Since tokamak fusion experimental reactors try to sustain 100-200 second D-T burning, helium exhaust from plasma core is one of the major issues of the tokamak reactors. There have been proposed two major concepts about helium exhaust from plasma. They are concepts of magnetic divertor and pumped limiter. In both cases, a very large pumping system is required for the helium and fuel pumping. The cryosorption pump with 4.2K panels is considered to be a prime candidate for these applications. When molecular sieves are used as the adsorbent, it has been recognized that the pump may not be able to accommodate helium-hydrogen isotopes mixtures, because condensed deuterium and tritium will block the adsorbent surface and prevent helium pumping. This means that the cryosorption panels (4.2K) for helium will be surrounded by two chevrons, one at 77K and the other at 4.2K. Recently, cryopumping has intensively studied in the TSTA project in the United States. It has been shown that cryosorption by charcoal and cryotrapping by argon condensed layers will appear to work successfully with tokamak reactors. In the cryopumps, it is necessary to recover the pumped gases as quickly as possible in order to achieve low inventories of tritium. The regencration cycle will be determined by considering several items which include tritium inventory and safety problems.
Frictional sliding occurs on both a microscopic and a macroscopic scale. Sliding on a microscopic scale appears as discrete events called microslips. Microslips are inherent in all sliding events and are quite different from macroscopic instabilities such as stick-slips. It is thought that the “training effect” observed in quench current data from a superconducting braid may be caused by microslips. The mechanism of sliding motion and its effects at 4.2K were studied in detail for a number of metal/insulator pairs that model superconducting magnet windings; the results impact the performance of superconducting magnets. Organic surface coating materials are generally effective in eliminating macroscopic instabilities. Instrumentation used in these experiments includes a high-resolution extensometer and an acoustic emission sensor, both with sensitivities capable of detecting microslips (-1μm).
Energy released following cracks and bond failures were measured for an EPON epoxy near 4.2K. Crack events were monitored with an acoustic emission sensor; the energy released by each crack or bond failure was calculated from the temperature rise measured with thermocouples. Cracking was observed to be load dependent; this may account in part for the training phenomenon observed in bringing epoxy-impregnated superconducting magnets to full design field.
Lorentz force induced strand motion in a superconducting cable (or bundle conductor) was investigated by a combined technique of voltage and acoustic emission measurement. Both types of motion, reversible and irreversible, were observed. Dissipation energy for one slip event was less than a few mJ.