Noriyoshi Nakajima

Large Helical Device (LHD) program

The largest superconducting fusion machine, Large Helical Device (LHD), is now under construction in Japan and will begin operation in 1997. Design and construction of related R&D programs are now being carried out. The major radius of this machine is 3.9 m and the magnetic field on the plasma center is 3 T. The NbTi superconducting conductors are used in both helical coils and poloidal coils to produce this field. This will be upgraded in the second phase a using superfluid coil cooling technique. A negative ion source is being successfully developed for the NBI heating of LHD. This paper describes the present status and progress in its experimental planning and theoretical analysis on LHD, and the design and construction of LHD torus, heating, and diagnostics equipments.

Effects of global MHD instability on operational high beta-regime in LHD

In the Large Helical Device (LHD), the highest operational averaged beta value has been expanded from 3.2% to 4% in the last 2 years by increasing the heating capability and exploring a new magnetic configuration with a high aspect ratio. Although the magneto-hydrodynamic (MHD) stability properties are considered to be unfavourable in the new high aspect configuration, the heating efficiency due to neutral beams and the transport properties are expected to be favourable in a high-beta range. In order to clarify the effect of the global ideal MHD unstable mode on the operational regimes in helical systems, especially the beta gradients in the peripheral region and the beta value, the MHD analysis and the transport analysis are performed in a high-beta range of up to 4% in LHD. In a high-beta range of more than 3%, the maxima of the observed thermal pressure gradients at a low order rational magnetic surface in the peripheral region are marginally unstable to the low-mode-number ideal MHD instability. Though a gradual degradation of the local transport in the region has been observed as beta increases, a disruptive degradation of the local transport does not appear in the beta range up to 4%.

Confinement physics study in a small low aspect ratio helical device: CHS

Variation of the plasma position relative to the centre of the helical coil winding is a very effective means of controlling the MHD stability and the trapped particle confinement in heliotron/torsatron systems, but improving one of these two characteristics with this parameter simultaneously has a detrimental effect on the other. The inward shifted configuration is favourable for drift orbit optimization but is predicted to be unstable according to the Mercier criterion. Various physics problems, such as electric field structure, plasma rotation and MHD phenomena, have been studied in the Compact Helical System (CHS) with a compromise intermediate position. With this standard configuration, CHS has yielded experimental results that contribute to the understanding of general toroidal confinement physics and low aspect ratio helical systems. In the recent experiments, it was found that a wide range of inward shifted configurations give stable plasma discharges without any restriction to the special pressure profile. Such an enhanced range of operation made it possible to study experimentally the drift orbit optimized configuration in heliotron/torsatron systems. The effect of configuration improvement was studied with plasmas in a low collisionality regime.

Theory of pressure‐induced islands and self‐healing in three‐dimensional toroidal magnetohydrodynamic equilibria

The role of singular currents in three‐dimensional toroidal equilibria and their resolution by magnetic island formation is discussed from both analytical and computational points of view. Earlier analytical results are extended to include small vacuum islands, which may, in general, have different phases with respect to pressure‐induced islands. In currentless stellarators, the formation of islands is shown to depend on the resistive parameter DR, as well as the integrated effect of global Pfirsch–Schluter currents. It is demonstrated that the pressure‐induced ‘‘self‐healing’’ effect, recently discovered computationally, is also predicted by analytical theory.

High beta discharges with neutral beam injection in CHS

High beta plasmas with a volume averaged equilibrium beta value of 2.1% were produced in CHS using tangential neutral beam injection. This beta value was achieved with the confinement improvement (reheat mode) observed after turning off strong gas puffing. Wall conditioning with titanium gettering was used to make high density operation (ne ? 8 ? 1019 m-3) possible for low magnetic fields (Bt = 0.6 T). The discharges start with the magnetic hill configuration (in vacuum) and finally achieve Mercier stable equilibrium owing to the self-stabilization effect given by the magnetic well which is produced by the plasma pressure. The Shafranov shift was about 40% of a plasma minor radius. Magnetic fluctuations did not increase with increasing plasma pressure when the beta value exceeded 1%. Dynamic poloidal field control was applied to suppress the outward plasma shift with increasing plasma pressure. Such operation gave an additional increase of beta value compared with the constant poloidal field operation

Neoclassical Flow, Current, and Rotation in General Toroidal Systems

Neoclassical theories for the parallel flow, current and rotation are extended to a multispecies plasma in general toroidal systems. The extended theory is constructed in the Boozer coordinate system to be common in all collisionalities (1/ν, plateau, Pfirsh-Schluter regime) in terms of the geometric factor. It is shown that the decisive difference between axisymmetric and non-axisymmetric toroidal systems is attributed to the geometric factor which reflects the breaking of axisymmetry. As a result it is first found that the neoclassical current driven directly by the radial electric field can exist in the non-axisymmetric device, whereas such the current vanishes in the axisymmetric torus. Explicit expressions for poloidal and toroidal rotations are first derived for a multispecies plasma in the non-axisymmetric toroidal system. In the limit of small geometric factor, the poloidal and toroidal rotations of ions are reduced to the usual diamagnetic and E × B drifts.

MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L?H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ? 0.75?T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the rational surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L?H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfv?n eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ?b?, and the ratio of energetic ion velocity to the Alfv?n velocity, Vb?/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

Superdense core mode in the Large Helical Device with an internal diffusion barrier

In reduced recycling discharges using a local island divertor in the Large Helical Device [O. Motojima, H. Yamada, A. Komori et al., Phys. Plasmas 6, 1843 (1999)], a stable high-density plasma develops in the core region when a series of pellets is injected. A core region with ∼5×1020m−3 and temperature of 0.85keV is maintained by an internal diffusion barrier (IDB). The density gradient at the IDB (r∕a∼0.6) is very high, and the particle confinement time in the core region is ∼0.4s. Because of the increase in the central pressure, a large Shafranov shift up to ∼0.3m is observed. The critical ingredients for IDB formation are a strongly pumped divertor to reduce edge recycling, and multiple pellet injection to ensure efficient central fueling. No serious magnetohydrodynamics activity and impurity accumulation have been observed so far in this improved discharge.

Physics and Engineering Design of the Low Aspect Ratio Quasi-Axisymmetric Stellarator CHS-qa

A low aspect ratio quasi-axisymmetric stellarator, CHS-qa, has been designed. An optimization code has been used to design a magnetic field configuration with evaluations of the following physical quantities: quasi-axisymmetry, rotational transform, MHD stability and alpha particle collisionless confinement. It is shown that the electron neoclassical diffusion coefficient is similar to that of tokamaks for the low collisional regime. A self-consistent equilibrium with bootstrap current confirms the global mode stability up to 130 kA for an R = 1.5 m and Bt = 1.5 T device. The neoclassical plasma rotation viscosity is greatly suppressed compared with that of conventional stellarators. The engineering design was completed with 20 main modular coils and auxiliary coils, which provide flexibility of configuration in experiments for confinement improvement and MHD stability.

Optimization of the bootstrap current in a large helical system with L = 2

Neoclassical bootstrap currents in the banana and plateau regimes are evaluated for a large helical system with L = 2 and with a major radius of 5 m. Various vacuum magnetic field configurations are studied with a view to optimizing the bootstrap current. In the banana regime, shifting of the magnetic axis and shaping of magnetic surfaces have a remarkable influence on the bootstrap current. It has been found that a small outward shift of the magnetic axis and vertically elongated magnetic surfaces are favourable for a reduction of the bootstrap current. In contrast, in the plateau regime the bootstrap current depends strongly on the toroidal pitch number of the helical winding coils, but shifting of the axis and shaping of magnetic surfaces are not effective. The magnitude of the bootstrap current is estimated for plasmas in a large helical system with parameters given by a one-dimensional radial transport code.