Tohru Matoba

Improvement of energy confinement time by continuous pellet fuelling in beam-heated Doublet III limiter discharges

A centrifuge injector that repetitively fires 1.3 mm deuterium pellets (1 torr⋅L per pellet) at a rate of 32 pellets per second was used to build up and maintain a Doublet III 2.4 MW neutral-beam-heated limiter discharge at a line-averaged density of 1 × 1014 cm−3. When compared to a conventional gas-fuelled plasma at similar density, the pellet-fuelled plasma was characterized by a factor-of-three reduction in edge neutral density and limiter recycling, a centrally peaked profile, a 70% increase in global energy confinement, and a tenfold increase in the fusion reaction rate.

Confinement and fueling studies during additional heating phase in the JFT-2M tokamak

Increments of peripheral hydrogen/deuterium neutral gas pressures (PH2/PD2) during the additional heating phase (neutral beam and RF heatings) have been observed in a D2 gas-puff fueled JFT-2M tokamak with H2-absorbed graphite limiters/divertor plates. In the beam heating phase, a large increment of PH2 raises the plasma density 2 times or more without the degrading energy confinement time. The D2 gas-puff valve is closed in this phase. This improvement is interpreted as being due to “wall fueling”, i.e., fueling by desorbed hydrogen from the graphite wall by energetic particles. The first observation of the H-mode in the INTOR-type stubby open divertor with a short divertor channel (1–8 cm) at the high density regime (4–7 × 1013 cm−3) enhanced by the wall fueling is presented. The improved energy confinement time is comparable to or higher than that of ohmically heated discharges.

Transition from the L-mode to the H-mode by electron cyclotron heating of a tokamak edge plasma

Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.

Analytical Approximations in the Theory of Relativistic Thomson Scattering for High Temperature Fusion Plasma

The light scattering spectrum from a high temperature plasma is calculated for the second order in v/c (c=light speed) and the second order approximate formula is obtained from the calculation. The blue side shift of the peak derived from the second order approximate formula agrees well with that of the full relativistic spectrum for an electron temperature of several tens of kilo electron-volts and less. The second order apparent temperature and density are deduced with an accuracy of better than 8 percent in the range up to 20 keV and 100 keV for scattering angles θ=90° and 50°, respectively, by application of the least-square procedure.

Improvement of plasma parameters by titanium gettering in the JFT-2 tokamak

Abstract Oxygen impurity has been reduced to about 1–2% of the electron density by titanium gettering onto the torus wall (~ 1 2 ) and the limiter. Radiation loss and effective ionic charge were decreased by a factor of ~2. As a result of reduced impurity influx, broader electron temperature profiles have been obtained. The energy confinement time and the scaling factor of the maximum electron density ( n e R/B t ) were improved by a factor of 1.6 with the titanium gettering. Limits on the density increase were investigated in connection with the radiation power. Mechanism limiting the density maximum is discussed.

Development of 180 GHz heterodyne radiometer for electron cyclotron emission measurements in JT‐60U

A 12‐channel heterodyne radiometer system has been developed to obtain electron temperature with high time and spatial resolution from the electron cyclotron emission (ECE) in JT‐60U. Resolutions of a single‐sideband‐type receiver are (1) Δt∼1 μs and (2) Δf=0.5 GHz. The second harmonic extraordinary mode of ECE is utilized for this system and the frequency band is 176–188 GHz. The noise level of the system is determined by unavoidable photon fluctuations, and the signal‐to‐noise ratio was measured to be about 50 with a video bandwidth of 100 kHz for a signal temperature above ∼200 eV. Details of the design, the obtained performance, and typical measurements are described.

Twenty‐channel grating polychromator diagnostic system for electron cyclotron emission measurement in JT‐60

A twenty‐channel grating polychromator diagnostic system has been built to measure the temporal evolution of local electron temperatures in JT‐60. A cross Czerny–Turner diffraction grating spectrometer is utilized for the measurement of second‐harmonic electron cyclotron emission with extraordinary modes in the range 85–300 GHz, in which a grating plate grooved on both faces with different grating periods is applied effectively to yield a wide coverage for the toroidal fields. The grating angle is automatically set up by control of a stepping motor according to the relation of the grating equation. The diffracted light is detected by 20 indium‐antimonide hot‐electron bolometers cooled at 4.3 K in a modified Solvay cycle cryogenic refrigerator. A typical resolving power of the instrument was measured to be λ/Δλ∼130, providing a spatial resolution of 2.3 cm at the plasma center. The transmission line over ∼38 m long is composed of oversized S‐band waveguides. The total transmissivity of this system is estim...

Diode-pumped 250-W zigzag slab Nd:YAG oscillator–amplifier system

A laser-diode-pumped zigzag slab Nd:YAG master oscillator power amplifier (MOPA) system featuring high pulse energy and high average power was developed for pumping of an ultrashort-pulse laser system. The MOPA system consists of an oscillator, a preamplifier, two postamplifiers, and image-relay telescopes. The postamplifiers have an angle-multiplexed ring-type double-pass configuration. A pulse energy of 1.26 J and an average power of 251 W were obtained at a repetition rate of 200 Hz. The frequency-doubled power when a LiB(3)O(5) crystal was used was 105 W at a repetition rate of 170 Hz. The intensity profiles of the fundamental and the second harmonic are nearly top-hat shaped and are suitable for pumping.

CO2 laser interferometer for electron density measurement in JT‐60U tokamak

A tangential chord CO2 laser interferometer for electron density measurement in the JT‐60U tokamak is described. To compensate for a phase shift due to mirror vibrations, an IR‐HeNe laser interferometer is introduced. Probe beams of the CO2 and the IR‐HeNe lasers travel in a similar form for a distance of 100 m by utilizing relay optics. The effective phase resolution of the system is estimated to be 1/68 fringe, which corresponds to a line integrated density of 3.1×1018 m−2. An ultra‐accurate phase comparator which has a 1/104 fringe resolution is proposed to improve the capability of present and future interferometers.

H-mode phenomena during ICRF heating on JFT-2M

Significant improvement of energy confinement has been observed on JFT-2M during ICRF heating. This improvement is preceded by a sudden drop in the Hα/Dα emission and a successive increase in stored plasma energy, electron density and radiation loss. This is believed to be the same phenomenon as the H-mode transition observed in ASDEX, and in PDX divertor experiments with neutral beam injection. However, in JFT-2M, this transition is observed both in limiter discharges and in open divertor configurations.