T. Satow

Coupling losses in cable-in-conduit conductors for LHD poloidal coils

Abstract Coupling losses in Large Helical Device (LHD) poloidal coils have been measured during operations with three different waveforms. The superconductors of the poloidal coils are cable-in-conduit conductors (CICC) cooled by supercritical helium. In the experiments, the operating currents were simultaneously changed with a given waveform, and the enthalpy increase due to the losses was observed at the inlet and outlet of the helium coolant. Inter-strand coupling currents through resistive contact points mainly caused the losses. Time constants of the coupling currents are estimated by using analytical expressions with a circuit model. The results indicate a broad distribution of the time constants from the order of 10–1000 s.

A large superconducting thin solenoid magnet for TRISTAN experiment(VENUS) at KEK

A 30 Gev e+e- collider TRISTAN is now under construction at KEK. The detector system VENUS in TRISTAN requires a large space of 7.5 kG magnetic field for the tracking of the particles while keeping the material thickness of the magnet as thin as possible. The superconducting thin solenoid magnet which is in the middle of the construction has 3.4m in warm bore diameter and 5.24m in usable length with material thickness of 0.52 radiation length. The geometrical thickness of the magnet is as thin as 208 mm while the outside dimension is as large as 4m phitimes 5.64 m. Since the electro-magnetic force is toward outside, the coil is supported by an aluminum case covering outside the coil instead of a bobbin in the coil. The water cooled welding technique and an expandable mandrel were developed to form the coil-case composite. The conductor is made of NbTi/Cu and pure aluminum stabilizer is extruded so as to contain the conductor in the stabilizer. The cooling of the magnet is provided by the forced flow of two phase helium in the cooling channel of the case. The cryostat uses CFRP (Carbon Fiber Reinforced Plastics) to reduce the material thickness of the magnet. The construction method and the design were confirmed by 1m^{phi} superconductive model and real size non-superconductive model. Major components such as conductors and cryostat have been completed and the magnet will be excited next spring.

Test results of high ramp rate pulsed superconducting coil for the reacting plasma tokamak

A model pulsed coil (RPC-II) with an average ramp rate of 200 T/s has been built and tested in order to meet the requirements of superconducting poloidal coils of medium size tokamaks. The inner and outer diameters of the coil are 19.0 and 47.2 cm, respectively, with an axial length of 43.1 cm. The coil has a solenoid winding with a layer to layer cooling spacing of 2.2 mm and insulation sheets between layers. The pulsing tests were performed by a capacitor discharge method with a crowbar circuit. By applying a terminal voltage of 7.6 kV, the field was raised up to 3.9 T in 19 ms, slightly increased up to the maximum field 4.0 T in about 120 ms from the start, and decayed with a time constant of 450 ms. This mode simulates the operation of ohmic heating coils. The average and maximum pulsing rates in charge-up period were 200 T/s and 314 T/s, respectively. The associated a.c. loss per pulse was about 400 J which was only 0.3 % of the energy stored in the coil. After twelve pulses with a repetition period of 15 min., no change in the pulsing characteristics of the coil has been observed. These results showed the scientific feasibility of superconducting poloidal coils to be used in long-pulse medium size tokamaks.

Superconducting poloidal coils for the reacting plasma project-test results of a model coil (RPC-I) at a 200 T/sec pulsing rate and design of a new coil (RPC-II)

A model pulse coil with a stored energy of 0.45 MJ at 6 T has been built and tested in order to meet the requirements of superconducting poloidal coils of a medium size tokamak of the Reacting Plasma Project. The inner and outer diameters of the coil (RPC-I) are 19.0 and 46.8 cm, respectively, with an axial length of 41.4 cm. The pulsing test was performed by a condenser discharge method with a clamp circuit. By applying a terminal voltage of 7.0 kV, the coil was charged up to 3.84 T in 26 msec and discharged to zero with a time constant of 100 msec. The average and maximum of pulsing rates in charge-up period were 147 T/sec and 231 T/sec, respectively. The associated a.c. loss per pulse was approximately 1 kJ which was only 0.57 % of the energy stored in the coil. These results have shown for the first time the scientific feasibility of superconducting pulse coils with a pulsing rate of about 200 T/sec. As the next step, we are constructing a new coil (RPC-II) with a winding composed of wider helium cooling channels and insulation sheets between layers.

Excitation of a superconducting large thin solenoid magnet

A superconducting large thin solenoid magnet was contructed for the VENUS detector in the TRISTAN 30 GeV e+ e- collider. The magnet has a 3.4 m warm bore diameter and a 5.24 m usable length with a material thickness of 0.52 radiation length. The first excitation was successfully performed up to 4000 A with no quench. Several forced quenches using a heater mounted on the coil were made to check the safety of operation. The quench propagation velocity was clearly measured in these quenches. The quench back phenomenon was observed about one second after the shut off of the power supply. The field quality of the magnet was found to be uniform within an accuracy of 0.3 per cent. The success of the first excitation was immediately followed by the assembly of the detector. VENUS was rolled into the TRISTAN collider storage ring and is now prepared for the first collision of e+ and e-.

SUPERCONDUCTING POLOIDAL COILS FOR THE REACTING PLASMA PROJECT I. A.C. LOSS AND STABILITY OF A PROTOTYPE CABLED CONDUCTOR

A cabled superconductor of current-carrying capacity of 10 kA at 6 T has been designed and fabricated to meet the requirements of the fast ramp-rate superconducting poloidal coils. The basic strand was a Cu/CuNi mixed matrix NbTi multifilamentary composite. The a.c. loss and the stability of subcables have been investigated under pulsing fields with a maximum of 9 T and ramp rates in excess of 200 T/sec. The temperature rise caused by a.c. losses was so small that the quench current in pulsing fields was close to the critical current at 4.2 K and considerably larger than the cold-end recovery current in static fields. These results confirm the scientific feasiblity of cryostable, low loss poloidal coils of pulsing rates of 200 T/sec.

Soundness evaluation of cryogenic support structure of large helical device

Abstract A fusion magnet system using superconducting magnets must have a cryogenic support structure to sustain huge electro-magnetic force. Since the structure is constructed by welding and it supports static and dynamic forces, the monitoring system for soundness evaluation of the support structure has been desired. In the Large Helical Device project, strain gages and wires were developed and applied to the strain monitoring system. The gages were attached on the weld line to investigate the change in strain due to crack initiation on the weld root. All gages are still active and the monitoring system is working on well. No significant change in strain has been observed and it results in revealing the soundness of the support structure of the Large Helical Device.

SUPERCONDUCTING POLOIDAL COILS FOR THE REACTING PLASMA PROJECT II. PERFORMANCE TEST OF A MODEL COIL UNDER A 200 T/sec PULSING RATE

A model pulse coil has been built and tested in order to meet the requirements of superconducting poloidal coils of a medium size tokamak of the Reacting Plasma Project. The inner and outer diameters of the coil are 19.0 and 46.8cm, respectively, with an axial length of 41.4cm. The pulsing test was performed by a condenser discharge with a clamp circuit. By applying a terminal voltage of 7.0 kV, the coil was charged up to 3.84 T in 26 msec and discharged to zero with a time constant of 100 msec. The average and maximum of pulsing rates were 14 7 T/sec and 2 31 T/sec, respectively. The associated a.c. loss per pulse was approximately 1 kJ which was only 0.57% of the energy stored in the coil. These results have proven for the first time the technological feasibility of a superconducting pulse coil with a pulsing rate of about 200 T/sec.

SMES-UPS for large-scaled SC magnet system of LHD

The LHD is an SC experimental fusion device of heliotron type. Eight sets of the helium compressors with total electric power of 3.5 MW are installed in the cryogenic system. The analytical studies of the SMES-UPS for the compressors under the deep voltage sag are reported in this paper. The amplitude and frequency of the voltage decrease gradually by the regenerating effect of the induction motors. The SMES-UPS system proposed in this report has the following functions; (1) variable frequency control, (2) regulations by ACR and AVR, and (3) rapid isolation and synchronous reconnection from the loads to grid line. We have demonstrated that SMES was useful for the large-scaled cryogenic system of the experimental fusion device.