T. Kondoh

Fast plasma shutdown by killer pellet injection in JT-60U with reduced heat flux on the divertor plate and avoiding runaway electron generation

A killer pellet is an impurity pellet that is injected into a tokamak plasma in order to terminate a discharge without causing serious damage to the tokamak machine. In JT-60U neon ice pellets have been injected into OH and NB heated plasmas and fast plasma shutdowns have been demonstrated without large vertical displacement. The heat pulse on the divertor plate has been greatly reduced by killer pellet injection (KPI), but a low-power heat flux tail with a long time duration is observed. The total energy on the divertor plate increases with longer heat flux tail, so it has been reduced by shortening the tail. Runaway electron (RE) generation has been observed just after KPI and/or in the later phase of the plasma current quench. However, RE generation has been avoided when large magnetic perturbations are excited. These experimental results clearly show that KPI is a credible fast shutdown method avoiding large vertical displacement, reducing heat flux on the divertor plate, and avoiding (or minimizing) RE generation.

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

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...

Characteristics of Alfvén eigenmodes, burst modes and chirping modes in the Alfvén frequency range driven by negative ion based neutral beam injection in JT-60U

The excitation and stabilization of Alfv?n eigenmodes and their impact on energetic ion confinement were investigated with negative ion based neutral beam injection at 330-360?keV into weak or reversed magnetic shear plasmas on JT-60U. Toroidicity induced Alfv?n eigenmodes (TAEs) were observed in weak shear plasmas with ?h ? 0.1% and 0.4 ? vb||/vA ? 1. The stability of TAEs is consistent with predictions by the NOVA-K code. New burst modes and chirping modes were observed in the higher ? regime of ?h ? 0.2%. The effect of TAEs, burst modes and chirping modes on fast ion confinement has been found to be small so far. It was found that a strongly reversed shear plasma with internal transport barrier suppresses AEs.

Investigation of high-n TAE modes excited by minority-ion cyclotron heating in JT-60U

Toroidicity-induced Alfven eigen (TAE) modes are observed during minority-ion cyclotron resonance heating (ICRH) in the JT-60U. The toroidal mode numbers of TAE modes are identified as 7, 8, 9, 10 and 11 from the Doppler shift in the TAE modes with scanning toroidal rotation at a plasma current of 3 MA. The toroidal mode number of TAE modes tends to increase during a giant sawtooth by ICRH with a decreasing safety factor for the central region. The TAE mode number increases with plasma current, so that nine TAE modes are observed sequentially during a giant sawtooth at a plasma current of 4 MA, where the maximum toroidal mode number is estimated to be at least 13. There are no Alfven continuum gaps for TAE modes in the safety-factor ranges of i-1/2n<q<i+1/2n, (i=1, 2, 3, ...), except for the gaps in ellipticity-induced Alfven eigen (EAE) modes, where n is the toroidal mode number of TAE modes. Therefore, control of the q profile might provide a means of avoiding TAE modes, as long as the pressure gradient of the high-energy ions is localized.

Temporal behaviour of the potential confined electrons in the central cell and in the plug region during a period with thermal barriers

An increase with time of the central-cell electron temperature during a period with thermal barrier potentials has been observed. The increase is explained by an improvement of the electron energy confinement due to the presence of thermal barriers. Different evolutions of the electron energy in the plug region have been observed; these are closely related with the variation in time of the confining potential of the plug electrons. The observed behaviour of the potential confined electrons provides the first data set for the evolution of the effect of thermal isolation caused by the formation of a thermal barrier in the kilovolt range.

Toroidal interferometer/polarimeter density measurement system on ITER

In order to measure the line integrated electron density on the International Thermonuclear Experimental Reactor (ITER), a vibration-compensated five-channel CO2 laser interferometer/polarimeter system is being developed. The design of the interferometer/polarimeter has been improved from the previous design, which used a CO2 laser (wavelength, λ=10.6 μm) and a CO laser (λ=5.3 μm), on the basis of experience gained with a dual CO2 system (λ=10.6 and 9.27 μm) on JT−60U. The JT−60U system gives good reliability for both interferometry and polarimetry operation. The performance of the dual CO2 laser is expected to meet the requirements for measurements on ITER (accuracy; 1%, time resolution; 1 ms).

Systematic study of toroidicity-induced Alfvén eigenmodes at low-q discharges in JT-60U

Toroidicity-induced Alfv?n eigenmodes (TAE) were studied systematically in low-q discharges in JT-60U. Low-n TAE modes appeared outside the q = 1 surface at a low internal inductance (li), while high-n multiple TAE modes appeared inside the q = 1 surface at high internal inductance. In addition, the low-n multiple TAE modes were observed outside the q = 1 surface in very low-li plasmas , while the bi-directional TAE modes were observed inside the q = 1 surface in high-li and relatively high electron density plasmas . The high-n multiple TAE modes observed inside the q = 1 surface are much more harmful for energetic ion confinement than the low-n TAE modes observed outside the q = 1 surface.

LHCD current profile control experiments towards steady state improved confinement on JT-60U

In JT-60U lower hybrid current drive (LHCD) experiments, a reversed magnetic shear configuration that was accompanied by internal transport barriers was successfully maintained in a quasi-steady state almost entirely non-inductively by the LH driven current and the bootstrap current. The normalized β was kept near 1 and the neutron emission rate was nearly steady as well, indicating no accumulation of impurities in the plasma. Diagnostics showed that the profiles of the electron and ion temperatures, the electron density and the current were almost unchanged during the LHCD phase. The applicability of LHCD to H mode plasmas has also been investigated. It was found that the LH waves can be coupled to an H mode edge plasma with a plasma-wall distance of up to about 14 cm.

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.