Atoms Experiment Vacuum Requirements

Ideal Vac Modular Vacuum Chamber:

Spec leak rate: 1*10^-8 mbar L/s = 10^-9 Pa m^3/s

This leak rate can be thought of as "being the amount of gas that flows through a leak at a given pressure differential per time"

TwisTorr 74 FS Turbo Pump pumping speed of N^2 below 10^-5 Torr: 70 L/s = 0.07 m^3/s

So the final pressure would be spec leak rate/pumping speed = 1.4*10^-8 Pa = 1*10^-10 Torr

In reality, the minimum pressure of the turbo is 3*10^-10 Torr, and the pumping speed gets worse near this pressure

In addition, ideal vac quotes the minimum pressure as 10^-8 Torr

 

How long does it take to reach this pressure? Let's use the Pfeiffer vacuum calculator

Volume: 6" tall, overhead view composed of an octagon with 6" sides and a 12" x 6" rectangle: 1,475 in^3 = 0.0242 m^3

Surface area: 960 in^2 = 0.6 m^2 (no internals assumed for this model, should be accurate to a factor of 2 when we stuff the chamber full of mirror mounts)

Assumed desorption rate: 10^-10 mbar L/(s*cm^2) = 10^-7 Pa m^3/(s*m^2), which Pfeiffier quotes as a moderately good baked system rate

Pump configuration: HiScroll 6 (~6 m^3/h), 1 m of 1.5" diameter tubing, HiPace 80 Neo DN63 (~80 L/s), pretty similar to our intended setup (we'll use Agilent)

1 hour to reach 10 Torr and turn on the turbo, 3 hours to reach 10^-8 Torr, final pressure 3*10^-10 Torr (this last figure is dubious since the ideal vac chambers aren't rated this low)

 

Ion pump requirements: 

spec leak rate/final pressure = 1*10^-8 mbar L/s  / 10^-8 Torr = 10^-9 Pa m^3/s / 10^-6 Pa = 10^-3 m^3/s = 1 L/s

So in theory we could get away with the VacIon 2 L/s Pump for ~$1000 (plus a $1700 controller?)

Since Nick's lab had trouble with these chambers, and I'm not sure Maria's lab saw great pressures either without cryopumping, it's probably wise to get a larger ion pump.

 

I'm pretty sure I set up the calculations right but someone with more experience should check