S. Maebara

Formation of spherical tokamak equilibria by ECH in the LATE device

The main objective of the Low Aspect Ratio Torus Experiment (LATE) device is to demonstrate the formation of spherical torus (ST) plasmas by electron cyclotron heating (ECH) alone without a centre solenoid and establish its physical bases. By injecting a 2.45 GHz microwave pulse for 4 s, a plasma current of 1.2 kA is spontaneously initiated by P = 5 kW under a weak steady vertical field of Bv = 12 G and then ramped up slowly with a slow ramp-up of Bv for the equilibrium of the plasma loop and finally reaches 6.3 kA by P = 30 kW at Bv = 70 G. This current amounts to 10% of the total coil currents of 60 kA flowing through the centre post for the toroidal field. Magnetic measurements show that an ST equilibrium, having the last closed flux surface with an aspect ratio of R0/a 20.4 cm/14.5 cm 1.4, an elongation of κ 1.5 and qedge 37, has been produced and maintained for 0.5 s at the final stage of discharge. Spontaneous formation of ST equilibria under steady Bv fields, where plasma current increases rapidly in the time scale of a few milliseconds, is also effective and a plasma current of 6.8 kA is spontaneously generated and maintained at Bv = 85 G by a 5 GHz microwave pulse (130 kW, 60 ms). In both cases, the plasma centre locates near the second or third harmonic EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the harmonic EC heating by the mode-converted electron Bernstein waves supports the plasma.

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.

Operation and commissioning of IFMIF (International Fusion Materials Irradiation Facility) LIPAc injector

The objective of linear IFMIF prototype accelerator is to demonstrate 125 mA/CW deuterium ion beam acceleration up to 9 MeV. The injector has been developed in CEA Saclay and already demonstrated 140 mA/100 keV deuterium beam [R. Gobin et al., Rev. Sci. Instrum. 85, 02A918 (2014)]. The injector was disassembled and delivered to the International Fusion Energy Research Center in Rokkasho, Japan. After reassembling the injector, commissioning has started in 2014. Up to now, 100 keV/120 mA/CW hydrogen and 100 keV/90 mA/CW deuterium ion beams have been produced stably from a 10 mm diameter extraction aperture with a low beam emittance of 0.21 π mm mrad (rms, normalized). Neutron production by D-D reaction up to 2.4 × 10(9) n/s has been observed in the deuterium operation.

High-power and long-pulse gyrotron development in JAERI

Abstract A maximum pulse duration of 1.3 s was achieved with a power of 410 kW at a frequency of 110 GHz and a maximum power of 550 kW was obtained with short-pulse operations (ca. 2 ms). The oscillation mode of the gyrotron is the TE22,2 whispering gallery mode, which is transformed into a gaussian-like beam by a built-in quasi-optical mode converter. R.f. power is extracted through a sapphire double-disc window cooled by FC-75. In long-pulse operation, no damage was observed in the gyrotron.

JAERI millimeter wave FEL experiments with a focusing wiggler

Abstract The first results of the JAERI millimeter wave FEL experiments with a focusing wiggler are presented. The induction linac delivers a 1 MeV, 1 ∼ 3 kA beam in a wiggler whose magnets have a curved surface; the wiggler has 30 periods of 5 cm. We found that such a focusing wiggler is essential for beam transport at low energy and high current. The power and spectrum of the superradiant emission have been measured. The growth rate of the radiation is 0.4 dB/cm.

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.

Development of plasma facing component for LHCD antenna

Abstract A mock-up module of an antenna tip made of carbon fiber composite (CFC) was fabricated for the development of a heat-resistive front of the lower hybrid current drive (LHCD) antenna. The CFC surface was coated with a thin Ti layer and was plated with Cu. The mock-up module has three waveguides and is designed to simulate the front tip of the 8 GHz LHCD antenna. The module has an active water cooling channel and the length is 200 mm, the mouth dimension is 19.5×50 mm 2 . The mock-up module was tested with heat fluxes up to 20 MW m −2 due to electron beam irradiation. No damage of the waveguide mouth was observed up to the heat flux of 3.2 MW m −2 for 2 min, though a maximum temperature of 762°C was measured at the septum plate. The obtained heat load of 3.2 MW m −2 is about 10 times higher than the steady-state surface-averaged heat load of the first wall required in the ITER design.

Development of a diffusion bonding method for fabrication of lower hybrid current drive grills

Abstract A diffusion bonding method has been developed to fabricate a LHCD (Lower Hybrid Current Drive) grill with a large number of waveguides. The grill is composed of several thin septum plates and pairs of rods, and is constructed by diffusion bonding. This method simplifies the fabrication process and the grill can have a high mechanical strength at high temperature. A 24×4 multijunction grill, which consists of 24 waveguides and 4 waveguides in the toroidal and poloidal directions, respectively, has been successfully fabricated by this method and has demonstrated a power handling capability of up to 26 MW/m2 on JT-60U. Moreover, the diffusion bonding method can make a grill composed of two different kinds of material: one has a high thermal conductivity for the septum plates to remove RF (Radio Frequency) losses, and the other has a low electrical conductivity for the rods to reduce eddy currents during a plasma disruption. A test module composed of Dispersion Strengthened Copper and Stainless Steel has been fabricated and has shown the practical application of diffusion bonding in the fabrication of a LHCD grill for steady-state operation.

Design of a 5 GHz window in a lower hybrid r.f. system

Abstract A new pill-box window at a frequency of 5 GHz, which has an oversized length in both the axial and the radial direction, has been designed to reduce the r.f. power density and the electric field strength at the ceramics. The dimension of the new pill-box is optimized from the numerical calculation and a voltage standing-wave ratio of less than 1.02 is obtained. The r.f. power density and the maximum electric field strength are reduced to about 40% and 66% of the standard pill-box window respectively. It is evaluated that the power capability of the new oversized pill-box window by cooling edge of ceramics is more than 500 kW with continuous-wave operation.