An insight into past, present and future cryogenic missions where the Cryogenics and Fluids Branch at NASA Goddard Space Flight Center had or has a significant role.
by Mark Kimball, Ph.D., Cryogenics and Fluids Branch, NASA Goddard Space Flight Center
Readers of this column may recall last year’s recounting of a past mission, the Superfluid Helium On-Orbit Transfer demonstration, or more succinctly, SHOOT. This year’s focus is on a mission in progress and another in formulation: COSI and PRIMA.
COSI
The COmpton Spectrometer and Imager (COSI) is a SMEX (Small Explorers) mission funded through NASA’s Explorers Program. Its purpose is to combine direct imaging, spectroscopy and polarization measurements of soft gamma rays emanating from various entities in the universe. All this is required to meet ambitious primary science goals: uncover the origin of galactic positrons, reveal galactic element formation, gain insight into extreme environments with polarization and probe the physics of multi-messenger events. Major science indeed.
To achieve these goals, the main COSI instrument uses an array of 16 germanium detectors that date back to the Lawrence Berkeley National Laboratory in California in the early 1990s. This technology has matured over the intervening decades, starting in the laboratory and then progressing through a series of stratospheric balloon missions to where we are today: a ticketed ride on a SpaceX Falcon 9.
To operate, the detectors must be cooled below 90 kelvins. I’ll let Howard Tseng from Goddard’s Cryogenics and Fluids Branch explain the cooling chain:
“GSFC is contributing the Cryostat Heat Removal Subsystem (CHRS). The CHRS uses radiators with a constant conductance heat pipe (CCHP) from the cryocooler to the radiator. The cryocooler is the Sunpower CryoTel CT-S, and the control electronics are provided by IRIS Technology. The heat pipes and radiator are provided by Advanced Cooling Technology (ACT), but the radiators are painted here at GSFC. Our system is providing the cooling to the cryostat, which is being developed by Berkeley. The cold head is kept optimally at 80 K and absorbing 7 W (current best estimate) from dissipation in the detector and parasitic heat flowing down wires from higher temperature.”
He goes a bit deeper into the ongoing technology development:
“The cryocooler has an active damper that is being controlled by a ‘new’ Cryocooler Control Electronics. ‘New’ being that this CCE has not been designed to do this, and the CCE has not been paired with this cryocooler before. So, there is a non-insignificant development effort needed to make sure the CCE algorithms work and that the CCE hardware can support driving both the cryocooler and the active balancer on the cryocooler.”
Never a dull moment around here. Anyhow, if you wish to learn more about COSI, I would start at https://cosi.ssl.berkeley.edu and go from there.
PRIMA
Another satellite being proposed now with cryogenic instruments at its core is PRIMA. That’s not a misprint. Instruments is correct, since there are two unique instruments cooled by a single sub-kelvin cooler on this 1.8-meter telescope. Oh, and the telescope itself is also cooled to cryogenic temperatures. Fun.
The PRobe far-Infrared Mission for Astrophysics is a collaboration between GSFC, JPL, SRON, CNES and Cardiff University. It’s currently in Phase A of the NASA lifecycle. If you are not well versed in NASA-speak, this means the project is in the concept and technology development phase. Here, top-level requirements drive designs that rely on calculations, simulations and models to prove feasibility. This process usually devolves into a tug-of-war between structural concerns, thermodynamic realities and mass constraints reminiscent of a freshman-level physics force diagram – but I digress.
The cryogenic system starts with a three-stage pulse tube cooler. This precools helium gas before it reaches the impedance within the separate Joule-Thomson cooler. Ultimately, the combination of these two coolers generates a temperature near 4.5 K with a heat lift of 50 mW. An interesting historic note: The particular cryocooler earmarked for use is the spare unit developed for the MIRI instrument that is performing flawlessly aboard the James Webb Space Telescope. This cools the infrared telescope to near 4.5 K and is also the heat sink for the multistage continuous adiabatic demagnetization refrigerator (CADR) being developed by the Cryogenics and Fluids Branch. Of the total available heat lift of 50 mW at 4.5 K, the CADR is allocated a mere 10 mW for its heat rejection. This becomes important later.
Both PRIMA instruments, PRIMAGER and FIRESS, require two stable temperatures to operate: 1 K and 0.120 K. The CADR provides these by linking together what is essentially two ADR systems running asynchronously (see Figure 4). The three stages left of the 1 K plate in Figure 4 work together to lift heat at 0.120 K from the detector and wiring transcending from higher temperature and pass it to the 1 K plate. The two remaining stages periodically swap roles, with one absorbing heat at 1 K and the other rejecting it to the cryocooler. This “second” ADR is not unprecedented. A system that operated in a similar fashion was presented in the April 2015 edition of Cold Facts.
ADRs providing continuous cooling at sub-kelvin temperatures have been demonstrated in the laboratories of the Cryogenics and Fluids Branch at GSFC for nearly three decades. Also, a pair of ADR stages that provide cooling near 1 K is not something “new,” as stated before. What is new here is the linking of these two CADR systems together to create a single cooler with two stable temperature platforms, ruggedized to survive a spaceflight launch. If this isn’t enough “newness” to keep your interest, remember that I mentioned earlier the total heat rejection from the CADR to the cryocooler may be no more than 10 mW. That’s right – not only must this ADR provide up to 9 µW of cooling at 0.120 K and roughly 1 mW of cooling at 1 K, but it has a ceiling on the heat rejection to the cryocooler. Talk about being confined inside a tight box. I’m feeling a bit claustrophobic.
Now, I realize I’ve spent half this article outlining the cryogenic system on PRIMA. This is Cold Facts, after all. However, if you want to learn more about the science, instruments or team associated with PRIMA, a good place to start is https://prima.ipac.caltech.edu. The CADR is described in more detail in the proceedings of the SPIE Astronomical Telescopes + Instrumentation Conference, 2024, Yokohama, Japan (https://doi.org/10.1117/12.3020300) or in a manuscript accepted for publication in the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS) titled “The Continuous Adiabatic Demagnetization Refrigerator for the PRobe far-Infrared Mission for Astrophysics (PRIMA).”
