Tc flattening

Before (left) and after (right) images showing the effect of Tc flattening on SCUBA-2 arrays.

This past week has been a busy one for the SCUBA-2 commissioning week. Using a technique, which we are calling “Tc flattening”, the team have shown that it is possible to return a larger number of bolometers in an array into transition. There are two issues: 1) the transition temperature of the bolometers varies slightly across the arrays, and 2) there is a small temperature gradient across the arrays. Combined, this means that for a given temperature of the focal plane arrays, although most of the bolometers will be in transition, some will be too warm and they will go normal, whilst others will be too cold and they go superconducting. In either case, those pixels are unresponsive. To determine the optimum setup that returns the highest yield of bolometers in transition is one of the main tasks of this phase of commissioning.

By turning on one of the SQUID bias supplies during an otherwise quiescent part of the readout cycle, it is possible to apply a different amount of heat to each row to counteract and flatten the variation in Tc and temperature across the arrays – we have dubbed this process “Tc flattening”.

The image above shows a first result of using this technique on array s8b (one of the 850µm arrays). The image on the left shows the number of usable pixels before Tc flattening and the image on the right shows the same after Tc flattening has been applied. We managed to get nearly 200 more bolometers into transition with this technique and we hope for more with further improvements.

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