Calorimetric AC Loss Measurement of 1.3-M Model Dipole Magnets by Using A 1.8 K Cryostat

Abstract

To measure AC losses of dipole magnet by the calorimetric method, the magnets were cooled to 1.8 K with 120 liters of pressurized superfluid helium, using a 1.8 K cryostat. Heat dissipated in the magnet during a ramp cycle was measured by observing the temperature of the superfluid helium. AC losses were evaluated by the production of the heat capacity and temperature rise of helium. The 1.8 K cryostat consisted of a 4.2 K helium vessel, a 1.8 K pressurized superfluid helium vessel, and heat exchangers (1.8 K saturated helium pot and precooler). Liquid helium from the 4.2 K vessel, cooled to 2.5 K in the pre-cooler, expanded in the 1.8 K saturated helium pot. It cooled the magnet and pressurized superfluid helium to 1.8 K. Evaporated helium gas in the pot was evacuated by a pumping unit. The magnets were model dipole magnets fabricated at KEK for SSC dipole magnet R&D. The overall length is 1.3 m and inner aperture of the coil is 50 mm. Two types of magnet were made to study the AC properties: Type 1 with 6 μm NbTi filament cables and Type 2 with 2.5 μm filament cables. AC losses were measured in a ramp cycle from 500 to 5(X)0 A with 5 second duration at the flat top. Ramp rates were varied from 70 to 400 A/s. Results show that the Type 2 magnet had smaller AC loss than the Type 1.