Thermal Loading Calculations (Updated January 12, 2009)
The thermal loads are calculated and shown in the file ThermalLoadTotals7pixel.XLS with the 15K to 300 K arrangement : 5" of copper WR42 output waveguide, 9" of SS WR42 output waveguide, a sliding waveguide joint, and a wr42 thermal gap (14 total).
Waveguide Window Leakage
The attached page summarizes the leakage rates for various waveguide window materials. A vacuum 10^-5 Torr increases the convection loading of Helium (1st stage loading of ~ 4 Watts). The table includes the number of days to reach this vacuum based on measured leak rates of materials and assumes a typical 0.51/101323 pressure of He. This may be an underestimate since He is the coolant for the GM refrigerators and may raise the ambient level. Charcoal traps effectively adsorb the He and extend this time
Waveguide Window Outgassing
April 2008 out gassing measurements of Ecco Foam PS102 are given here :TestSummary.xls
. The test dewar measured 12.75" diameter x 21.5" height. The sample was cylindrical with dimensions 6.25" diameter x 3" height. From these measurements the rate is: 1.1 10^-3 atm cm^3/s or flux assuming STP as 1.41772*10^-6 atm cm^3/cm^2-s.
Taking into account the surface area of all the windows(14 wr42 windows and 7 circular windows) and the presumed outgassing ratio of 0.51/101323 for He gives 3.7 atm cm^3/s leak rate, or 236 days to reach 10^-5 Torr( 13 10^-9 atm)
This paper charcoalcurves.pdf
gives Langmuir curves for charcoal adsorbent. The fractional coverage for a monolayer of adsorbed gas as a function of pressure P is given by the equation, v/vm = B P/(1 + BP). A fit to the Langmuir curves gives B = 300 atm^-1 and vm = 50 atm cc/g of material for He adsorption at 10 K. The constant B scales exponentially with inverse temperature giving a correction of .8 for B at 20K. A total of 67 grams for one year of adsorbate is needed. With a bulk density of 0.4 g/cm^3, a total of 838 cm^2 area of 2 mm charcoal is required, or a ~6 inch square plate with charcoal on both sides.