S. Kitajima

Proposal of mechanically jointed superconducting magnet using high critical temperature superconductors

Abstract For future design of fusion reactors, development of the remountable superconducting magnet can become one of the important technical issues, which will bring huge benefit to reduce both costs of reactor construction and maintenance. In the conventional design, however, there is a fatal problem of heat generation occurring at the joints, which causes quenching of the magnet. On the other hand HTc superconductors, which has large specific heat, are now being used for magnets. In the previous research [International Journal of Applied Electromagnetics and Mechanics, IOS Press, in press. Joint Conference of the 12th International Toki Conference on Plasma Physics and Controlled Nuclear Fusion and The Third General Scientific Assembly of Asia Plasma Fusion Association, Toki. 2001, in press], the butt jointing method of HTc S.C. tapes had been proposed and its contact performance had been reported to be relatively excellent. In this research, therefore, the current distributions in the tape are calculated by numerical analysis and then the heat generations in the jointing region are estimated to evaluate the difference of contact performance in some jointing cases. The results show that the contact performance does not dependent on relative location of each S.C. filaments.

Frequency Spectrum of Fluctuations Near a Rational Surface in a Toroidal Heliac

Density and space potential fluctuations have been studied in electron cyclotron resonance heating (ECRH) plasma of a helical axis stellarator TU Heliac using Langmuir probe techniques. These fluctuations are coherent and global, which can be explained by a drift instability model in cylindrical geometry. A particular fluctuation mode vanishes inside a rational surface. The ratio of this mode, n/m, corresponds to the rotational transform ι/ of this rational surface, (m, n: poloidal, toroidal fluctuation modes, respectively). This phenomenon near the rational surface can also be explained by a drift instability theory.

Observation of multi-scale turbulence and non-local transport in LHD plasmas

We have studied two types of spatio-temporal turbulence dynamics in plasmas in the Large Helical Device, based on turbulence measurements with high spatial and temporal resolution. Applying conditional ensemble-averaging to a plasma with Edge-Localized Modes (ELMs), fast radial inward propagation of a micro-scale turbulence front is observed just after ELM event, and the propagation speed is evaluated as ∼100 m/s. A self-organized radial electric field structure is observed in an electrode biasing experiment, and it is found to realize a multi-valued state. The curvature of the radial electric field is found to play an important role for turbulence reduction.

LH transition by a biased hot cathode in the Tohoku University Heliac

In the Tohoku University Heliac (TU-Heliac), a helical axis stellarator, an electron injection electrode using a hot cathode made of LaB6 was developed and the transition mechanism to an improved mode has been intensively studied. In the electrode current-sweep mode of biasing experiments, a bifurcation phenomenon, i.e. a negative resistance feature in the electrode characteristics was observed accompanied with transition to an improved mode (H-mode) or transition from H-mode to L-mode, in helium plasma discharges of a wide range of collisionality. The ion viscous damping force was estimated from the J ? B driving force for poloidal rotation. The local maxima in viscosity were found at the poloidal Mach number around ?Mp ~ 1?3, as predicted by neoclassical theory. It was also found that the negative resistance (a bifurcation phenomenon) was observed when the poloidal viscosity showed a local maximum.

Confined alpha particle diagnostic system using an energetic He0 beam for ITER

The beam neutralization system for measurement of the spatial and velocity distributions of alpha particles of ITER plasmas was studied. As forward angle detection against the beam injection direction is required for effective neutralization, arrangement of the measurement system using possible ports in ITER configuration is proposed. The count rate of neutralized alpha particles produced by the double charge exchange interaction with energetic He0 beam particles injected is estimated. The ratios of signal to neutron-induced noise are evaluated. When a He0 beam produced by autodetachment from a 1–1.5MeV He− beam of 10mA is injected, the signal to noise ratio becomes greater than 1 at ρ<0.4, even without beam modulation. Usage of a lock-in technique at the frequency of radio-frequency quadrapole accelerator will make measurement at the outer region possible.

Optimization of a compact multicusp He+ ion source for double-charge-exchanged He− beam

Preliminary test bench results to study the beam quality extracted from a compact multicusp He+ ion source for He− beam production are reported. The bench is a part of the beam diagnostic system equipped with energy analyzers, emittance meters, focusing beam optics, an alkali-metal charge-exchange cell, a neutral particle energy analyzer, a double focusing magnetic momentum analyzer, a postaccelerator, and a drift tube. Utilizing the front end of the bench, the transverse emittance and the energy distribution function of a He+ beam extracted from a multi-line-cusp magnetic-field ion source 8cm in diameter and 9cm in length were measured. The results indicated that improvements in both formation of the plasma meniscus and reinforcement of pumping in the extraction region are necessary to produce a higher brightness He+ beam.

A New Capacitive / Resistive Probe Method for Studying Magnetic Surfaces

A new capacitive/resistive probe method for mapping the magnetic surfaces from resistance or capacitance between a magnetic surface and a vacuum vessel was developed and tested. Those resistances and capacitances can be regarded as components of a simple electrical bridge circuit. This method exploits electrical transient response of the bridge circuit for a square pulse. From equiresistance or equicapacitance points, the magnetic surface structure can be deduced. Measurements on the Tohoku University Heliac, which is a small-size standard heliac, show good agreement with numerical calculations. This method is particularly useful for pulse-operated machines.

Full orbit calculation for lost alpha particle measurement on ITER

An orbit following calculation code with full gyromotion under the ITER magnetic field configuration has been developed to investigate escaping alpha particle orbits in ITER and to determine the geometrical arrangement for alpha particle detection. The code contained the full geometrical information of the first wall panels. It was carefully investigated whether an alpha particle escaping from the plasma through the last closed flux surface does not touch or intersect the first wall boundary before reaching the detection point. Candidates of blanket module modification have been studied to achieve effective measurement geometry for escaping alpha particle detection. The calculations showed that direct orbit loss and banana diffusion can be detected with a probe head recessed from the first wall surface.

Development of neutron measurement system for nd∕nt fuel ratio measurement in ITER experiments

The possibility of measurement of fuel ratio from the DT/DD reaction ratio with a neutron spectrometer in the International Thermonuclear Experimental Reactor was studied. The results of neutron transport calculations using the Monte Carlo N-particle [MCNP, a general Monte Carlo N-particle transport code, version 4C, LA-13709, edited by J. F. Briesmeister, Los Alamos National Laboratory, 2000] code system indicated the possibility of DD spectrum separation from the scattered/energy-degraded neutrons derived from DT neutrons by selection of the measurement location and collimator design. A time-of-flight spectrometer was used in this study. An experiment using DD and DT neutrons from an accelerator was conducted at the Japan Atomic Energy Agency Fusion Neutronics Source facility. Using a sophisticated circuit with three discriminators for the first detector, separation of DD and DT neutron components of the spectrum in the megahertz region was confirmed experimentally.

Characterization of scintillators for lost alpha diagnostics on burning plasma experiments

The characteristics of light output by ion beam irradiations under high ion fluxes have been measured for three kinds of scintillators: ZnS:Ag deposited on the glass plate, Y3Al5O12:Ce powder stiffened with a binder, and Y3Al5O12:Ce ceramics sintered at high temperature. The ion beam flux in the range from 1012to1013ions∕(cm2s) is irradiated to simulate the burning plasma experiments. The decrease of light output has been observed by long time ion irradiation. The deterioration of ZnS:Ag deposited scintillator is most serious. The deterioration has been improved for the scintillators of Y3Al5O12:Ce with a binder and that sintered. Their applications to ITER lost alpha diagnostics are discussed.