S. Shinozaki

Initial results of electron cyclotron range of frequency (ECRF) operation and experiments in JT-60U

Abstract The 110 GHz 1 MW electron cyclotron range of frequency (ECRF) system was designed and constructed on JT-60U to locally heat and control the plasmas. The gyrotron has a diamond window to transmit RF power with Gaussian mode, which is easily transformed to HE11 mode for the transmission line of the corrugated waveguide. The second diamond window is installed at the inlet of the antenna for a vacuum seal between the transmission line and the JT-60U tokamak. The total length of the transmission line from the gyrotron to the antenna is about 60 m including nine meter bends, The antenna has a focusing mirror and a flat steerable one to focus and to control the RF beam angle mainly in the poloidal direction. In the initial operation, the power of PEC∼0.75 MW for 2 s was successfully injected into plasma when the gyrotron generated the power up to 1 MW. The total transmission efficiency from the gyrotron to the plasma was about 75%. A controllability of local electron heating with the deposition width of =15 cm was well demonstrated by using the steerable mirror. A large downshift in the deposition position was observed at the high Te plasma. Strong central electron heating was obtained from 2.2 to 6.6 keV for PEC∼0.75 MW, 0.3 s at the optimized polarization. An effective electron heating was also obtained up to ∼10 keV during EC injection for ∼1.6 s in the high βp H-mode plasma produced by NBI.

Simple multijunction launcher with oversized waveguides for lower hybrid current drive on JT-60U

Abstract A multijunction technique with oversized waveguides has been developed for the lower hybrid current drive launcher on JT-60U. The launcher consists of 4 (toroidal) × 4 (poloidal) multijunction modules. RF power in the module is divided toroidally into 12 sub-waveguides at a junction point through an oversized waveguide. This method simplifies the structure of the multijunction launcher with a large number of subwaveguides. A maximum power density up to 25 MW m −2 has been achieved with a low reflection coefficient of less than 2%. The coupling and current drive efficiency are well explained by the designed wave spectra without taking account of higher modes in the oversize waveguides. Thus, the simple multijunction launcher has been demonstrated to excite expected wave spectra with high power handling capability.

The 110-GHz Electron Cyclotron Range of Frequency System on JT-60U: Design and Operation

The electron cyclotron range of frequency (ECRF) system was designed and operated on the JT-60U to locally heat and control plasmas. The frequency of 110 GHz was adopted to inject the fundamental O-mode from the low field side with an oblique injection angle. The system is composed of four 1 MW-level gyrotrons, four transmission lines, and two antennae. The gyrotron is featured by a collector potential depression (CPD) and a gaussian beam output through a diamond window. The CPD enables JAERI to drive the gyrotron under the condition of the main DC voltage of 60 kV without a thyristor regulation. The gaussian mode from the gyrotron is effectively transformed to HE11 mode in the 31.75 mm diameter corrugated waveguide. About 75% of the output power of the gyrotrons can be injected into plasmas through the waveguides about 60 m in length. There are two antennae to control the deposition position of the EC wave during a plasma discharge. One is connected with three RF lines to steer the EC beams in the poloidal direction. The other is to control the EC beam in the toroidal and poloidal directions by two steerable mirrors. On the operation in 2000, the power of 1.5 to 1.6 MW for 3 s was successfully injected into plasmas using three gyrotrons. Local profile control was demonstrated by using the antennae. This capability was devoted to improve the plasma performance such as high Te production more than 15 keV and suppression of the MHD activities. In 2001, the fourth gyrotron, whose structure was improved for long pulse operation, has been installed for a total injection power of ~3 MW.

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.

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.

Operational progress of the 110GHz-4MW ECRF heating system in JT-60U

The JT-60U electron cyclotron range of frequency (ECRF) heating system is utilized to realize high performance plasma. Its output power is 4 MW at 110 GHz. The output power of the gyrotron used in the heating system can be controlled by changing its anode voltage. Then, a compact anode voltage controller was developed to modulate the injected power into plasmas. This controller achieved the modulation frequency of 12 - 500 Hz with modulation factor of 80 % at 0.7 MW of output power. Additional function of this controller also could make the pulse duration longer from 5 s to 16 s at 0.5 MW. For the long pulse operation, temperature rise of the DC break made of Alumina ceramics in the gyrotron was estimated from the measured temperature rise of the coolant and its maximum temperature was about 140 deg. From the analysis of this temperature rise, DC break materials should be changed to low-loss materials for extending to 30 s of the pulse duration. The stabilization of neoclassical tearing mode (NTM) was demonstrated by ECRF heating using the real-time system in which ECRF beams were injected to the NTM location predicted from ECE measurement every 10 ms. The ECRF beams were scanned poloidally with the steering mirrors of the antennas.

Development and Operation of the Lower Hybrid Range of Frequency System on JT-60U

Development and operation of a lower hybrid range of frequency (LHRF) system for JT-60U are presented. The LHRF system was constructed in 1986 to study current drive and plasma heating at high injection power. Its main specifications are the total output power 24 MW with 24 high power klystrons, the frequency 1.74 to 2.23 GHz, and the injection power ~10 MW with three conventional antennas. To improve the antenna capabilities such as the current drive efficiency, N//peak controllability and the power injection properties, a 3-divided multi-junction type (CD1’ launcher) and a 12-divided multi-junction type (CD2 launcher) are developed. The CD2 launcher can also reduce the number of the transmission lines to one fourth of the original system. The injection power ~7 MW is attained, and then the highest current drive efficiency 3.5 × 1019 m-2AW-1 and the highest non-inductive driven current 3.6 MA are achieved. The high power klystron capable of the cathode-heater operation times more than 3000 hours is improved. The outgassing rate is estimated with the CD2 launcher as 1-10 × 10-6 Pam3/sm2, which is sufficiently small not to need the vacuum pumping system for the launcher. Heat load onto the launcher due to the ripple enhanced banana drift loss is first observed in NBI or ICRF heating. From investigation on antenna-plasma coupling, the gas puffing improves distant coupling.

Performance of the JT-60 ICRF antenna with an open type Faraday shield

Abstract Performance of the JT-60 ICRF antenna in second and third harmonic heating schemes (f = 120, 131 MHz) over past four years of operation is presented. The antenna is mainly composed of phased 2 × 2 loops, an open type Faraday shield and a metallic casing, forming a plug-in type. The antenna is operated for wide plasma parameters: n e = 1–7×10 19 m −3 , I P = 1–2.8 MA and B T = 2.2–4.8 T . The open type Faraday shield shows no deterioration for impurity production and heating efficiency up to the maximum injected power of 3.1 MW (the power density of 16 MW/m2) except the following particular condition. Only for (0, 0) phasing and less than 30 mm of the distance between the outermost magnetic surface and the antenna guard limiter, the radiation loss increases abruptly from ΔP rad P IC ∼ 0.3 to ΔP rad P IC ∼ 4 in carbon limiter discharges when the injected power exceeds a threshold value of ∼ 0.5 MW. Strong titanium impurity release from the Faraday shield is observed in coincidence with the increase in the radiation loss. This suggests that strong ion sputtering is induced on the Faraday shield by RF sheaths.

Recent RF activities on JT-60U

Recently, the JT-60U has employed ECRF system in addition to LHCD and ICRF systems. Three types of RF experiments on JT-60U have been performed. A reversed magnetic shear (R/S) configuration with internal transport barrier (ITB) was successfully maintained for 4.7 s in a quasi-steady state by LHCD in addition to the bootstrap current, where all profiles of temperature and plasma current were almost stationary. On the normal operation without heating during plasma current ramp-up, high electron temperature (Te0∼10 keV) with a strongly peaked profile was produced by LHCD alone at the density of ∼0.5×1019 m−3. This peaked temperature profile was featured by a reduction in core density. The central electron temperature and density slowly changed in several seconds. The performances of ICRF (2ωCH) on the R/S configuration were also studied. Degradation of confinement of energetic particles was observed probably due to the large banana orbit loss. An 110 GHz ECRF system has been operated on JT-60U. Its gyrotron...

Operation and control of JT-60U ECRF system

This paper describes the operation and control for the 110 GHz ECRF system for JT-60U, which has started operation since March in 1999. This system is composed of a 1 MW gyrotron, its high voltage power supply, a transmission line about 60 m in length and a quasi-optical antenna using a steerable mirror. Key issues of the ECRF system are to drive the gyrotron stably and to control the local RF deposition profile in a plasma.