I've modeled the theoretical "best" achievable configuration in Megastat, replacing the flexible strap with a solid copper element, and leaving all but one aperture open in the inner shield [Attachment 1]. In this configuration, the wafer can reach 123K in ~20 hours.
- The difference in time constant between the copper bar end and cold head (purple and blue curves) is due to the thermal resistance across the bar and 2 joints, plus realistic radiative losses from bar.
- The difference in time constant between the cold plate and copper bar end (brown/green and purple) is the thermal resistance across the additional solid copper finger and 2 joints.
- The stead-state temperature difference between the inner shield and the wafer (orange and green) is maintained by heat leaking from 1 aperture in the shield. (We've closed up 4, but best case there will need to be 1 open for the copper bar and leads.
Attachment 2 shows power budgeting of heating and cooling power delivered to various components: 1. copper bar, 2. cold plate, 3. inner shield, 4. Si wafer. The points of intersection of the heating and cooling power curves correspond to the steady-state temperature for each component, which can be verified by Attachment 1. The total heat load on the copper bar and cold plate is just over 70W.
The ideal model shows that there are two areas of potential gains:
Analyzing the most recent cooldown data will hopefully validate the efforts of closing up apertures and painting the entire wafer enclosure with aquadag.