H. Tongu

Real-Time Optimization of Proton Production by Intense Short-Pulse Laser with Time-of-Flight Measurement

A scheme of the real-time optimization of proton production by an intense short-pulse laser interacting with a foil target was developed using a time-of-flight measurement with a plastic scintillator. Owing to special treatments, the detection of protons using a scintillation counter has become possible under heavy backgrounds such as laser light itself, laser-generated hard X-ray, self-emission light, and electrons from the laser-produced plasma. With such a real-time measurement of protons, the energy spectrum of protons could be obtained shot by shot, and the experimental conditions for optimal proton production could be determined very efficiently.

Phase rotation scheme of laser-produced ions for reduction of the energy spread

In order to widely spread out particle beams utilized in cancer therapy, laser-produced ions are developed as the injection beam for an ion synchrotron dedicated for cancer therapy. Such a laser ion source is expected to contribute largely to the realization of compactness of the size and low cost of the cancer therapy accelerator. The energy spectrum of the laser-produced ions, however, has no peak, but their intensities decrease exponentially according to the increase of the energy. For the purpose of modifying such a situation, we have proposed a scheme to rotate the beam in the longitudinal phase space with the use of the RF electric field, which is phase-adjusted with the pulse laser. We aim for a reduction of the energy spread of ± 5% selected by an energy analyzer and slits to ±1% by such phase rotation. For this purpose, a quarter wavelength resonator with two gaps with the same resonant frequency as the source laser has already been fabricated, together with its RF power source. The above phase rotation system and its recent experimental realization are presented.

High-Quality Laser-Produced Proton Beam Realized by the Application of a Synchronous RF Electric Field

A short-pulse (~210 fs) high-power (~1 TW) laser was focused on a tape target 3 and 5 µm in thickness to a size of 11×15 µm2 with an intensity of 3×1017 W/cm2. Protons produced by this laser with an energy spread of 100% were found to be improved to create peaks in the energy distribution with a spread of ~7% by the application of the RF electric field with an amplitude of ±40 kV synchronous to the pulsed laser. This scheme combines the conventional RF acceleration technique with laser-produced protons for the first time. It is possible to be operated up to 10 Hz, and is found to have good reproducibility for every laser shot with the capability of adjusting the peak positions by control of the relative phase between the pulsed laser and the RF electric field.

Observation of strongly collimated proton beam from Tantalum targets irradiated with circular polarized laser pulses

High-energy protons are generated by focusing an ultrashort pulsed high intensity laser at the Advanced Photon Research Center, JAERI-Kansai onto thin (thickness <10 μm) Tantalum targets. The laser intensities are about 4 × 10 18 W/cm 2 . The prepulse level of the laser pulse is measured with combination of a PIN photo diode and a cross correlator and is less than 10 −6 . A quarter-wave plate is installed into the laser beam line to create circularly polarized pulses. Collimated high energy protons are observed with CH coated Tantalum targets irradiated with the circularly polarized laser pulses. The beam divergence of the generated proton beam is measured with a CR-39 track detector and is about 6 mrad.

Efficiency Enhancement of Indirect Transverse Laser Cooling with Synchro-Betatron Resonant Coupling by Suppression of Beam Intensity

The efficiency of indirect transverse laser cooling with synchro-betatron resonance coupling has been improved with the reduction in beam intensity by scraping the tail part of the beam. In order to measure the beam size at a low beam intensity, a new scheme to measure the beam profile by observation of the survival ratio with changing the scraper position has been established. With 104 particles, the transverse cooling time was reduced to 1.2 s, and the cooled horizontal and vertical beam sizes were 0.19 and 0.61 mm, corresponding to temperatures of 20 and 29 K, respectively, which is largely improved compared with that in our previous experiment [Nakao et al.: Phys. Rev. ST Accel. Beam 15 (2012) 110102].

Longitudinal and Transverse Coupling of the Beam Temperature Caused by the Laser Cooling of 24Mg

A laser-cooling experiment of a 40 keV 24Mg+ beam was carried out in the small laser-equipped storage ring (S-LSR). A laser co-propagating with the beam and an induction accelerator were utilized in the experiment. The lowest longitudinal temperature achieved in the present experiment was 3.6 K for 3?104 ions stored in the ring. It was found that the number of stored ions is related to the temperature at the final equilibrium state of the laser cooling. This relation shows that the longitudinal temperature of the laser-cooled beam linearly couples with the transverse one through intra-beam scattering.

Laser Cooling for 3-D Crystalline State at S-LSR

At ICR, Kyoto University, an ion storage and cooler ring, S‐LSR has been constructed. Its mean radius and maximum magnetic rigidity are 3.6 m and 1.0 Tm, respectively. 24Mg+ ions with the kinetic energy of 35 keV are to be laser‐cooled by the frequency doubled ring dye laser with the wavelength of 280 nm. In order to avoid the shear heating, dispersion compensation is planned by the overlap of the electric field with the dipole magnetic field in all 6 deflection elements. Intermediate electrodes, which can be potential adjusted, are to be utilized so as to realize a uniform electric field radial direction within a rather limited vertical gap, 70 mm of the dipole magnet. Synchro‐betatron coupling needed for 3‐dimensional laser cooling is to be realized by placing the RF cavity at the siraight section with finite dispersion for the normal mode lattice, which is expected to realize 1 dimensional string. For the case of dispersion compensated lattice to suppress the shear heating, possibility of realizing “ta...

Practical Applications of Permanent Magnet Multipoles

Permanent magnets are superior to electromagnets in generating strong multipole magnetic fields. Their fields are sometimes stronger than those that can be generated by superconducting magnets with the same bore radius, when the number of poles are higher. Three fabricated multipoles including a quadrupole magnet for the ILC (International Linear Collider) final focus doublet, a quadrupole magnets as a spin filter for cold neutrons, and a sextupole magnet for neutron beam focusing are described.

Compact permanent magnet H+ ECR ion source with pulse gas valve

Compact H(+) ECR ion source using permanent magnets is under development. Switching the hydrogen gas flow in pulse operations can reduce the gas loads to vacuum evacuation systems. A specially designed piezo gas valve chops the gas flow quickly. A 6 GHz ECR ion source equipped with the piezo gas valve is tested. The gas flow was measured by a fast ion gauge and a few ms response time is obtained.

Variable Permanent Magnet Multipoles

Permanent magnet quadrupoles (PMQ) and sextupoles (PMSx) have been fabricated and tested. Their effective strengths are variable. The PMQ has been developed as a final focus magnet for a linear collider. There is enough space to put Glucksterns 5-ring singlet structures in each incoming beam line. The magnetic center offsets and higher harmonic components can be adjusted by adjusting the positions of the magnet pieces with screws on holders. The measured integrated gradient can be reduced down to less than 1% of the maximum strength. The PMSx has been developed to focus pulsed cold neutrons for a small angle neutron scattering (SANS) experiment. The strength is modulated by a so called double ring structure, where the inner ring is fixed and the outer ring is rotated. A torque canceller can reduce the torque to rotate the outer ring by one third of its original magnitude. The focal length of the fabricated triplet lens is less than 50 cm for Very Cold Neutrons (VCN). It is demonstrated that VCN-SANS can shorten a SANS beamline, which usually has a few tens of meters, down to 2~3 meters.