Chapter 1 Introduction
6.2 An asymmetric cavity system
6.2.5 Vacuum chamber
Figure 6.8: Accelerometer measurements of the mini-GAS suspension. The suspen- sion mount is displaced and its ringdown as a function of time is recorded (inset). A Fourier transform of this data reveals expected resonances at around 2 Hz and 8 Hz, corresponding to the pendulum mode and the leaf spring resonance. Vibrations at higher frequencies are quickly damped by the spring system.
would be to use the suspension to support future microtoroid experiments. As light is coupled in and out of the microtoroids via optical fiber rather than in free space, the motion of the cavity with respect to the vacuum chamber is no longer a cause for concern, and perhaps low-frequency active damping could be avoided entirely.
Figure 6.9: Vacuum chamber design for a new asymmetric cavity. Much of the earlier lab 1 design remains intact, with an upper and lower chamber connected by a differential pumping tube and brought to UHV with two Varian ion pumps. New components include a residual gas analyzer (RGA), a titanium sublimation pump, and a multiplexer port on the upper chamber in order to incorporate getters.
custom-built by NorCal and was replaced with a Kimball Physics 6” spherical oc- tagon of similar dimensions. The upper chamber was again a 2.75” Kimball Physics spherical hexagon. The 55 l/s and 20 l/s Varian VacIon Plus Starcell ion pumps that had been used for the lower and upper chambers were also replicated. The new dif- ferential pumping tube was identical to the older one, as it was a backup version that had been machined at the same time. Both chambers now had nude Bayert-Alpert ionization gauges from Varian, model UHV-24 for the upper chamber and UHV-24p (lower pressure limits 2×10−11 torr and 5×10−12 torr, respectively) for the lower chamber. All-metal gate valves from VAT could be used to pump down the upper and lower chambers simultaneously.
From his experience building vacuum chambers at Michigan, Russ Miller suggested that we include a residual gas analyzer (RGA200, SRS) in order to diagnose any leaks we encountered and a titanium sublimation pump (Lesker) to reduce the final system pressure by an order of magnitude. As we were concerned about the possibility of titanium sputtering near our mirrors, there is no line of sight between this pump and the cavity.
The previous chamber had used a cesium ampoule to load the upper chamber MOT from a reservoir of background gas. We decided instead to implement the newer technique of “getters,” alkali metal dispensers which emit a vapor when they are resistively heated [112, 113]. Using getters is simpler, more compact and avoids problems associated with successfully breaking the glass ampoule under vacuum. It also permits a lower background pressure in the chamber, especially if the getters are operated in a pulsed configuration. However, the metal in each dispenser eventually becomes depleted; how long this takes depends on the current at which the dispenser is operated, as well as whether it has been subjected to short bursts of high current.
We have seen getters in use in the atomic ensembles experiment (lab 2) last only several months. In comparison, the amount of cesium provided by the lab 1 ampoule has been sufficient for the past ten years. Cindy Regal reports that at JILA, getters are used for glass chambers, but in stainless steel chambers (where alkali atoms are absorbed by the walls), ampoules are still used. Our getter design replaced a 2.75”
window on the upper chamber with a five-port 2.75” multiplexer (Kimball Physics);
the center port was for a MOT beam, while the others could be used for getters. With two getters per port, we could in principle stock our chamber with eight getters and hope for several years of operation before they were all depleted. One encouraging factor is our chamber geometry, which allows us to place the getters ∼ 2 cm from the upper MOT. Thus, we are able to form a bright MOT with lower currents than have been necessary in lab 2 and, more recently, in the microtoroid experiment: Dal and summer student Jie Wu measuredNatoms ∼108 with 3.25 A through the getters, well below the threshold current of ∼ 4 A where the response becomes nonlinear [114]; labs 2 and 11a operate at or above this threshold. (It is, however, necessary to
Vacuum Components ◦C Stainless steel chambers, upper and lower 450 Stainless nipples and flanges 450 Stainless tees, crosses, multiplexers 450
Differential pumping tube 450
Getter feedthroughs 450
PZT feedthrough 450
Valves for roughing pumps 450 open 350 closed Viewports, without AR coating 400
Ion pumps without magnets 350
Titanium sublimation pump cartridge 350 Residual gas analyzer (RGA) 300
Table 6.2: Maximum baking temperatures for vacuum chamber components operate the getters briefly above threshold during bake-out as part of an “outgassing”
process to remove impurities from the surface. This process should be repeated after each instance in which the getters are exposed to air [115].)
In Table 6.2, I list the maximum baking temperatures for our vacuum chamber components, also reproduced in Yat’s thesis. In the past, we have obtained UHV viewports from Larsen with anti-reflection (AR) coatings applied by Guernsey. The uncoated viewports have a baking temperature of 400◦C limited by the glass-to- metal transition, but according to Guernsey engineers, the coatings could not be baked above 250◦C. As this would potentially limit our bake-out temperature, we arranged for a coating run by Advanced Thin Films (Longmont, Colorado) on bare Larsen viewports. These coatings reflect less than 0.5% of light between 800 and 950 nm and no longer limit the viewport baking temperature.
In order to determine our target bake-out temperature, Yat was able to find measured thermal desorption spectra for 316LN stainless steel. [116] These indicated a desorption peak for water molecules at around 300◦C. Thus, even if our cavity mount assembly required lower temperatures, we would at least plan to pre-bake the chamber itself above 300◦C for several days. We purchased a large custom oven from Milmetco which could reach temperatures of 600◦F (316◦C). We threaded a 48 inch braided bellows through a hole in the side of the oven so that we could bake the
chamber under vacuum. Inside the oven, one end of the bellows attached to a tee from which two smaller bellows connected to the upper and lower all-metal valves of the chamber. Outside the oven, the other end of the bellows allowed us to pump either with a turbo pumping station (Pfeiffer TSU071) or, at lower pressures, with an ion pump (VacIon Plus 40 Starcell). The chamber, of course, has its own ion pumps attached, but as their magnets have to be removed temporarily for baking, we are not able to use them inside the oven.