## 3 Applications with J-KAREN Laser System
### Laser-Accelerated Ions from Cluster-Gas Targets  
In laser-driven ion accelerations, diagnosis of laser-accelerated ion beams attracts much attention and a precise characterization of accelerated ions is a crucial issue to develop a compact laser ion accelerator for medical and industrial applications. Solid state nuclear track detectors such as CR-39 detectors have been so far extensively used to detect laser-accelerated ions [18]. It is, however, impossible to detect ions with energies higher than the detection threshold limit of the CR-39, where the detection threshold limit is defined as the particle energy where the stopping power in the detector becomes smaller than the sensitivity of the detector. For example, in the case of BRYOTRAK (Fukuvi Chemical Industry, Fukui, Japan), it cannot detect He ions having energies of 10 MeV per nucleon and more.  
Therefore, new methods to detect ions whose energies are far beyond the detection threshold limit of the CR-39 are required to further facilitate the laser ion acceleration experiments. We have demonstrated a new simple diagnosis method for high energy ions utilizing a single CR-39 detector mounted on plastic plates to identify the presence of the high energy component beyond the detection threshold limit of the CR-39 [79]. In addition, we have applied this method to the laser-driven ion acceleration experiments using cluster-gas targets [20].  
The proof-of-principle experiments for the new ion detection method were carried out at TIARA facility in JAEA-TAKASAKI, which delivered \({}^{4}\)He\({}^{2+}\) ion beam with an energy of 100 MeV (25 MeV per nucleon). In contrast to the fact that the stopping power of the incident ion beam is too small to create etchable tracks in the 100-\(\upmu\)m thick CR-39, a number of etch pits having elliptical openings were created on the rear surface of the CR-39. The growth curve analyses using the multi-step etching technique revealed the following scenario: The incident He ions once passed through the CR-39 without creating any etchable track, and various nuclear reactions such as nuclear spallation  
Figure 12: Proton beam footprints on the CR39 plates for a 2 \(\upmu\)m thick Stainless (SUS) target irradiated at the intensity of \(1\times 10^{21}\) W/cm\({}^{2}\). The proton energy range detected with each layer is also indicated.  
were induced by incident He ions in the plastic plates. As a result, the backscattered He and other heavy ions hit into the CR-39 from the rear surface and created the etchable tracks on the rear surface [79].  
This method in combination with a magnetic energy spectrometer is applied to the laser-driven ion acceleration experiments using the cluster-gas target, which consists of submicron-size CO\({}_{2}\) clusters and a background He gas, with J-KAREN laser system in JAEA. Figure 13a shows the whole view of etch pit distribution on the rear surface of CR-39. Experiments are conducted at a laser intensity of \(7\times 10^{18}\) W/cm\({}^{2}\) (40 fs, 1 J pulses) with a contrast ratio of \(10^{-10}\). Etch pit radii indicate that ion species are He and other heavy ions. Detailed microscope investigations show that almost all of the etch pits on the rear surface have elliptical openings (as seen in Figure 13b,c) to demonstrates that laser-accelerated ions with energies up to 50 MeV per nucleon were generated [20].