ITER CS Model Coil Research Articles

Superconducting Property and Strain Effect Study of the ${\rm Nb}_{3}{\rm Sn}$ Strands Developed for ITER

Japan Atomic Energy Agency has developed Nb3Sn strands for the ITER magnet, whose critical current density is about 1.4 times as large as that of ITER CS Model Coil. The magnetic field, temperature, and strain dependence on the critical current (Ic) of the strands were measured with apparatus that were developed by the authors. The strain properties in particular were investigated in detail, with strain tensor analysis revealing that the upper critical field dependence on strain can be naturally formulated using a high order polynomial terms of strain taking into account residual strain due to thermal contraction difference inside the strand and strain applied externally. A correlation formula for field, temperature, strain and Ic of the strand is discussed and presented. Recent results of fabricating strands for ITER TF coil are then finally provided.

Measurement of High Frequency Impedance of ITER CS Model Coil and the Simulation Analysis of Internal Resonance Phenomenon

This paper describes the internal resonance phenomenon of a large scale superconducting coil. The frequency dependence of electric impedance defined at the coil terminal and the voltage distribution of the layer winding were measured at the ITER CS model coil under the superconducting state in advance of the pulse operation test. As a result, large change of the impedance and unbalance of the layer voltage were observed. The AC circuit analysis was carried out in order to evaluate the measurement results and to calculate the voltage of layer insulator. It was concluded that the higher voltage, which may appear on the layer insulation, is about a half of the coil terminal voltage in high frequency.

Progress in the Superconducting Magnet Technology Through the ITER CS Model Coil Programme

The world's largest high-field pulsed super conducting magnet, the CS model coil, was manufactured and tested by the international and participant teams for the verification of new design principles and fabrication concepts proposed for the ITER CS. State-of-the-art technologies were developed through the CS model coil program including high-performance Nb/sub 3/Sn strand, 46 kA cable-in-conduit conductor, and coil fabrication using "wind, react, insulate and transfer" method that enables Nb/sub 3/Sn to be applicable to a large magnet with mechanically rigid and high-voltage insulated windings. The CS model coil reached its design point of 13 T and 46 kA and demonstrated the pulsed operation of 0.6 T/s to 13 T, achieving all the development goals. The experiments of the CS model coil and three single-layer inserts have provided detailed knowledge on the performances of the coils and conductors including the hydraulic behavior, AC losses and critical currents. In particular, significant effects of large electromagnetic forces acting on the conductor were observed in these performances and the movement of cable inside the conductor was identified to be important. Results and experiences from the program are not only useful for the design optimization of the ITER magnets, but also applicable to other areas such as high energy physics.

Toward an accurate scaling relation for the critical current in niobium-tin conductors

Until a few years ago, a set of equations commonly referred to as the Summers relations gave the most accurate description of the critical current in Nb/sub 3/Sn conductors as a function of applied field, temperature and axial strain. Although highly empirical, they describe reasonably well the critical current data of past Nb/sub 3/Sn conductors. New data from various types of Nb/sub 3/Sn conductors, as well as recent analysis of the ITER CS model coil results reveal however, that this description lacks the precision, required to correlate the conductor data to the model coil results. This discrepancy, attributed to the highly empirical background for the relations, manifests itself mainly in the strain- and temperature dependence. The development of an alternative, more accurate description of the behavior of the critical current, starting from a more fundamental description of the strain dependence, has been initiated. At the moment, the development concentrates around the improvement of the temperature dependency relations to achieve a better accuracy of the overall descriptions, especially in the high temperature region.

Pulsed operation test results of the ITER-CS model coil and CS insert

The ITER-Central Solenoid (CS) model coil, which was wound with forced cooled Nb/sub 3/Sn cable-in-conduit conductors, was fabricated and tested to demonstrate the justification of CS design for ITER. The CS model coil, whose magnetic stored energy at generation of 13 T is 640 MJ, is the largest one among superconducting pulsed coils constructed so far. The target of pulsed operation is to generate 13 T with ramp rate of 0.4 T/s. The CS model coil and also CS insert were achieved the target without quench. One of the most interesting test items for these coils is to know the dependence of quench current on ramp up rate in pulsed operation. Another is to investigate temperature margin to the target, which was made by changing ramp up rate and inlet temperature. In case of inlet temperature of 4.5 K, the CS model coil and CS insert were operated up to 13 T without quench with a rate of 0.6 T/s and 1.2 T/s, respectively. In case of 6.5 K, however, quench current of the coils was decreased as ramp rate was increased. These results are discussed in comparison with the critical current and the limiting current.

ITER CS model coil and CS insert test results

The inner and outer modules of the central solenoid model coil (CSMC) were built by US and Japanese home teams in collaboration with European and Russian teams to demonstrate the feasibility of a superconducting central solenoid for ITER and other large tokamak reactors. The CSMC mass is about 120 t; OD is about 3.6 m and the stored energy is 640 MJ at 36 kA and peak field of 13 T. Testing of the CSMC and the CS insert took place at Japan Atomic Energy Research Institute (JAERI) from mid March until mid August 2000. This paper presents the main results of the tests performed,.

First Test Results on ITER CS Model Coil and CS Insert

The Inner and Outer modules of the Central Solenoid Model Coil (CSMC) were built by US and Japanese home teams in collaboration with European and Russian teams to demonstrate the feasibility of a superconducting Central Solenoid for ITER and other large tokamak reactors. The CSMC mass is about 120 t, OD is about 3.6 m and the stored energy is 640 MJ at 46 kA and peak field of 13 T. Testing of the CSMC and the CS Insert took place at the Japan Atomic Energy Research Institute (JAERI) from mid March until mid August 2000. This paper presents the main results of the tests performed.

Experimental verification of the temperature and strain dependence of the critical properties in Nb/sub 3/Sn wires

The critical current density in Nb/sub 3/Sn conductors is described with an improved scaling formula for the temperature, magnetic field and strain dependence. In an earlier study, it is concluded that the largest uncertainties in this description arise from the temperature dependence that is described with various slightly different empirical relations. For the optimization of the numerical codes, used to predict the stability of large magnet systems, a more accurate description is required. Therefore, two different bronze processed conductors for the ITER CS model coil are analyzed in detail. The critical current is measured at temperatures from 4.2 K up to the critical temperature, in magnetic fields from 1 T to 13 T and with an applied axial strain from -0.6% to +0.4%. The axial strain is applied by a U-shaped bending spring and a comparison is made between brass and Ti-6Al-4V, as substrate material.

Development of 46-kA Nb/sub 3/Sn conductor joint for ITER Model Coils

The conductor joint is one of the key technologies for superconducting coils. A butt type joint has been successfully developed for the ITER magnets. The 46 kA Nb/sub 3/Sn conductors are connected by the diffusion bonding technique in vacuum, after the reaction of Nb/sub 3/Sn. The advantage of this joint is low losses against pulse field, because the compacted part is very small compared with other types of joint. 15 butt joints have already been fabricated in the ITER CS Model Coil. According to the test results of the full-size conductor samples, these butt joints will be operated stably in the pulse operation, because the temperature increase due to ac losses and Joule heating by joint resistance is very small and the joint has a sufficiently high temperature margin.

Preparations for pulse operation tests of the ITER CS model coil using the JT-60 power supply

Some preparations for pulse operation tests of the ITER CS model coil in the JT-60 power supply system have been performed. Since the ITER CS model coil takes a longer current ramp-up time, the pulse duration is extended up to 70 s from 15 s by decreasing the output voltage of the motor-generator. In the protection system a new grounding resistor with higher resistance has been installed and a new protection make-switch has also been developed. An analysis for the local resonances in the coil has been performed, and measurements of the coil impedance is planned to confirm the validity of the analysis results.

Qualification of joints for the inner module of the ITER CS model coil

The US-ITER home team has constructed a prototype, low-loss, low-resistance lap joint sample using materials, fabrication techniques, machine tooling and quality assurance procedures identical to those for the layer-to-layer joints in the inner module of the ITER Central Solenoid Model Coil. The joint sample was tested at the MIT Pulse Test Facility to 50 kA current, in both parallel and transverse time-varying fields at ramp rates from 0.05 to and background induction to 4T, to qualify performance to ITER-relevant operating conditions. The joint shows DC below 2.5 nOhm and pulsed, transverse field losses for a 1.5 ramp at 0.4 T/s below 90 J; both values are well within target limits for the ITER-CS joints.

Final design of a cryogenic system for the ITER CS model coil

The final design of a 5-kW / 800-L/h cryogenic system was completed and the construction has been started. Cloude cycle with double JT valve system is adopted. The system will be available for 8-kW refrigeration in addition of a cryogenic turbine. High energy efficiency is expected, where 1/346 (Watt 4.5K/Watt 300K) and 1/248 at 5kW and 8kW, respectively. Re-installation of the existing cryogenic components has achieved the cost reduction of the system by more than 30%.