I ended the first cooldown of the run at 12:49 today, approximately 92 hours after it had started. Workpiece temperature was 100 K - more data and model fitting will reveal more insights on the results.
I started a 5 W injection through the heater (dog-clamped to baseplate with indium gasket), which I intend to leave running until steady state temperature is reached, at least overnight. This power level will not present a risk to the system, so I feel comfortable with this static power input even though it is not interlocked.
Update after 25 hours [ATTACHMENT 1]
The 5 W heating of the shields has not quite leveled off yet, and the workpiece temperature resumed falling, showing the temperature had not yet reached a steady state. I will leave 5 W on until I see a steady state, then I will plan to turn off 5 W and allow the workpiece trend to level off fully. Summary:
See attachment 1 for data viewer screenshot which reflects the above summary.
Update after 145 hours [ATTACHMENT 2]
Continued injecting 5 W for the whole weekend. Over the last 24 hours, the workpiece temperature seems to have leveled off into a new cooling rate, while both shields alternated between heating and cooling. I think this can be described as a steady-enough state that I'm ending the cooldown.
Now, I'll let the chamber come up to temperature with the help of the 5 W load.