An ETF TF-coil concept employing NbTi alloy, bath cooled with superfluid helium

Under subcontract to the MIT/FBNML, General Atomic Company has performed a preliminary study to assess the feasibility and engineering considerations of employing NbTi alloy conductor, bath cooled with superfluid helium (He II), in an Engineering Test Facility (ETF) toroidal field (TF) coil. This study indicates that saturated superfluid helium (He II) merits serious consideration as an alternative to the use of He I for high field (11-12 tesla) NbTi alloy TF-coils, which require bath temperatures below 4 K. The primary advantages of He II over reduced temperature (2.5-3 K) He I are two: (1) Due to the extremely high thermal conductivity of He II, almost all of the sub-lambda enthalpy is available to absorb local or transient heat loads. Further, this He II characteristic results in vapor evolution only at the bath surface, thus obviating concerns over vapor locking within the heated centerpost region; and (2) The relatively high surface heat transfer results in substantially improved conductor stability characteristics. As a result, coil protection limits (dump voltage and conductor temperature) become the limiting factors in allowable substrate current density. The disadvantages of He II relative to reduced temperature He I are increased refrigeration power and pumping requirements, and some additional system complexity. This study affords a direct comparison with the He I bath cooled TF-coil concept developed by Team One (GA/MCA) for the DOE 12 Tesla Coil Development Program.