Cryostat design for the "bucket test" of the NHMFL 900-MHz NMR magnet

The NHMFL 900-MHz magnet is a 40-MJ, 7-t solenoid designed for NMR spectrometry in a 105-mm warm bore. Prior to installation in its final cryostat, it was decided to test the magnet in an available "bucket" dewar to confirm proper operation of all components, requiring design and construction of a special cryostat for this purpose. The primary requirements for the cryostat were to use the existing bucket dewar (1.52-m i.d. x 3.66-m i.h.) and cool the magnet to near Hell temperatures as efficiently as possible. With the large and relatively short conduction path formed by the inner dewar wall and the 2 in 3 vapor volume above the liquid capped by a 2 in 2 surface at room temperature, the latter requirement was not an easy one to meet. In this paper we describe a cryostat design based on an internal, subatmospheric bath continuously fed by an external reservoir of normal liquid. The vapor stream evolved from the internal bath was used to intercept heat in various paths, e.g.: the incoming LHe stream, a system of radiation baffles in good thermal contact with the dewar wall, the vertical supports for the magnet, the main current leads for the magnet, and the multitude of minor leads for instrumentation. We report details of this design, its performance in tests of the 900-MHz magnet, and the potential for application of this design topology for cooling other magnets.