Vitaly Vysotsky

New method of current distribution studies for ramp rate stability of multistrand superconducting cables

The ramp rate limitation phenomena were studied using local field sensors to observe the intrinsic processes within the cable. Sensitive miniature Hall sensors and small pick-up coils placed around cable-in-conduit superconductor were used to measure local magnetic fields and field derivatives associated with currents in the cable. Using this method, both fast jumps and slow changes in local magnetic fields at different conditions mere observed. First jumps occured during ramping background magnetic field and may indicate a fast current redistribution processes. Slow changing of local fields may be associated with current loops closed through the current lead joints. Such current loops may also indicate the nonuniformity of current distribution in the cable strands. The new method is a promising tool for future investigations of stability of multistrand cables.<<ETX>>

Spike voltages seen during "quick charge" ramp limitation tests on Nb/sub 3/Sn cable-in-conduit conductors

Spike voltages observed during ramp rate limitation tests on sub-sized Nb/sub 3/Sn cable-in-conduit superconductors are analyzed using current loop model. The effects of loop currents on the ramp limitations of multi strand superconducting cables are discussed. Current loops existing in multi strand cables generate excess local currents that quench strands and produce voltage spikes. Experimental results previously reported as abnormal ramp rate limitations are explained by loop current phenomena.

Ramp-rate limitation experiments using a hybrid superconducting cable

Abstract Ramp-rate limitation experiments were done in a new facility at the MIT (Massachusetts Institute of Technology) Plasma Fusion Center. The features of this new facility include (1) a superconducting pulse coil that can superimpose high ramp-down rates, up to 25 T s−1, (2 T in 80 ms) at a background field up to 5 T, (2) new power supplies that can supply high rates of dl dt and dB dt to the sample under test and (3) a forced-flow supercritical helium system for cooling CICCs (Cable-In-Conduit Conductors). This paper discusses the results of the ramp-rate limitation experiments on a 27-strand hybrid Nb3Sn cable. The cable was tested under field ramps of up to 2.5 T s−1 with various operating currents. It did not quench with dB dt , field and average strand currents that were simultaneously above the operating range of TPX-PF (Tokamak Physics Experiment Poloidal Field) coils. Further ramp-rate limitation experiments revealed that the tested 27-strand hybrid cable has very high transient stability at ramped fields, extending out to average strand currents that are nearly triple the TPX-PF operating current.

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.

Current distribution in a 12 strand Nb/sub 3/Sn CICC and its influence on ramp rate limitation

Direct measurements of the current in each strand of a chrome coated Nb/sub 3/Sn CICC were performed during current and/or external magnetic field ramps. Severe non-uniformity of the strand currents was found immediately before quench. The currents in some of the strands were also observed to change abruptly at various points during external field ramps. The observed peculiarities of the strands current behaviors are considered in the present paper and a simple ad hoc model of the ramp rate limitation phenomenon is proposed based on the hypothesis that current non-uniformity is a main cause of RRL. This model is compared with the experimental results.

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.

On the Position of the Apparent Current Center Inside CICC during External Magnetic Field Ramp

We believe that ramp rate limitation phenomena (RRL) are strongly influenced by nonuniform current distribution between strands. To check this assumption, local field sensors such as Hall probes were used to study current redistribution inside of an ITER-type cable-in-conduit (CICC) subcable during fast field ramps.1 It is possible to determine the position of the apparent magnetic center of all currents in the cable by combining signals from at least two Hall sensors located in one cross-section around the CICC The apparent current center is a point where all current may be placed to create the same local field intensity measured at the sensors. We often found that the current center does not match the geometrical center of the cable. This definitely shows that the current inside the conduit is distributed non-uniformly Moreover, during magnetic field ramp the apparent current center demonstrates a sophisticated movement inside the CICC. Similar displacement of the current center was found in the sample whose conduit was tilled by oil and frozen at 4.2 K to exclude possible mechanical motion of the strands. Detailed experimental procedures and data about the position of the apparent current center are presented. A two-stream model is proposed to estimate the current distribution inside the conduit.

Ramp-rate limitation experiments in support of the TPX magnets

Fast magnetic field change is required for full-size tokamak reactors. The poloidal field magnets are usually ramped to full field at 1.2 T/s, and see pulsed fields of up to 20 T/s during plasma initiation. A new facility has been constructed at M.I.T. that simulates the expected operating conditions of the Tokamak Physics Experiment (TPX) magnets. New features in this facility include (1) a superconducting pulse coil that can superimpose high ramp-down rates, up to 25 T/s, (2 T in 80 msec) on a background field up to 5 T, (2) new power supplies that can supply high rates of dI/dt and dB/dt to the sample under test and the pulse coil, and (3) a forced-flow supercritical helium system that can simulate cooling conditions within the winding pack. The first sample tested in the facility is a 27-strand sub-cable, using 3.1:1 copper/non-copper ratio Nb/sub 3/Sn superconductor, typical of the strands to be used in ten of the poloidal field system magnets. This paper presents the first experimental results on the ramp rate limitation of the sub-size cable sample of TPX PF coil conductor. The transient stability at high ramp rate fields will be discussed.

Jumps of the local magnetic field near cicc during external magnetic field ramp and their connection with the ramp rate limitation

A new method has been developed to study Ramp Rate Limitation (RRL) phenomena.1 Samples of ITBR-type cable-in-conduit (CICC) subcable were instrumented with local field sensors such as Hall probes and pick-up coils and then subjected to rapidly changing external magnetic field. We found that during fast field sweeps some discontinuous changes, or jumps occur in the local field. We believe that these jumps indicate a fast current redistribution processes inside CICC. Detailed information about local magnetic field jumps during changing field is presented. Possible origin of the jumps and their connection with RRL are discussed.

Superconducting pulse coil set for stability test of superconducting cables

A superconducting pulse coil set was constructed for transient stability experiment of CICC (cable-in-conduit conductor). The pulse coil set was composed of an inner and an outer coil connected in series. With 5 T background field, the pulse coil produced an additional 2 T on the CICC. In order to simulate the TPX (Tokamak Physics Experiment) plasma initiation scenario, 2 T field drop from 7 T was tried with an approximate ramp rate of -25 T/s. This paper describes the design for the coil and presents the experimental results of its successful AC operation.