Mikio Tanabe

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.

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...

Experimental approach to ultra-cold ion beam at S-LSR

At S-LSR, abrupt reduction of momentum spread of 7 MeV proton beam to ~2 times 10-6 at proton number of -2000 has been observed, which indicates phase transition to 1 dimensional ordered state. Attained proton temperatures after transition are 26 mueV and 1 meV for longitudinal and transverse directions, respectively, which compared with the corresponding values of 20 mueV and 34 meV for electron beam, indicates the magnetization of electron. Laser cooling of 24Mg+ has also been started and momentum spread of ~108 ions is reduced to 2 times 10-4, saturated with the momentum transfer from transverse degree of freedom by intra-beam scattering.

S-LSR Cooler Ring Development at Kyoto University

A compact ion cooler ring, S‐LSR is under construction in Kyoto University. One of the subjects of S‐LSR is a realization of the crystalline beams using the electron beam and the laser cooling. The ring is designed to be satisfied several required conditions for the beam ordering, such as a small betatron phase advance, a small magnetic error and a precise magnet alignment. The design phase advance per a period is less than 127 degree. The calculated closed orbit distortion and the stopband is less than 1 mm and 0.001 without correction, respectively.

MeV quasi‐mono‐energetic proton beam created by a combination of a laser‐plasma ion accelerator and synchronous rf cavity

MeV quasi‐mono‐energetic proton beam is produced by a combination of a synchronous radio frequency (rf) electric field and laser‐plasma ion acceleration. The experiment was carried out at the Kansai Photon Science Institute, JAEA, using the Ti:Sapphire laser system called J‐KAREN. The proton beam is emitted normal to the rear surface of the thin polyimide target of the thickness 7.5 μm irradiated at peak intensity of 4×1018 W/cm2. The energy spread is compressed from 100% to less than 11% at FWHM by the rf field. The focusing and defocusing effect of the transverse direction is also observed. These are also studied by a Monte Carlo simulation. The relation between the transverse focusing and the energy spectrum of the phase‐rotated beam is systematically shown by the simulation.

Experimental Study of Dispersion Control Utilizing both Magnetic and Electric Fields

An experiment to control dispersion of beams in one bending section has been carried out. This experiment is based on a theory that the dispersion of accumulated beams can be controlled, if they are bent by a cross field composed of magnetic and an electric fields. Suppression of the dispersion can ease a shear which affects the 3‐dimensionally ordered structure of the ultimate‐low‐temperature beams. In order to realize this scheme experimentally, we have manufactured a set of electrodes to create precise electric fields whose strength is 6.6 × 104 V/m for 24Mg+, 35keV beam. The electrodes have been inserted to the gap of dipole magnet. 3‐dimensional field calculation shows that the error of the electric fields is less than 0.1% within ± 5mm from the reference orbit. We also tested the effect of the electric field using a single set of bending elements. The result showed that the linear dispersion can be controlled or canceled by changing the ratio of magnetic and electric fields.