Y. Kusama

Ripple induced fast ion loss and related effects in JT-60U

Experiments have been carried out in JT-60U to verify the modelling of fast ion ripple transport. The ripple induced loss was estimated from the neutron decay following neutral beam pulse injection and the loss related heat load on the first wall. Comparison of the lost fraction and the hot spot positions between measurements and orbit following Monte Carlo calculations exhibited good agreement, indicating that the ripple transport governing fast ion losses is explained within the framework of existing theory. Neutral beam heating experiments in JT-60U also indicate that H modes free of ELMs are still obtainable for ripple amplitudes of up to 2.2%

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.

Fast ion losses due to toroidal field ripple in JT-60U

A previous experiment in JT-60U supported an orbit following Monte Carlo (OFMC) calculation regarding ripple trapped loss, and the present experiment, furthermore, suggests that the OFMC also predicts banana drift loss fairly well. In the experiment presented, the total fast ion losses due to toroidal field ripple were estimated from the decay in neutron emission following a short neutral beam injection (90 keV, D). The neutron decay for co-passing beam injection showed a diffusivity of about 0 m2/s, which indicates no fast ion loss. In contrast, the neutron decay for trapped particle injection exhibited characteristic enhancement of fast ion losses due to toroidal field ripple: the fast ion losses consisted of ripple trapped convection and ripple banana diffusion in the low collisionality regime. The OFMC calculation reconstructed completely the experimental neutron decay irrespective of the total ripple losses and the fraction of banana drift loss. Considering the previous work on ripple trapped loss and this result, it can be concluded that the OFMC code gives a good quantitative estimation of banana drift loss as well as ripple trapped loss

Study of plasma termination using high-Z noble gas puffing in the JT-60U tokamak

Argon, krypton and xenon were puffed with and without simultaneous hydrogen gas puffing into Ohmically heated plasmas of the JT-60U tokamak with low plasma currents in order to study the capability of disruption mitigation. It was found that krypton gas puffing can provide a plasma termination with smaller amounts of runaway electrons in comparison to argon and xenon gas puffing.

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.

Excitation of high n toroidicity-induced Alfvén eigenmodes and associated plasma dynamical behaviour in the JT-60U ICRF experiments

Abstract High frequency MHD activities observed during second harmonic ICRF heating are identified to be toroidicity-induced Alfven eigenmodes (TAE) driven by MeV protons produced by ICRF heating. Correlation between MeV protons and TAE modes is clearly observed. TAE mode amplitude increases exponentially with increasing toroidal mode number up to more than ten. The tendency cannot be explained by present local TAE stability theories. Long suppression of TAE modes after a giant sawtooth crash can be explained by fast ion loss due to the sawtooth crash and evolving q -profile.

Effects of complex magnetic ripple on fast ions in JFT-2M ferritic insert experiments

In JFT-2M, ferritic steel plates (FPs) were installed on almost the whole inner surface of the vacuum vessel. This arrangement is called the ferritic inside wall (FIW), and is the third step of the advanced material tokamak experiment programme. The toroidal field (TF) ripple was reduced by optimizing the thickness of FPs but the total ripple structure has become more complex, with a non-periodic feature in the toroidal direction, because of the existence of ports and other components that limit the periodic installation of FPs. We investigated the effect of this complex ripple on the heat flux onto the first wall due to fast ion loss. The ripple trapped loss was reduced as a result of the reduced magnetic ripple of the FIW. Additional FPs were also installed outside the vacuum vessel to produce a localized larger ripple. A small ripple trapped loss was observed when the shallow ripple well exists in the poloidal cross section, and a large ripple trapped loss was observed when the ripple well extends deep into the plasma region. Experimental results were almost consistent with computation with a newly developed fully three-dimensional magnetic field orbit-following Monte-Carlo code including the three-dimensional complex structure of the TF ripple and the non-axisymmetric first wall geometry.

Recent progress of Alfvén eigenmode experiments using N-NB in JT-60U tokamak

Bursting modes in the frequency range of the toroidicity induced Alfv?n eigenmode are observed in the plasma to which the negative-ion-based neutral beam (N-NB) is injected. A bursting mode changes its frequency by 10-20?kHz in 1-5?ms and is called fast frequency sweeping (fast FS) mode. Another bursting mode evolves explosively in ?400??s and is called an abrupt large-amplitude event. The dependence of their saturation level was compared?with the experimentally observed growth rate and damping rate. The mode amplitude increases with the observed growth rate for fast FS modes. The modes with large amplitude and a large enhanced transport were observed when a large neutron emission rate was observed. The burst period increases as the drop ratio of the neutron emission increases.

Heating and non-inductive current drive by negative ion based NBI in JT-60U

The current drive and heating properties of negative ion based NBI have been studied comprehensively in JT-60U. It has been confirmed from shine-through measurements of the injected beam (350 keV) that multistep ionization processes are essential in the ionization processes of high energy particles. The profile of the current density driven by a negative ion based NB (N-NB) has been determined experimentally. This is in good agreement with the theoretical prediction, and N-NB driven current reached 0.6 MA with EB = 360 keV and PINJ = 3.7 MW. The current drive efficiency ηCD is increased by increasing electron temperature and improved by increasing beam energy. The fast ions from N-NBs are well confined in the enhanced confinement core by the weak poloidal magnetic field of reversed shear plasmas. A clear H mode transition was obtained with N-NB dominant heating, where the net absorbed power required for an H mode transition seemed similar to the previous result obtained in JT-60U using a low energy beam (90 keV). With the strong electron heating by N-NBI (80% absorbed by electrons), an H factor ( = τE/τITER-89PLE) of 1.64 with Te(0) = 1.4Ti(0) was obtained in the steady state ELMy phase.

Attenuation of high-energy neutral hydrogen beams in high-density plasmas

Beam stopping cross section and shine-through for neutral hydrogen beam injection into fusion plasmas have been calculated by using recommended cross sections presently available for atomic processes including multistep collision processes involving excited states. The shine-through thus obtained agrees well with recent experiments of JT-60U. The present calculations show that the multistep processes play a crucial role in the stopping of high-energy neutral hydrogen beams in high-density plasmas. Analytical fits to the stopping cross sections and fitting parameters are also presented for plasma impurities with nuclear charge and Z = 26.