## 3 Applications with J-KAREN Laser System  
### Laser-Accelerated Protons from Thin Foils  
Since the first observation of the energetic ions from the interaction between the high-intensity laser pulses and thin-foil target at about three decades ago, many investigations have been carried out in this field. This is because the laser-driven energetic ions show many peculiar characteristics compared with those of the ion beams from conventional accelerators [70, 71], thus many potential fields of applications are proposed, including medical application [72, 73, 74]. For the medical applications, there still exist many issues which we have to overcome. One of those is the extension of the maximum energy of the ions.  
The maximum proton energy ever achieved in the last decade was \(\sim\)60 MeV by using 400 J subpicosecond laser pulse from the laser system [75] which is basically single shot based operation, interacting with 100 \(\upmu\)m foil. On the other hand, 24 MeV protons were demonstrated with the interaction between the 10 J, 100 fs laser pulses from the relatively compact laser system which can carry out repetitive operation, interacting with 3 \(\upmu\)m target at the intensity of \(10^{20}\) W/cm\({}^{2}\)[76]. One of the important points for the application, especially for the medical application, the energetic ions should be accelerated repetitively with relatively small laser energy from a compact laser system. For obtaining higher energy of ions with relatively small laser energy, the increase of the intensity of the laser pulses on target is necessary.  
The J-KAREN laser pulses are controlled temporally and spatially for achieving as high intensity as possible with the limited energy of laser pulses. The sub-ns contrast of the laser pulses is routinely obtained to \(10^{10}\) without the plasma mirror system. The temporal profile of the main pulse and contrast in sub-ps time scale are optimized with the AOPDF using feedback from transient grating frequency-resolved optical gating (TG-FROG) apparatus. The pulse width and contrast, which are used in this experiment, are 36 fs (FWHM) and \(10^{10}\), respectively. The derived on-target intensity is greater than \(10^{21}\) W/cm\({}^{2}\) with the focal spot size of 3 \(\upmu\)m \(\times\) 2 \(\upmu\)m in FWHM [19]. The observed electron spectra show the slope temperature of more than 10 MeV. By assuming the ponderomotive scaling of the electron temperature [77], a normalized vector potential of the laser is a\({}_{0}\) = 0.85 \(\times\)\(10^{-9}\) (I [W/cm\({}^{2}\)])\({}^{1/2}\)\(\lambda\) [\(\upmu\)m]) = 20, thus consistent with the estimated intensity of the laser pulse on target. The 7 J laser pulses are focused from the incidence angle of 45 degree onto the Stainless (SUS) Steel 2 \(\upmu\)m tape target, which is also suitable for the repetitive operation. The proton detection is conducted with CR39 stacks which enable to detect the protons with energies up to 40 MeV. After the etching process with the 6 N KOH solution at 70 \({}^{\circ}\)C, clear etch pits caused by protons are revealed on the last layer of CR39, showing that the acceleration of protons is up to 40 MeV (as seen in Figure 12). The conversion efficiency of the laser energy into protons above 15 MeV is 0.1%. The estimate using the strong charge separation model [78] with the observed hot electron temperature of 10 MeV predicts the maximum proton energy of 54 MeV, which is in reasonable agreement with observed results.  
As a summary, the maximum proton energy of 40 MeV is obtained using only 7 J pulses from a Ti:sapphire laser system focused to very high intensity This proton energy enhancement and efficiency indicate the potential for simple thin metallic foils as sources for the laser-driven accelerator for medical application.