Robert F. Boyle

NASA advanced cryocooler technology development program

Mechanical cryocoolers represent a significant enabling technology for NASAs Earth and Space Science Enterprises. Over the years, NASA has developed new cryocooler technologies for a wide variety of space missions. Recent achievements include the NCS, AIRS, TES and HIRDLS cryocoolers, and miniature pulse tube coolers at TRW and Lockheed Martin. The largest technology push within NASA right now is in the temperature range of 4 to 10 K. Missions such as the Next Generation Space Telescope (NGST) and Terrestrial Planet Finder (TPF) plan to use infrared detectors operating between 6-8 K, typically arsenic-doped silicon arrays, with IR telescopes from 3 to 6 meters in diameter. Similarly, Constellation-X plans to use X-ray microcalorimeters operating at 50 mK and will require ~6 K cooling to precool its multistage 50 mK magnetic refrigerator. To address cryocooler development for these next-generation missions, NASA has initiated a program referred to as the Advanced Cryocooler Technology Development Program (ACTDP). This paper presents an overview of the ACTDP program including programmatic objectives and timelines, and conceptual details of the cooler concepts under development.

High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission for the next (2000) solar maximum

The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) Small Explorer mission selected by NASA is to investigate the physics of particle acceleration and energy release in solar flares. Observations will be made of x-rays and (gamma) rays from approximately 3 keV to approximately 20 MeV with an unprecedented combination of high resolution imaging and spectroscopy. The HESSI instrument utilizes Fourier- transform imaging with 9 bi-grid rotating modulation collimators and cooled germanium detectors. The instrument is mounted on a Sun-pointed spin-stabilized spacecraft and placed into a 600 km-altitude, 38 degrees inclination orbit.It will provide the first imaging spectroscopy in hard x-rays, with approximately 2 arcsecond angular resolution, time resolution down to tens of ms, and approximately 1 keV energy resolution; the first solar (gamma) ray line spectroscopy with approximately 1-5 keV energy resolution; and the first solar (gamma) -ray line and continuum imaging,with approximately 36-arcsecond angular resolution. HESSI is planned for launch in July 2000, in time to detect the thousands of flares expected during the next solar maximum.

SHOOT cryogenic components: testing and applicability to other flight programs

Cryogenic components and techniques for the superfluid helium on-orbit transfer (SHOOT) flight demonstration are described. Instrumentation for measuring liquid quantity, position, flow rate, temperature, and pressure has been developed using the data obtained from the IRAS, Cosmic Background Explorer, and Spacelab 2 helium dewars. Topics discussed include valves and burst disks, fluid management devices, structural/thermal components, instrumentation, and ground support equipment and performance test apparatus.

Flight Hardware Implementation of a Feed-Forward Vibration Control System for Space Flight Cryocoolers

A simple control system using force sensors and non-real time signal processing has been designed and tested which reduces vibration levels to 0.1 Newtons or less at the fundamental drive frequency and the 2nd through the 10th harmonics. The NASA/GSFC control algorithm is briefly discussed.

Overview of NASA space cryocooler programs

Mechanical cryocoolers represent a significant enabling technology for NASA’s Earth and Space Science Enterprises, as well as augmenting existing capabilities in space exploration. An overview is presented of ongoing efforts at the Goddard Space Flight Center and the Jet Propulsion Laboratory in support of current flight projects, near-term flight instruments, and long-term technology development.

A High Tc Superconducting Current Lead Assembly for the XDS Helium Cryostat

The X-ray Spectrometer Detector System (XDS) helium cryostat consists of a tank of pumped liquid helium at about 1.3 kelvin suspended inside a seventeen kelvin cylindrical support structure. The tank is a heat sink for an adiabatic demagnetization refrigerator (ADR) and its superconducting magnet. The cryostat’s small initial helium volume and mission lifetime goal of 2.5 years require that the average total heat load to the helium be less than about 800 microwatts. During the mission the superconducting magnet requires a current of 2 amps with a three percent duty cycle. In addition, wires capable of carrying up to 1 amp are needed for cryogenic valve operations during the cryostat’s ground servicing. The best optimized conventional current leads between the 17 kelvin stage and the magnet and valves would contribute an average heat load to the helium of about 3 milliwatts. An assembly of superconducting YBaCuO fibers bonded to a fiberglass tube and suspended by a Kevlar* braid was developed to conduct the current from the 17 kelvin support structure to a vapor-cooled 4 kelvin stage. NbTi wires provide a superconducting path from the 4 kelvin stage to the magnet and valves on the 1.3 kelvin helium tank. This paper describes the assembly’s fabrication and suspension and presents the results of its performance and vibration tests.

Lab Tests of a Thermomechanical Pump for SHOOT

Laboratory tests of a thermomechanical (TM) pump utilizing a commercially available porous disk have been conducted. Various size disks, heater configurations and outlet flow impedances have been used to characterize scale models of the pump proposed for the Superfluid Helium On-Orbit Transfer (SHOOT) Flight Experiment. The results yield the scalability of the TM pump to larger diameters and hence larger pumping rates, the dependance of flow rate on back pressure and heater power, and the limits of pumping speed due to internal losses within the porous disk due to mutual and superfluid friction. Analysis indicates that for low back pressures the flow rate is limited by the superfluid friction rather than the mutual friction. For the porous plug used in the early tests this amounts to a practical limit of 4.4 liters per hour per square centimeter. For our baselined flight plug area of 180 cm2 this yields 790 liters per hour.

The XRS low temperature cryogenic system: Ground performance tests results

Abstract The X-Ray Spectrometer (XRS) instrument is part of the Astro-E mission scheduled to launch early in 2000. Its cryogenic system is required to cool a 32-element array of X-ray microcalorimeters to 60–65 mK over a mission lifetime of at least 2 years. This is accomplished using an adiabatic demagnetization refrigerator (ADR) contained within a two-stage superfluid helium/solid neon cooler. Goddard Space Flight Center is providing the ADR and helium dewar. The flight helium dewar was assembled in Sept. 1997 and subjected to extensive thermal performance tests. This paper presents test results at both the subsystem and component levels. In addition, results of the low temperature topoff performed in Japan with the engineering unit neon and helium dewars are discussed.

Design of the XRS helium insert

The X-Ray Spectrometer (XRS) instrument has gone through numerous iterations, first as an instrument on NASA’s Advanced X-Ray Astrophysics Facility (AXAF), then on AXAF-S, and now scheduled to fly on the Japanese Astro-E satellite. The Astro-E XRS is a high precision x-ray spectrometer with better than 20 eV resolution for x-ray energies from 0.3 to 10 keV. The requirement to obtain a lifetime greater than two years within the weight constraints of Astro-E has presented quite a challenge in the design of the cryogenic system. The design of the superfluid helium insert is described, with emphasis on innovative approaches taken to meet the requirements.

Design and Test of Potential Cryocooler Cold Finger Interfaces

NASA/Goddard Space Flight Center (NASA/GSFC) is investigating advanced thermal interface techniques between a cryocooler coldfinger and the load to be cooled. The fundamental goal was to develop a six degree of freedom, flexible cryogenic thermal strap which would mitigate the propagation of residual vibration from the cryocooler to the load while keeping strap thermal resistance to a minimum. Nine different straps have been fabricated and evaluated as of June 1993. Several new concepts will be tested in late 1993 or early 1994. A cryogenic test dewar was fabricated to permit accurate temperature measurement at typical cryocooler operating temperatures of 80K and 30K. A random frequency electromagnetic shaker was then used to accelerate one end of each thermal strap; the other end was fixed. A three axis accelerometer was mounted on the moving end of each thermal strap and three force transducers were mounted orthogonally at the fixed end of each strap. These three acceleration inputs and three force outputs were used to calculate each thermal straps transfer function.