Theory - Vacuum Chambers (Cleaning & Assembly)

Goals

By the end of this module, you should

• know the procedures for cleaning the vacuum chamber.

• understand the importance of the bake-out process.

Deliverables

• things they should have (worked in to the content)

Creating a vacuum

A vacuum in essence is a region/space where we have removed gaseous atoms and molecules. The higher the vacuum, the more atoms removed relative to atomspheric pressure (760 Torr).

There are often multiple pressure ranges that people discuss when it comes to vacuum, where the division is arbitrary and different geographies and companies have slightly different definitions.

For the sake of our discussion, we will define it as the following

Description	Value	Unit
Rough vacuum	1000 - 1	Torr
Medium vacuum	1 - $10^{-3}$	Torr
High vacuum (HV)	$10^{-3}$ - $10^{-7}$	Torr
Ultrahigh vacuum (UHV)	$10^{-7}$ - $10^{-14}$	Torr

For atomic physics experiments, we need to sufficiently isolate our atomic sample trapped by the MOT (at mK) from the background gas atoms/molecules that exists at room temperature. Practically, this means that we have a requirement to achieve UHV for our experiment on the order of $10^{-10}$ Torr to have lifetimes of over 1s, with the higher vacuum the better.

There is a clear and dedicated process to achieving this, first by cleaning all vacuum facing pieces to ensure that they are UHV-compatible, followed by vacuum pumpdown that will be discussed in a later section.

Cleaning and Bake-out

From a practical perspective, since we are dealing with an experiment at the atomic scale, we need to make sure that our equipment is cleaned, prepped, and ready.

The parts for the vacuum chamber will undergo a journey to arrive at your lab station. To start, make sure you clean the parts with soap, water, and acetone.

Our goal is to create a vacuum, so we next need to remove any compounds or gasses that are trapped within the vacuum chamber or parts of the vacuum chamber. To do so, we will perform a bake-out. This is done by heating the vacuum chamber to high temperatures.

Bake-out Procedure

The process for the bake-out is as follows.

• Wrap all the vacuum joints with heat tape, and also place the vacuum chamber into an oven.

• Heat the oven at a rate of about 10 Celsius / hour up to 150 degrees.

  • Concept Check: Why do we work at such a slow rate? We use this controlled rate because all the components will have a different rate of thermal expansion. Heating too quickly could result in components breaking.

• Hold the 150 degree temperature for about 6 hours.

References

Fundamentals of Vacuum Technology