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Superconducting magnet research and development program

Description: Viewgraphs for the conference presentation are given. The author reports technical progress in the research and development of cable-in-conduit internally-cooled superconductors; the design, construction and test of EBT-P magnets; the tests of Westinghouse/Airco Nb/sub 3/Sn conductors; partial-array test in IFSMTF; the evaluation of pressure taps on the Westinghouse conductor sheath; and IFSMTF vapor-cooled-lead tests. 12 figs., 1 tab. (WRF)
Date: January 1, 1985
Creator: Lue, J.W.
Partner: UNT Libraries Government Documents Department

Oak Ridge Tokamak experimental power reactor study, 1976. Part 3. Magnet systems

Description: This part of the ORNL EPR composite report documents last year's design effort in the magnet systems. The discussion covers both the toroidal field (TF) coil system and the poloidal field (PF) coil system. Consideration of coil support structure, cryogenic requirements, and power supplies is included. Particular attention is also paid to the protection scheme of both TF and PF coils. The conductor design is based on forced-flow supercritical helium-cooled cable in conduits. A hybrid system is proposed for the TF coil system. It uses Nb/sub 3/Sn in the high field region for a designed maximum field of 11 T, and NbTi in the low field region. Conductor stability analysis in the forced-flow scheme is discussed. Coil connection options to obtain desirable current and voltage ratings in PF coils are described. The refined design in the magnet systems is more self-consistent in every respect.
Date: February 1, 1977
Creator: Lue, J. W. (ed.)
Partner: UNT Libraries Government Documents Department

Design of force-cooled conductors for large fusion magnets

Description: Conductors cooled by supercritical helium in forced convection are under active consideration for large toroidal fusion magnets. One of the central problems in designing such force cooled conductors is to maintain an adequate stability margin while keeping the pumping power tolerably low. A method has been developed for minimizing the pumping power for fixed stability by optimally choosing the matrix-to-superconductor and the metal-to-helium ratios. Such optimized conductors reduce pumping power requirements for fusion size magnets to acceptable limits. Furthermore, the mass flow and hence pumping losses can be varied through a magnet according to the local magnetic field and magnitude of desired stability margin. Force cooled conductors give flexibility in operation, permitting, for example, higher fields to be obtained than originally intended by lowering the bath temperature or increasing the pumping power or both. This flexibility is only available if the pumping power is low to begin with. Scaling laws for the pumping requirement and stability margin as functions of operating current density, number of strands and such physical parameters as stabilizer resistivity and critical current density, have been proved. Numerical examples will be given for design of conductors intended for use in large toroidal fusion magnet systems.
Date: January 1, 1977
Creator: Dresner, L. & Lue, J.W.
Partner: UNT Libraries Government Documents Department

Design of force-cooled conductors for large fusion magnets

Description: One type of conductor under consideration for tokamak toroidal field (TF) magnets is a cable-in-conduit cooled by supercritical helium in forced convection. The main problem is designing such force-cooled conductors (fcc) is to maintain adequate stability while keeping the pumping power tolerably low. The transit time of the helium through a coil is many minutes. Since recovery of the conductor from a thermomechanical perturbation takes on the order of tens of milliseconds, for purposes of calculation, the inventory of helium available to promote recovery is finite. This means that a large enough perturbation will quench the conductor. We can then judge the stability of a fcc by the maximum perturbation of some specified type against which the conductor is stable, i.e., can still return to the superconducting state. The simplest type of perturbation is a sudden, uniform heat input over the entire length of the conductor. The maximum, sudden, uniform heat input per unit volume of metal ..delta..H is called the ''stability margin.''
Date: January 1, 1977
Creator: Dresner, L. & Lue, J. W.
Partner: UNT Libraries Government Documents Department

Advanced designs for highly stable superconductor systems

Description: Experimental evidence is mounting for enhanced stability against pulsed heat loads in cable-in-conduit conductors brought about by transient pressure waves in helium. It has been suggested that this enhancement derives from improved heat transfer caused by pressure induced flow and from extra heat absorption capability due to the thermodynamic path followed by helium in the course of pressure rise and release. A basic conductor design is given to take advantage of these phenomena yet avoiding the difficulties encountered in the force-cooled conductor presently under development. The design is discussed in terms of manufacturability, performance, and applicability in large fusion magnets. A few small scale test conductors have been constructed. Preliminary test results on the performance of one of them is included. Possible variations offered by the flexibility of the basic design is also discussed.
Date: January 1, 1979
Creator: Lue, J.W. & Miller, J.R.
Partner: UNT Libraries Government Documents Department

Fault Current Tests of a 5-m HTS Cable

Description: The first industrial demonstration of a three-phase high-temperature superconducting transmission power cable at the Southwire manufacturing complex is in progress. One crucial issue during operation of the 30-m HTS cables is whether they could survive the fault current (which can be over an order of magnitude higher than the operating current) in the event of a short-circuit fault and how HTS cables and the cryogenic system would respond. Simulated fault-current tests were performed at ORNL on a 5-m cable. This single-phase cable was constructed in the same way as the 30-m cables and is also rated for 1250 A at 7.2 kV ac line-to-ground voltage. Tests were performed with fault-current pulses of up to 15 kA (for 0.5 s) with pulse lengths of up to 5 s (at 6.8 kA). Although a large voltage drop was produced across the HTS cable during the fault-current pulse, no significant changes in the coolant temperature, pressure, or joint resistance were observed. The cable survived 15 simulated fault-current shots without any degradation in its V-I characteristics.
Date: February 19, 2001
Creator: Lue, J.W.
Partner: UNT Libraries Government Documents Department

Parametric study of the stability margins of cable-in-conduit superconductors: experiment

Description: In a previous experiment on the stability of cable-in-conduit superconductors, we sometimes observed multivalued stability margins, which we attributed to strong heating-induced transient flows. We proposed a schematic theory from which we derived a scaling relation for the limiting current below which the stability margin is always singlevalued. Measurements at different magnetic fields are used to test the scaling with critical temperature and resistivity. We also examine the scaling with heated length and heat pulse duration. The results of these experiments are given and compared with theory.
Date: January 1, 1980
Creator: Lue, J.W. & Miller, J.R.
Partner: UNT Libraries Government Documents Department

Performance of an internally cooled superconducting solenoid

Description: A superconducting magnet constructed of an internally cooled solenoid has been produced and tested. Particular emphasis was placed on measurement of the stability margins of the magnet. Test results showed that this magnet has demonstrated the following merits: (1) it reached short sample critical current limit with no training; (2) no degradation due to strand motion in fields was apparent; (3) it can be charged and discharged to high current densities and high fields at a fast rate; (4) it has reasonable stability margin up to 90% of the critical current; (5) stability measurements verify the scaling relationship for a limiting current below which there is single valued high stability margin; (6) it is not really necessary to have bulk helium flow to ensure high stability; and (7) very high pulse field changes can be accommodated as long as the energy deposition is within its stability margin.
Date: January 1, 1981
Creator: Lue, J.W. & Miller, J.R.
Partner: UNT Libraries Government Documents Department

Interaction and protection of superconducting poloidal field coils and toroidal field coils in a Tokamak Experimental Power Reactor

Description: The protection problems of superconducting coils in a large tokamak device are delineated. Effects of the plasma discharge on the first wall, the induced voltage, and the temperature rise during the quench of a superconducting coil, as well as the mechanical load on coils due to their mutual interaction under normal or fault conditions, are discussed. Various design choices and protection schemes are used to ensure the integrity of the coils during quench. For the Oak Ridge EPR design, a scheme of connecting symmetrically-located toroidal field coils in groups and isolating and discharging the fault coil only gives satisfactory results.
Date: September 1, 1976
Creator: Yeh, H. T. & Lue, J. W.
Partner: UNT Libraries Government Documents Department

Toroidal field coil system of the Oak Ridge EPR reference design

Description: A refined design of the toroidal field (TF) coil system for the Oak Ridge Tokamak Experimental Power Reactor (EPR) study is presented. This design is based on cable conductor cooled by force-flow supercritical helium. It uses superconducting multifilamentary Nb/sub 3/Sn for a maximum design field of 11 T at the coil windings. A hybrid system which uses NbTi at low field regions is recommended. The coil structure consists of stainless steel segments welded together to form a continuous stiff honeycomb. Conductor optimization and stability analysis specifically applicable to the forced-flow cooled conductors are given.
Date: January 1, 1976
Creator: Lue, J. W. & Luton, J. N.
Partner: UNT Libraries Government Documents Department

Investigation of current transfer in built-up superconductors

Description: Superconductors carrying 10 kA or more have been widely suggested for use in fusion research and reactor magnets. Built-up or cable conductors have been proposed in which superconductor is concentrated in part of the conductor or part of the strands while the stabilizer occupies the rest. This scheme leads to substantial saving in manufacturing cost and to reduction of ac losses. Simplified analysis indicates that the current transfer from superconducting wire to normal wire takes place over a characteristic length depending on the resistivity of the contact barrier, the resistivity of the stabilizer, and the geometry of the conductor. Furthermore, the cold-end recovery suffers a reduction. Two types of conductors were constructed for the experimental test. Triplex conductors consisting of either three superconducting wires or two superconducting plus one copper wire were used to simulate cables. Laminated superconductor and copper strips with different soldering bonds were used for build-ups. Normal zone propagation and recovery experiments have been performed and results are compared with the theory.
Date: January 1, 1977
Creator: Miller, J.R.; Dresner, L. & Lue, J.W.
Partner: UNT Libraries Government Documents Department

High-field, high-current-density, stable superconducting magnets for fusion machines

Description: Designs for large fusion machines require high-performance superconducting magnets to reduce cost or increase machine performance. By employing force-flow cooling, cable-in-conduit conductor configuration, and NbTi superconductor, it is now possible to design superconducting magnets that operate a high fields (8-12 T) with high current densities (5-15 kA/cm/sup 2/ over the winding pack) in a stable manner. High current density leads to smaller, lighter, and thus less expensive coils. The force-flow cooling provides confined helium, full conductor insulation, and a rigid winding pack for better load distribution. The cable-in-conduit conductor configuration ensures a high stability margin for the magnet. The NbTi superconductor has reached a good engineering material standard. Its strain-insensitive critical parameters are particularly suitable for complex coil windings of a stellarator machine. The optimization procedure for such a conductor design, developed over the past decade, is summarized here. If desired a magnet built on the principles outlines in this paper can be extended to a field higher than the design value without degrading its stability by simply lowering the operating temperature below 4.2 K. 11 refs., 3 figs.
Date: January 1, 1989
Creator: Lue, J.W.; Dresner, L. & Lubell, M.S.
Partner: UNT Libraries Government Documents Department

Internally Cooled Cable Superconductor (ICCS) for TF and PF coils of FED

Description: Internally Cooled Cable Superconductor (ICCS) concepts developed for TF and PF coils of FED are described. These concepts represent one of the options for FED, and other conductor concepts are still being explored, i.e., no decision has been made for the conductor concepts to be utilized for FED. The TF coil conductor design is based on an ICCS successfully used in a small test magnet at ORNL. The conductor consists of triplets of NbTi strands loosely packed in a stainless steel conduit similar to the Westinghouse LCP coil. The operating current for the conductor is 25.5 kA at l0T and 3.1/sup 0/K. The conductor is co-wound with a stainless steel C-shaped channel to provide a direct load path to the coil case for the accumulated magnetic loads in the winding. The strand diameter in the conductor is optimized to reduce the eddy current losses. The nuclear heating in the winding is the most dominant heat load. In order to remove these heat loads due to nuclear heating and ac losses in the winding, it is necessary to lower the inlet temperature of helium to 2.2/sup 0/K. The conductor has a thermal capacity of approx. 200 mJ/cc, which provides a comfortable stability margin under the operating conditions.
Date: January 1, 1982
Creator: Srivastava, V.C.; Lue, J.W. & Lubell, M.S.
Partner: UNT Libraries Government Documents Department

Development and testing of toroidal field superconductors for fusion devices

Description: Superconductors rated at 10-16 kA in a magnetic field of 8 T are being designed, manufactured, and tested in cooperative development programs between Oak Ridge National Laboratory (ORNL) and conductor manufacturers. Conductor specimens based on ideas previously submitted by the manufacturers were fabricated and tested along with others designed and fabricated at ORNL. The tests measure short sample currents, stability, pressure drop, and heat transfer. Conductor parameters include strand size, cable configuration, superconductor distribution within the conductor, and compaction.
Date: January 1, 1978
Creator: Fietz, W A; Lue, J W & Miller, J R
Partner: UNT Libraries Government Documents Department

Normal zone propagation and Thermal Hydraulic Quenchback in a cable-in-conduit superconductor

Description: When a local normal zone appears in a cable-in-conduit superconductor, a slug of hot helium is produced. The pressure rises and the hot helium expands. Thus the normal zone propagation in such a conductor can be governed by the hot helium expansion, rather than the heat conduction along the conductor. The expansion of the hot helium compresses the cold helium outside of the normal zone. This raises th@ temperature of the cold helium. When the temperature rise reaches the current sharing limit, the superconductor in contact goes normal. Thus a rapid increase in normal zone propagation occur. This phenomenon is termed Thermal Hydraulic Quenchback (THQ). An experiment was performed to investigate this process. The existence of THQ was verified. Thresholds of THQ were also observed by varying the conductor current, the magnetic field, the temperature, and the initial normal zone length. When THQ occurred, normal zone propagation approaching the velocity of sound was observed. A better picture of THQ is obtained by a careful comparison of the data with analytical studies.
Date: November 1, 1993
Creator: Lue, J. W. & Dresner, L.
Partner: UNT Libraries Government Documents Department

Vapor locking as a limitation to the stability of composite conductors cooled by boiling helium

Description: Vapor locking was observed to limit the stability of superconductors in narrow channels. The limitation is consistent with an analysis based on reduction of cooled surface by vapor formation. In very long channels (L/D >> 50), however, it was found that stability increases. When the superconductor inside long sheathed sections recovers from the resistive state, it recovers faster than in the open section. This is believed due to increase of the heat transfer coefficient and heat capacity caused by pressure build-up and relief inside the sleeve.
Date: January 1, 1977
Creator: Lue, J. W.; Miller, J. R. & Dresner, L.
Partner: UNT Libraries Government Documents Department

Investigation of stabiity of composite superconductors in typical coil configurations

Description: The stability of various composite conductor designs in realistic coil environments has been examined. We measure the velocity of propagation or contraction of a normal region, the full recovery current, and the minimum propagating current in a coil segment. We examine the dependence of these measurements on background field, transport current, electrical insulation, cooling passage size and orientation, and proximity of other conductors. Comparison of experiment and calculation provides indirect information about local heat transfer to the helium bath and direct information about safe operating current limits for particular coil designs.
Date: January 1, 1976
Creator: Miller, J. R.; Lue, J. W. & Dresner, L.
Partner: UNT Libraries Government Documents Department

Pressure rise during the quench of a superconducting magnet using internally cooled conductors

Description: Superconducting magnets cooled by supercritical helium flowing through internal conductor passages are an alternative to magnets cooled in a boiling pool. This alternative involves a possible large pressure increase in the captured volume of helium during a quench. In the US Large Coil Program (LCP), three of six coils to be tested will use internally cooled conductors. This paper describes experiments performed to understand the quench behavior of the Westinghouse coil. Agreement between experiment and theory is good. Also discussed is the extension of this work to the EURATOM coil and the Swiss coil, as well as to any coils wound with internally cooled conductors.
Date: January 1, 1980
Creator: Miller, J.R.; Dresner, L.; Lue, J.W.; Shen, S.S. & Yeh, H.T.
Partner: UNT Libraries Government Documents Department

Single-magnet results of the first EBT-P development magnet

Description: The main purpose of the present test was to see if the coil still operates reliably, especially under the simulated x-ray heating load. Thermal performances of the dewar, stack, and support structure shielding were also being investigated.
Date: January 1, 1983
Creator: Lue, J.W.; Shen, S.S.; Ballou, J.K.; McManamy, T.J. & Wilson, C.T.
Partner: UNT Libraries Government Documents Department

Simulation of the quenching of an internally cooled superconducting magnet

Description: Stability measurements on cable-in-conduit internally cooled superconductors have shown that the heat transferred to helium earlier will cause a pressure rise and induce a transient flow inside the conduit. In turn, this will enhance the heat transfer and help the superconductor to recover to its superconducting state. However, if the heat input is very high or if the heated length is very long, the pressure rise can be tremendous and the helium expulsion can be excessive. This paper describes the experimental results of simulating the quench of an entire hydraulic path. The measured thermal expulsion of helium and peak pressure during the quench are compared favorably with a similarity theory and a simple scaling relation.
Date: January 1, 1982
Creator: Lue, J.W.; Miller, J.R.; Dresner, L. & Shen, S.S.
Partner: UNT Libraries Government Documents Department

Test results of the vapor-cooled leads for the IFSMTF

Description: A vapor-cooled lead system that used six pairs of leads in twelve separate dewars was built for the International Fusion Superconducting Magnet Test Facility (IFSMTF). Each of these leads was cooled by helium vapor from a reservoir separate from the coil, in pool boiling or by force flow. Satisfactory acceptance tests were performed on the early production runs of the leads. Inconsistent and anomalously high heat losses were observed on the two pairs of lead assemblies used in the partial-array test. Additional tests on the leads and their associated dewars confirmed satisfactory performance of the lead and dewar and offered an explanation for the excessive losses during the partial-array test.
Date: January 1, 1985
Creator: Lue, J.W.; Fietz, W.A.; Stamps, R.E. & Zahn, G.R.
Partner: UNT Libraries Government Documents Department

Test of a cryogenic helium pump

Description: The design of a cryogenic helium pump for circulating liquid helium in a magnet and the design of a test loop for measuring the pump performance in terms of mass flow vs pump head at various pump speeds are described. A commercial cryogenic helium pump was tested successfully. Despite flaws in the demountable connections, the piston pump itself has performed satisfactorily. A helium pump of this type is suitable for the use of flowing supercritical helium through Internally Cooled Superconductor (ICS) magnets. It has pumped supercritical helium up to 7.5 atm with a pump head up to 2.8 atm. The maximum mass flow rate obtained was about 16 g/s. Performance of the pump was degraded at lower pumping speeds. (LCL)
Date: January 1, 1981
Creator: Lue, J.W.; Miller, J.R.; Walstrom, P.L. & Herz, W.
Partner: UNT Libraries Government Documents Department

Stability measurements of a large Nb/sub 3/Sn. [8-T fields]

Description: The Westinghouse coil for the Large Coil Program (LCP) at ORNL will use a cable-in-conduit conductor made of copper-stabilized, multifilamentary (MF) Nb/sub 3/Sn strands enclosed in a stainless steel jacket. The operating current will be 16 kA with an 8-T maximum field. The stainless steel jacket provides a channel around the conductor to allow forced cooling by supercritical helium. This study investigates the performance of a subsize conductor similar in construction, but with only one-third as many active strands in the cable.
Date: January 1, 1979
Creator: Miller, J R; Lue, J W; Shen, S S & Dresner, L
Partner: UNT Libraries Government Documents Department

Measurments of stability of cabled superconductors cooled by flowing supercritical helium

Description: The concept of stability in superconductors cooled by forced flow of supercritical helium is somewhat different from conductors cooled by pool-boiling helium. The crucial point is whether such a conductor can recover from a large deposition of energy before the oxygen is heated to a level that prohibits recovery. In an investigation of stability of forced flow conductors it is usually necessary to use indirect methods of heating the conductor initially, such as by an external pulse coil. However, in using such indirect methods it is essential to determine accurately the time development and magnitude of energy deposition. We use ac loss techniques to examine pulse coil heating and compare those results with extensive measurements on a specially constructed sample containing an embedded heater.
Date: January 1, 1978
Creator: Miller, J.R.; Lue, J.W.; Shen, S.S. & Lottin, J.C.
Partner: UNT Libraries Government Documents Department