C.Y. Gung

Measurements of ramp-rate limitation of cable-in-conduit conductors

The ramp-rate limitation found in the US-demonstration poloidal coil (US-DPC) test was studied in a laboratory scale experiment. The ramp-rate sensitivity has been identified on a 27-strand cable-in-conduit conductor at a background ramped field to 9.5 T with various ramp rates of 0.5 T/s to 2 T/s, simulating the US-DPC test conditions. A model assuming the existence of periodic disturbances is proposed in which the disturbance frequency is directly proportional to the ramp rate of the square of field. A semi-empirical formula was developed which fits the ramp-rate limitation data of both the US-DPC large coil and the 27-strand cable. The ramp-rate limitation does not occur for currents below the conventional limiting current.<<ETX>>

Effect of conduit material on CICC performance under high cycling loads

Recent International Thermonuclear Experimental Reactor (ITER) Model Coils and tests on Nb/sub 3/Sn Cable in Conduit Conductors (CICC) showed a significant and unexpected increase in the broadness of the transition to the normal state, resulting in degradation of superconducting properties. To investigate these phenomena, two CICC samples were built with identical 144 strand cables but different conduit materials. One sample had titanium conduit with low coefficient of thermal expansion, the other had stainless steel conduit. The purpose of this experiment was to study changes in strand properties in the cable (critical current, current sharing temperature, n-value), the effects of cycling and high electromagnetic load, and the effect of the conduit on the CICC performance.

Test Results of the First US ITER TF Conductor in SULTAN

The US Domestic Agency is one of six parties supplying TF cable-in-conduit conductors (CICCs) for ITER. Previous tests have shown that measured performance of the TF CICCs can be much lower than expected from the strand properties at the projected uniaxial strain and that the cabling pattern may also be an important factor. Worst of all, voltage signals well below the expected critical surface could not be reliably interpreted or canceled, making test results very suspect. The TFUS1 sample was prepared to achieve multiple goals: 1) to ensure uniform current distribution and to eliminate parasitic voltage signals by improving joints, 2) to explore the potential benefits of a different cabling pattern for better support of strain-sensitive strands, and 3) to explore the source of voltage development in the cable through the use of innovative penetrating diagnostics. Test results of the first US-made samples are presented and discussed.

Design and manufacture of the US-ITER pre prototype joint sample

The US-ITER pre prototype joint sample which has been fabricated at the MIT Plasma Fusion Center is the first attempt to fabricate an optimized full size joint which can be stably operated in ITER required AC background fields at reduced coupling losses. This paper presents an overview of the joints construction and fabrication, highlighting some of the procedural steps that have since been incorporated into fabrication of current terminations for the inner module ITER central solenoid (CS) model coil.

Progress in the Development of Superconducting Quadrupoles for Heavy-ion Fusion

The Heavy Ion Fusion program is developing single aperture superconducting quadrupoles based on NbTi conductor, for use in the High Current Experiment at Lawrence Berkeley National Laboratory. Following the fabrication and testing of prototypes using two different approaches, a baseline design has been selected and further optimized. A prototype cryostat for a quadrupole doublet, with features to accommodate induction acceleration modules, is being fabricated. The single aperture magnet was derived from a conceptual design of a quadrupole array magnet for multi-beam transport. Progress on the development of superconducting quadrupole arrays for future experiments is also reported.

Real-time simultaneous temperature and strain measurements at cryogenic temperatures in an optical fiber

A novel fiber optic sensor has been developed to be used in superconducting magnets for fusion reactors and other large cable-in-conduit superconductor (CICC) magnet applications. These large superconducting magnets need a diagnostic that can measure the temperature and strain throughout the magnet in real-time, which was not possible until now. Simultaneous temperature and strain measurements at cryogenic temperatures have been demonstrated, using spontaneous Brillouin scattering in an optical fiber. Using an extremely narrow (100 Hz) linewidth Brillouin laser with very low noise as a frequency shifted local oscillator, the frequency shift of spontaneous Brillouin scattered light was measured using heterodyne detection. A pulsed laser was used to probe the fiber using Optical Time Domain Reflectometry (OTDR) to determine spatial resolution. The spontaneous Brillouin frequency shift and linewidth as a function of temperature agree with previous literature on stimulated Brillouin scattering data from room temperature down to 4 K. For the first time, the spontaneous Brillouin frequency shift, linewidth, and intensity as a function of strain have been measured down to 4 K. Analyzing the frequency spectrum of the scattered light after an FFT gives the Brillouin frequency shift, linewidth, and intensity of the scattered light. 65,000 pulses, with 53 ns pulse widths, were averaged in under one second, providing a 5 meter spatial resolution along a fiber that was about 100 m long. Measuring these three parameters allow the simultaneous determination of temperature and strain in real-time throughout a fiber with a spatial resolution on the order of several meters.

Experimental Studies of Transverse Stress Effects on the Critical Current of a Sub-Sized

Large superconducting magnets will play a central role in the success of the International Thermonuclear Experimental Reactor (ITER) and for the future of fusion energy. Cable-in-conduit conductors (CICC) will be used for the ITER magnets. As a CICC is energized, electromagnetic forces accumulate across the conductor, pressing strands transversely against one side of the conduit. We have developed a device to study the effect of transverse stress on a sub-sized cable using a mechanical load that simulates the Lorentz loads in the ITER Central Solenoid conductor. The test sample is a single turn (about 110 mm diameter) circular cable composed of 36 superconducting strands (cabling pattern of 3 x 3 x 4 ). The transverse stress is applied to the cable using a conical wedge that converts a vertical force into a radial (transverse) force. The vertical force is provided by a linear actuator. The stress on the cable is measured using two independent techniques: (1) strain gages applied directly to the ring where the cable is located and (2) a load cell located outside the cryostat. The device was successfully tested using a 20T, 190 mm bore Bitter magnet at National High Magnetic Field Laboratory (NHMFL), FSU, and a first series of tests were carried out showing a degradation of the critical current as a function of transverse stress. The maximum transverse stress was 100 MPa, for which the cable critical current degraded to roughly 30% of its initial value.

{\rm Nb}_{3}{\rm Sn}

The Central Solenoid Model Coil (CSMC) was built by the US and the Japanese home teams as part of an international collaboration which also involved the European Union and Russian Federation. The CSMC and CS insert coil were installed at a purpose build a test facility at the Japan Atomic Energy Research Institute in Naka, Japan and tested from March through August 2000. With over 500 sensors installed on the coils, bus bars, plumbing, facility and the structure, the CSMC and CS insert were instrumented to obtain valuable data about their performances, and to reveal the qualification of the large state superconducting solenoids for the nest fusion machine. This paper describes the instrumentation inside the vacuum chamber used in various stages of the CSMC and the CS insert coil operation.

Superconducting Cable

This paper demonstrates a practical process for evaluating AC losses of a full-size chrome-plated Nb/sub 3/Sn cable with key parameters determined from a laboratory-scale single-wire experiment. By using wire specifications given in the engineering design phase as experimental information, this process was applied to estimate AC losses in the inner-most layer of the ITER central solenoid.

Instrumentation of the Central Solenoid Model Coil and the CS insert

A local maximum at a slow field variation was measured in the AC-loss profile vs. field ramping rate in a double-stacked wire, which is not predicted by the existing AC-loss models. The AC losses of three important superconducting wires, manufactured mainly for superconducting magnets running at large amplitudes and slowly ramping fields, have been measured. These three single-strand samples, including a NbTi, an internal-tin, and a modified jelly-roll internal-tin wire, have different cross sectional constructions. A model taking into account the saturation of the filament bundle walls caused by strong local interbundle coupling currents in the inner multifilament region is proposed to provide a possible explanation to this special loss dependency.<<ETX>>