M. Ohya

Cooling stability test of He II cooled LHD conductor (1) - current supply and measuring method

This paper deals with experimental methods to perform the dynamic stability test of large-scale superconductors cooled by He II. The large helical device (LHD) conductor used in this work has a critical current of dozens of kA. It is difficult to supply such a large current to the conductor immersed in pressurized He II by use of an external power supply in a laboratory scale. The transformer type current supplying method was proposed. The small test coil was wound and short-circuited with the LHD conductor on a stainless steel bobbin. The test coil was set in the center of a superconducting coil coaxially, which supplies an external magnetic field to the test coil. The test coil was inductively coupled with the field coil at a large turn ratio. The large current through the short-circuited test coil was excited by changing the outer field coil current at a constant rate and was kept constant to keep the field coil current. In order to obtain the required set of the test coil DC current and the magnetic fields for the stability tests, the test coil current was attenuated at an appropriate time by energizing a heater that attached on the test coil. The short-circuited test coil current was monitored from measured data of the magnetic field by a Hall element. The calculated current was calibrated by use of a Rogowski coil set to the conductor. The cooling stability test of the LHD conductor can be carried out successfully by use of the proposed method.

Cooling stability test of He II cooled LHD conductor (2) - experimental results

Cooling stability tests of the Large Helical Device (LHD) conductor immersed in pressurized He I and He II were carried out. A small test coil wound and short-circuited with a LHD conductor on a stainless steel bobbin was used. The test coil was set coaxially in the center of a superconducting magnet (field magnet), which supplies a certain magnetic field to the test conductor. A large current was supplied to the test coil conductor by use of a transformer effect, that is, the test coil current was induced by increasing the current of the field magnet. Stability tests of the LHD conductor at a certain pulse heat input were performed for the magnetic flux densities from 1.2 T to 6.8 T and the bulk liquid He temperatures from 2.0 K to 4.2 K at atmospheric pressure. Experimental results can be classified into three groups. The normal zone arose only around the heater and disappeared after the heat pulse (Group I). The normal zone moved to only one side direction and disappeared (Group II). The normal zone spread on both sides and the conductor current decreased greatly (Group III). The maximum conductor current without a quench at the pulse heat input increased greatly by shifting to He II regime from He I regime. The advances of the He II cooling on the stability of the LHD conductor were confirmed.

Numerical Analysis on Dynamic One‐side Propagation of Normal Zone Observed in LHD Conductor

The dynamic one‐side propagation of a normal zone, which was initiated by a thermal disturbance, was observed in the superconducting helical coils of the Large Helical Device (LHD). The initiated normal zone propagated only to one‐side along the conductor, and shrank after a few seconds. The dynamic one‐side propagation phenomenon was not expected during the design stage of the LHD, and it has not been explained yet. In this paper, a new numerical code is proposed for predicting the dynamic one‐side propagation phenomenon. As a result of the transient stability simulation, the asymmetrical propagation of a normal zone was seen. At the early stage of the current distribution, symmetrically circulating Hall currents flowed to the longitudinal direction with respect to the center of the initiated normal zone. These Hall currents cause the asymmetrical Joule heat distribution, with the result that the asymmetrical propagation is generated. This is the first time that the dynamic one‐side propagation phenomeno...

Stability of Superconducting wire with various surface conditions in pressurized He II (2)-numerical analysis

We developed a computer code to analyze the stability of a superconducting wire in He II based on the one-dimensional heat equation. We applied to this code our experimental data on heat transfer characteristics of He II, such as Kapitza conductance, critical heat flux and heat transfer curve in film boiling. Using this code, we simulated the recovery or quench process of a normal zone that was initiated by a thermal disturbance for the liquid helium temperatures from 1.8 K to 2.0 K at atmospheric pressure. The numerical solutions agreed with our experimental data for the same characteristics of the wire (bare and oxidized surfaces) and corresponding experimental conditions. We clarified the influences of the surface insulation and the liquid helium temperature on the stability of the superconducting wire.

Effect of Surface Oxidation on Stability of LHD Conductor Immersed in Pressurized

Stability tests were performed on two small test coils wound with the Large Helical Device (LHD) conductors with different surface conditions. One used the conductor with a chemically oxidized copper surface, which is the same as that used for the LHD helical coils. Another used the same structural conductor with a polished copper surface. The stability tests were performed for the magnetic flux densities from 3 T to 7 T and at the bulk liquid helium temperatures from 2.0 K to 4.2 K at atmospheric pressure. Stability limit current for the LHD test coil was defined as a minimum dynamic propagation current, beyond which a short normal zone initiated by a thermal disturbance dynamically propagated to both sides along the conductor. There was a slight difference in the stability limit currents for the two test coils in case of He I cooling. However, the stability limit current for the polished coil was significantly improved as compared with that for the oxidized one in case of He II cooling. The Kapitza conductance on an oxidized copper surface is known to be quite smaller than that on a polished copper surface. One of the main factors that govern the stability of the LHD conductor cooled by He II is the Kapitza conductance

rm He

Stability tests were performed for two small test coils wound with NbTi/Cu composite superconducting wires with different surface conditions respectively. One is a 0.5 mm-diameter bare wire with the copper ratio of 1.3. Another is the same wire with chemically oxidized copper surface. The stable limit current under constant magnetic field greatly increased by shifting to He II cooling from He I cooling for both wires, but stationary normal zones were observed in wide current area lower than the stable limit currents for the oxidized wire. The characteristic of the stability of a superconducting wire is more deeply dependent on the conductor surface condition cooled by He II than that in case of He I cooling.

II

Numerical simulations were carried out on the transient stability of large-scale composite superconductors against a thermal disturbance, that is, a LHD conductor, which consists of a NbTi/Cu Rutherford cable, a pure aluminum stabilizer, and a copper sheath around the composite. The simulations were also performed on an Al-less test conductor, which is a LHD conductor without the Al stabilizer and a half of the copper sheath. The recovery and propagation characteristics of an initiated normal zone were simulated to know the effect of the Al stabilizer on the transient stability of the LHD conductor cooled by Liq.He II. The normal zone propagation initiating current at a certain magnetic flux density for the LHD conductor was compared with that for the Al-less test conductor. Asymmetrical propagation of the normal zone appears even in the LHD based conductor without the Al stabilizer. The range of the transport current, which lead to the one-side propagation, is narrower than those for the LHD conductor. It is confirmed that the Al stabilizer in LHD conductor plays main role in the asymmetrical normal zone propagation. The high performance of the He II cooling in the transient state for the Al-less test conductor makes the normal zone initiating current up to the same level of that for the super-stabilized LHD conductor. It is confirmed that only a slight area of the thick aluminum works as a stabilizer at the transient state because of its low magnetic diffusion factor.

Stability of superconducting wire with various surface conditions in pressurized He II (1)-experimental results

Stability tests were performed of small test coils using two kinds of superconducting wire wound respectively on a FRP or a SUS bobbin. One wire is 0.50 mm‐diameter NbTi composite wire with the copper ratio of 1.3, and with no insulation film. The other wire is a 0.80 mm‐diameter NbTi composite wire with the copper ratio of 6.5, and with the PVF insulation. The stability limit was determined as the maximum direct current that could be applied to the test coil without spreading of a normal zone after giving a pulse current to a small heater located at a center part of the test coil winding. The stability limits were obtained for magnetic fields from 1.1 T to 7.6 T and bulk liquid temperatures from 1.6 K to 4.2 K at atmospheric pressure. The critical current at the stability limit under a constant magnetic field increased slightly with the decrease of liquid He temperature from 4.2 K down to near the λ‐temperature. The stability limit increased dramatically by shifting to He II cooling from He I cooling. Th...

Transient Stability of Large Helical Device Conductor With and Without Aluminum Stabilizer (1)—Experimental Results

An analytical code to simulate the dynamic one-side propagation of a normal zone observed in the LHD conductor was developed. The dynamic one-side propagation phenomenon is the one in which a short normal zone initiated by a thermal disturbance dynamically propagates only to one side along the conductor and shrinks after a few seconds. This code was based on the two-dimensional heat balance equation, current diffusion ones, Hall effect and database on the cooling properties of He II. Using the proposed code, numerical analyses were performed on two LHD conductors with different surface conditions (oxidized and polished). The asymmetrical propagation phenomenon was successfully simulated. The analytical results showed that the stability of the LHD conductor becomes better with making Kapitza conductance large. One of the main factors that govern the stability of the LHD conductor cooled by He II is Kapitza conductance.