Cryogenics 38 (1998) 1123–1134  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0011-2275(98)00099-X 0011-2275/98/$ - see front matter The electrical insulation system of a forced flow cooled superconducting (sc) magnet K. Bauer, S. Fink, G. Friesinger, A. Ulbricht* and F. Wu ¨chner Institut fu ¨r Technische Physik, Forschungszentrum Karlsruhe, Hermann-Helmholtz- Platz 1, D-76344 Leopoldshafen, Germany Received 25 May 1998; revised 17 August 1998 The discharge of stored magnetic energy of sc magnets in the GJ range requires dis- charge powers up to several hundreds of MW. The present state-of-the-art of sc high current cables in the 50 kA range leads to voltages of several tens of kV. Forced flow cooled conductors allow a rigid winding construction with higher discharge voltages. At Forschungszentrum Karlsruhe a sc poloidal field model coil ([ 3 m) was constructed and tested at about 20 kV. The typical components of the electrical insulation system were developed.Partial discharge measurements and pulse voltage were applied. It was demonstrated that the engineering basis of the insulation system for about 20 kV was developed. 1998 Elsevier Science Ltd. All rights reserved Keywords: high voltage components; superconducting magnets; cryogenic tempera- tures;force flow cooling; insulation breaks; high voltage instrumentation cable; cur- rent feedthroughs; test facilities The emerging ideas of application of superconducting (sc) magnetic energy storage in the early seventies soon showed that in magnet technology high voltage was also needed for transferring the stored magnetic energies within a certain time interval. This development was speeded up by the fact thatplasmaphysicsexperiments needed morefficient pulse power supplies for which the release of stored mag- netic energy from sc magnets was one option. In the frame of these discussions a small pool cooled power pulse gener- ator (sc coil with sc switch) was constructed for single shot operation with non helium transparent winding, and suc- cessfully tested up to a pulse power of 40 MW and 47 kV 1 . The development of forced flow cooled sc magnets in the frame of the ‘Large Coil Task’ (1976–1987) was the basis for the development of the application of higher rated volt- ages 2 . Considering all boundary conditions for the oper- ation of a pool cooled helium transparent winding, gaseous helium has to be taken into account up to 10 K at 0.1 MPa pressure, especiallyin interfaceareas(e.g. winding terminals). Operation pressure at 1 MPa and 10 K increases the helium density and therefore the dielectric strength by a factor of 10 for forced flow cooled coils 5 . Voltages up to 2.5 kV were applied for the forced flow cooled LCT coils. The voltage limit was given at this time by the compati- bility of the torus operation with the pool cooled coils and the experimental surrounding (instrumentation, dump circuit). In a special project ‘POLO’ assigned to the devel- opment of sc poloidal field coils for tokamak machines at *To whom correspondence should be addressed. Cryogenics 1998 Volume 38, Number 111123 the Forschungszentrum Karlsruhe (FZK) the developmen was pushed in the typical voltage range of 20 kV which has been required for furtherapplications 3 . A seriesof components had to be developed for making this type of coil suitable forvoltage ratings of about20 kV.These developments cover the insulation system of the coilin respect to the helium supply, the electrical supply and the instrumentation. Basic consideration about design ofthe electrical insulationand electrical insulationmaterials applicable in sc magnet technology have been published Schwenterly and Gerhold 4,5 . There was a poloidal field coil projectatJAERI,Japan,too,butwith more emphasis on the conductor development 6 . Therefore the most advanced frontierknown in this field is presently the development performed in the frame of the POLO Project. That will be treated in the article. An extended description of the doc mentation relating to POLO component development and the partial discharge insulation diagnostics was published by Irmisch 7,8 . Interesting aspects with potential for further developments coming from other projectswillbe men- tioned in the corresponding sections. Most of the develo technology will be applied to the ITER toroidal field mod coil and the test facility TOSKA. Components of the electrical insulation of forced flow cooled sc magnets The electrical insulation system of a forced flow cooled s magnet can be described by the electrical insulation sys of the winding. This is the turn insulation and the insulat