Y. Oda

Development of pellet injector system for large helical device

A multi-pellet injector system has been developed as a fundamental fueling tool for the Large Helical Device (LHD). Since a primary goal of the LHD project is demonstration of steady-state high-temperature plasmas, highly reliable and reproducible performance is prerequisite for the LHD pellet injector. The helium Gifford-McMahon (GM) cycle cryogenic refrigerator and conventional pipe-gun mechanism have been employed along with these requirements. Since the triple point temperature of hydrogen is lower than those of isotopes; deuterium and tritium, a careful and steady temperature control is necessary in pellet formation. The amount of propellant gas has been minimized by small reservoir to reduce heat load and inflow into the plasma vacuum chamber. Both robustness and flexibility have been realized by using the programmable logic controller combined with GUI on WindowsNT. The pellet injector has demonstrated continuous hundreds of injection without any fail and has been applied for the LHD experiment successfully.

Repetitive fueling pellet injection in large helical device

Abstract A repetitive pellet injector has been developed for investigation of fueling issues towards the steady-state operation in Large Helical Device (LHD). The goal of this approach is achievement of the plasma operation for longer than 1000 s. A principal technical element of the pellet injector is solidification of hydrogen and extrusion of a solid hydrogen rod through a cryogenic screw extruder cooled by Giffard–McMahon (GM) cryo-coolers. Continuous operation of more than 10 000 pellet launches at 10 Hz has been demonstrated. The reliability of pellet launch exceeds 99%. The pellet mass and velocity, the consumption of propellant gas and quality of pellets have been successfully tested to fit the experimental requirement in LHD.

Laser‐prearc railgun: Development for the application to a fuel pellet injector of a nuclear fusion reactor

The laser‐prearc railgun, that utilizes the phenomenon of laser‐induced arc formation, was constructed and tested with plastic pellet projectiles. We envision our railgun as especially well suited as a solid hydrogen pellet injector for magnetic confinement fusion. The system consisted of a gas gun for preacceleration of a pellet and a railgun for its primary acceleration. A Q‐switched ruby laser was used to induce electrical breakdown of propellant helium gas behind a dielectric pellet in the railgun. The present railgun was shown to accelerate a plastic pellet up to a velocity of 2.4 km/s.

Engineering Design of a Spallation Reaction-Based Neutron Generator for Boron Neutron Capture Therapy

An engineering design of an epithermal neutron generator for boron neutron capture therapy (BNCT) has been completed, which utilizes the spallation reaction by protons accelerated to 50 MeV. The critical issues for realization of the neutron generator are the mechanical structure of a target with cooling capability and its integrity under operating conditions with powers as high as 50 MeV × 300 μA. The integrity of a target structure design has been confirmed by thermal and stress analyses with a finite element method code ANSYS. Moreover, a target replacement strategy is also studied based on a radioactivity evaluation performed by the IRACM code system. In addition to the target structure design, the neutronics design has been optimized with the Monte Carlo code MCNPX. A high epithermal neutron flux of 1.8×109 cm−2.s−1 has been achieved at the aperture of the collimator, which allows a RBE dose of over 30 Gy-eq to be delivered to a brain tumor within 5.9 cm in phantom depth for a therapeutic time of 31 min.

Development of repetitive railgun pellet accelerator and steady-state solid hydrogen extruder

Development of a railgun pellet accelerator and a steady-state solid hydrogen extruder has been conducted. A railgun accelerator has been investigated for a high-speed repetitive pellet acceleration. The final objective is to develop a railgun system that can achieve a 5km/s speed-class repetitive (2Hz) pellet injection. Improvement in the acceleration efficiency showed a pellet velocity of more than 2km/s using augment rails and a ceramic insulator applied to a 1m-long railgun. The other investigation focused on the development of a steady-state solid hydrogen extruder for continuous pellet injection. Screw-driven extruding system has been chosen to extrude the solid hydrogen filament continuously. Theoretical considerations suggest that temperature control of the system is important in future research.

Railgun pellet injection system for fusion experimental devices

Abstract A railgun pellet injection system has been developed for fusion experimental devices. Using a low electric energy railgun system, hydrogen pellet acceleration tests have been conducted to investigate the application of the electromagnetic railgun system for high speed pellet injection into fusion plasmas. In the system, the pellet is pre-accelerated before railgun acceleration. A laser beam is used to induce plasma armature. The ignited plasma armature is accelerated by an electromagnetic force that accelerates the pellet. Under the same operational conditions, the energy conversion coefficient for the dummy pellets was around 0.4%, while that for the hydrogen pellets was around 0.12%. The highest hydrogen pellet velocity was 1.4 km s−1 using a 1 m long railgun. Based on the findings, it is estimated that the hydrogen pellet has the potential to be accelerated to 5 km s−1 using a 3 m long railgun.

Refueling for Steady-State Plasma by Repetitive Pellet Injection in Large Helical Device

A repetitive pellet injector has been developed for investigation of refueling issues towards the steady-state operation in Large Helical Device (LHD). Continuous operation of more than 10000 pellet launching at 10 Hz has been demonstrated. The maximum repeating rate is 11 Hz. No technical constraint for longer operation has been found. The reliability of pellet launch has exceeded 99.9%. The initial application to the NBI-heated plasmas has been successful in the last experimental campaign of LHD. Although the pulse length is limited by the operational constraint of NBI, the plasma with a density of 8 × 1019 m-3 has been sustained for 2 s by the pellet injection at 10 Hz. A prospect for the future experiment is discussed on the basis of the initial result.

Development of Dust Removal System for Fusion Reactor

In the future fusion reactor, dust control may become more important for safety, than for existing fusion facilities. Some estimations show more than hundreds kg/yr dust will be generated from the plasma facing materials in the vacuum vessel. If we consider continuously operating plant, dust should be monitored and removed during the operation time. Optical monitoring methods and electrostatic removal methods are useful approaches to accomplish this. An investigation of the development of the dust removal system for a fusion reactor is reported in this paper.

Development of Railgun Pellet Injector Using a Laser-Induced Plasma Armature: Results of Dummy Pellet Acceleration Tests

Using the low electric energy railgun system, dummy pellet acceleration tests have been conducted to investigate the application of the electromagnetic railgun system for high-speed pellet injection into fusion plasmas. The primary objective of the development is to improve the pellet acceleration efficiency and durability of the rail materials. In the system, the pellet is pre-accelerated before railgun acceleration. A laser beam is used to induce plasma armature. The ignited plasma armature is accelerated by an electromagnetic force that accelerates the pellet. As low electric energy was used, rail materials were used for multiple operations. Tungsten-alloy rail provided longer durability and slightly higher energy conversion coefficient than copper rail. The energy conversion coefficient was from 0.3 to 0.5% using a plastic insulator. A ceramic insulator improved the energy conversion coefficient by 80%. The highest pellet velocity was 1.7km/s using wooden pellets accelerated by 1m-long railgun. Based ...

DEVELOPMENT OF RAILGUN SYSTEM FOR HIGH-SPEED PELLET INJECTION

The application of the electromagnetic railgun system for high-speed pellet injection has been investigated. A pulse laser beam is employed to induce an initial plasma armature between rails. This unique feature provides the reduction of the supplied voltage to the rails, in order to avoid any unnecessary breakdown between the rails and to reduce the erosion of the rails. The ignited plasma armature is accelerated by an electromagnetic (Lorentz) force, that accelerates the pellet to high velocity. A series of experiments using dummy pellets have been conducted to examine the system, along with theoretical study. This report presents the current results of our research and development on the railgun system.