Fusao Saito

Design and tests of 168-GHz, 500-kW gyrotrons and power supply system

Abstract A practical multi-gyrotron oscillation system, using collector-potential depression, composed of six gyrotron tubes and 3 U of power supplies, was designed, fabricated and tested. This system was designed to generate power levels of 3 MW for pulse duration of 1-s at ≈168 GHz for electron cyclotron heating of LHD at the National Institute for Fusion Science. The all-solid-state power supply unit can drive a maximum of three gyrotrons by equipping the collector power supply with three pairs of the anode and body power supplies. The gyrotrons used a TE31,8,1-mode interaction cavity. A single-stage depressed collector with sweeping coils was employed to increase system efficiency and reduce the heat flux to the collector surface. An internal converter produced a flattened Gaussian profile at a single-disk silicon-nitride window. The output mode was reconverted into the HE11 mode by an MOU. We reconstructed a main circuit of the power supply unit because of stray capacitors in the actual circuit. There were some differences between the designed and measured output wave profiles. The tubes were tested for 1-s pulse with power levels of 500 kW; system efficiencies were 30% at the peak and 28% at the average and temperatures of the windows were ≈200°C.

A 250 MVA motor generator for large-scale plasma heating in the large helical device

Abstract For pulsed mode plasma experiments in the large helical device (LHD), high pulsive power has to be fed to the heating devices to produce currentless high-temperature and high-beta plasmas. The peak energizing power for these heating devices exceeds 200 MVA, and the energy consumed is 1000 MJ per 10 s pulse. To feed such enormous power, a motor generator (MG) system has been constructed and tested successfully. The MG is a vertical-shaft a.c. synchronous generator with nominal capacity 250 MVA. The maximum ejection energy is 1400 MJ per pulse at a repetition time of 5 min. The operational system is composed so as to deliver the power to various different loads, and the sequence is controlled voluntarily by operational computers near the load devices. The operational loss of the MG is reduced by using a computer network, so that an appropriate rotating speed and start time of acceleration of the MG are set automatically at every shot.

Development of plasma facing component for LHCD antenna

Abstract A mock-up module of an antenna tip made of carbon fiber composite (CFC) was fabricated for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. The CFC surface was coated with a thin Ti layer and was plated with Cu. The mock-up module has three waveguides and is designed to simulate the front tip of the 8 GHz LHCD antenna. The module has an active water cooling channel and the length is 200 mm, the mouth dimension is 19.5×50 mm 2 . The mock-up module was tested with heat fluxes up to 20 MW m −2 due to electron beam irradiation. No damage of the waveguide mouth was observed up to the heat flux of 3.2 MW m −2 for 2 min, though a maximum temperature of 762°C was measured at the septum plate. The obtained heat load of 3.2 MW m −2 is about 10 times higher than the steady-state surface-averaged heat load of the first wall required in the ITER design.

Stabilization of power system for large-scaled experimental fusion facility

Abstract In the early experiments of the LHD, the harmful third harmonic currents had been observed in the power line of the NIFS substation, when the fly-wheel motor generator (MG) for the neutral beam injection (NBI) reaccelerates between the plasma productions. The variable harmonic currents, which were estimated to be 1±6 and 1±12 ns, were induced through a Scherbius device. They extremely resonated to the circuit impedance of the power line. To stabilize electrical power in the site, following modifications were conducted: (1) reduction of the harmonic current from the Scherbius device; (2) reconnection of the power supplies of the SC coils and heating devices to suitable power feeders; (3) compensation of lagging reactive power of the MG. As a result, harmful harmonic currents were suppressed.

Beam acceleration experiment with developed 10 MW class high-precision power supply for accelerator electromagnets

A 10 MW-class high-precision power supply was developed for the electromagnets of a large proton accelerator (J-PARC). This power supply consists of an advanced self-commutated current-source type converter (ACSC) using IEGTs and a hybrid filter. The current ripple was less than 1×10−6, and the tracking error was less than ±0.5×104. With this power supply, the proton beam was successfully accelerated to 30 GeV, which is the target energy of the first stage of J-PARC.

A power supply system for a 125 keV/250 keV neutral beam injector test stand based on negative ion sources

Abstract A design for a power supply system for the neutral beam injection (NBI) test stand of the large helical device (LHD) is presented. The acceleration power supply of −250 kV, 50 A is composed of three power supplies connected in series to deliver two intermediate outputs. It has a −40 kV power supply, a −90 kV power supply and a −120 kV power supply. A beamline and the major part of the power supply system for the NBI test stand were constructed in 1992. The −120 kV power supply was moved from Higashiyama to Toki in 1994. Another power supply of −40 kV will be added for −250 kV acceleration in the future. A brief outline of the control system of the NBI test stand is also presented.