We’ve been very busy in the last few months, and although we’ve taken a lot of photos there hasn’t been much time to write an update. At the end of May we received the BA2 cryostat from the California Institute of Technology. This cryostat was the second to come off the production line and had been shipped to Caltech after being outfitted here at Minnesota. At Caltech, the sub-Kelvin insert and cryogenics were integrated, these allow our detectors to reach their final operating temperature of ~270mK (just 0.27 degrees above absolute zero!) They tested out its performance, then sent it back to us so that we could integrate it into the mount while they continue to work on developing, fabricating, and testing the batch of detector tiles we will be deploying this coming winter.
The BA2 cryostat shipped in a disassembled state, so the first task was to rebuild it. For most of us locally, this was the first time we’d been able to interact directly with this part of the receiver so we jumped at the chance and took plenty of photos. For this post I’m going to focus more on the receiver and less on the mount.
First a quick breakdown:
The detectors we use for our experiment are superconductors. This requires that they be maintained at an extremely cold temperature, just a fraction of a degree above absolute zero (around -459 degrees F). However, for something that cold anything at room temperature shines like the sun and provides a lot of heat. Therefore we need to provide a lot of shielding, we call this structure the cryostat (shown below).
The largest (by volume) portion of the cryostat consists of three nested shells, color-coded red, green and blue from warmest to coldest in the image above. The red shell is our vacuum jacket, it sits at about room temperature and is air-tight which allows us to put everything inside it under vacuum which means there is no gas bouncing around to transfer heat to the cold components. The green shell sits at a nominal temperature of about -370F and serves mostly to intercept the heat radiated inwards by the outermost (red) vacuum shell. The blue shell sits at a nominal temperature of about -452F and serves both to block heat radiated inwards by the green shell as well as a place to attach our optics. The green and blue shells are both cooled by a pulse tube cryocooler which sits inside the smaller canister on the right hand side of the image. Although the shells need to be rigidly supported inside each other, heat conduction across any support system must be very small if we want to keep our internal components cold. Our support system is made out of a number of fiberglass and carbon fiber rods at the bottom end and very thin titanium strips at the top end.
This pulse tube can only cool us down to about 4 Kelvin however, which isn’t cold enough for our detectors. Therefore, inside our “4 Kelvin” stage (blue) we have an additional “sub-Kelvin” stage (below left) which is cooled by a 3-stage Helium Sorption Fridge (below right)
The sub-Kelvin stage is similar to the larger nested shells, in that its tiered structure provides a way to shield the coldest elements of the receiver from as much heat as possible. To that end it has three primary tiers, each cooled by one of the fridge stages. The first at about 2 Kelvin (-456 F) the second at about 0.340 Kelvin (-459.05 F) and the coldest at about 0.250 Kelvin (-459.2F). It is to this last stage that we attach our detector modules.
Each part of the receiver has to be installed carefully and precisely, and there are massive numbers of cables that carry data between different elements. And of course, with such a complex piece of hardware, plenty of tests happen between each individual component installation to ensure that nothing was accidentally bumped or disconnected. The whole process can take 1-2 weeks to build up a receiver from all its composite parts. Below is a collection of images from that process:
Unpacking the sub-Kelvin stage from its shipping container and installing the fridge
Installing the heat straps between the fridge and the three sub-Kelvin stages. Checking the correct installation of our temperature sensors.
Installing the pulse tube cryocooler and the heat straps that go the the 50 Kelvin and 4 Kelvin shells
A frequent sight: running electrical tests between each subcomponent’s installation to ensure nothing has become accidentally disconnected.
Installing a single detector module (covered with a copper plate) and shorting boards in the rest of the slots. We only had one module available for us to use here at Minnesota for testing in the mount, the green shorting boards are just place-holders in the electrical system. After installation, everything is covered with reflective material which serves as both a shield to electronic interference and to reflect radiated heat from warmer components.
Left: The Niobium (superconducting) magnetic shield that helps to keep our detectors from picking up stray magnetic signals. Right: The Vacuum jacket, 50 Kelvin and 4 Kelvin shells waiting to be installed. The two cold shells are coated in reflective material to help them reflect radiated heat. Bottom: looking downwards from the top of the cryostat after the three shells have been installed.
For this run, we didn’t have any optics available which is why the focal plane can actually be seen from the top of the tube. For a science run there would be two filters and two lenses blocking the view. For our run we ran “dark” which means we blocked off the line of sight from the detectors with two aluminum plates.
Finally, the cryostat operating on the ground and in the mount.
After installation in the mount, we spent a significant amount of time getting all of the data acquisition and control software working so that we can read out data reliably.