Brent A. Warner

Magnetic shielding for sensitive detectors

Abstract This paper describes the design and test of shielding for a superconducting magnet. The magnet is part of an Adiabatic Demagnetization Refrigerator (ADR) for the Submillimeter and Far Infrared Experiment (SAFIRE), a payload on the Stratospheric Observatory for Infrared Astronomy (SOFIA). SAFIRE will use superconducting detectors which must be shielded from the magnet cooling system. Because the detectors use Superconducting Quantum Interference Devices (SQUIDs), the field at the detector package must remain at or below the 10−7 T level while the detectors are operating. We discuss laboratory tests of the passive shielding and simulations using Poisson group programs.

Thermal design of the XRS helium cryostat

Abstract The required lifetime for the Astro-E X-Ray Spectrometer (XRS) is 2 years, with a goal of 2.5 years. To meet this requirement, significant advances in state-of-the-art longlife cryogenic systems are required. The XRS system is a hybrid neon/helium system with a final stage of cooling provided by an adiabatic demagnetization refrigerator. The thermal design of the helium cryostat is described in this paper. To achieve a lifetime of 2.5 years with a helium volume of approximately 20 litres, the heat load on the helium must be of the order of 800 μW or less. The expected lifetime and sensitivity of the lifetime to changes in the design or external heat loads is modelled. Results of preliminary thermal conductivity measurements are presented and future tests are identified. A study of heat loads that were small enough to be neglected in previous designs of long-life cryogenic systems was undertaken. A summary of the findings is presented.

Progress on a 4 K to 10 K Continuously Operating Adiabatic Demagnetization Refrigerator

An adiabatic demagnetization refrigerator (ADR) is under development for use in cooling relatively large loads (10–100 mW) at 4 K and rejecting that heat to a cryocooler operating at 10 K. ADRs can operate in this temperature range with an efficiency of 75% of Carnot, saving as much as 2/3 of the required overall input power. In addition this ADR can provide cooling down to 0.4 K. The ADR magnet consists of 8 short coils wired in series and arranged in a toroid to provide self‐shielding of its magnetic field. Eliminating passive or active shields saves 30% of the mass of the system. The average field is 3 Tesla using 5 amps. In the first model the coils are wound with ordinary NbTi superconducting wire and operated at 4 K. A second version will then use fine Nb3Sn wire to provide complete 10 K operation. As a refrigerant for this temperature range we are using readily available gadolinium gallium garnet (GGG) crystals, which provide suitable performance. In the future we will switch to either GdLiF4 or Gd...

The HAWC and SAFIRE adiabatic demagnetization refrigerators

Abstract High-resolution airborne wide-band camera (HAWC) and sub-millimeter and far-infrared experiment (SAFIRE) are far-infrared experiments which will fly on the stratospheric observatory for infrared astronomy (SOFIA) aircraft. HAWCs detectors will operate at 0.2 K, while those of SAFIRE will be at 0.1 K. Each instrument will include an adiabatic demagnetization refrigerator (ADR) to cool its detector stage from the liquid helium bath temperature (HAWCs at 4.2 K and SAFIREs pumped to about 1.3 K) to its operating temperature. Except for the magnets used to achieve the cooling, the magnetic shielding, and a slight difference in the heat switch design, the two ADRs are identical. We describe the ADR design and present the results of performance testing.

Passive magnetic shielding for the submillimeter and far infrared experiment

Abstract Goddard Space Flight Center is developing the submillimeter and far infrared experiment (SAFIRE). SAFIRE will use SQUIDs as amplifiers for detectors, which must be shielded from the magnet cooling system, an adiabatic demagnetization refrigerator (ADR). The magnetic field at the detector package must remain at or below the 10 −7 tesla level while the detectors are operating. We discuss laboratory tests of the passive shielding and simulations.

Thermal performance of the XRS helium cryostat

The Astro-E/X-Ray Spectrometer (XRS) is required to operate for 2 years on-orbit with approximately 20 liters of superfluid helium. The total heat load on the helium must be less than 1 mW, which is significantly smaller than any previous spaceborne helium system. Consequently, the XRS cryostat incorporates several new techniques that have not been used before in spaceborne cryogenic systems, including high temperature superconducting leads, a film flow suppressor in the vent line, and kevlar suspension systems. The thermal design of the cryostat is described and results of the ground tests of both an engineering model cryostat and the flight cryostat are presented. Tests of the flight cryostat are incomplete at the writing of this paper, so only preliminary results are given.

Magnetic Shielding for a Spaceborne Adiabatic Demagnetization Refrigerator (ADR)

The Goddard Space Flight Center has studied magnetic shielding for an adiabatic demagnetization refrigerator. Four types of shielding were studied: active coils, passive ferromagnetic shells, passive superconducting coils, and passive superconducting shells. The passive superconducting shells failed by allowing flux penetration. The other three methods were successful, singly or together.

Spaceworthy ADR: recent developments

Recent developments concerning the performance and reliability of a spaceworthy adiabatic demagnetization refrigerator (ADR) for the AXAF X-ray spectrometer are considered. They include a procedure for growing the salt pill around a harness made up of 6080 gold-plated copper wires, a totally modular gas gap heat switch, and a suspension system utilizing Kevlar fibers.

Geometry dependence of superconducting shielding for sensitive detectors

Superconducting shielding is a powerful technique for shielding detectors. However, it is sensitive to details of geometry. This paper uses the geometry of the shielding of the submillimeter and far infrared experiment (SAFIRE) as a basis for study. Shielding tests are performed on three different geometries of niobium - disk, ring and tube. Attempts to model the superconducting shielding are also made and compared with measurements.

Electromagnetic Interference (EMI) and Thermal Studies of Flat Conductor Cable (FCC)

The Cryogenics Section at the Goddard Space Flight Center is studying Flat Conductor Cable (FCC) to determine whether it is suitable for spaceflight cryogenic systems. FCC could replace conventional cryogenic wiring, such as coaxial cables and single strand stainless steel and manganin.