Superconducting Magnet Coils Research Articles

KSTAR Magnetic Field Measurement During the Commissioning and Remanent Field Evaluation

An in situ measurement of the magnetic field generated by the assembled superconducting magnet coils was held by using precision Hall sensors during the commissioning of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. This was done in order to investigate the magnetic influence of Incoloy 908, which is the jacket material for the cable-in-conduit conductors (CICCs) of the Nb3Sn coils. After the PF coils were discharged from 1 kA the vertical remanent field at the plasma center was more than 10 G, while the TF coils were not energized. The vertical magnetic field generated by the PF coils had a discrepancy of up to 50 G between measurement and the calculation assuming no magnetic influence of Incoloy 908. Thus, non-negligible ferromagnetism was identified and attributed to Incoloy 908. In contrast, most of the hysteresis observed in the magnetic measurements was eliminated when the Incoloy 908 of the TF windings was saturated by the TF coil charging.

Young's Modulus and Thermal Expansion of Filled Cyanate Ester and Epoxy Resins

During the winding of high field superconducting magnet coils, small gaps inevitably form between the wire and the supports. These gaps must be filled with a material that will fully support the superconductor and prevent cracking of the epoxy or cyanate ester resin. Many times the simplest solution is to manually chop some glass fiber and pack it into the space hoping it will be uniformly infiltrated with epoxy. But what are the properties of this makeshift filler? Two epoxy resin systems and one cyanate ester resin were highly filled with ceramic and glass particles. Measurements of the thermal expansion show that they can approach the expansion values of typical superconducting magnet supports and cable, but are still 20% too high. Young's modulus was also measured at room temperature and in liquid nitrogen.

COMMISSIONING RESULT OF THE KSTAR HELIUM REFRIGERATION SYSTEM

To keep the superconducting (SC) magnet coils of KSTAR at proper operating conditions, not only the coils but also other cold components, such as thermal shields (TS), magnet structures, SC bus-lines (BL), and current leads (CL) must be maintained at their respective cryogenic temperatures. A helium refrigeration system (RRS) with an exergetic equivalent cooling power of 9 kW at 4.5 K without liquid nitrogen () pre-cooling has been manufactured and installed. The main components of the KST AR helium refrigeration system (HRS) can be classified into the warm compression system (WCS) and the cryogenic devices according to the operating temperature levels. The process helium is compressed from 1 bar to 22 bar passing through the WCS and is supplied to cryogenic devices. The main components of cryogenic devices are consist of cold box (C/B) and distribution box (D/B). The C/B cool-down and make the various cryogenic helium for the KSTAR Tokamak and the various cryogenic helium is distributed by the D/B as per the KSTAR requirement. In this proceeding, we will present the commissioning results of the KSTAR HRS. Circuits which can simulate the thermal loads and pressure drops corresponding to the cooling channels of each cold component of KSTAR have been integrated into the helium distribution system of the HRS. Using those circuits, the performance and the capability of the HRS, to fulfill the mission of establishing the appropriate operating condition for the KSTAR SC magnet coils, have been successfully demonstrated.

Globally optimal superconducting magnets Part II: Symmetric MSE coil arrangement

A globally optimal superconducting magnet coil design procedure based on the Minimum Stored Energy (MSE) current density map is outlined. The method has the ability to arrange coils in a manner that generates a strong and homogeneous axial magnetic field over a predefined region, and ensures the stray field external to the assembly and peak magnetic field at the wires are in acceptable ranges. The outlined strategy of allocating coils within a given domain suggests that coils should be placed around the perimeter of the domain with adjacent coils possessing alternating winding directions for optimum performance. The underlying current density maps from which the coils themselves are derived are unique, and optimized to possess minimal stored energy. Therefore, the method produces magnet designs with the lowest possible overall stored energy. Optimal coil layouts are provided for unshielded and shielded short bore symmetric superconducting magnets.

Observations of Beam Losses Due to Bound-Free Pair Production in a Heavy-Ion Collider

We report the first observations of beam losses due to bound-free pair production at the interaction point of a heavy-ion collider. This process is expected to be a major luminosity limit for the CERN Large Hadron Collider when it operates with (208)Pb(82+) ions because the localized energy deposition by the lost ions may quench superconducting magnet coils. Measurements were performed at the BNL Relativistic Heavy Ion Collider (RHIC) during operation with 100 GeV/nucleon (63)Cu(29+) ions. At RHIC, the rate, energy and magnetic field are low enough so that magnet quenching is not an issue. The hadronic showers produced when the single-electron ions struck the RHIC beam pipe were observed using an array of photodiodes. The measurement confirms the order of magnitude of the theoretical cross section previously calculated by others.

Fabrication, Evaluation and Radiation Behavior of S2-Glass Fiber Reinforced Polyimide Laminates for Cryogenic Applications

A number of thermosetting polyimide resin formulations have been prepared evaluated and used in the resin transfer molding (RTM) fabrication of S2-glass reinforced composite laminates. The resin formulations are based on combining varying amounts of 3, 4'-oxydianiline and 4, 4'-(1,3-phenylenedioxy) dianiline with 4, 4'-bisphenol A dianhydride and 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride and terminated in 4-phenylethynyl phthalic anhydride. The initial viscosity and working life of the resins were measured to determine suitability for RTM. Composite laminates were prepared by degassing and then transferring the resins at 260 or 270 °C. A laboratory RTM system with process capabilities to 300°C and 2000 psi, was built and utilized. The polyimide matrix was cured at 320 and 340°C, with a dwell for 3 h at each temperature, under 280 psi hydrostatic pressure. The effect of three different Co-60 gamma irradiation doses on cryogenic (77K) mechanical and thermal properties was determined. Performance was compared to that of laminates prepared from a commercially available bismaleimide (RTM-651®). The composites are being studied for their potential use as the electrical insulation in the super-conducting magnet coils of future fusion reactor devices such as the International Thermonuclear Experimental Reactor.

Review of Quench Performance of LHC Main Superconducting Magnets

The regular lattice of the large Hadron collider (LHC) will make use of more than 1600 main magnets and about 7600 corrector magnets, all superconducting and working in pressurized superfluid helium bath. This complex magnet system will fill more than 20 km of the LHC underground tunnel. In this paper an overview of the cold test program and quality assurance plan to qualify all LHC superconducting magnets will be presented. The quench training performance of more than 1100 LHC main dipoles and about 300 main quadrupoles, cold tested to date, will be reviewed. From these results an estimate of the number of quenches that will be required to start operation of the whole machine at nominal energy will be discussed. The energy level at which the machine could be operated at the early phase of the commissioning without being disturbed by training quenches will be addressed. The LHC magnet program required the development of many new tools and techniques for the testing of superconducting magnet coils, magnet protection systems, cryogenics, and instrumentation. This paper will also present a summary of this development work and the results achieved.

Thermal stability analysis of high temperature superconducting magnet coils under overcurrent pulses

This paper presents some numerical simulation results of thermal behavior on the high temperature superconducting (HTS) magnet coils wounded by Bi2223/Ag multifilamentary tapes subjected to an overcurrent pulse using the finite element method (FEM). The analytical results show that the time-varying spatial temperature distribution of the HTS magnet coils under the condition of different overcurrent pulses has an important influence on its stability estimation, and the temperature dependence of minimum propagation current plays a key role in the stability analysis of the HTS magnet. Through the FEM analysis, the permitted duration time of 5-multiple overcurrent pulse has been obtained for the HTS magnet under the various operating conditions and cooling conditions. In this paper, an analytical method for determining the stability of the HTS magnet coils subjected to an overcurrent pulse has been proposed and a stability criterion has been presented. The critical current and minimum propagation current can be chosen as the restrictive condition of operating current in the HTS magnet coils subjected to an overcurrent pulse.

Mechanical performance of cyanate ester based insulation systems under ITER relevant conditions

Epoxy based glass fiber reinforced composites are candidate insulation systems for the superconducting magnet coils of fusion devices, e. g. of ITER. Because of their relatively low radiation hardness, the radiation-harder cyanate ester based systems have become of special interest for application in ITER and future fusion devices.This paper presents recent results obtained on pure cyanate ester and cyanate ester/epoxy blends reinforced with R-glass fibers/Kapton H foil. The systems were investigated at 77 K prior to and after irradiation at ambient temperature (340 K) to a fast neutron fluence of 1 × 1022 m−2 (E > 0.1 MeV) in the TRIGA reactor (Vienna). The mechanical material performance was characterized under static load conditions in tension and interlaminar shear. In addition, tension-tension fatigue measurements were carried out to simulate the pulsed operation of ITER and to assess the lifetime performance of these materials. The results show only slight mechanical degradations (by about 5 to 10%) after irradiation.

Buckling behavior of a superconducting magnet coil

The buckling behavior of a superconducting magnet coil has been investigated for out of plane perturbations. It has been shown that such a coil becomes unstable due to its own magnetic field for out of plane bending at the operating values of electric currents. A technique devised by the author that computes the inductance change due to deformation, has been incorporated in this study. The evaluation shows that although the magnetic fields external to the coil are important and that there is additional structural stiffness provided by the surrounding structure, the effects of the self magnetic field should be evaluated and incorporated in the design of such coils. However, based on previous studies, both analytical and experimental, it has been established that the out-of-plane deformation is the most critical mode from a stability standpoint. Therefore, this mode has been exclusively addressed in this study.

The batch production for superconducting magnet coils of EAST (HT-7U)

The experimental advanced superconducting tokamak (EAST) is being constructed at the Institute of Plasma Physics of Chinese Academy of Sciences (CASIPP). The EAST project, approved by the Chinese government as a national mega project of science research is a fully superconducting tokamak. The most key component for EAST is the superconducting magnet coils (SMCs), which consists of 16 toroidal field coils (TFCs) and 14 poloidal field coils (PFCs). In 2003, three prototypes, one TFC and two PFCs, were successfully completed and passed a series of cryogenic tests. Batch production, needed for the SMCs has begun at CASIPP since 2002. Up to now, all 58 CIC conductors with a total length of 32 km, 12 TFCs out of 16 and 10 PFCs out of 14 have been fabricated. This paper emphasizes on the various technology issues that must be faced and solved for four R&D lines of SMCs after transforming to batch production. Quality control methods in process are also described.

Mechanical performance of magnet insulation materials fabricated by the “Insulate-Wind-and-React” technique

Superconducting magnet coils are usually fabricated according to the “Wind-React-Insulate-and-Transfer” technique. However, the alternative “Insulate-Wind-and-React” technique could simplify the coil manufacturing process considerably. An insulation system designed for this process has been investigated. It consists of a R-glass fiber reinforcement heat treated at 650 °C and impregnated afterwards with epoxy. For the mechanical material characterization, tensile, short-beam shear (SBS) and tension–tension fatigue tests were employed at 77 K. In addition, half of the SBS samples were reactor irradiated to a neutron fluence of 1 × 10 22 m −2 ( E > 0.1 MeV) to check for radiation induced material degradation.

Improved structure and long-life blanket concepts for heliotron reactors

New design approaches are proposed for the LHD-type heliotron D–T demo-reactor FFHR2 to solve the key engineering issues of blanket space limitation and replacement difficulty. A major radius of over 14 m is selected to permit a blanket-shield thickness of about 1 m and to reduce the neutron wall loading and toroidal field, while achieving an acceptable cost of electricity. Two sets of optimization are successfully carried out. One is to reduce the magnetic hoop force on the helical coil support structures by adjustment of the helical winding coil pitch parameter and the poloidal coils design, which facilitates expansion of the maintenance ports. The other is a long-life blanket concept using carbon armour tiles that soften the neutron energy spectrum incident on the self-cooled flibe-reduced activation ferritic steel blanket. In this adaptation of the spectral-shifter and tritium breeder blanket (STB) concept a local tritium breeding ratio over 1.2 is feasible by optimized arrangement of the neutron multiplier Be in the carbon tiles, and the radiation shielding of the superconducting magnet coils is also significantly improved. Using constant cross sections of a helically winding shape, the ‘screw coaster’ concept is proposed to replace in-vessel components such as the STB armour tiles. The key R&D issues for developing the STB concept, such as radiation effects on carbon and enhanced heat transfer of Flibe, are elucidated.

Radiation shielding calculations for the vista spacecraft

Abstract The VISTA spacecraft design concept has been proposed for manned or heavy cargo deep space missions beyond earth orbit with inertial fusion energy propulsion. Rocket propulsion is provided by fusion power deposited in the inertial confined fuel pellet debris and with the help of a magnetic nozzle. The calculations for the radiation shielding have been revised under the fact that the highest jet efficiency of the vehicle could be attained only if the propelling plasma would have a narrow temperature distribution. The shield mass could be reduced from 600 tons in the original design to 62 tons. Natural and enriched lithium were the principle shielding materials. The allowable nuclear heating in the superconducting magnet coils (up to 5 mW/cm3) is taken as the crucial criterion for dimensioning the radiation shielding structure of the spacecraft. The space craft mass is 6000 tons. Total peak nuclear power density in the coils is calculated as ∼5.0 mW/cm3 for a fusion power output of 17 500 MW. The peak neutron heating density is ∼2.0 mW/cm3, and the peak γ-ray heating density is ∼3.0 mW/cm3 (on different points) using natural lithium in the shielding. However, the volume averaged heat generation in the coils is much lower, namely 0.21, 0.71 and 0.92 mW/cm3 for the neutron, γ-ray and total nuclear heating, respectively. The coil heating will be slightly lower if highly enriched 6Li (90%) is used instead of natural lithium. Peak values are then calculated as 2.05, 2.15 and 4.2 mW/cm3 for the neutron, γ-ray and total nuclear heating, respectively. The corresponding volume averaged heat generation in the coils became 0.19, 0.58 and 0.77 mW/cm3.

Radiation effects on the mechanical integrity of novel organic insulators for the ITER magnet coils

High quality glass fiber reinforced composites will be used as electrical insulation systems for the windings of the superconducting magnet coils of the ITER fusion device. Recently, considerable interest in coil technology has focused on improving the radiation hardness of the organic impregnation resins, in order to avoid serious material damage, such as fiber-matrix debonding (delamination). This paper reports on first screening tests of novel cyanate-ester based glass fiber reinforced composites, which were developed especially for ITER-relevant applications. The material behavior was investigated at 77 K prior to and after reactor irradiation to a neutron fluence of 1 × 10 22 m −2 ( E>0.1 MeV) using the tensile and the short-beam-shear test, respectively. Furthermore, tension–tension fatigue measurements were carried out at low temperatures, in order to assess the mechanical performance under the pulsed operating conditions of ITER.

Radiation hardness of newly developed ITER relevant insulation systems

The application of glass fiber reinforced plastics (GFRPs) as insulating materials for the superconducting (SC) magnet coils of the International Thermonuclear Experimental Reactor (ITER) fusion device imposes high demands on the material performance. Therefore, GFRPs with improved mechanical and electrical properties are of special interest. One of the most promising matrix materials for higher radiation resistance is the cyanate ester system. Two innovative insulation systems with boron-free S-glass fiber reinforcements and cyanate ester based matrix compositions were irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 1×10 21, 1×10 22 and 5×10 22 m −2 ( E>0.1 MeV). After irradiation, swelling and weight loss were measured prior to static and dynamic tensile as well as interlaminar shear testing at 77 K.

Optimisation of the reaction heat treatment cycle of internal tin and bronze Nb 3Sn strands for ITER

The heat treatment cycles of Nb 3Sn superconductor strands are mainly designed to achieve the highest possible critical currents and are, therefore, very long (up to nearly 600 h including ramping times). Such long interruptions would delay the manufacturing cycle of large superconducting magnet coils considerably. Therefore, it was of interest to investigate if the same or nearly the same performance of the strands could be achieved with shorter heat treatment cycles. Strand material used for the ITER CS and TF model coils from two European suppliers were submitted to various heat treatments and subsequently characterised with respect to critical current, hysteresis losses, residual resistance ratio (RRR) and magneto-resistance. The measurements were supplemented by metallurgical and microscopic investigations. This paper summarises the promising results opening the possibility to shorten the total heat treatment time for the ITER TF coils by approximately 1 year.

Development of CCVD ceramic insulation for Bi-2212 superconducting wires and rutherford cables

A low cost, open-atmosphere deposition technique, Combustion Chemical Vapor Deposition (CCVD) is used to deposit zirconia-based, multi-component insulation films on Bi-2212 tapes for fabricating high temperature superconducting magnet coils. The insulation coatings were investigated in detail in terms of composition, microstructure, and insulation strength versus the deposition temperature and other parameters. Dielectric and mechanical properties and chemical compatibility with BSCCO have been characterized and optimized. The advantage of the CCVD technique in producing multi-component insulating materials with the desired properties are described. A brief evaluation of this insulation coating on Rutherford cables fabricated from Bi-2212 wires is also reported.

Low-temperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation

The windings of the superconducting magnet coils for the ITER-FEAT fusion device are affected by high mechanical stresses at cryogenic temperatures and by a radiation environment, which impose certain constraints especially on the insulating materials. A glass fiber reinforced plastic (GFRP) laminate, which consists of Kapton/R-glass-fiber reinforcement tapes, vacuum-impregnated in a DGEBA epoxy system, was used for the European toroidal field model coil turn insulation of ITER. In order to assess its mechanical properties under the actual operating conditions of ITER-FEAT, cryogenic (77 K) static tensile tests and tension–tension fatigue measurements were done before and after irradiation to a fast neutron fluence of 1×10 22 m −2 ( E>0.1 MeV), i.e. the ITER-FEAT design fluence level. We find that the mechanical strength and the fracture behavior of this GFRP are strongly influenced by the winding direction of the tape and by the radiation induced delamination process. In addition, the composite swells by 3%, forming bubbles inside the laminate, and loses weight (1.4%) at the design fluence.

Optimization of the radiation shielding mass for the magnet coils of the VISTA spacecraft

An attempt has been made for the optimisation of the radiation shielding of a spacecraft design concept with inertial fusion energy propulsion for manned or heavy cargo deep space missions beyond earth orbit. Rocket propulsion is provided by fusion power deposited in the inertial confined fuel pellet debris, and with the help of a magnetic nozzle. The allowable nuclear heating in the super conducting magnet coils (up to 5 mW/cm 3) is the crucial criterion for the dimensioning of the radiation shielding structure of the spacecraft. The optimized design reduced the shield mass from 600 tons to 93 and 88 tons with natural and enriched lithium, respectively. The space craft mass was 6000 tons. Total peak nuclear power density in the coils is calculated to be 5.0 mW/cm 3 for a fusion power of 17,500 MW. Peak neutron heating density is 2.6 mW/cm 3 and peak γ-ray heating density is 2.9 mW/cm 3 (all on different points). However, volume averaged heat generation in the coils is much lower, namely 0.30, 0.73 and 1.03 mW/cm 3 for neutron, γ-ray and total nuclear heating, respectively.