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Phil Richerme | CERN | Switzerland

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The Emotional Rollercoaster of Experimental Physics

Emotions around an experimental physics lab are very tightly coupled to the status of the experiment. It is a singularly wonderful and proud feeling when an experiment is running smoothly, and a startlingly sinking feeling when the experiment breaks for no discernible reason.

Rime ice forming on the cryogen lines during our cooldown

At the ATRAP experiment, our official data-taking run starts in early May, when CERN begins to deliver low energy antiprotons. However, we like to have our apparatus cold and tested well before this time.
Our cooldown process takes roughly a week, start to finish. This is short compared with the several-weeks timescale to cool down the LHC magnets, though long compared with the human patience timescale. We start by cooling with liquid nitrogen (temperature = 77 Kelvin), and the experiment reaches 77 K after a few days. We then switch to liquid helium (temperature = 4 Kelvin), which is less efficient, but can cool us the rest of the way. Having the experiment at 4 K helps us in two major ways. First, particles in our trap will come into equilibrium with the trap temperature; having the coldest possible particles is important for making trappable antihydrogen (I have discussed this in more detail previously). Second, the low temperature acts as a “cryopump” – background gas molecules will eventually collide with a cold wall and freeze, effectively decreasing the pressure. This is particularly important for antimatter research, since collisions of antimatter with any background gas will lead to annihilations. In the past, we’ve used these annihilations as a measurement of the background gas pressure, and have shown it to be less than 5e-17 Torr – one of the best vacuums in the world.

Looking down the center of the positron entry tube. The black, funny-shaped piece in the center is blocking the path.


A highly sophisticated piece of scientific apparatus

Two weeks ago, our cooldown was complete with no problems, and the emotion was one of cautious optimism. We still needed to test the trap wiring, since any poor soldering job, improperly strain-relieved wires, etc. can all lead to broken or shorted connections when the experiment cools. Thankfully everything passed, and spirits were pretty high.
Then, the unexpected happened: positrons, which we load through the top of our apparatus, were not making their way into the trap. Looking down from above revealed why: a thin piece of insulator had fallen down the positron entry tube, and was blocking passage into the trap. Just like that, our high spirits hurtled back down towards the earth; we would likely need to warm up the experiment, remove the positron tube, remove the blockage, put the experiment back together, and cool down once more.
However, improvisation – a necessary tool in the kit of an experimental physicist – was put to good use here. We were able to remove the blockage without warming up the apparatus by using – and I swear I’m not making this up – a stick with some tape on the end. OK, I’m being a bit simplistic. We used all cryogenic-compatible materials, a thin-walled fiberglass tube to reduce heat transfer from room temperature to 4 K, and set up an airlock system to avoid ruining the vacuum – but, the main idea is the same. We were able to grab the blockage with the tape and lift it straight out the top. It’s gratifying to think that some simple ingenuity saved us over 2 weeks of work and thousands of dollars worth of cryogens.
Needless to say, our emotions once again swung upwards. After this episode, we’re back to running smoothly, and are beginning to perform some diagnostic experiments with electrons and positrons to prepare for our antiproton beam run. Undoubtedly, we’ll all remain in good humor until the next mini-crisis appears.

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