Yoshio Wada

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.

Radio-frequency quadrupole trap as a tool for experimental beam physics

In recent papers [Okamoto, Hiroshima University Preprint HUBP-01/98 (1998); Okamoto and Tanaka, Nucl. Instr. Meth. A 437 (1999) 178], we proposed a novel experimental approach to investigate various collective effects in space-charge-dominated beams. It was demonstrated that either a radio-frequency quadrupole trap or a solenoidal trap could reproduce non-linear processes equivalent to those in a beam transport channel. In the present work, we study the former trap configuration in more detail, specifying a typical parameter range for beam-physics applications. A three-dimensional Maxwell-equation solver is employed to show the basic characteristics of the plasma confinement field. After deriving several useful formulae, we discuss possible trap experiments that greatly deepen our current understandings of collective beam instabilities including coherent resonances, halo formation, etc.

Optimum target position of ion production in ultrashort-pulse laser interaction with foil targets under the existence of prepulses

It was found that ion production during ultrashort-pulse laser interaction with foil targets is most efficient when the target is positioned behind the waist of the laser, and if the main laser pulse is accompanied by prepulses that evaporate the target. The maximum proton energy was 1.2 MeV when the target was positioned 0.3 mm behind the waist, while it was 0.25 MeV at the waist. A model is proposed in which a prepulse ablates the surface of the target. The resultant neutral gas guides the head of the main pulse caused by the Kerr effect while it is ionized, so the ion generation is more efficient than that without a prepulse.

Neutral Beam Generation by Laser Irradiation of Thin Foils

We irradiated thin (< 10 μm) plastic and metal foils by a 1 TW, 50 fs, T3 laser. The foil surface was located at π/4 to the laser injection. Particle beams were obtained at both sides of the foil with respect to the laser injection. The beam at the rear side, flowing to the direction perpendicular to the foil surface, was insensitive to the magnetic field. Measurements using the track detector CR39 tell that it has small divergence (∼ 200 mrad) and suggest that their particles could have high energies (up to 1 MeV). We therefore conclude that these particles make a neutral beam. To the contrary, ions were obtained at the same side of the foil with respect to the laser injection. They also have energies up to ∼ 1 MeV, but larger angler distribution.

Interpersonal synchrony-based dynamic stabilization of the gait rhythm between human and virtual robot — Clinical application to festinating gait of Parkinson's disease patient

Recently, robotics attracts attention in field of not only industry and production but also assistive technology and rehabilitation method. It is required how the system generate assistive functions in interaction with the user, while lots of one-sided help forms are in previous researches. From these backgrounds, we have focused on cooperative gait between humans as interpersonal synchronization, and modeled the mechanism of footstep rhythm synchronization. Moreover, we developed an interpersonal synchrony emulation robot named Walk-Mate, which was biped virtual robot synchronizing with the users footstep rhythm via walking together. In this study, we evaluated the effect of Walk-Mate in stabilizing gait with Parkinsons disease (PD), which previously displayed disturbances in rhythm formation and festinating gait (accelerating footsteps). The results showed that the festinating gait, evaluated by stride time reduction rate, significantly stabilized and accelerated less with Walk-Mate compared to unassisted walking. Additionally, carry-over effects were significantly observed. After termination of the auditory stimulation, the gait remained stabilized. These suggested that gait with PD was dynamically stabilized by the interpersonal synchrony process between timing of humans gait and of external auditory cues. In this paper, we showed significant improvement for the festinating gait in the PD patients.

Ion and neutral beam generation by 1 TW, 50 fs laser irradiation of thin foils

Thin ( 45 ◦ ) divergence. The components of the neutrals and ions were contaminants of the foil surface. To the contrary, mainly ions were observed on the laser-illuminated side, which were components of the target foils. The most energetic particles were protons on both sides, whose energy was about 550 keV.

Laser driven high energy ion generation for a compact cancer therapy accelerator

We started a project to develop a very compact accelerator for cancer therapy. To reduce the size of the system, we adopted a laser plasma ion source using a compact ultra-high intensity laser. We have performed ion generation experiments in which the laser parameters were as follows: The wave length and the pulse duration were 800 nm and 50 fs, respectively. Peak power was 4 - 5TW. The laser pulse with normal incidence angle to the target was focused onto the target with 15 μm diameter giving power density of 3 - 4x1018W/cm2. The thin foil metals (Ti, Al) and plastics (polypropylene, polyethylene) with the thicknesses of 4 - 100 μm were used for targets. We found that the angular distribution of ions with an energy of ~0.1 MeV had a significant peak in the backward and forward in respect to the laser incidence direction.

Ion Generation via Interaction between Intense Ultra‐short Laser Pulse and Solid Target for Application to Cancer Therapy

We started a project to develop a very compact accelerator for cancer therapy. To reduce the size of the system, we adopted a laser plasma ion source using a compact ultra‐high intensity laser. We have performed ion generation experiments in which laser parameters were as follows: The wave length and the pulse duration were 800nm and 50fs, respectively. Peak power was 4–5TW. The laser pulse with normal incidence angle to the target was focused onto the target with 15μm diameter giving power density of 3–4×1018W/cm2. Target foils were metals (Ti, Al) and plastics (polypropylene, polyethylene) with the thicknesses of 4–100μm. We found that the angular distribution of ions with an energy of ∼0.1MeV had a significant peak in the backward and forward in respect to the laser incidence direction.

POSSIBILITY OF A SMALL NEUTRON SOURCE BY LASER-PLASMA INTERACTION

Cold neutrons are good probe for nano meter scale objects. While some huge facilities as spallation neutron sources are under construction, small local (satellite) neutron sources will help to test new ideas on research. Such small ones are considered to be complementary parts of the huge facilities. Because the threshold energy of the d-d reactions is fairly low, irradiation of deuterated polyethylene (CD2) target by laser induced energetic deuterons may generate neutrons. A preliminary experiment performed with 1TW laser irradiating CD2 foil just before another thick CD2 target is described.

Ion Production Enhancement by Rear-Focusing and Prepulse in Ultrashort-Pulse Laser Interaction with Foil Targets

It was found that the maximum ion energy produced in ultrashort‐pulse laser interaction with foil targets is enhanced by a factor of five when the target is positioned before the focal point of the laser and the laser is accompanied by prepulses. A model is proposed in which the prepulse ablates the surface of the target. The resultant neutral gas guides the head of the main pulse by the Kerr effect. This finding opens up the possibility of MeV proton production by a TW‐class laser.