40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
 ATF eLog Not logged in
This is a draft message, edit and submit it to make it permanent
Message ID: 2790     Entry time: Mon Jun 27 10:59:04 2022     In reply to: 2774
 Author: Radhika Type: DailyProgress Category: Emissivity estimation Subject: Si wafer emissivity testing

Here I analyze potential gains from using LN2 to kick off a Megastat cooldown, and transition to a cryocooler to cool under 77K. I've removed the heat loads in the system by modeling an empty chamber (no wafer) and no exposed apertures in the inner shield. I've also modeled the flexible strap is a solid copper element, for an ideal cooldown scenario.

Attachment 1 is a reminder of the cooldown of the cold head of the cryocooler. The cooling power of the cryocooler is a function of the temperature at the cold head, and this is tabulated in its manual. The cold head reaches 77K at ~3.8 hours from past data. This provides an upper limit on gains from using LN2 - if the cold head is cooled to 77K instantly, its cooldown curve will be shifted to the left by 3.8 hours, and thus the system will reach steady state 3.8 hours faster. In reality, there would be a time constant for the cold head to reach 77K due to the LN2-copper interaction, and the mass/heat capacity of the cold head.

Attachment 2 is a plot from Ekin which shows the heat transfer rate (Qdot) per m^2, for the boundary between LN2 and a solid. The different curves correspond for different $\Delta T$ between the LN2 and cold head, and for a majority of the cooling from RT, we will use the curves for $\Delta T$ ~ 100K. These have lower heat transfer because film boiling is occurring, and a layer of film "insulates" the surface of the solid from the 77K bath. These curves vary in the diameter of the contact area, and we use the bottom curve corresponding to D >= 1.0 cm. Once $\Delta T$ reaches 10K, the heat transfer rate jumps up to the nucleate boiling curves. We use the one corresponding to 1atm of pressure.

 Attachment 1: coldhead_vs_LN2.pdf  13 kB  Uploaded Mon Jun 27 12:03:01 2022
 Attachment 2: thermal_transfer_rate_LN2.png  244 kB  Uploaded Tue Jun 28 12:57:19 2022
ELOG V3.1.3-