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2012-07-12 Deflux SQUID

Another update on the routines that I'm writing for the SAFARI project. I've already explained that we work with a sensor type called TES, and that to read out this sensor, we use a device called a SQUID. Our SQUID is basically a 3x3mm chip, Basically, the TESes and the SQUID are put together in an electronic circuit, which is then immersed in liquid helium.

Since at this temperature (at or below 4 Kelvin), the circuit is superconducting, the SQUID can trap flux because a superconducted current is flowing around in the SQUID.

Since the SQUID is basically a magnetometer, we need trapped flux just as much as we need a punch in the face. The flux can be removed by heating the SQUID a little but, but only just so it no longer superconducts. This is a very common thing for superconducting sensors, so our readout electronics have firmware which contains a "remove flux" procedure.

However, you can't just put the complete procedure in firmware, because we have only one version of electronics and we have multiple different setups in the labs. Each of these setups has a different cryostat with different temperatures (anything between 4 Kelvin and 50 milliKelvin). So the firmware deflux routine has a bunch of parameters which should be controlled through a software routine, which the user can then configure and kick off.

You might wonder why we don't just do it all in software. The reason is, that for the best result, the heating current must be stopped abruptly and the timing aspect is much more tightly controlled by using firmware.