Theory - Ultrahigh Vacuum

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

< INSERT TABLE FROM WEEK 6 - for some reason it isn't letting me copy >

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.

Creating the UHV

< Written in OpenQuantum Google doc. Needs final approval >

Ion Pumps and Creating the UHV

Our goal is to create an ultra-high vacuum. This means we need to achieve roughly $10^{-7}$ torr. With the vacuum chamber prepared, we now need to create the actual vacuum.

Concept Check: Why do we need a vacuum for our experiment? We need to avoid atmospheric collisions as that will heat up our atomic vapor. In the end we need to cool and trap our atoms. In this experiment, we will be using an ion-pump. It can be used continuously to create and maintain an ultra-high vacuum. The ion-pumps have a low power usage. They also have long lifetimes and low noise which are perfect for our experiment.

<SCHEMATIC FOR ION PUMPS>

The ion pump will produce a cloud of electrons within the chamber. When we aren’t at UHV, there will be ambient molecules in the chamber. The electron cloud will ionize the ambient molecules in the chamber. Due to the large voltage difference, the ionized particles will be accelerated into the cathode of the capacitor. With a strong enough voltage difference, the particles will be embedded on the capacitor plate, emptying the vacuum chamber over time.

On paper, this is all we need. However, there are some practical limitations with the ion pump. The ion pump acts also as the disposal for the ionized particles. If we start the vacuum at a standard room pressure, the disposal will saturate immediately. Therefore, we break apart the pumping in a few stages. We first start to initially decrease the pressure to about $10^{-4} torr$. At this point, the ion pump can be used for a while without fear of saturation.