ADR

X-ray Astronomy

The night sky looks very different with x-ray vision. Compared to visible light, x-rays are much higher energy, and require different techniques to gather and record incident light. One method of x-ray detection is known as a microcalorimeter. These devices detect the heat change when an x-ray is absorbed, and can therefore accurately record time, position, and energy level of the x-ray. However, these detectors must be kept at a very stable, low temperature, such as 50 milliKelvin.

Adiabatic Demagnetization Refrigerators

Currently, this may be done with an adiabatic demagnetization refrigerator. ADRs use the process of misaligning magnetic dipoles to cool a thermal body, rather like conventional refrigerators use the decompression of gas to reduce temperature. Here is the website of a previous Olin team explaining in detail the workings of an ADR.

Multiple ADR Cooling Systems

A number of techniques were developed to improve the utility of the ADR. A single ADR must cycle through a rapid cooling stage, an extended stage where the magnetic field is being slowly decreased to balance incoming heat flow, then reheating and remagnetization, during which the detector is unusable.One solution to this process is instituting a string of ADRs. The first ADR may be maintained at a constant temperature. During remagnetization, heat may be dumped to a second ADR, which cylces normally. This ADR may then, in turn, dump heat to a third, etc. The working temperatures ( minimum and maximum for each ADR) need to have some overlap to allow heat to flow. Additionally, the cycles must be precisely timed so that by the time the first ADR needs to dump, the next ADR is at its coldest.

CADR

The CADR system has a number of advantages over non-continuous systems. While this does provide the ability to continuously detect, more important considerations are weight and expense. The inidividual ADRs can operate over a smaller temperature band, and can have a shorter demagnetizing period than a single ADR. This means that the salt pills and superconducting magnets may be smaller, since less heat needs to be stored or converted to entropy. Also, since the overall system can span a larger temerpature band, mechanical coolers can be used to dump heat into space, rather than liquid helium and solid neon, both of which must be bulky to lengthen the lifespan of the device in space. Though multiple ADRs do need magnetic shielding to prevent inadvertant magnetizing of nearby slatpills, the entire assembly still weighs less, and has a longer usable lifespan, than a single ADR. However, controlling all four ADRs optimally is more difficult than it first appears.

The goal of this project is to improve the stability of a continuous ADR system by a factor of ten. Current technology provides a continuous temperature of 50 mK, but the thermal noise between stages in the cooling cycle is too large to allow continuous measurements. This project will hopefully reduce these fluctuations and allow a greater proportion of usable time for the x-ray sensors.