Kotaro Kondo

Fast ignition realization experiment with high-contrast kilo-joule peta-watt LFEX laser and strong external magnetic field

A petawatt laser for fast ignition experiments (LFEX) laser system [N. Miyanaga et al., J. Phys. IV France 133, 81 (2006)], which is currently capable of delivering 2u2009kJ in a 1.5 ps pulse using 4 laser beams, has been constructed beside the GEKKO-XII laser facility for demonstrating efficient fast heating of a dense plasma up to the ignition temperature under the auspices of the Fast Ignition Realization EXperiment (FIREX) project [H. Azechi et al., Nucl. Fusion 49, 104024 (2009)]. In the FIREX experiment, a cone is attached to a spherical target containing a fuel to prevent a corona plasma from entering the path of the intense heating LFEX laser beams. The LFEX laser beams are focused at the tip of the cone to generate a relativistic electron beam (REB), which heats a dense fuel core generated by compression of a spherical deuterized plastic target induced by the GEKKO-XII laser beams. Recent studies indicate that the current heating efficiency is only 0.4%, and three requirements to achieve higher efficiency of the fast ignition (FI) scheme with the current GEKKO and LFEX systems have been identified: (i) reduction of the high energy tail of the REB; (ii) formation of a fuel core with high areal density using a limited number (twelve) of GEKKO-XII laser beams as well as a limited energy (4u2009kJ of 0.53-μm light in a 1.3u2009ns pulse); (iii) guiding and focusing of the REB to the fuel core. Laser–plasma interactions in a long-scale plasma generate electrons that are too energetic to efficiently heat the fuel core. Three actions were taken to meet the first requirement. First, the intensity contrast of the foot pulses to the main pulses of the LFEX was improved to >109. Second, a 5.5-mm-long cone was introduced to reduce pre-heating of the inner cone wall caused by illumination of the unconverted 1.053-μm light of implosion beam (GEKKO-XII). Third, the outside of the cone wall was coated with a 40-μm plastic layer to protect it from the pressure caused by imploding plasma. Following the above improvements, conversion of 13% of the LFEX laser energy to a low energy portion of the REB, whose slope temperature is 0.7u2009MeV, which is close to the ponderomotive scaling value, was achieved. To meet the second requirement, the compression of a solid spherical ball with a diameter of 200-μm to form a dense core with an areal density of ∼0.07u2009g/cm2 was induced by a laser-driven spherically converging shock wave. Converging shock compression is more hydrodynamically stable compared to shell implosion, while a hot spot cannot be generated with a solid ball target. Solid ball compression is preferable also for compressing an external magnetic field to collimate the REB to the fuel core, due to the relatively small magnetic Reynolds number of the shock compressed region. To meet the third requirement, we have generated a strong kilo-tesla magnetic field using a laser-driven capacitor-coil target. The strength and time history of the magnetic field were characterized with proton deflectometry and a B-dot probe. Guidance of the REB using a 0.6-kT field in a planar geometry has been demonstrated at the LULI 2000 laser facility. In a realistic FI scenario, a magnetic mirror is formed between the REB generation point and the fuel core. The effects of the strong magnetic field on not only REB transport but also plasma compression were studied using numerical simulations. According to the transport calculations, the heating efficiency can be improved from 0.4% to 4% by the GEKKO and LFEX laser system by meeting the three requirements described above. This efficiency is scalable to 10% of the heating efficiency by increasing the areal density of the fuel core.

Compact pulse power device for generation of one-dimensional strong shock waves

A compact pulse power device is proposed for generation of one-dimensional strong shock waves, and preliminary experimental results are reported. The cross section of the discharge area is decreased by tapered electrodes to prevent the decay of the shock wave. This new device drives a quasi-one-dimensional strong shock with the front speed 45km∕s in the acrylic guiding tube filled with Xe gas. When the front speed is more than the critical speed Drad, an interesting structure is observed at the shock front, which indicates that the radiative energy transport affects the shock structure.

Laser ion source with solenoid field

Pulse length extension of highly charged ion beam generated from a laser ion source is experimentally demonstrated. The laser ion source (LIS) has been recognized as one of the most powerful heavy ion source. However, it was difficult to provide long pulse beams. By applying a solenoid field (90u2009mT, 1u2009m) at plasma drifting section, a pulse length of carbon ion beam reached 3.2u2009μs which was 4.4 times longer than the width from a conventional LIS. The particle number of carbon ions accelerated by a radio frequency quadrupole linear accelerator was 1.2u2009×u20091011, which was provided by a single 1u2009J Nd-YAG laser shot. A laser ion source with solenoid field could be used in a next generation heavy ion accelerator.

Experimental and numerical study of dose distribution around a syringe needle-type proton-induced X-ray source for radiotherapy

A syringe needle-type proton-induced monochromatic X-ray source was proposed to solve the issue that could occur in practical brachytherapy, such as loss of seed sources and radiation exposure to surgical staff. This paper discusses comparison between experimental results and a Monte Carlo numerical simulation of the dose distribution around the needle. Some simulation results for different source designs are presented as a first step of the design optimization.

Production of quasimonochromatic X-ray microbeams using MeV-protons and a polycapillary X-ray half lens

In this paper, we have proposed monochromatic X-ray microbeams, produced by a proton-induced X-ray technique and a polycapillary X-ray half lens, as a tool for micro-X-ray fluorescence (XRF) analysis. A 30 μm thick planar Cu target was irradiated by a 2.5 MeV proton beam to produce Cu Kα X-rays (8.0 keV), and a polycapillary X-ray half lens was utilized to focus the X-rays emitted behind the Cu target. The focal spot size of the focused X-ray beam was 250 μm at full width at half maximum, which was verified using a knife-edge scanning method. The output focal distance and the depth of focus of the optics were measured to be 47 mm and 1 mm, respectively. A square grid pattern of Co thin films, formed on a thick Cu substrate by thermal evaporation, was used as a test sample for evaluation of the analytical performance of the micro-XRF setup. Two-dimensional mapping of the Co distribution on the Cu substrate was successful, and the spatial resolution was consistent with the beam spot size. For this Co layer, a minimum detection limit of 2.3 ng was achieved.

Relaxation layer in electro-magnetically driven strong shocks

A study on electro-magnetically driven shock in laboratory experiments is reported. The shock waves are driven by a compact pulse device at 40 km/s into a high-Z gas with 1 μg/cm3. The pulse power device with tapered electrodes can generate a quasi steady and one-dimensional shock. Ion-electron and ionization relaxation processes, based on a steady and 1-Dimensional shock condition, are calculated. Comparison of the experimental result to calculation one indicates that the shocks produced by the pulse power device have potential to observe ion-electron and ionization relaxation layer with an appropriate spatial scale.

Velocity evolution of electro-magnetically driven shock wave for beam-dissociated hydrogen interaction experiment

We present the velocity measurements in electro-magnetic shock tube for beam interaction experiment by three methods; laser refraction, photodiode for self-emission, and high speed framing camera. The laser refraction showed that the average shock velocity was 6.7 km/s when the initial pressure was 1000 Pa and the initial charging voltage was 16 kV. The self-emissions from piston discharge plasma were measured by photodiodes and by high speed framing camera. The measurements showed that the duration between shock and piston was up to 8 microseconds with a 400-mm propagation in the shock tube, which is enough time as dissociation target for beam interaction experiment.The complementary velocity measurement is significant for understanding the electro-magnetically driven shock physics.

Direct observation of dose distribution around a syringe-needle type proton-induced X-ray source using liquid scintillator and a CCD camera

This paper discusses a method to directly observe the dose distribution around a syringe-needle-type proton-induced monochromatic X-ray source proposed to avoid the risk that could exist in conventional brachytherapy. Images taken by using liquid scintillator and a high sensitivity CCD camera are presented and the dose distribution is qualitatively evaluated by comparing it with a Geant4 Monte Carlo simulation result.

Selective internal radiotherapy using proton-induced monochromatic X-rays and cancer-targeting nanoparticle sensitizers

In this paper, we propose a highly-selective radiotherapy based on monochromatic X-rays and cancer-targeting gold nanoparticle (GNP) sensitizer. In order to deliver the low-energy monochromatic X-rays which selectively ionize the Au L-shell into the cancerous tissue deep inside the patient’s body, we employ a syringe-needle type X-ray source driven by an MeV proton beam. From a simple numerical evaluation, we found that optimization of the primary X-ray energy was essential to enhance the dose around the nanoparticle. In order to confirm the above idea qualitatively, we performed a simulation experiment in the atmosphere, where 100 nm Au foils were used instead of the GNPs. The experimental result showed that the dose around the Au foils was much higher than that at positions away from the foils, owing to short-range secondary electrons from the foils.

Digital subtraction cineangiography using proton-induced quasi-monochromatic pulsed X-rays

Two different kinds of metallic plates on a rotating disk target were irradiated with a MeV proton beam, and quasi-monochromatic pulsed X-rays with different energies around the absorption edge of the contrast medium were alternately produced. By using these dual-energy X-rays and a high-sensitivity X-ray movie camera, we took a motion picture of the transmission image of a periodically moving phantom, which simulated a rat heart as a test animal. We found that the enhanced movie imaging of the contrast agent is available by subtraction between adjacent picture frames.