Masahiro Ikegami

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.

Laser-driven proton acceleration and plasma diagnostics with J-KAREN laser

We describe results of experiments on laser-driven proton acceleration and corresponding laser-plasma diagnostics performed with the multi-10-TW J-KAREN laser. The laser consists of a high-pulse-energy oscillator, saturable absorber, stretcher, Optical Parametric Chirped Pulse Amplifier (OPCPA), two 4-pass Ti:Sapphire amplifiers, and compressor. The final amplifier is cryogenically cooled down to 100 K to avoid thermal lensing. The laser provides ~30 fs, ~ 1 J, high-contrast pulses with the nanosecond contrast better than 1010. The peak intensity is 1020 W/cm2 with the 3- 4 μm focal spot. Using few-μm tape and sub-μm ribbon targets we produced protons with the energies up to 4 MeV. The tape target and repetitive laser operation allowed achieving proton acceleration at 1 Hz. We found significant differences in stability and angular distribution of proton beam in high-contrast and normal-contrast modes. The plasma diagnostics included interferometry and measurement of the target reflectivity. The latter provides convenient diagnostics of the laser contrast in the ion acceleration, harmonics generation, and other laser - solid target interaction experiments.

Feedback Damping of a Coherent Instability at Small-Laser Equipped Storage Ring, S-LSR

A beam test for the feedback damping of a coherent instability at small-laser equipped storage ring (S-LSR) was carried out. The instability caused beam loss, which limited the stored beam current to be about 600 µA during beam stacking with electron cooling. A vertical feedback damping system (VFS) using an electro-static beam position monitor (ESBPM) and a vertical kicker was applied to suppress this beam instability. The VFS realized not only suppressing the vertical coherent instability, but also revealing the characteristics of the feedback damping.

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...

Design and measurement of the S-LSR quadrupole magnet considering the influence of a neighboring field clamp

The design and the magnetic field measurement of the quadrupole magnet for the compact ion ring, S-LSR, are presented. Because the S-LSR is a compact ring accelerator, whose components take up most of the space in the arc sections. Therefore, we have to optimize the integrated field gradient in the quadrupole magnetic field taking account of the effect of the neighboring field clamp. Under these conditions plus requirements from accelerator parameters, the design of the S-LSR quadrupole magnet was optimized by means of 2D and 3D modeling calculations. A magnet field measurement by a Hall probe is carried out together with the field clamp and S-LSR bending magnet. Both results of the calculation and the measurement present the almost same result as that for the effect of the field clamp.

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.

Three-Dimensional Laser Cooling of Ion Beams Using a Wien Filter

Laser cooling of ion beams generally operates only upon the longitudinal motion of an ion beam. In order to extend this powerful dissipative force to the transverse directions, we consider introducing a Wien filter in a cooing straight section. It is theoretically shown that longitudinal cooling within a Wien filter naturally develops a transverse (mostly horizontal) cooling effect as well. Molecular dynamics simulations demonstrate that this method enables us to reach an ultralow-temperature state of a stored ion beam.