and collimated proton beams (~1° divergence). However, acceleration and bunching remain limited. We recently demonstrated that these limitations stem from current dispersion effects during its propagation along the helix. To address this, a new target design was proposed, incorporating a tube around the helix to reduce dispersion and significantly enhance bunching [6].
To further improve post-acceleration, it is necessary to synchronize proton propagation with the current pulse over a longer helix length. For this purpose, we developed a new electromagnetic model of a helix with variable pitch and diameter [7]. Using this model, novel variable-pitch helices were manufactured and tested in recent experiments at the ALLS facility (INRS, Canada). We will present the experimental results obtained that reveal effects not only on protons but also on carbon ions [8].
Since this experiment, new helical targets have been designed to bunch and accelerate alpha particles for scandium radioisotope production. We will present the results of simulations (Particle-In-Cell and Monte Carlo) that predict that these targets could increase radioisotope production by a factor of 10 to 3000 [9].

Cryogenic hydrogen targets [10] represent an appealing possibility to answer the need for high repetition-rate laser-plasma acceleration. Contrary to plastic or metallic targets, hydrogen targets leave no debris behind. This is because hydrogen is gaseous at room temperature, and can simply be evacuated by the pumping system. Also, since they are formed directly inside the high vacuum of the interaction chamber, no surface contaminants are deposited on the target before being irradiated. They can be prepared in an ultra-pure state, generating pure proton beam [11], which is an advantage when the accelerated ion beam needs to be conditioned and used for other experiments as a single specie greatly simplifying the beam shaping to be done. They can be fed continuously in-situ inside the high vacuum of the interaction chamber, which removes the need of venting and opening the chamber to replace the target. This specific characteristic makes them ideal for operation with modern high-repetition rate lasers [12]. Finally, the low density of solid hydrogen minimizes the amount of hard-radiations (X-rays) generated by the laser-driven hot electrons, through Bremsstrahlung and synchrotron processes. This reduces radio-biological hazards and the likeliness of hampering active detectors, easing operation and safety.
We performed Particle-In-Cell simulations of the interaction of high intensity laser pulses using the parameters of the Apollon laser with thin cryogenic hydrogen ribbons. Collimated jets of protons with maximum energies higher than 200 MeV can be produced in this regime. The preparation of experiments on Apollon using the cryogenic system developed at CEA IRIG-DSBT will be discussed.
 

High Intensity Laser,Laser-ion acceleration,Micro-coils,Cryogenic targets