R. Maekawa

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.

Extra AC losses for a CICC coil due to the non-uniform current distribution in the cable

Extra AC losses were observed during the Experiments on a Single Inner Vertical coil (EXISV). The Inner Vertical (IV) coils are the smallest poloidal coils for the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with cable-in-conduit conductor (CICC) whose strands are NbTi/Cu without any surface coating. Many causes for the extra AC losses were considered, such as the decrease of a contact resistance between strands due to the large electromagnetic force in the conductor or due to the stress during the coil winding process, etc. and possibilities were investigated from the experimental data. Finally, we found that a coupling current with a very long time constant of 124 s caused the AC loss increase. The coupling current with such a long time constant cannot be explained from the symmetric twisting configuration of the CICC but can be explained as a local loop current corresponding to a cyclic change of the non-uniform current distributions in the cable. The non-uniform current distribution could be induced by an asymmetry of the strand transposition in the cable. To verify the above reasoning, we did fundamental experiments on a two-strands-cable, which has an intended asymmetry in the cable twisting. Extra AC losses were also observed for an asymmetric two-strands-cable, and it was demonstrated that the non-uniform current distribution causes an increase of AC losses.

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.

AC loss measurements of the experiments on a single inner vertical coil (EXSIV) for the Large Helical Device

The AC losses of the Inner Vertical (IV) coil have been measured during the Experiments on a Single Inner Vertical coil (EXSIV). The IV coils are the smallest poloidal coils of the Large Helical Device (LHD) and their inner and outer diameters are 3.2 m and 4.2 m, respectively. The coil consists of 16 pancake coils wound with the cable-in-conduit conductors (CICC) whose strands are NbTi/Cu without any surface coating. The AC losses were measured with a calorimetric method during excitation tests and current shut-off tests. In addition to the usual inter-strand coupling losses with short time constants, unexpected coupling losses were observed due to the coupling current of a long time constant.

Achievement of high availability in long-term operation and upgrading plan of the LHD superconducting system

The Large Helical Device (LHD) is not only the largest stellarator for the research of fusion plasma near a reactor region but also the largest superconducting system. Availability higher than 98% has been achieved in the long-term continuous operation both in the cryogenic system and in the power supply system. It is due to the robustness of the systems and efforts of maintenance and operation. One big problem is the shortage of cryogenic stability of a pair of pool-cooled helical coils. Composite conductors had been developed to attain sufficient stability at high current density. However, it was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into a thick pure aluminium stabilizer. Besides, a novel detection system with pick-up coils along the helical coils revealed that normal-zones were initiated near the bottom of the coil where the field is not the highest. Therefore, the cooling condition around the innermost layers, the high field area, will be deteriorated at the bottom of the coil by bubbles gathered by buoyancy. In order to raise the operating currents, methods for improving the cryogenic stability have been examined and stability tests have been carried out with a model coil and small coil samples. We selected a method to lower temperatures of the coil and an additional cooler has been installed at the inlet of the coil. The outlet temperatures of the coil have been successfully lowered to 3.8 from 4.4 K of the saturated temperature, as planned.

Kapitza conductance of an oxidized copper surface in saturated He II

The heat transfer from a metal to He II is determined by Kapitza conductance at the interface. Surface temperature estimation of the metal is used for the study of Kapitza conductance. The surface temperature is usually extrapolated from the temperature gradient in the metal. In the case of the metal with a coated layer, however, it is difficult to estimate the surface temperature. A similar case is applied for the superconductor of the helical coil of the large helical device (LHD). The copper surface is chemically treated by oxidation in order to improve the heat transfer characteristics in He I. It is planned that the helical coil will be cooled by He II in the phase II upgrade. The Kapitza conductance of the conductor has to be estimated for a stability analysis. Therefore, the heat transfer from the oxidized copper surface to the saturated He II has been measured using an oxygen-free copper cylinder. To investigate the thermal resistance in the oxidized layer, two coating surfaces are prepared: (a) coated with Stycast; (b) coated with Stycast on the oxidation layer. In this paper, the Kapitza conductance of the chemically oxidized copper surface is discussed.

Overview of long pulse operation in the Large Helical Device

The Large Helical Device is the worlds largest heliotron type helical system, with the plasma confining magnetic field being generated by only external superconducting coils. One of the main objectives of the LHD project is to sustain high temperature plasmas for a long time in steady state. The plasma vacuum vessel and the divertor are water cooled, and a heat load of 3 MW can be removed continuously. The NBI, ECH and ICRF heating systems, diagnostic instruments and data acquisition system are designed for long pulse operation. The present status of these systems and the recent experimental results of long pulse operation are reviewed. A steady state discharge with NBI was obtained for 35 s. The ECH discharge duration was extended to 120 s with a duty factor of 95%. Plasma sustainment by ICRF alone was achieved for 2 s. The performance of these long pulse operations is summarized.

Summary of a 1 MJ Conduction-Cooled LTS Pulse Coil Developed for 1 MW, 1 s UPS-SMES

The development study of a 1 MJ conduction-cooled low temperature superconducting (LTS) pulse coil used for a 1 MW, 1 s UPS-SMES is summarized. We have developed a conduction-cooled LTS pulse coil as a key technology for the UPS-SMES. The AC loss reduction and the high stability are required for the SC conductor for a LTS pulse coil because of a limited cooling capacity of 4 K cryocooler. The conductor of a NbTi/Cu compacted strand cable extruded with an aluminum was designed to have the anisotropic AC loss properties to minimize the coupling loss. The coil was wound, utilizing a specially developed automatic winding machine which enables an innovative twist-winding method. The Dyneema FRP (DFRP) spacers and the Litz wires (braided wires of insulated copper strands) were inserted in each layer in order to enhance the heat transfer in the coil windings. The coil was installed in the test cryostat and was connected to three GM cryocoolers, which have a total cooling capacity of 4.5 W at 4 K and 240 W at 50 K. The coil was cooled conductively without liquid helium by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. The cooling and excitation test of the 1 MJ coil has been done successfully. The test results validated the high performance of the conduction-cooled LTS pulse coil, because the high thermal diffusivity resulted in the rapid temperature stabilization in the coil.

Validation of the High Performance Conduction-Cooled Prototype LTS Pulse Coil for UPS-SMES

A conduction-cooled low temperature superconducting (LTS) pulse coil has been developed as a key technology for UPS-SMES. We have been developing a 1 MW, 1 s UPS-SMES for a protection from a momentary voltage drop and an instant power failure. A conduction-cooled LTS pulse coil has excellent characteristics, which are adequate for a short-time uninterruptible power supply (UPS). The LTS coil has better cost performance over the HTS coil at present and the conduction cooling has higher reliability and easier operation than the conventional cooling schemes such as pool boiling with liquid helium or forced flow of supercritical helium. To demonstrate the high performances of the LTS pulse coil, we have fabricated a prototype coil with stored energy of 100 kJ and have conducted cooling and excitation tests. The successful performance test results including current shut-off test with a time constant of 1.3 s and repeated excitation of a triangular waveform with high ramp rate are reported

Experimental evaluation of loss generation in HTS coils under various conditions

To develop the high performance HTS coil operated in persistent-current mode, loss mechanisms of the HTS coil have been studied. We consider that the loss generation is associated with the shielding current and its temporal variations. To investigate the shielding current characteristics, the decay of shielding currents were measured under various conditions, and we evaluated these effects in simple experiments.