Vertical access zero boiloff rampable superconducting magnet system with horizontal field for semiconductor crystal growth

Abstract

A magnet system has been designed and built to minimize convection mixing during the growth of 0.2 m (8 inch) diameter semiconductor crystals. Due to the size requirements of the crystal growth furnace, the magnet has a room temperature vertical access bore of diameter 1.27 m and height 1.2 m. The cold bore of the magnet is diameter 1.4 m and height 0.8 m with a 280 liter liquid helium bath. The field is perpendicular to the cylindrical bore and is provided by two saddle shaped NbTi coils. The two coils, each with 1200 turns operating at 200 A, produce a 0.2 T horizontal field at the center of the bore. The series inductance is 0.3 H for a total stored energy of 6 kJ. High Tc (BISCO in silver tape) leads were used to allow the system to be operated without persistent current switches, for ease of ramping. A 100 W at 80 K, 5 W at 20 K shield GM cooler plus a 45 W at 50 K, 1.5 W at 4.2 K helium recondensing G-M cooler were used to achieve zero boiloff. The measured shield temperatures in operation are 65 K and 17 K. Design considerations, including thermal staging of the silver tape/BISCO (HTC) current leads, lead quench protection circuit, low thermal conduction mechanical support structure and magnet construction, are discussed.