ITER Magnet System Research Articles

Analytical studies on the hotspot temperature of cable-in-conduit conductors

This paper describes an analytical study to review the hotspot temperature design criterion of the cable-in-conduit conductors for the ITER magnet system. The ITER magnet system uses three kinds of cable-in-conduit conductors for the Toroidal Field (TF) coils, the Central Solenoid (CS) and the Poloidal Field (PF) coils. The amount of copper in the superconducting cable has been defined by using the classical hotspot temperature design criterion that is based on the adiabatic condition. In the current design, ITER superconducting cables include a large number of pure copper strands to satisfy the classical criterion. However, the temperature and stress in the conduit and insulations after quench can be simulated with the quench simulation program and stress analysis program using the latest analysis tools. This analysis shows that the strand temperature is dominated by the conduction along strands and the heat capacity of other conductor materials and coolant. The hotspot temperature depends strongly on the delay time for quench detection. This analysis provides an estimation of the delay times for quench detection. The thermal and stress analysis can provide the maximum allowable temperature after quench by determination of a failure or a functional disorder condition of the jacket material and turn insulation. In conclusion, it is found that the current density of the cable space can be increased, by reducing the extra copper strand, thereby, allowing a reduction of the coil radial build.

Analytical Studies on Hotspot Temperature of Cable-in-conduit Conductors

This paper describes an analytical study to review the hotspot temperature design criteria of the cable-in-conduit conductors for the ITER magnet system. The ITER magnet system uses three kinds of cable-in-conduit conductors for the Toroidal Filed (TF) coils, the Central Solenoid (CS) and the Poloidal Filed (PF) coils. The amount of copper in the superconducting cable has been defined by using the classical hotspot temperature design criteria that is based on the adiabatic condition. In the current design, ITER superconducting cables include a large amount of pure copper strands to satisfy the classical criteria. However, temperature and stress in the conduit and insulations after quench can be simulated with the quench simulation program and stress analysis program using the latest analysis tools. This analysis shows that the strand temperature is dominated by the conduction along strands and the heat capacity of other conductor materials and coolant. The hotspot temperature depends strongly on the delay time for quench detection. This analysis provides an estimation of delay times for quench detection. The thermal and stress analysis can provide the maximum allowable temperature after quench by determination of a failure or a functional disorder condition of the jacket material and turn insulation. In conclusion, it is found that the current density of the cable space can be increased, by reducing the extra copper strand, thereby, allowing a reduction of the coil radial build.

Qualitative analysis of accidents possible in iter magnets

Abstract The ITER magnet system main elements are concisely described. The superconducting magnets of Russian tokamaks T-7, T-15 and Stand for Investigation of superconducting Magnet Systems (SIMS) are considered as the representative models of the ITER magnet system and the basie characteristics of their designs are presented. The failures and accidents of superconducting magnets that have taken place in these representatWe models are analyzed. The potential initial events that may cause accident situations in the ITER magnets are identified taking into consideration the experience gained under superconducting magnet operation at Russian experimental facilities.

Three dimensional numerical structural analysis of the iter magnet system at fault event

Abstract An accident has been considered when a short circuit takes place in the single ITER toroidal coil. The case has been studied when the current in the shorted coil is 250% of the coil current at normal operation. The current in the rest of the coils drops to 50% of the normal current. The 3D FE stress analysis has been carried out and the main analysis results relating to the structure deformation and the stresses state in the system components are presented. It is shown that the main stress occur in the shorted coil and the value does not exceed ∼ 1000MPa.

Analysis of Selected Mechanical Details of the ITER Magnet System

The magnet system of the full sized ITER (Final Design Report (FDR) version), has progressed to the point where basic sizing has been accomplished and local details are being addressed. The ITER US Home Team has worked on global structural models of the magnet system as well as local models of components that are strongly coupled with the global behavior. Areas of interest include the crown – monorail -slot interaction, proposed outerintercoil structure (OIS) cover plates, and case wall thickness studies. The root of the crown slot is highly stressed. Specialized fabrication techniques will be needed to eliminate flaws from this area to meet fatigue criteria. The fluted geometry of the outer cylinder was not found to contribute to the Out-of-Plane (OOP) support of the The inner leg. TF case thickness was studied and it was concluded that the sidewalls of the case may be thinned up to 7cm in selected regions between the OIS. A cover added to the outside of the middle OIS was found to improve the shear loads at the flange pins. The use of 20 vs. 10 support structures for PF 1 and 9, is recommended.

Investigation of cryogenic irradiation influence on mechanical and physical properties of ITER magnetic system insulation materials

A set of methods of cryogenic irradiation influence test on mechanical and physical properties of insulation of ITER magnetic system are presented in this paper. Investigations are carried out without intermediate warming up of samples. A Russian insulating composite material was irradiated in the IVV-2M Reactor. The ratio of energy absorbed by insulation materials from neutron irradiation to that from gamma irradiation can be varied from ∼(25:75) % to ∼(50:50) % in the Reactor. The test results on the thermal expansion, thermal conductivity and gas evolution of the above material are presented. It was shown, that cryogenic irradiation up to the fluence ∼2 × 10 22 n/m 2 ( E⩾0.1 MeV) leads to 0.27% linear size changes along layers of fiber-glass, the thermal conductivity coefficient is decreased on 15% at 100 k in perpendicular direction to fiber-glass plane, and thermal coefficient of linear expansion (TCLE) has anomalous temperature dependence.

Analysis of electric arc short-circuit consequences in ITER toroidal field coils

Abstract Based on the results of the ITER magnet system structure analysis the thermal analysis of a toroidal field coil in the case of short-circuit has been performed in view of the normal zone propagation (quench), as well as the conductor temperature and coil resistance rise. The electric arc behaviour within the magnet system is considered. The main results of calculations and theoretical study of the electrical arc under conditions typical for short-circuits in the magnet system coils are outlined. Theoretical physical models for an arc discharge burning within the insulation and conductor bulk are developed and described. Energy release in the arc is a matter of concern. Dangerous consequences of the arc discharge burning in the magnet system are estimated.

The ITER magnet system

The main components of the ITER magnet system include 20 toroidal field (TF) coils, a central solenoid (CS), seven poloidal field (PF) coils and structural elements. The TF coils operate at a maximum field of 12.5 T with a total stored energy of about 100 GJ. These coils are enclosed in stainless steel cases that form the major part of an integrated mechanical structure and are bucked on the CS. The CS provides about 140 MA turns at a maximum field of 13 T. In the reference design, the CS is a monolithic, 12 m tall coil. An alternative segmented CS design is being considered to extend the range of operational flexibility and plasma control capability. The PF coil system comprises seven coils, with diameters up to 30 m, for plasma and shape control. The salient activity of the ITER R&D programme is the manufacture of model coils. The conductor production for these coils is well underway and construction of the coils has started. The initiation of the model coil test programme is scheduled for mid 1998.

Magnet Safety Assessment for ITER

The ITER magnet system consists of structurally linked sets of toroidal (TF) and poloidal (PF) field coils, central solenoid (CS), and various support structures. The coils are superconducting, force flow Helium cooled with a Kapton-Glass-Epoxy multilayer insulation system. The stored magnetic energy is about 100GJ in the TF system and 20GJ in the PF-CS. Coils and structure are maintained at 4 K by enclosing them in a vacuum cryostat. The cryostat, comprising an outer envelope to the magnets, forms most of the second radioactivity confinement barrier. The inner primary barrier is formed by the vacuum vessel, its ports and their extensions. To keep the machine size within acceptable bounds, it is essential that the magnets are in close proximity to both of the nuclear confinement barriers. The objective of the magnet design is that, although local damage to one of the barriers may occur in very exceptional circumstances, large scale magnet structural or thermal failure leading to simultaneous breaching of both barriers is not credible. Magnet accidents fall into three categories: thermal (which includes arcing arising from insulation failure and local overheating due to discharge failure in the event of a superconductor quench), structural (which includes component mechanical failure arising from material inadequacies, design errors and exceptional force patterns arising from coil shorts or control failures), and fluid (Helium release due to cooling line failure). After a preliminary survey to select initial faults conceivable within the present design, these faults are systematically analyzed to provide an assessment of the damage potential. The results of this damage assessment together with an assessment of the reliability of the monitoring and protective systems, shows that the magnets can operate with the required safety condition.

Design features of internal tin superconductors for ITER magnetic system

The influence of parameters of internal tin superconductor design on the main working characteristics such as critical current density and hystereris losses were analyzed. It was shown that having the value of hysteresis losses at the acceptable level of 400-600 mJ/cm/sup 3/, the critical current density 20 30% higher than the value typical for bronze route processed superconductors was attainable in principle. The results or experimental work on the design of new types of internal tin superconductors for ITER magnetic system are given.

Mechanical modeling of the ITER toroidal field coils shear keys behavior. Design and choice of key mock-up for electrical insulation testing on the basis of numerical models

The ITER shear key system is designed to interconnect neighboring toroidal field coils at their inner portions. Due to these keys the coils inner portions resist the torque as a whole structure. On the other hand, the ITER magnet system design provides supporting of the centering load on the TFC by a bucking cylinder. Hence, the keys should not produce any significant wedging during the radial movement of the coils. Numerical modeling of the so called scissors' action keys that satisfy above requirements has been performed with assembly gaps being taking into account. Cooldown, TFC energizing and poloidal fields coils pulse regimes have been studied. Since the TFC torsion produces high cyclic compression on the key ground insulation, the electrical insulation testing is required. Two mock-up systems for this testing have been proposed and modeled. One system models an effect of the TFC radial movement resulting in higher peak compression on the insulation.

Alternative strand material for the toroidal field coils of the ITER magnetic system: assessment of Nb/sub 3/Al

Designers of conductors for ITER TF coils selected Nb/sub 3/Sn as the strand material. As the influence of stress and strain on the superconducting property in Nb/sub 3/Al conductors is smaller than in Nb/sub 3/Sn conductors, Nb/sub 3/Al has become a potentially attractive candidate material. However, its industrial production has not yet been developed at the level of Nb/sub 3/Sn. Of interest is the evaluation of the use of cable-in-conduit-conductors for ITER TF coils. The assessment is carried out considering alternative jacket material (stainless steel), manufacturing approaches (winding of reacted conductors) and performance of the final conductor in the operating conditions of ITER TF coils.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of quality of high-resistive coatings of superconducting wires with acoustic emission method

The mechanism and kinetics of fracture of electrolytic chrome coatings on multifilamentary Nb/sub 3/Sn-based superconducting wire have been investigated with the methods of acoustic emission (AE) and electron microscopy when developing superconductors for the ITER magnetic system. Coatings 1, 3, and 8 /spl mu/m thick of the following three types were obtained by varying the electrolyte composition and the mode of precipitation process: milk chrome (Cr); hard Cr; and black Cr. The detected acoustic signal and AE spectra were processed in digital form with a specially designed analyzer of AE signals. Milk Cr coatings are shown to have the greatest crack resistance at any coating thickness, the same is for black Cr, but at thickness of 1 to 3 /spl mu/m. In these coating the microcracks formation is going by the ductile mechanism. Hard Cr coating have the least resistance to brittle cracks at any thickness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development and investigation of multifilamentary Nb/sub 3/Sn conductors to be used in the ITER magnetic system

Problems of manufacturing wires for the ITER magnetic system are considered. Results for Nb/sub 3/Sn wires 0.80-0.85 mm in dia. manufactured using bronze technology under commercial conditions are given. Wires containing up to 45,000 filaments with volume fractions of copper stabilizer up to 56%, chromium coatings 2 mm thick and having critical current densities (non-Cu) up to 650 A/mm/sup 2/ (12T, 4.2K) are discussed. Wires with extended tin sources and volume fractions of copper stabilizer equal to 60% are investigated. J/sub c/ (non-Cu) for such wires 0.85 mm in dia. reaches 1000 A/mm/sup 2/ (12T, 4.2K) with hysteresis losses <550 mJ/cm/sup 3/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Material science problems in manufacturing of superconducting wire for iter magnetic system winding

Abstract The paper discusses material science problems in producing a superconducting wire for ITER magnetic system winding. Results of investigation and development of methods for producing single superconductors containing 45 000 niobium filaments 2 mm diameter including made up of largely composite precursors up to 300 mm diameter, are presented. The problems are considered related to the manufacture of copper channel shapes with a resistive barrier. Data on the development of soldering schedules for winding wire elements and leaktight welding of a steel sheath containing soldered transposed multifilamentary superconductors and copper channel shapes are given. The need is indicated to develop non-standard equipment lines for manufacturing long pieces of a winding wire.