K. Yokokura

Electron cyclotron resonance discharge cleaning of JFT-2 Tokamak (Jaeri)

Abstract A discharge cleaning experiment was carried out in JFT-2 tokamak by using a hydrogen plasma produced by electron cyclotron resonance (ECR) discharge. By the use of a 2.45 GHz, 2 kW CW microwave, a plasma with electron temperature of 4eV and density of 1.2 × 10 10 cm −3 was produced. Under quasi-equilibrium conditions, a comparison between the ECR-DC and the Taylor discharge cleaning (TDC) was done by observing the partial pressures of CH4, H2O, and CO gases. The TDC was carried out with a plasma with electron temperature of 4.1 eV and density of 3.5 × 10 12 cm −3 , produced with a pulse width of 10 ms and a repetition frequency of 0.67 Hz. Both cleaning methods were found to have an approximately equal cleaning effect. This shows the applicability of the ECR-DC to advanced tokamaks.

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.

Efficient lower hybrid current drive using a multijunction launcher on JT-60

A multijunction launcher was designed and constructed to improve the directivity of launched waves with a narrow wave number spectrum by dividing each main waveguide into three secondary waveguides. The coupling characteristics agree fairly well with theory, and a power of up to 2 MW was obtained after a few days of conditioning. The dependence of the current drive efficiency ηCD (= neIRFR/PLH) on the wave spectrum was studied by changing the phase difference between the adjacent main waveguides. It was found that high current drive efficiency is obtained by waves interacting with fast electrons as far into the plasma as the waves travel. A maximum current drive efficiency of 2.8 × 1019 AW−1m−2 was achieved with this launcher at a plasma current of 1 MA. The efficiency of the multijunction launcher was higher by 40% than that of the previous conventional launcher on JT-60.

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 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.

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.

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.

FIRST RESULTS OF JT-60U ICRF HEATING SYSTEM

The modified ICRF heating system for JT-60 Upgrade has started its operation in January of 1992. The coupling resistance of the new antenna at (π,0) phasing is sufficiently high, ˜ 5 Ω, even with 0.1m of the distance between first walls and separatrix at line averaged electron densities of 1 - 3 × 1019m−3. RF power up to 3.6 MW for 1.4 sec at 116 MHz has been coupled to the plasma in relatively short operation period.

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.