T. Tsuzuki

Progress summary of LHD engineering design and construction

In March 1998, the LHD project finally completed its eight year construction schedule. LHD is a superconducting (SC) heliotron type device with R = 3.9 m, ap = 0.6 m and B = 3 T, which has simple and continuous large helical coils. The major mission of LHD is to demonstrate the high potential of currentless helical-toroidal plasmas, which are free from current disruption and have an intrinsic potential for steady state operation. After intensive physics design studies in the 1980s, the necessary programmes of SC engineering R&D was carried out, and as a result, LHD fabrication technologies were successfully developed. In this process, a significant database on fusion engineering has been established. Achievements have been made in various areas, such as the technologies of SC conductor development, SC coil fabrication, liquid He and supercritical He cryogenics, development of low temperature structural materials and welding, operation and control, and power supply systems and related SC coil protection schemes. They are integrated, and nowadays comprise a major part of the LHD relevant fusion technology area. These issues correspond to the technological database necessary for the next step of future reactor designs. In addition, this database could be increased with successful commissioning tests just after the completion of the LHD machine assembly phase, which consisted of a vacuum leak test, an LHe cooldown test and a coil current excitation test. These LHD relevant engineering developments are recapitulated and highlighted. To summarize the construction of LHD as an SC device, the critical design with NbTi SC material has been successfully accomplished by these R&D activities, which enable a new regime of fusion experiments to be entered.

Plasma confinement studies in LHD

The initial experiments on the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW produced plasmas with a fusion triple product of 8 × 1018m-3keVs at a magnetic field strength of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV were achieved simultaneously at a line averaged electron density of 1.5 × 1019 m-3. The maximum stored energy reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to β = 0.7%. Energy confinement times reached a maximum of 0.17 s. A favourable dependence of energy confinement time on density remains in the present power density (~40 kW/m3) and electron density (3 × 1019 m-3) regimes, unlike the L mode in tokamaks. Although power degradation and significant density dependence are similar to the conditions on existing medium sized helical devices, the absolute value is enhanced by up to about 50% from the International Stellarator Scaling 95. Temperatures of both electrons and ions as high as 200 eV were observed at the outermost flux surface, which indicates a qualitative jump in performance compared with that of helical devices to date. Spontaneously generated toroidal currents indicate agreement with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to the finite β effect was well described by 3-D MHD equilibrium analysis. A density pump-out phenomenon was observed in hydrogen discharges, which was mitigated in helium discharges with high recycling.

High frequency ion Bernstein wave heating experiment in the JIPP T-IIU tokamak

An experiment in a new regime of ion Bernstein wave (IBW) heating was carried out using 130 MHz high power transmitters in the JIPP T-IIU tokamak. The heating regime utilized the IBW branch between the 3rd and 4th harmonics of the hydrogen ion cyclotron frequencies. This harmonic number is the highest one used in IBW experiments conducted previously. The net radiofrequency (RF) power injected into the plasma is around 400 kW and is limited by the transmitter output power. Core heating of ions and electrons was confirmed in the experiment and density profile peaking was found to be a special feature of IBW heating. Peaking of the density profile was also found when IBWs were injected into neutral beam heated discharges. An analysis, using a transport code with these experimental data, indicates that particle and energy confinement should be improved in the plasma core region upon application of IBW heating. It is also found that the ion energy distribution function observed during IBW heating has a smaller high energy tail than those observed in conventional fast magnetosonic wave ICRF heating regimes. The ion energy distribution function obtained during IBW heating is in reasonable agreement with that calculated using the quasi-linear RF diffusion/Fokker-Planck model

Observation of toroidal plasma rotation driven by the electric field induced by loss of ions

Toroidal plasma rotation driven not by the momentum input but by the electric field induced by loss of ions has been observed for plasmas heated by perpendicular neutral beam injection. The rotation is found to increase in the direction opposite to the plasma current when the plasma ions are heated by ion cyclotron resonance frequency waves. The rotation driven by the electric field is quantitatively consistent with that inferred from the ambipolarity of particle fluxes.

Fast potential change during sawteeth in JIPP T-IIU tokamak plasmas

Fast changes of electric potential with different polarities are observed during sawtooth oscil- lations in the core region of a tokamak plasma using a heavy ion beam probe. Near the inversion radius the polarity of the observed change of the potential is found to be dependent on the swift movement of the hot core at the crash and is consistent with the prediction of one- fluid MHD theory. Near the magnetic axis the change of the potentials is positive and outside the inversion radius the change is negative. This is in contradiction with the MHD prediction. A periodic crash of the central electron tempera- ture called a sawtooth oscillation is a common fea- ture of tokamak plasmas (l-61. After the crash, a flattening of the pressure throughout the q = 1 sur- face is observed. Kadomtsev proposed a theoretical model of the resistive reconnection (7). The theory predicted the flattening of the q profile as well as the pressure profile inside the q = 1 surface. The predic- tion was, however, found to be in contradiction with recent measurements of the current density in toka- mak plasmas (8). In addition, experiments in large machines such as JET and TFTR showed much faster crashes than those predicted by resistive reconnection (2-61. More refined theories, which take account of the kinetic effect and ergodicity on the reconnections, have now been proposed (9-121. The present Letter reports the first measurement of potential changes during sawtooth oscillations and discusses the impli- cations of the results. The experiment is performed with the JIPP T-IIU tokamak (13). Its major radius is 93 cm and the maximum toroidal field is 3 T. The experiment is conducted at a relatively low electron density of about 2 x 1013 cm3, so that good beam penetra- tion into the centre of the plasma could be obtained. A 450 keV singly ionized thallium beam is injected

Density fluctuations in JIPP T-IIU tokamak plasmas measured by a heavy ion beam probe

Multiple and small sample volume measurements of the density turbulence and potential profile measurements in tokamak plasmas were conducted using a heavy ion beam probe. The obtained wavenumber-frequency spectrum S(k, omega ) shows that the cross-sections of neutral beam injection (NBI) heated plasmas are divided into three regions of different turbulence characteristics outside the layer where the direction of the density turbulence propagation reverses, a low frequency and low wavenumber mode travelling in the ion diamagnetic drift direction dominates. The region enclosed by this reversal layer is divided into two parts during nearly perpendicular NBI heating: a region where the propagation velocity is near the Etau /Bt poloidal rotation velocity and a bad curvature region of very small wavenumber and high propagation velocity. The region of high propagation velocity, found in NBI plasmas, disappears in the ohmic plasmas. In addition, a small component that propagates in the ion diamagnetic drift direction is observed in NBI plasmas

Application of the intermediate frequency range fast wave to the JIPP TII-U plasma

A series of experiments has been conducted on the JIPP TII-U tokamak since 1989, using the newly constructed 130 MHz radiofrequency system. It has been predicted theoretically that the fast wave in this frequency range interacts weakly with particles. Two mechanisms of wave absorption have been identified in the experiment: electron Landau damping/transit time damping and 3rd harmonic ion cyclotron heating. The former mechanism is intimately connected with fast wave current drive and the latter can provide a new regime of plasma heating or a possible method of controlling the transport of alpha particles. It is found that the efficiency of the 3rd harmonic ion cyclotron heating is improved by using it in combination with neutral beam injection and ion cyclotron range of frequency heating. The heating efficiency obtained is as high as that of conventional heating. The experimental results are also analysed on the basis of a global wave theory which takes into account wave-particle interactions. These mechanisms of interaction are competing with each other; this will also be the case under more realistic reactor conditions.

Limiter H-mode and other improved confinement regimes with ICRF and NBI heating in JIPP T-IIU

The H-mode, an improved confinement regime, was attained recently in JIPP T-IIU high power heating experiments, in a limiter configuration without any shaping of the plasma cross-section. This H-mode is unusual because it was obtained with heating in the ion cyclotron range of frequencies (ICRF). It was also attained with combined ICRF and neutral beam injection (NBI) heating. The threshold power level obtained with ICRF alone is similar to or less than that obtained in the combined heating case. The dependence of the power threshold of the H-mode on various plasma parameters has been studied. It increases with the plasma current and is insensitive to the plasma density, and there is an optimum value of the toroidal field intensity. The power deposition profile for ICRF heating has been analysed with a ray tracing code and used to explain the observed dependence on the toroidal magnetic field. The paper also discusses a class of discharges with improved confinement observed in the same series of experiments. These discharges had a power level close to the H-mode threshold power and exhibited a marked improvement of confinement. They were, however, different from H-mode discharges in the time evolution of the profiles.

Fast wave heating at intermediate ion cyclotron harmonics on the JIPP T-IIU tokamak

Fast wave absorption and heating at a relatively high ion cyclotron harmonic (ω ΩD) were studied on JIPP T-IIU. In spite of the low absorption efficiency predicted by a simple theory for present experimental parameters, appreciable electron and ion heating was observed. Heating is most effective at relatively low densities (e 1 × 1019 m−3). An ion tail on the majority deuterium energy spectrum was observed, but not an ion tail on the minority hydrogen spectrum, which suggests the possibility of direct absorption of mode converted ion Bernstein wave power by deuterium ions.