Ballooning instabilities in a Heliotron J plasma

Recent progress of superconducting magnet system design for Large Helical Device (LHD) of National Institute for Fusion Science (NIFS) is described. LHD is one of large scale nuclear fusion devices which will be completed in the 20th century, and fully superconducting coil system to create magnetic field. LHD is composed of a plasma vacuum vessel, superconducting helical and poloidal coils, cryogenic supporting structures, and an outer vacuum vessel, and surrounded by utilities of coil power supply, helium refrigerator and nitrogen supply, and vacuum pumping system. Cryogenic mass of the system is estimated as 860tons including coil and supporting structure. Cooling method of the superconducting coil is decided as bath cooled of liquid helium for the helical coil, and force cooled of supercritical helium for the poloidal coil. Major and minor radii of the helical coil are 3.9m and 0.975m, respectively. The conductor material of Nb-Ti alloy is used for both helical and poloidal coils from the point of strain effect. Conductor maximum field will be 6.9T for the plasma center field of 3T at the first phase with saturated boiling helium and 9.2T for 4T at the second phase with superfluid helium, Electromagnetic force and electrical insulation of the coil are the most important items to design the coil. Research and development work is intensively continued now, including conductor stability, helical and poloidal winding, elasticity of the coil, and cryogenic handling.

Review of SC magnet technologies developed in LHD project

Magnetohydrodynamic (MHD) stability studies of low-mode-number free-boundary kink modes in a finite-β Large Helical Device (LHD) [A. Ilyoshi, et al. Fusion Technology 17, 169 (1990)] equilibrium series with self-consistent bootstrap current find some serious free-boundary modes. They indicate that care must be taken in the design of high-β experiments. Since the LHD configuration is flexible, with the possibility of introducing or modifying dipole and quadrapole fields, unbalancing the currents in the helical coils, operating with a high-temperature divertor, and changing the collisionality regime by working with different values of temperature and density, paths to high-β operation should exist. Comparison of the experimental results with these theoretical predictions for the studied equilibrium sequence will provide understanding of the MHD stability properties of LHD.

Vertical profiles of edge impurity emissions have been measured in upper half region of elliptical plasmas at horizontally elongated plasma cross section in large helical device (LHD). The vertical profiles near upper O-point located just below helical coil are analyzed to study the plasma edge boundary of the ergodic layer consisting of stochastic magnetic field lines with connection lengths of 30 ≤ Lc ≤ 2000 m. As a result, C3+ ion emitting CIV spectrum is identified as the ion existing in the farthest edge of the ergodic layer. The peak position of CIV (312.4 Å: 1s23p 2P1/2,3/2-1s22s 2S1/2) vertical profile does not change at all in a wide temperature range of 150 ≤ Te(ρ = 1) ≤ 400 eV, whereas it moves inside the ergodic layer when Te(ρ = 1) is reduced below a threshold temperature, e.g., 130 eV at Rax = 3.75 m configuration. It is found that the C3+ ion exists at the boundary between ergodic layer and open magnetic filed layer at which the Lc distributes in lengths of 5 to 30 m. The result indicates that the edge boundary near the O-point in LHD is determined by a starting point of the open filed layer, where a tokamak-like steeper edge temperature gradient is formed, although the edge boundary is quite obscure at the X-point region. Any plasma does not exist between the edge boundary and the vacuum vessel. The CIV profile at the O-point is simulated using a three-dimensional edge transport code of EMC3-EIRENE in which the magnetic field structure in vacuum is used for the ergodic layer. A clear discrepancy of 8 mm is found in the peak positions of CIV between measurement and simulation for magnetic configurations with thick ergodic layer, i.e., Rax =3.90 m, while only a small discrepancy of 3 mm is observed for those with relatively thin ergodic layer, i.e., Rax = 3.75 m. It suggests that the discrepancy is caused by a modification of the magnetic filed due to the presence of plasma pressure.

Field-aligned-impedance-transforming ICRF antenna in the LHD

Performance of the superconducting helical coils of the Large Helical Device (LHD) during the past 12 cooling cycles is reviewed. The pair of helical coils are pool cooled by liquid helium and wound with aluminum-stabilized NbTi/Cu composite-type superconductors. Intensive efforts have been made to reliably carry out excitations, as more than 20 temporary normal transitions were observed. It was found that the minimum propagation current was about 10% lower than the nominal operation current. To improve the cryogenic stability, subcooled liquid helium has been supplied since 2006 using cold compressors, and the inlet temperature is lowered to be 3.2 K. The toroidal magnetic field has been raised by 5% and the plasma parameters are being enhanced. Pulse-height analysis is successfully applied on the balance voltage and acoustic emission signals to investigate the mechanical properties of the windings and their changes in years of operation. Short-duration normal transitions are automatically detected using a sophisticated monitoring system and careful operations are continued.

The Large Helical Device (LHD) is a plasma physics experimental device with a magnetic stored energy of 960 MJ, consisting of two superconducting (SC) helical coils and six SC poloidal coils. The trial operation and the first plasma discharge of the Phase I project for LHD were completed on 31 March 1998 as initially planned. The second experimental campaign was conducted with additional heating using two NBI devices. The third campaign started in June 1999 and was completed in January 2000. Many plasma heating tests with a plasma field of up to 2.90 T were carried out. Major test results on the SC magnet system for LHD are as follows: (1) The LHD cryogenic system was successfully operated for 13 400 h and proved its high reliability. (2) A central field of 2.91 T at a radius of 3.60 m was achieved at an inner helical coil current of 11.08 kA, a middle helical coil current of 11.83 kA and an outer helical coil current of 12.02 kA. (3) All six poloidal coils were excited stably. (4) Nine flexible SC bus lines with a total length of 497 m were operated stably and safely.

Helical coils of the Large Helical Device (LHD) are large scale superconducting magnets for fusion plasma experiments. The cooling system was upgraded in 2006 to improve the cryogenic stability of the coils by subcooling supplied helium. After the upgrade, the subcooled helium of designed mass flow rate of 50 g/s at designed temperature of 3.2 K has been supplied to the coils stably during steady state subcooling operations. The supplied helium is subcooled at a heat exchanger in a saturated helium bath. A series of two centrifugal cold compressors are utilized in the subcooling system to reduce the bath pressure and temperature. It is necessary to operate the cold compressors within a range of appropriate mass flow rate in order to escape from surge and choke. When the rotational speed of the cold compressors increased/decreased, the mass flow rate of the cold compressors varied. Thus, it is important to comprehend the transient performance of the cold compressors in those cases and to control the mass flow rate for the purpose of the safe operations. In the present study, the characteristics of the system were investigated and automatic operating methods with a heater in the bath and a bypass valve were developed during the change of the rotational speed of the cold compressors. Consequently, the reliable subcooling operations of the system have been achieved over 6,000 hours for three years.

The three-dimensional (3D) plasma response to the magnetic field structure is studied for high-β plasmas in the Large Helical Device (LHD). For the vacuum field in the LHD configuration, the boundary of the plasma is defined by the last closed flux surface. However, for high β, the magnetic field structure is changed by the 3D plasma response and the plasma boundary is shifted in the radial direction. The radial electric field, Er, to identify the plasma boundary, is measured in the peripheral region. The radial electric field shear from negative to positive appears in the region and this suggests the boundary between open and closed field lines. The position of the Er shear is always outside the vacuum boundary. A 3D magnetohydrodynamic modelling predicts the expansion of the stochastic region by the 3D plasma response. However, since the connection length of the stochastic field lines is larger than the electron mean free path, a confinement is expected in the region. In such a case, the edge of the stochastic region can be defined as the effective plasma boundary. The position of the appearance of the strong Er shear is almost comparable to the edge of the stochastic region of the modelling. That is, the 3D plasma response is identified in LHD experiments.

Mechanical analysis and fabrication of the R&D forced-flow helical coil (TOKI-PF)

The radial electric field in the plasma edge is studied in the Large Helical Device (LHD) experiments. When magnetic field lines become stochastic or open at the plasma edge and connected to the vessel, electrons are lost faster than ions along these field lines. Then, a positive electric field appears in the plasma edge. The radial electric field profile can be used to detect the effective plasma boundary. Magnetic topology is an important issue in stellarator and tokamak research because the 3D boundary has the important role of controlling MHD edge stability with respect to ELMs, and plasma detachment. Since the stochastic magnetic field layer can be controlled in the LHD by changing the preset vacuum magnetic axis, this device is a good platform to study the properties of the radial electric field that appear with the different stochastic layer width. Two magnetic configurations with different widths of the stochastic layer as simulated in vacuum are studied for low-β discharges. It has been found that a positive electric field appeared outside of the last closed flux surface. In fact the positions of the positive electric field are found in the boundary between of the stochastic layer and the scrape-off layer. To understand where is the boundary of the stochastic layer and the scrape-off layer, the magnetic field lines are analyzed statistically. The variance of the magnetic field lines in the stochastic layer is increased outwards for both configurations. However, the skewness, which means the asymmetry of the distribution of the magnetic field line, increases for only one configuration. If the skewness is large, the connection length becomes effectively short. Since that is consistent with the experimental observation, the radial electric field can be considered as an index of the magnetic topology.

Averaged flux surfaces with respect to the toroidal angle in l =2 heliotron/torsatrons are deformed in D-shape or inverse D-shape by the hexapole field. D-shape deformation of vacuum flux surfaces enhances magnetic well or reduces magnetic hill in Heliotron-E and Large Helical Device (LHD) configuration. However, complete elimination of the magnetic hill region is not obtained by the D-shape deformation without substantial reduction of plasma radius. The inverse D-shape deformation degrades MHD stability against ideal interchange modes in the central region of both devices for fairly peaked pressure profiles. In the LHD configuration, a broad pressure profile proportional to (1-ψ 2 ) 2 degrades the MHD stability and the hexapole effect is almost irrelevant, where ψ is a poloidal flux function; however, second stability appears against both the local and non-local interchange modes.

Kinetic Alfvén and pressure gradient driven instabilities are very common in magnetized plasmas, both in space and the laboratory. These instabilities will be easily excited by energetic particles (EPs) and/or pressure gradients in present-day fusion and future burning plasmas. This will not only cause the loss and redistribution of the EPs, but also affect plasma confinement and transport. Alfvénic ion temperature gradient (AITG) instabilities with the frequency and the toroidal mode numbers are found to be unstable in NBI internal transport barrier plasmas with weak shear and low pressure gradients, where and are the frequencies of the beta- and toroidicity-induced Alfvén eigenmodes, respectively. The measured results are consistent with the general fishbone-like dispersion relation and kinetic ballooning mode equation, and the modes become more unstable the smaller the magnetic shear is in low pressure gradient regions. The interaction between AITG activity and EPs also needs to be investigated with greater attention in fusion plasmas, such as ITER (Tomabechi and The ITER Team 1991 Nucl. Fusion 31 1135), since these fluctuations can be enhanced by weak magnetic shear and EPs.

Stability and quench experiments of a Nb-Ti cable-in-conduit superconductor with bare strands have been carried out to determine the performance of the poloidal coil for the Large Helical Device (LHD). The conductor was formed into a double-pancake coil named IV-S and was mounted on TOKI-PF-a previously tested R&D coil. In excitation tests at 7.4-8.3 K, the IV-S coil reached the critical currents without premature quenches. Stability tests at 7.5 K indicated that the limiting current exceeds 20 kA, which is the nominal operating current of the LHD poloidal coils. These results demonstrated that the stability of the chosen conductor with bare strands is high enough for the LHD.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Beam emission spectroscopy is widely used as local density fluctuation measurement of fusion plasmas. We have installed a beam emission spectroscopy system in the large helical device (LHD) with radially and poloidally elongated optical configuration to enhance both signal-to-noise ratio and wavenumber sensitivity in radial and poloidal directions. The calculation considering the integral effect of the line of sight showed the radially and poloidally elongated sightlines have increased sensitivity compared with the square shaped bundle design of the same sampling area.