T. Hatae

YAG laser Thomson scattering diagnostic on the JT-60U

A YAG laser Thomson scattering system has been developed to measure the time evolution of electron temperature (Te) and density (ne) profiles in the JT-60U. Presently this system provides Te and ne periodically every 100 ms (20 ms at the minimum) at 15 spatial channels. A six spectrum channel polychromator, which is a successive interference filter type, was designed to measure a wide Te range (20 eV–20 keV) by using five channels. The sixth channel is used for calibration by Rayleigh scattering. However, we also used the Rayleigh channel to measure Te and ne since the stray light level was very low. As a result, we were able to measure temperatures below 20 eV. The temperature of the silicon avalanche photodiode (APD) is stabilized by a Peltier element so that the gain is kept constant within ±0.5%. The stable performance of the APD enables accurate Te and ne measurement. During plasma operation, the electron temperature is measured over a wide range from the low temperature plasmas in the runaway plasma...

Applications of phase conjugate mirror to Thomson scattering diagnostics (invited)

A high performance phase conjugate mirror based on stimulated Brillouin scattering (SBS-PCM) has been applied to the Thomson scattering system in the JT-60U tokamak for the first time in order to improve the measurement performance. A SBS-PCM realized a high reflectivity of 95% at a high input power of 145W (2.9J, 50Hz). Using the SBS-PCM, two methods have been developed to increase the intensity of scattered light. For the first method, we have developed a new optical design to provide a double-pass scattering method with the SBS-PCM. A laser beam passing through the plasma is reflected by the SBS-PCM. The reflected beam passes the plasma again along the same path by means of the phase conjugation of the optically nonlinear stimulated Brillouin scattering process. The double-pass Thomson scattering method using the SBS-PCM has demonstrated an increase of the scattered light by a factor of 1.6 compared with the single-pass scattering method in JT-60U. A multipass Thomson scattering method in which the las...

Pedestal characteristics and extended high-βp ELMy H-mode regime in JT-60U

With the high-field-side multiple pellet injection into high-triangularity high-βp type I ELMy H-mode discharges, JT-60U has extended the density range with a favourable integrated performance. The pellet injected discharge has a high pedestal pressure compared with the gas fuelled discharges, and the pedestal temperature does not decrease even at high pedestal density. At high triangularity, the pedestal βp and the edge α-parameter increase with increasing total βp. Possible linkage among pedestal and core parameters has been proposed based on a variety of JT-60U experiments. Grassy ELMy discharges have been achieved at the low safety factor regime q95<4 for the first time in JT-60U with high triangularity (~0.6). The full non-inductive operation has been demonstrated with grassy ELMs.

Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27 μm for electron density measurement on large tokamaks

A new CO2 laser interferometer has been developed to measure the electron density of tokamak plasmas. Two different wavelength oscillators of 10.6 and 9.27 μm are utilized for simultaneous measurement of the density component and the optical path length change. A new technique using a common frequency shifter for two color lasers improves the stability of the system by a complete matching of both beat frequencies. The system provides advantages for practical problems of large tokmaks related to window darkening and large mechanical vibrations of reflection mirrors and offers improved laser beam monitoring and a simplified optical layout by using closer wavelengths. The electron density of JT‐60U is successfully measured for a plasma current of up to 3 MA. The density behavior during a fast major disruption is also diagnosed without a fringe loss. An effective density resolution is observed to be 2×1019 m−2, which corresponds to about 1/10 of a fringe. A very high resolution (1/104 fringe) phase comparator...

Attainment of high fusion reactivity under high bootstrap current fraction in JT-60U

The fusion performance of high βp plasmas with significant D-D neutron production up to (5.6 ± 0.6) × 1016 n/s, has been investigated. The attainment of a high fusion triple product, nd(0) τEtotalTi(0), up to (1.1 ± 0.3) × 1021 m-3.s.keV is confirmed by both time dependent and time slice analysis. The highly enhanced confinement of up to 3.6 times ITER-89P scaling is dominated by the improved thermal confinement, and one half of the D-D neutrons come from the thermal reactions. The highest equivalent QDT of approximately 0.56 was obtained. The ratio QDT/QDD is in the range 133-151. More importantly, a high bootstrap current of up to approximately 1 MA (~50% of the total current) was obtained simultaneously with the high fusion performance. These results are quite encouraging as far as the realization of a steady state tokamak reactor is concerned

Heating and current drive by electron cyclotron waves in JT-60U

The results of studies on heating and current drive by the electron cyclotron (EC) waves in JT-60U are presented. An electron temperature of up to 26 keV, as measured by ECE diagnostics, was achieved by injecting EC waves into the centre of a reversed shear plasma produced by the lower hybrid (LH) waves. The electron temperature Te exceeds 24 keV in the region ρ < 0.3, where ρ is the normalized minor radius. The EC driven current profile was measured at high Te up to 21 keV without using LH waves, and was found to be spatially localized. The ECCD (current drive) efficiency ηCD increased with Te and reached 0.42 × 1019 A W−1 m−2 at Te = 21 keV. The dependence of normalized CD efficiency on deposition location was also studied to optimize the CD efficiency, since the trapped particle effect, which depends strongly on deposition location, is expected to reduce ζ. The effect was detected from a significant decrease in ζ in the lower magnetic field deposition, which is consistent with a linearized Fokker–Planck calculation. The measured CD efficiency ζ increased with electron density ne for the same Te, which showed a stronger dependence on ne compared with the linear calculation. Further off-axis ECCD profile at about half of the minor radius was measured, showing fairly good agreement with linear calculation.

Two-Beam-Combined 7.4 J, 50 Hz Q-switch Pulsed YAG Laser System Based on SBS Phase Conjugation Mirror for Plasma Diagnostics

The phase conjugation of the optically nonlinear stimulated Brillouin scattering (SBS) process using heavy-fluorocarbon materials effectively compensated thermal degradation at two amplifier lines, resulting in an output energy of 7.4 J at 50 Hz. When combining two beams, the beat wave generation caused by the temporal coherence between the two beams can be prevented because the two beams leave at intervals longer than the coherence length of the pulse. The use of different Brillouin frequencies of reflection from different SBS substances can also be effective in preventing the interference effect caused by the spatial coherence between the two beams.

Approach to a window coating problem by in situ transmission monitoring and laser blow-off cleaning developed in the JT-60U Thomson scattering system

For solving a window coating problem, an in situ window transmission monitoring by inferring it precisely from a known attenuation of the deposited film has been developed in the JT-60U Thomson scattering system. Also the practicability of an in situ window cleaning based on a laser blow-off technique has been investigated extensively. The coated film composition could be guessed to be a resin such as polyacrylonitrile with a small amount of metals. The existence of chromatic upper limit has been found in the recovered transmission after the blow-off cleaning, which gives systematic errors only of less than 3% to an apparent measurement of both electron density and temperature at 10 keV or less. The attenuation itself is unchanged before and after the laser blow-off cleaning. A complementary use of both methods will provide the Thomson scattering measurement of high Te plasmas with durable reliability and sufficient precision in the existing tokamaks and the International Thermonuclear Experimental Reactor.

Formation of internal and edge transport barriers in JT-60U

Spontaneous formation of internal transport barrier was observed in JT-60U high- beta p discharges, where q=3 surface is the most likely explanation for its radial location. Further improved confinement was brought about by the subsequent transition to the edge transport barrier.

YAG Laser Perfomance Improved by Stimulated Brillouin Scattering Phase Conjugation Mirror in Thomson Scattering Diagnostics at JT-60

An improvement of the output energy of a multistage YAG laser system by using a stimulated Brillouin scattering (SBS) phase conjugation mirror (PCM) was achieved. The phase conjugation of the optically nonlinear SBS process in a liquid material effectively compensated thermal degradation at an average/peak power amplifier, resulting in an average power increase from 1.5 J at 30 Hz repetition rate to 2.6 J at 50 Hz drive. The beam quality was also recovered without wave front deformation and depolarization resulting in a transfer-limited divergence with a good flat-top pattern in a near field.