XRS Heat Loads

The XRS cryostat was challenging to design, because the required lifetime is long (2 years), while the supply of liquid helium coolant is small (20 liters). To avoid evaporating the liquid helium before the end of the 2-year lifetime, the heat flow into the cryostat must stay below 800 microwatts. No one before this time had built a space flight cryostat with such a low heat leak.

It is especially difficult to keep the heat load low on a space flight cryostats (as opposed to cryostats for use on the ground), because space flight cryostats need to be sturdy to survive the stress of launch. Thus space flight cryostats need to be strongly built.

To make sure that the heat load stays below the 800 microwatt limit, we investigated many different possible heat loads. Here is a partial list of the possible heat loads we studied:

Chemical reactions (such as ortho/parahydrogen conversion).: Any such reactions should be complete within a few days of cooldown to helium temperature and thus would not be a problem on orbit.
Vibrational heating during launch.: By comparison with the experience of the COBE launch, we expect that launch will add 8 1/2 Joules to the XRS helium bath. This value is only approximate. The heating of the COBE helium bath during launch is only approximately known. Also, COBE launched on a Delta, while XRS will launch on an M V.
Mass gauge heat.: Mass gauging is a technique for measuring the quantity of helium in the helium tank while the cryostat is in orbit. On earth, it's relatively easy: just check how far up the level of liquid comes. But in orbit, with zero gravity, there is no top; the liquid floats wherever it wants to. In mass gauging, an electric heater puts a known amount of heat into the helium. The heat spreads throughout the helium, heating it all equally. It there's a lot of helium, the temperature change is small. If there is only a small amount of helium, the temperature change is larger. For more details, see mass gauging.
Diamagnetic properties of helium.: Helium is diamagnetic, that is, it experiences a force pushing it away from a magnetic field. In theory, this force could accelerate the helium, adding kinetic energy which could be converted to heat energy when the helium hit the wall of the helium tank. However, the calculated accelerations are small, on the order of 10^-7 g. Thus, only a small amount of energy could be added to the helium via magnetic force. The susceptibility of helium is 2 x 10^-6 cm³/mole (cgs) or 9 x 10^-7 (mks).
Heating by radioactivity of the materials used to build the cryostat.: We judged this heat load to be negligible. Radioactivity levels in structural materials are generally so low that they would not be a problem. In fact the levels are usually so low that the only people interested are scientists building instruments to measure radioactive particles. They need to know levels of radioactivity in the materials they use in their instruments so that they can judge how much radioactivity is coming from the samples they're trying to measure. They have found that materials which are nominally the same may differ in the amount of radioactivity present. Here are a few references.
High-energy particles from outside the instrument absorbed by the cryostat.: We calculated that the average heat load delivered to the helium will be 16 microwatts. We based our calculations on NASA Technical Memorandum X-73358, Charged Particle Radiation Environment for the Spacelab and Other Missions in Low Earth Orbit, Rev. A. (1976),Appendix C: Dose rates behind spherical aluminum shell shield of various thicknesses as a function of orbital altitude and inclination.
Superconducting hysteresis loads in the magnet.: For magnet designers, a big advantage of superconducting wire is that a steady current in a superconducting wire generates no heat. A current flowing in a non-superconducting wire does create heat; a lot of heat in the high resistance wires of an electric heater, but even some heat in wires made of good conductors, such as copper. High field magnets of copper wire have so much wire packed in so close together that cooling is a major problem. Thus, by using superconducting wire, the magnet designer avoids a major headache.
However, a changing current in a superconductor does generate heat. The largest heating effect is called hysteresis. For details, see M. N. Wilson, Superconducting Magnets, Clarendon Press, Oxford, 1983. We have measured the heat generated in two of our magnets while ramping the current from zero to 2 amps and back to zero. The amount of heat generated was 4 Joules for one magnet and 6 Joules for the other.
Heat resulting from possible magnet quench.: Under special circumstances, a superconducting magnet can suddenly cease to be superconducting. (For technical details, see, for example, M. N. Wilson, Superconducting Magnets, Clarendon Press, Oxford, 1983.) As the wires suddenly jump in resistance, the current flow begins to generate resistive heating. The amount of heat generated resistively is just the amount stored in the magnetic field, given by 0.5 LI² , where I is the current and L is the inductance. Our magnet has an inductance of about 1000 Henries and a maximum current of about 2 amps. If it quenched, it would vaporize about 80 grams of helium, or about 1/40 of our total supply.
Another hazard of a quench is the danger of creating a short circuit in the magnet. As the current drops, the inductance of the magnet creates large voltages. These voltages may be large enouogh to spark through the insulation of the wire, shorting out the magnet. Therefore, superconducting magnet manufacturers have developed circuits that dissipate these voltages quickly. Early in the ADR development program, we deliberately quenched one of our superconducting magnets by disconnecting the power supply while the magnet was powered up. The magnet warmed up and blew off lots of helium coolant, but the quench protection circuit kept it from being destroyed. That magnet, as well as the XRS flight magnet, was built by Cryomagnetics, Inc., of Oak Ridge TN.

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Curator: Mark O. Kimball
NASA Official: Eric A. Silk
Last Updated: 09/11/2014