M. Saigusa

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.

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.

Current drive and confinement studies during LHRF experiments on JT-60

Results are presented of the first Lower Hybrid Current Drive (LHCD) experiments in JT-60. 2 MA of RF driven current is successfully produced for the first time in a reactor grade tokamak. The magnetic divertor works quite well in eliminating the impurities released by the current carrying fast electrons which have allowed the generation of the reactor relevant RF current in a very low density plasma. The efficiency which is defined as ηCD = eRIRF/PLH(1019 m−3 AW−1), reaches values of 0.8 to 1.7. NBI heating enhances the current drive efficiency by a factor of 1.5, and LHCD improves the confinement time of high power NBI heated plasma. The key to confinement improvement is found to be the active control of the current profile by LHCD.

The JT-60 radio-frequency heating system: description and R&D results

A system feature of the radio-frequency (RF) heating system for JT-60 is described. This system contains three units Lower Hybrid Range of Frequencies (LHRF) and one unit Ion Cyclotron Range of Frequencies (ICRF) heating systems. The LHRF heating system produces about 24 MW at 2 GHz of RF power using 24 high power klystrons and the ICRF heating system does 6 MW at 120 MHz of RF power using 8 high power tetrodes. Development of a high power klystron for the LHRF heating system and the coupling system for high power density of the transmission are performed for the construction of the RF heating system for JT-60. Japan Atomic Energy Research Institute (JAERI) has already constructed the RF heating system for JT-60 and is now performing the coupling test to JT-60 plasma.

Invited paper: Interaction between RF and edge plasma during ICRF heating in JT-60

Abstract Heating experiments in the second harmonic ion cyclotron range of frequencies (ICRF) have been performed with a phased array of 2×2 loop antennas in JT-60. Properties of antenna-plasma coupling are examined by phasing antenna currents in the toroidal direction. In particular, it is first found that the antenna-plasma coupling resistance increases after the H-mode transition in out-of-phase excitation of antenna currents. This result is well explained with the cold plasma coupling theory which takes into account a change in the edge density profile at the transition. Two types of parametric decay instabilities near the plasma edge are observed. One type is decay into an ion Bernstein wave (IBW) and an ion cyclotron quasimode (IQM) and the other into an ion Bernstein wave and a cold electrostatic ion cyclotron wave (CESICW) or an electron quasimode (EQM). Intensity of IBW detected by a probe near the antenna in the decay into IBW and IQM increases with reduction of B T and I p . The decay instabilities are observed only in the case of in-phase excitation. The edge plasma is heated by the decay instability and the radiation loss during ICRF heating increases with the decay activity.

Effect of shear in toroidal rotation on toroidicity induced Alfven eigenmodes

It is found that low-n toroidicity induced Alfven eigenmodes (TAEs) excited by ion cyclotron resonance heating can be stabilized by counter-tangential neutral beam injection in JT-60U. The deformation of the radial structure of the TAE mode due to the shear in toroidal rotation is a plausible stabilizing mechanism. This mechanism is also expected to be effective in suppressing high-n TAE modes. Real time control of TAE mode amplitudes using toroidal rotation control is possible for burn control in fusion reactors

Test results of X2242 and X2274 high power tetrodes with the JT-60 ICRF amplifier in a frequency range of 110-130 MHz

Abstract This paper reports the test results of newly developed Varian tetrodes, X2242 and X2274, using a JT-60 ICRF amplifier under the US-Japan collaboration program. The JT-60 ICRF amplifier was designed to deliver 0.75 MW at 110 to 131 MHz with the Varian EIMAC 8973 tetrode. Although the new tetrodes are similar to the 8973 in all dimensions, they have pyrolitic graphite grids for higher screen and control grid dissipation capability. The new tetrodes require only the following amplifier system modifications: (a) new filament, bias, and screen grid power supplies, (b) a second output port for reduction of rf voltage in the output cavity. The objective for the tests are to confirm 1.5 MW output at 130 MHz for 5 seconds, and to check the reliability of both the tube and the amplifier with a mismatched load which simulates power transmission to an antenna coupled to the plasma. The first test with an X2242 demonstrates that excessive screen dissipation limits the output power. The second test with an X2274, whose improved screen grid reduces rf heating to 50% of that of the X2242, achieves 1.7 MW at 131 MHz for 5.4 seconds. This is not only a power higher than the objective but also the highest long pulse VHF power level for fusion research above 110 MHz. The modified amplifier with the X2274 also shows good, stable performance in the mismatched load tests. As the theory predicts, the maximum anode dissipation is 1.4 times higher with a VSWR = 1.5 than with the previous VSWR ≅ 1.0.

Experimental study on beam acceleration with combined NBI heating and second-harmonic ICRF heating in JT-60

Beam acceleration by heating in the second-harmonic ion cyclotron range of frequency (ICRF) in combination with heating by hydrogen neutral beam injection (NBI) was investigated in the JT-60 tokamak. The energy spectra of the accelerated fast ions were measured by a charge exchange neutral energy analyser whose line of sight was intersected by specific beam lines in the plasma core in order to obtain the required information. The dependences of the tail ion temperature on various parameters (electron density, NBI power and the toroidal phasing of the antenna) were examined. Optimized conditions for beam acceleration were evaluated. For combined ICRF heating and NBI heating, an incremental energy confinement time of 210 ms was achieved, which was three times higher than that obtained with NBI heating alone or with ICRF heating alone. This improvement of the energy confinement during combined NBI and ICRF heating can be explained by a build-up of the fast ions accelerated by the ICRF wave. The scaling of the incremental energy confinement time during combined ICRF+NBI heating was obtained.

Electrical design and test of ICRF antenna for JT-60U

Abstract An antenna array is designed for ion cyclotron resonance heating (ICRH) for JT-60U. The coupling properties of the antenna are calculated by using the variational method for obtaining a consistent current profile on the current strap. The antenna consists of a 2 × 2 poloidal and toroidal phased loop array with two solid septa between each two antennas. The spectrum of the radiated parallel refractive index is optimized for out-of-phasing in order to obtain a high heating efficiency and a high loading resistance, and to suppress the impurity production. The predicted loading resistance is sufficient for ICRH experiments as long as the distance between the Faraday shield and plasma surface is smaller than 6 cm in the H-mode and 8 cm in the L-mode. The scattering parameters of the JT-60U antenna are measured in a low power test. The maximum with-standing RF voltage in the test stand is 38 kV which is limited by generator power and circuit losses.