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Long pulse plasma operation on large magnetic fusion devices require multiple forms of cryogenically formed pellets for plasma fueling, on-demand edge localized mode (ELM) triggering, radiative cooling of the divertor, and impurity transport studies. The solid deuterium fueling and ELM triggering pellets can be formed by extrusions created by helium cooled, twin-screw extruder based injection system that freezes deuterium in the screw section. A solenoid actuated cutter mechanism is activated to cut the pellets from the extrusion, inserting them into the barrel, and then fired by the pneumatic valve pulse of high pressure gas. Fuel pellets are injected at a rate up to 10 Hz, and ELM triggering pellets are injected at rates up to 20 Hz. The radiative cooling and impurity transport study pellets are produced by introducing impurity gas into a helium cooled section of a pipe gun where it deposits in-situ. A pneumatic valve is opened and propellant gas is released downstream where it encounters a passive punch which initially accelerates the pellet before the gas flow around the finishes the pellet acceleration. This paper discusses the various cryogenic pellet production techniques based on the twin-screw extruder, pipe gun, and pellet punch designs. [ABSTRACT FROM AUTHOR]

With recommendations from wire manufacturers as a starting point, a series of solder joints were fabricated and characterized to determine the best method for producing repeatable, low-resistance and high-mechanical-strength splices in as-manufactured, stabilized YBCO coated conductors. From the 2.54 cm long splice joints that were fabricated, parameters such as solder material, stabilization material, fabrication method, and conductor geometry were varied to determine the impact of each on the properties of splice joints. Results indicate that the splice joints of lowest resistance were influenced primarily by the tape orientation in the joint and the stabilization material. The lowest resistances were between 2×10-8 Ω and 1.0×10-7 Ω in 4-mm wide tapes and were obtained from pure copper-stabilized tapes oriented with the YBCO layers in closest proximity. The voltage drop along the splice length indicated that only a fraction of the splice length contributes to the splice joint resistance. Mechanical characterization of splice joints showed that the joint resistance remained unchanged under axial stress up to a stress level at which the critical current of the tapes forming the joint degrades irreversibly. [ABSTRACT FROM AUTHOR]

The tri-axial HTS cable design uses three concentric superconducting layers for the phase conductors separated by a cold dielectric material. It offers an efficient HTS cable configuration by reducing the amount of superconductor needed, and placing all three phases in a single cryostat. Ultera and ORNL tested a 5-meter long tri-axial HTS cable and terminations designed to operate at 3 kA ac and 13.2 kV. Test results, including the thermal loads on the system, will be reported. An existing liquid nitrogen skid that circulates subcooled liquid nitrogen through the cable was initially designed for the heat loads on single phase cables at lower current ratings. The refrigeration needed for the 3 kA tri-axial cable configuration made it necessary to upgrade the nitrogen system to increase the cooling capacity. A description of the upgrades and performance of the system is provided. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Following the positive results of a 1.5-m long prototype cable, the Ultera and Oak Ridge National Laboratory (ORNL) team built and tested a 5-m triaxial cable rated at 1.3-kA rms, 15-kV ac. The three concentric superconducting phases are made of BSCCO-2223 high-temperature superconducting (HTS) tapes, separated by layers of cold-dielectric tapes. A copper braid is added as the grounding shield on the outside of the three active phases. To facilitate the calorimetric ac loss measurement, the cable was cooled by liquid nitrogen flowing in the annulus between the cable and the cryostat only. Tests of this cable were performed at temperatures ranging from 76 to 84 K. AC loss data reconfirmed the previous gratifying result on the 1.5-m prototype cable that the total three-phase loss is approximately the sum of the calculated ac losses of the three separate phases. No additional loss is due to possible coupling among the three phases. This and other test results of the 5-m, 1.3 kA triaxial cable are reported here. The next step in the project is to build and install a triaxial cable rated at 3-kA rms, 13-kV ac at a utility site. A single-phase 3-kA cable prototype was also built and tested for use as the most demanding inner phase of the triaxial cable. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

A series of BSCCO-2223 and YBCO tapes were subjected to burnout tests in a liquid-nitrogen bath to observe operational stability limits when different layers of dielectric tape are added to the sample surface. In this study, the BSCCO tapes were composed of a silver/alloy sheath with nickel/copper plating, while the YBCO tapes had a 50-μm layer of copper attached to the silver surface. After attaching the tapes to a thermally insulated G-10 holder, the stability of the tapes was found by applying current greater than the critical current and holding it constant for up to 1 min. If the sample voltage increased rapidly during this period, the tape was considered unstable at this current. This was repeated at different layers of Cryoflex™, and the results were compared to a numerical simulation of the energy balance equation. This simulation was also utilized to investigate the effect of the layers on the stability limit and estimate the thermal conductivity of the Cryoflex™. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The role of silver in the quench dynamics of RABiTS-processed YBCO tapes was examined. The voltage distribution along the 20 cm long YBCO tapes with silver thickness between two and eight micrometers was measured when different transport current pulses were applied. Measurements on each sample were performed in a conduction-cooling environment at approximately 50 K. After normal regions were induced in the sample by short over-current pulses, an operating current was applied to monitor the sample recovery or quench. When scaled to the lowest critical current, a thermal runaway current was identified and found to increase with increasing silver thickness. A simple one-dimensional model of the system supported this trend. [This paper is also published in Advances in Cryogenic Engineering Volume 47, AIP Conference Proceedings Volume 613, pp. 449-456. [ABSTRACT FROM AUTHOR]

A 20-cm-long YBCO tape fabricated by the Rolling Assisted Bi-axially Textured Substrate (RABiTS) technique was used to study the quench behavior of a secondgeneration high-temperature superconductor (HTS). Quench tests of the sample were performed in a conduction cooling condition with a cryocooler. The sample was subjected to a short overcurrent pulse to create normal zones, followed by a longer operating current up to the critical currents of the weaker zones of the tape. Distinctive normal zone propagation was observed for the first time on a YBCO tape. Propagation velocities of 1.4 to 8.3 mm/s were measured, which increase linearly with the current. Stability margins and a minimum propagation current were also observed. One-dimensional thermal modeling showed good agreement with the test results. [This paper is also published in Advances in Cryogenic Engineering Vol. 47, AIP Conference Proceedings Vol. 613, pp. 457-464. [ABSTRACT FROM AUTHOR]