The design and development of novel quench detection systems for the International Thermonuclear Experimental Reactor (ITER) and the Tokamak Physics Experiment (TPX) has advanced our knowledge of design principles governing quench detection systems. Design studies have quantified the detection signal-noise ratios for several types of quench detector, including external and cowound voltage sensors, fiber optic temperature sensors, and optical and piezoresistive flow meters. The effects of sensor placement and subdivision have also been studied. Sensor topologies with the highest signal-noise ratios are identified. Fiber-optic and voltage sensors have been fabricated for the ITER QUELL experiment, that have demonstrated the capacity to survive cabling and compaction, heat treatment, and cooldown to low temperature with tight conduit bend radii. Extraction techniques have been developed that use redundant seals and coefficient-ofexpansion matching to guarantee leaktightness. Electrical integrity is ,guaranteed in a design through the control of eiectrical fields in the feedthrough geometry and insulation material selection. I, INTRODUCTION Superconducting magnets have traditionally used relatively simple methods for detecting a quench.Voltage taps on the surface of a winding are the most common, and changes in temperature, pressure, and flow have also been used. These methods become inadequate when a magnet has to operate in a strongly pulsed field. The problem of induced noise voltage is exacerbated by large size and large transients in flow or temperature.
VOLTAGE SENSORSThe ability to cancel inductive voltages is greatly improved by the use of internal voltage sensors. A cowound sensors has been used previously on the US-DPC coil, where an insulated wire was wound along an edge of the conduit on the outside [ll. If the sensor is cabled on the inside of the conductor, the degree of noise cancellation will improve by at least another order of magnitude.In a large CICC magnet, the terminal voltage during pulsing may be as high as 10-15 kV. The POLO coil set a world's record for CICC at 23 kV [21, while recent designs of large coil systems, such as ITER (10 kv), TPX (7.5 kV), and SMES (10 kV) have specified voltages in this range. By contrast, a large number of quench simulations have shown that in order t o hold hot-spot temperatures to 150 K, a quench must be detected at a threshold voltage of 0.2-.0 V [3], [4]. If, as in the TPX design, a desired value of signavnoise of 10:1 is specified 151, the quench detection sensors must be capable of reducing noise levels to -20-100 mV. This implies that the voltage rejection capability of the quench detection system should be on the order of 100,000-500,OOO:l. Although this may seem optimistic, recent experiments at M.I.T., LLNL, and EPFL CRPP have demonstrated the feasibility of such high levels of voltage noise rejection [31, [61.Several noise rejection techniques must be used simultaneously in order to obtain high system signahoise ratios. The individual concepts that...