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Message ID: 765     Entry time: Thu Dec 22 20:28:58 2011
Author: Frank 
Type: DailyProgress 
Category: RefCav 
Subject: copper shields 

tapped a few #2-56 holes in the top of the copper tube which will be used as a radiative shield and heater for the cavities. The holes are needed to fix the heating wire (AWG32, NiCr80, 11.3Ohms/ft, w/Heavy ML Enamel,  0.0080" Diameter). Polished and cleaned the tube ready for in-vacuum use (not baked, does not fit in chamber). Teflon pieces and screws for mounting the thing got cleaned and are currently vacuum-baked at 80degC over night.
Here a list of thoughts/facts:

  • Wire resistance is about pi*2.5in/12*11.3 = 7.4Ohms/turn.
  • The total heating power for the chamber to bring it from RT to 35degC is about 7W.
  • I would like to drive the heater directly with an low-noise opamp to not have to build a low-noise, high power driver
  • Using standard opamps the maximum voltage will be somewhere around 12V.
  • Assuming 1W of power for each heater is more than plenty we need to go for 20 turns.
  • 20 turns would get us 148 Ohms, so maximum current @12V would be 80mA
  • LT1028/1128 can only drive 25mA, AD797 can directly drive 50mA
  • 50mA and 20 turns would allow (only) 370mW of heating
  • if we go to 25 turns we will have 185Ohms, giving us 465mW of heating power using 50mA @9.25V
  • The whole shield is 10 inches long (the cavity is 8 inches). 
  • A uniform distribution is different as the shield has two cut-outs for the cavity supports.
  • The frequency difference of both cavities at 35C is ~160MHz
  • tuning factor for the cavity is ~155MHz/K (or 6.4mK/MHz)
  • required temp difference will be 1K if we use both AOMs
  • 1K temp gradient requires 22 mW heating power for polished copper cylinder in polished SS cylinder (no conductivity assumed, is wrong but have to measure contact area between PTFE mounts and top stack plate)
  • conductive PTFE pieces are 0.188" thick, assuming an touching area of 0.5"x0.125" this results in a conductive heat transfer of 2.2mW per mount, so a total of 6.6mW for all three mounts
  • --> total heat transfer assuming 1K temperature difference : ~30 mW

 

picture shows one of the shields with some copper wire for test-wrapping

 P1810530.JPG

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