Philip C. Michael

Burnishing and adhesive wear of an electrically conductive polyester-carbon film

Abstract Experiments were carried out to examine the friction and wear behavior of an electrically conductive polyester-carbon film during small amplitude reciprocating sliding against some typical noble metal alloys. The effects of normal load and environmental humidity on friction and wear were examined. A method was devised to monitor continuously the wear of the film based on the increase in its bulk electrical resistance. The frictional force was also monitored during sliding, providing a simultaneous record of both friction and wear. A linear regression analysis of the test results yielded a fourth power relation between wear and friction, indicating the probable occurrence of an adhesive wear mechanism. During sliding at low humidity, the sliding surfaces acquired a relatively smooth, polished appearance and the wear coefficient of the film was of the order of 10 −8 . These observations suggest that burnishing, resulting from the loss of material on a molecular scale, was the wear mechanism at low humidity. At relative humidities above about 40%, the wear mechanism shifted towards moderate adhesive wear; friction was higher, the wear surfaces were noticeably rougher and the wear coefficient of the film was in the range 10 −7 /210 −6 . Implications of these findings for the use of the unlubricated or poorly lubricated polyester-carbon film in demanding position control applications are discussed.

Voltage spike observation in superconducting cable-in-conduit conductor under ramped magnetic fields: 1. Experiment

A 27-strand hybrid superconducting cable-in-conduit conductor (CICC) was fabricated and tested under quickly-ramped high magnetic fields. When the field increased linearly on the CICC, the voltage signal showed several intermittent spikes before it quenched. This paper describes an observation of peculiar voltage spikes during these ramp-rate limitation experiments. The voltage spikes are interpreted as quench precursors and understood as current redistribution events within the local cable inside the conduit. A quantitative correlation is obtained for the magnetic field at which the first voltage spike occurs during ramping fields. The non-uniform current distribution among the strands and the induced loop current in the cable, which is generated by ramped fields, are found to be responsible for the voltage spikes.

Design, fabrication and test of the react and wind, Nb/sub 3/Sn, LDX floating coil conductor

The Levitated Dipole Experiment (LDX) is a novel approach for studying magnetic confinement of a fusion plasma. In this approach, a superconducting ring coil is magnetically levitated for up to 8 hours a day in the center of a 5 meter diameter vacuum vessel. The levitated coil, with on-board helium supply, is called the floating coil (F-Coil). Although the maximum field at the coil is only 5.3 tesla, a react-and-wind Nb/sub 3/Sn conductor was selected because the relatively high critical temperature will enable the coil to remain levitated while it warms from 5 K to 10 K. Since prereacted Nb/sub 3/Sn tape is no longer commercially available, a composite conductor was designed that contains an 18 strand Nb/sub 3/Sn Rutherford cable. The cable was reacted and then soldered into a structural copper channel that completes the conductor and also provides quench protection. The strain fabrication steps such as: soldering into the copper channel, spooling, and coil winding, to prevent degradation of the critical current. Measurements of strand and cable critical during state of the cable was continuously controlled currents are reported, as well as estimates fabrication, winding and operating strains on critical current.

Voltage spikes in superconducting Cable-In-Conduit Conductor under ramped magnetic fields. Part 2: Analysis of loop inductances and current variations associated with the spikes

A 27 strand hybrid superconducting Cable-In-Conduit Conductor (CICC) sample (so-called TPX-PF model sample) has been fabricated and tested in quickly ramped background magnetic fields. The voltage spikes that appeared in the samples terminal voltages during magnetic field sweeps at DC transport current are analyzed using a model that calculates the magnitude of individual strand current drops and the strand to strand/cable inductances associated with each voltage spike. Dependencies of the strand inductances and current variations with consecutive voltage spike numbers, total transport current in the cable and background magnetic field are analyzed and discussed. The analysis confirms previously reported suggestions that voltage spikes and the corresponding rapid variations, or jumps, observed in the conductors local magnetic field are indications of rapid redistribution of current from one of the cables strands in which the current reached its critical level. It is shown that rapid current redistributions which are too small to initiate total cable quench lead to more uniform distribution of current among the strands in the CICC. Therefore, it may be possible to apply small disturbances to a CICC to improve its strand to strand current distribution in a cable and to stabilize its Ramp Rate Limitation behavior.

The ITER Central Solenoid

The central solenoid for the International Thermonuclear Experimental Reactor (ITER), a fusion tokamak experiment with the goal of generating 500 MW of fusion power with high gain (Q>10), must provide most of the volt-seconds needed to induce and sustain a 15 MA plasma for burn times of >400 s. The 6.4 GJ central solenoid design requires a 45 kA conductor and has a peak field of 13 T. The central solenoid consists of six pancake-wound modules, stacked vertically, and held in axial compression by an external structure. The five-stage cable has 1/3 copper and 2/3 advanced Nb3Sn strands in a thick superalloy conduit and is cooled by the forced-flow of supercritical helium through the cable space. Key design issues include the qualification of a conduit with adequate fatigue strength, avoiding filament damage from transverse Lorentz loads, eliminating axial tension in the winding insulation, and qualification of space-saving intramodule butt joints

Measurements of current distribution in a 12-strand Nb3Sn cable-in-conduit conductor

*Institute of Superconductivity and Solid State Physics of Russian Research Center ‘Kurchatov Institute’, 123182 Moscow, Russia tPlasma Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 18 November 1996; revised IO February 1997 Experiments were performed to measure directly the current in each strand of a 12- strand Nb,Sn cable-in-conduit superconductor during current and/or external mag- netic field ramps. The goal of the experiment was to get straightforward evidence of current maldistribution in a cable-in-conduit conductor (CICC). A heavily instrumented sample coil from Nb,Sn TPX-TF strands was specially prepared. Severe non-uniformity of the strand currents were found during field ramp. Immediately before a quench the individual strand currents within a triplet differed by as much as an order of magni- tude. During field ramps with constant transport current, the currents in some strands were observed to drop rapidly and then recover. The data show that quench develop- ment in the CICC is a complicated phenomenon involving dynamic redistribution of current among the strands. Non-uniformity of current along the strands during quench was also observed. 0 1997 Elsevier Science Ltd.

Qualification of joints for the inner module of the ITER CS model coil

The US-ITER home team has constructed a prototype, low-loss, low-resistance lap joint sample using materials, fabrication techniques, machine tooling and quality assurance procedures identical to those for the layer-to-layer joints in the inner module of the ITER Central Solenoid Model Coil. The joint sample was tested at the MIT Pulse Test Facility to 50 kA current, in both parallel and transverse time-varying fields at ramp rates from 0.05 to and background induction to 4T, to qualify performance to ITER-relevant operating conditions. The joint shows DC below 2.5 nOhm and pulsed, transverse field losses for a 1.5 ramp at 0.4 T/s below 90 J; both values are well within target limits for the ITER-CS joints.

Stabilization of dry-wound high-field NbTi solenoids

Dry-wound, Formvar-insulated NbTi superconducting solenoids are frequently used for small-bore and low-field applications because of their low manufacturing costs. The more widespread use of dry-wound coils for large-bore and high-field applications has been limited by their tendencies toward conductor-motion-induced degradation. Simple models for estimating the occurrence of conductor-motion disturbances in dry-wound solenoids are described. These models were used to design and analyze the training behaviors of three small, high-field NbTi test coils. The experimental results suggest that stabilization is best achieved by forcing all potential conductor motions to occur during low-field portions of the magnets energization sequence, where its stability margin is greatest.<<ETX>>

Persistent-current switch for pancake coils of rare earth-barium-copper-oxide high-temperature superconductor: Design and test results of a double-pancake coil operated in liquid nitrogen (77–65 K) and in solid nitrogen (60–57 K)

We present design and test results of a superconducting persistent current switch (PCS) for pancake coils of rare-earth-barium-copper-oxide, REBCO, high-temperature superconductor (HTS). Here, a REBCO double-pancake (DP) coil, 152-mm ID, 168-mm OD, 12-mm high, was wound with a no-insulation technique. We converted a ∼10-cm long section in the outermost layer of each pancake to a PCS. The DP coil was operated in liquid nitrogen (77-65 K) and in solid nitrogen (60-57 K). Over the operating temperature ranges of this experiment, the normal-state PCS enabled the DP coil to be energized; thereupon, the PCS resumed the superconducting state and the DP coil field decayed with a time constant of 100 h, which would have been nearly infinite, i.e., persistent-mode operation, were the joint across the coil terminals superconducting.

Design and Test of a Prototype 20 kA HTS DC Power Transmission Cable

We present an innovative design for a high temperature superconductor (HTS) dc power transmission system containing an HTS cable connected to optimized multistage current leads and cooled by a multistage turbo-Brayton cryocooler. Analytical results indicate that our proposed 5 MW transmission system can provide a 35% reduction in transmission losses and 93% reduction in system weight, compared with ambient-temperature copper bus. We describe a test stand to assess the performance of the proposed design and discus modifications to the facility to permit cable testing at up to 20 kA current. Efforts to demonstrate full current operation in the simulated system were hampered by higher than expected electrical resistance at the cable joints.