Fridge torture

Here is a picture of what I did yesterday.

Torturing of the dilution refrigerator is the only way to diagnose what is one of the experimentalist’s worst nightmares…

‘The Intermittent Fault’…

I have had intermittent faults with this system for about 3 years now. The main problem arises from the use of tiny stainless steel coaxial cables (0.3mm outer diameter). You can’t solder to stainless steel without using an acid flux. If you use this, the connectors fall apart due to corrosion after the first few thermal cycles. You can crimp the connectors onto the coax (it’s fiddly) but then the problem is that thermal cycling can cause mechanical stresses which then just break the inner wire, (not difficult considering it’s thinner than a hair).

Anyway back to the torturing. The faults in the wiring only occur at low temperatures <- That’s the important point by the way. You can’t check continuity of a wire when it’s deep down in a sealed dewar immersed in Liquid Helium.

There are 4 signal lines along which these awkward faults can occur. The way to find them involves dipping the lower part of the dilution refrigerator slowly into an open bucket of Liquid Nitrogen until the fault appears. The temperature gradient above and below the liquid surface is high so you can accurately pinpoint the problem area in terms of vertical position. Using a heat gun (which looks similar to a hairdryer, but goes up to 650C, hehehe) on the part just above the liquid nitrogen surface helps increse the temperature gradient. It creates pretty Nitrogen fog too. (a bit wasteful, but LN2 is cheap).

Poor fridge.

The coaxes have now been temporarily replaced with twisted pair wiring as multiple connectors on 3 of the 4 lines were found to be faulty. It’s also a really great way to find dry solder joints. NEVER rely on the fact that you can create 100 or so solder joints that will all remain robust – even if you’ve been soldering since you were about 8 years old 🙂

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4 thoughts on “Fridge torture

  1. quantummoxie says:

    Looks a little like the flux-capacitor in the back of the DeLorean in Back to the Future.

  2. physicsandcake says:

    I wish it did allow me to travel through time 🙂

  3. rrtucci says:

    What are you planning to use your dilution fridge for (besides storing cake)? How come your fridge looks so “basic”, compared to the one pictured in this wikipedia article. The wikipedia one has a nice circuit diagram for those going blind.

  4. physicsandcake says:

    I use the fridge mainly to characterise Josephson Junctions at temperatures from 1.5K down to 30mK or so. Specifically, I look at their current-voltage (IV) characteristics and in detail at the onset of the critical current (in a stochastic way).

    There is a lot of information available from the IV curve. For example, using an AC voltage signal to take the differential of the IV curve gives you the conductance of the junction as a function of the voltage, which is directly related to the density of states. From this you can determine whether or not there are non-superconducting ‘quasiparticles’ present in your device, which are a major source of decoherence in qubits.

    I also perform experiments which give information about how the phase tunnels in the junctions, and at what temperatures the devices will be operating in the fully quantum (coherent) regime. I work with novel types of Josephson Junctions made with materials other than the standard ‘Niobium’ technologies, and therefore it is important to determine at what temperature the quantum regime occurs in these devices, as the physics is somewhat different.

    As for the equipment itself, these pictures only show the lower part of the experimental insert, which is the actual fridge itself. The picture on wikipedia shows a (rather old) gas handling unit which is used to control the flow of cryogens around the system. You can see the gas handling system in the photograph on my about page, and there is a picture of the entire system on my Research Page about half way down. We actually do have some fridges with gas systems that look like the on on wikipedia, but they were built in the 80’s!

    Usually the more modern the fridge design, the more basic and simple it looks. Although admittedly mine does not reach the coldest temperatures (about 2mK I think is the record for a dil fridge) hence it is rather more ‘compact’.

    Hope that explains a bit 🙂

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