[Editor’s note: These parting words are from Justin Jessup shortly after graduating and joining Stoke Space.]

“You can’t spell ‘cryogenics’ without ‘cry’” – Jake Leachman

This common saying amongst cryogenic researchers refers to the difficulties working with and designing cryogenic systems. However, a little over 5 years in the lab has prompted me to revise this saying to something truer of the engineering process:

“You can’t spell cryogenics without ‘cry-oh’” – Justin Jessop, HYPER Alum

I’ve found that the ‘oh’ of realization and learning is just as (if not more) notable than the initial ‘cry’ of your first attempt going awry. Getting from the ‘cry’ moment to the next big ‘oh’ moment takes determination, patience, and often a few cycles of ‘cry-oh’. Continually working to get to the next ‘oh’ is a mark of a successful cryogenic engineer. To help you get past some of the ‘cry’ moments I’d like to share a few of my ‘oh’ realizations and hopefully save you some tears.

Before digging down into specifics, some general rules to live by:

  1. Maintain alertness and anticipate danger – working sustainably means working safely. Safety Plans for new builds and experiments handle the large safety concerns, but it’s often on the smaller tasks with unanticipated hazards that cause harm.
    1. Wear PPE
    2. Have training on and knowledge of your tools
    3. Always be thinking about what you’re about to do, the potential hazards, and your hazard mitigations
  2. You will never regret doing things right the first time – when encountering an experimental challenge there can be a knee-jerk temptation to go with the quickest and easiest solution possible at the cost of reliability. The time spent repairing, redoing, and lost data or wasted tests will quickly cost more than going with the reliable solution from the start.
  3. If you don’t know, ask. If you do know, ask – building off the first two rules, sometimes you haven’t used a tool or process, sometimes you don’t know what solution to go with, and even if you think you do, you will miss the incredible and unforeseen insights from your peers and advisors by assuming you’ve got it.
  4. Trust the engineering process – you probably have great ideas for a design. Or maybe you know exactly what equation you need to solve a problem. You’ve probably also overcomplicated, over-simplified, overlooked, or just been flat out wrong. The engineering design process minimizes these common pitfalls and can leave you impressed by what you were able to accomplish.

I’ve broken the next lessons into three categories: Electrical, Vacuum, and Cryostat.

Electrical

At the heart of any experiment is electrical power and signals recording data, supplying heat, and providing power. As a mechanical engineer, it can be easy to take one circuits course and conclude you simply aren’t up to snuff to handle electrical work. Don’t succumb to this thinking. With some fundamentals, careful technique, and willingness to troubleshoot (cry-oh), anyone can wire a cryostat.

Soldering:

There will be a lot of soldering to make an experiment work. Take the time to get good at making solid connections and you will only have to do them once.

  1. Flux is the key to making any good solder connection – yes, even if the solder is rosin core. Don’t believe me? Take two stripped wires, dip one in flux, then try tinning (coating in solder) the fluxed wire and then the non-fluxed. The results will speak for themselves.
  2. Use a solder bead, don’t feed – when I was first taught to solder, the process was to hold the iron to the wire, getting it hot enough to melt the solder, then feed solder into the wire with another hand. With this method, the surface area to conduct heat from the iron to the wire is very small, just two cylinders contacting along a line. Instead, dip your wire in flux then feed solder directly onto your iron until there is a small ball (bead) hanging off the tip. When you contact this bead to your wire, the bead creates a much larger surface area to conduct heat from the iron. A bonus to this method is that if you don’t need much solder, you also now have a free hand to hold things.
  3. Shrink wrap is essential for any connection – not just for electrical isolation on a tight pass through, but for mechanical support. You can check the size of the shrink wrap is right by hitting a small section with a heat gun then comparing it to your wire or solder cup. Slightly smaller in diameter means it’s going to hold on tight.
  4. Stress test the solder joint while it’s easy to get to – slip on some shrink wrap before making your connection, solder your wire, then give the wire a small tug to be sure it’s strong. I’ve caught a lot of bad connections before shrink wrapping this way. Once the connection is good, push the shrink wrap all the way onto the cup or wire and hit it with a heat gun.
  5. Strength in numbers – once you’ve finished all your connections, take a zip tie (or if you can, a large piece of shrink wrap) and tighten it around all the wires. This distributes any tension on your wires across all the connections instead of one or two.
  6. Make a pinout diagram – if you are making a pass through for an experiment, take the time to create a pinout diagram of what goes where. You’ll thank yourself both while soldering and down the road when you’re tracking down a discontinuity or short.
  7. Make your pass through as modular as possible – soldering inside a tight chamber is not fun and poses a ventilation hazard. I have found that 15-pin d-subs can pass through a KF-40 or KF-50 vacuum fitting, allowing for the pass-through to be soldered on a bench and then installed.

If all goes well, you will end up with successful wiring:

 

Sourcing Components:

Ohm’s Law is your friend – often cartridge heaters are used for their simplicity, and it’s easy to think that your DC power supply can provide 50 W, so you just need a 50 W heater. Voltage multiplied by current gives power. Voltage drives current, not the other way around. Know the resistance of your heater (before you buy it), what voltage you have available to you, and what current will result. Then, calculate your power before buying anything. This goes for any component being sourced.

Troubleshooting:

Discontinuities can be obnoxious, but they happen. If you’re lucky, it will be as obvious as a hanging wire. If you’re not lucky, you can feel lost not knowing where to begin with the 4 + connections between your device and the power supply or controller. A multimeter and some basic knowledge of your devices is always the way out. To begin, you need to figure out where the discontinuity or short is. Take the farthest possible connection in your cryostat to your device and the connection to the power source (temperature controller or power supply) and perform a continuity check. Once you find the pins that are causing issues, start with the smallest possible verifiable section of your pass through and perform a continuity check. Keep adding on sections of the pass-through until you find where the short or disconnect is occurring.

A note here: heaters have a known resistance. If this resistance goes from 50 ohms to 5 mega ohms, there’s a good chance that it got burnt out or there was a disconnect. Temperature sensors use 4 wires to eliminate the voltage drop across the wires. There are two sets of wires that are continuous, and you can use this to diagnose bad sensors or connections.

Vacuum

The first thing you need to accept about vacuum is that you likely know much less about it than you think you do. The second thing is that achieving a stable “leak-tight” vacuum is much harder than it looks. With transitions from a continuum to the intermolecular flow regime, leak rate units of , off-gassing, and more, it can become confusing quickly. I’m going to tell you what I wish I had learned (or listened to) when I started.

  1. Use as few non-welded connections as possible – all connections are a potential leak point and if they can be removed or simplified, it will make your life easier.
  2. If you do have to make a connection for vacuum this is the order I would recommend – welded connections, Kf vacuum fittings/O-ring, VCR, Compression, Epoxy, Indium, NPT. The size of seal you need, how often it gets connected and disconnected, cryogenic compatibility, and the vacuum level you need to achieve will all determine which fitting you go with.
  3. Use large, short pipes to pull vacuum – in the high vacuum range, molecules are no longer influenced by one another and randomly bounce around until they eventually interact with your turbo pump and are wacked out of the system. Using large pipes with short distances means molecules are more likely to bump into your turbo, resulting in quicker times to higher vacuums.
  4. Vacuum grease doesn’t make the seal – it’s often misconstrued that you need a lot of vacuum grease on an O-ring to make a leak tight connection. Really, a thin film is all that’s needed, and any more is just making a mess and wasting a resource.
  5. Helium is a useful tool in small quantities – when leak checking with helium, you tend to think you need to spritz an audible amount of it to get picked up. The spectrometer can count individual molecules; any excessive amount will just disperse to other connections/leak sites confusing your leak check process. Use the smallest possible amount (hardly audible up to your ear or felt when sprayed at your lips) to make your life easier and conserve helium.
  6. Leak check as you assemble – just like the electrical pass-through, validate one component at a time, then increase complexity during your assembly. There is nothing worse than putting everything together and not being able to even get down into the range that your helium mass spectrometer can detect accurately.
  7. Leaks become increasingly difficult to find as the vacuum pressure decreases – it can sometimes take up to 2 minutes to see a measurable rise in leak rate if the pressure is low and helium can diffuse to other connections in that time. I utilize tin foil and tape to isolate connections from the initial helium spritz, so you know more confidently what connections are good and which aren’t. Keep in mind that helium is buoyant, so start high and go low.
  8. Sometimes the issue is your vacuum gauge, not your system – if you can’t find any large leaks, it never hurts to borrow a kind neighbor’s vacuum gauge and a KF T to validate that your gauges are reading the same thing. I once had a power outage throw a gauge out of calibration, sparking a 2-day wild goose chase that could have been avoided had I first checked my instruments.
  9. Sometimes the issue is your vacuum pump, not your system – like gauges, pumps can go bad. You can always validate yours by seeing if a known, reliable pump works better or the same as yours.
  10. Epoxied vacuum seals can be improved – if you are finding that a vacuum epoxy connection/pass through is leaking, you can mix up a batch of epoxy, pull vacuum, allow the epoxy to thicken up (about an hour or longer after mixing), then apply a thin coat. The vacuum will pull epoxy into the leak points and fix your issue.
  11. Off-gassing can make it seem like a leak is occurring when it’s not – I had a volume that I was pulling vacuum on that could be isolated from the pump via valve. I closed the valve and found the pressure would rise and concluded it was a leak. When I couldn’t find the leak, I let it sit for a day and found that the pressure hardly rose at all when closing the volume. This meant the leak I was looking for was coming from inside the volume itself and disappeared when I finally pulled all the excess gas from the material.

Cryostat

Most of the design of your cryostat comes down to engineering calculations, but there are a few general rules and tips that can save you time.

  1. ETP Copper (RRR-50) is good enough – while it’s desirable to get the highest possible thermal conductivity, machining and working with OFHC copper is tricky and expensive. If you’re not able to get the performance needed with ETP, I’d suggest tweaking your design to make it work.
  2. Practice your ABC’s – materials shrink when they get cold. Some materials shrink more than others. In order from most to least: Plastics, Aluminum, Copper, and Steels. This simple mnemonic allows you to quickly parse out CTE issues.
  3. Belleville washers are your friend – if you need a fastener in your cryostat, you could run the CTE calcs on using brass, or you could just use stronger stainless steel fasteners with Belville washers.
  4. Thermal dead ends – with one end isolated from any heat sources, and a cryocooler on the other, everything will equilibrate to the same temperature. This set up also means you don’t have to deal with the CTE issues of connecting something on both sides of your cryostat.
  5. Contact resistance can help or hurt you – chain supports enable the weight of your test cell to be supported from a lid while minimizing the heat load due to stacked contact resistances from link to link. On the flip side, contact resistance between two surfaces intended to conduct heat can create large temperature gradients instead. In the latter case, a thin film of Indium goes a long way in preventing large temperature gradients.
  6. Cylindrical vacuum chambers simplify your cryostat – premanufactured cylindrical vacuum chambers with lip seals are great because they require no bolted connections. Additionally, and in the event of over pressurization in your cryostat, they can simply detach themselves from the lid.
  7. You will never regret designing for assembly – a design that works but is incredibly difficult to assemble will cost you immense amounts of time and frustration. Put in the time up front to consider assembly. This might even mean 3D printed mockups to ensure things will go together the way you anticipate. You and everyone else that uses the experiment will appreciate this foresight.
  8. You need a copy of Jack Ekin’s Experimental Techniques for Low Temperature Measurements –

material properties, experimental techniques, charts, tables and graphs, this book is an invaluable tool that is well worth having.

  1. NIST has 90 % of the material data you need – NIST has created curve fits for most materials used in cryogenics. Conveniently, they also have calculators to find properties at specified temperatures or over specified ranges (aka Integrated Properties).

Available here: https://trc.nist.gov/cryogenics/calculators/graphcalc.html

 

This list is not exhaustive but nevertheless distills down a lot of the lessons I had to learn the hard way. If you found something you weren’t familiar with while reading through this list, consult a trusted resource and learn. By the time you have a few years under your belt, you’ll be able to grow the list or even create an entirely new one of your own.

Best of luck on your journey below -150°C!