This picture.(37KB)

Fig. 5-4 Principle of proton generation The laser pulse is focused on the tantalum (Ta) target. Protons are accelerated from the surface layer of the hydrogen-stored Ta target. The angular and the energy distributions of generated protons are measured with five track detectors and the magneto-electric ion energy analyzer, respectively. On the right hand side, a proton signal is shown as an example.

This picture.(50.4KB)

Fig. 5-5 Schematic view of laser-driven ion source Ions such as protons or carbon ions produced at the laser irradiated tape target are transferred into the phase rotation device in which the ion energy distribution is compressed. The ions are then transferred into the bending magnet where ions having the desired energy component are selected. Finally, the quality of the ion beam is determined by the emittance measurement system.

This research has been performed to develop a very compact ion source to reduce the size of an ion accelerator for cancer therapy. This collaborative effort involves several domestic laboratories and is organized by the National Institute of Radiological Sciences. To generate protons having an energy > 1 MeV efficiently, an ultra-short (50 fs) high-intensity laser pulse from a compact laser system was focused on a thin metal foil target with a hydrogen stored layer, giving an irradiance of 3×1018 W/cm2 (Fig. 5-4). The resulting maximum proton energy was 1 MeV and the number of protons whose energy was > 100 keV per single laser shot was 109. This result was obtained using a pre-pulse that arrived at the target earlier than the main high-intensity laser pulse. The purpose of the low-intensity pre-pulse is to hit the metal foil target to form expanding preformed plasma. The main laser pulse then interacts with the preformed large-scale plasma. Although this phenomenon had been predicted earlier by computer simulation, this was the first time it was demonstrated experimentally. This demonstration also verified the validity of the simulation model. Based on the results obtained, target irradiation with an intensity 5 ~ 10 times higher will make possible an ultra-compact ion source (Fig. 5-5) with sufficient quality to function as an ion injector for accelerators. Development of a laser-driven heavy-ion generator, such as for carbon ions, may follow this effort.

Reference K. Matsukado et al., Energetic Protons from a Few-Micron Metallic Foil Evaporated by an Intense Laser Pulse, Phys. Rev. Lett., 91(21), 215001 (2003).

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