Sangkwon Jeong

Optimum temperature staging of cryogenic refrigeration system

Abstract Optimum temperature staging is investigated to minimize entropy generation in a multi-stage cryogenic refrigeration cycle. It is found that the best intermediate temperature distribution is to have the same high to low temperature ratio at each stage of the system. As an example, the result is applied to the design of a cryogenic cascade thermoelectric cooler to find the optimum size distribution of each stage.

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.

Ramp-rate limitation test of cable-in-conduit conductors with supercritical helium

It has been found on the United States Demonstration Poloidal Coil (US-DPC) and in 27 strand subsized cables of pool boiling cable-in-conduit conductor (CICC), that there is critical current degradation due to fast ramping of the magnetic field. The characteristics of this ramp-rate limitation phenomenon are investigated by using a 27 strand Nb/sub 3/Sn cable in supercritical helium at 6 atm. A 3 m long cable-in-conduit conductor is prepared noninductively and tested in a background field up to 9.5 tesla with maximum ramp rate of 1.6 tesla/second. The ramp-rate limitation results are compared with results of the ramp rate test of the US-DPC and previous experiments. The experimental data are analyzed to identify and understand possible sources of ramp-rate limitation.<<ETX>>

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.

Tandem magnetic refrigerator for 1.8 K

Abstract A tandem magnetic refrigerator has been developed for continuous operation between 4.2 and 1.8 K. This paper presents the design, computer simulation and construction of the prototype machine. The regenerative concept is employed in the magnetic refrigerator to obtain a cascaded magnetic Carnot cycle effect along the temperature axis from the cold end to the warm end. Entropy pumping action from the cold heat reservoir to the warm heat reservoir occurs in the active magnetic regenerator. The inherent sources of irreversibility in regenerative magnetic refrigerators, i.e. heat capacity imbalance between magnetic refrigerant and heat transport medium, helium entrainment and dead volume effect, have been minimized by design optimization of the system components. The magnetic system of the tandem refrigerator has two virtually identical units, each consisting of a gadolinium gallium garnet (GGG; Gd 3 Ga 5 O 12 ) magnetic core, a superconducting magnet, a warm end heat exchanger and a cold end heat exchanger. These components are united by a cryogenic displacer which shuttles the heat transport medium, subatmospheric 3 He gas, between the two units. The prototype of the designed magnetic refrigerator has operated continuously, producing a net refrigeration rate of 12 mW per magnetic core at 1.8 K.

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.

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.

Magnetically Augmented Regeneration in Stirling Cryocooler

This paper describes how the magneto-caloric effect can be employed with direct entropy coupling between the magnetic entropy of the magnetic material and the pressure entropy of the gas in refrigeration cycle. A magnetic refrigerator generally works in the low temperature range below 4.2 K, rejecting heat to the higher temperature that is considered to be the lowest temperature stage of a reliable gas compression refrigerator. A Stirling type Gifford-McMahon cryocooler is usually operated above 8 K. Cooling at lower temperatures with a Stirling type cryocooler has been difficult because of the low specific heat of the regenerator material at temperatures below 8 K. To overcome the limitation of the small lattice specific heat of the regenerator material, a material with large magneto-caloric effect is proposed for the regenerator packing and external magnetic field change is incorporated in coordination with the gas flow through the regenerator. The virtual heat capacity of the paramagnetic material in the magnetically augmented regenerator is increased by variation of the magnetic field. The material is demagnetized for heat storage and magnetized for heat release. The net magnetic work is ideally zero. The paper describes a first order analysis of the combined gas-magnetic system and concludes with a design idea for the refrigerator operating between 300 K and 4.2 K. The objective of this paper is to demonstrate the potential of the entropy coupled cycle for further development and to provide the basis for a decision on the construction of a prototype.

Experimental Study of AC Losses in a Rutherford Type Cable

This paper describes measured AC losses of NbTi Rutherford type cable when the wide side of the cable is parallel to the field variation. In the case of a Rutherford type cable, the orientation of the cable in the magnetic field is an important factor for the interstrand coupling loss which can be dominant over other AC losses such as hysteresis loss or intrastrand coupling loss. A calorimetric method was used to quantify experimentally the AC losses of the cable. The measurement results were essential to the thermal design of a superconducting pulse coil set for use in stability experiments on the TPX (Tokamak Physics Experiment) subsize conductors. AC losses were also measured with transport current to see its effect on this loss. The transport current effect on the interstrand coupling loss was generally negligible when the 0.086 T ripple field at 7.5 Hz was applied to the cable on top of the dc bias field. However, when the field was varied in the slow continuous triangular wave between 0 to 7 T, the AC loss increased noticeably due to transport current.

Persistent Current Decay of a Superconducting Coil in an AC Field

The superconducting magnets for a magnetically levitated vehicle are often designed to operate in persistent current mode. Ideally, the magnets carry DC transport current in a steady magnetic field. The magnet, however, will experience transient fields and thus induce transient transport current due to mechanical and electrical perturbations. Under these conditions, it is possible for the AC losses in the superconductor to result in a decay of the persistent transport current. This paper presents a unique method of measuring the transport current loss component of the hysteretic AC loss by measurement of the decay of the persistent current. The experimental apparatus consists of a pair of matched solenoidal coils. These coils, after being charged by an energizing coil, are subject to magnetic field variations. The current decay rate in the persistent-mode coil pair is measured as a function of the strength and frequency of the external field. This current decay rate is directly related to the loss of energy. The experimental data are presented primarily for large field variations, and these results are compared to theoretical prediction.