Progress in Superconductivity and Cryogenics (한국초전도ㆍ저온공학회논문지)

The Korean Society of Superconductivity and Cryogenics (한국초전도저온학회)
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Current overshoot operation of a REBCO magnet to mitigate SCF

  • Lee, Changhyung (School of Mechanical Engineering, Changwon National University) ;
  • Hahn, Seungyong (Department of Electrical and Computer Engineering, Seoul National University) ;
  • Bang, Jeseok (Department of Electrical and Computer Engineering, Seoul National University) ;
  • Cho, Jeonwook (Korea Electrotechnology Research Institute (KERI)) ;
  • Kim, Seokho (School of Mechanical Engineering, Changwon National University)
  • Received : 2018.12.11
  • Accepted : 2018.12.21
  • Published : 2018.12.31
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Abstract

Due to large in-field current carrying capacity and strong mechanical strength, a REBCO wire has been regarded as a viable high temperature superconductor (HTS) option for high field MRI and > 1 GHz (>23.5 T) NMR magnets. However, a REBCO magnet is well known to have an inherent problem of field inhomogeneity, so-called 'Screening Current induced magnetic Field (SCF)'. Recently, 'field shaking' and 'current overshoot operation' techniques have been successfully demonstrated to mitigate the SCF and enhance the field homogeneity by experiments. To investigate the effectiveness of current overshooting operation technique, a numerical simulation is conducted for a test REBCO magnet composed of a stack of double pancake coils using '2D edge-element magnetic field formulation' combined with 'domain homogenization' scheme. The simulation result demonstrates that an appropriate amount of current overshoot can negate the SCF. To verify the simulation results, current overshoot experiments are conducted for the REBCO magnet in liquid nitrogen. Experimental results also demonstrate the possible application of current overshoot technique to mitigate the SCF and enhance the field homogeneity.

Keywords

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Fig. 1. Generation of non-uniform current distribution by induced screening current in a REBCO tape.

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Fig. 2. Example of current overshoot operation.

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Fig. 3. Double pancake REBCO coils by co-winding of polyimide film.

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Fig. 4. Fabricated REBCO magnet by stacking 3 DPCs and external joints.

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Fig. 5. Critical current measurement of the REBCO magnet.

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Fig. 6. Equivalent current distributions at transport current of 40 A: (a) uniform current distribution; (b) after simple ramp up; (c) after 10 % current overshoot; (d) after 20 % current overshoot.

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Fig. 7. Calculated SCF at the magnet center for different current overshoots and target currents.

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Fig. 8. Experimental results of simple linear ramp up and down.

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Fig. 9. Measured SCF at the magnet center for different current overshoots and target currents.

TABLE I SPECIFICATIONS OF REBCO WIRE AND KEY PARAMETERS OF REBCO MAGNET.

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TABLE II COMPARISON OF ANALYSIS AND EXPERIMENTAL RESULTS.

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