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 atmospheric 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.

Creating the UHV

As alluded to before, we need to prepare a vacuum so that we can effectively cool and trap our atoms. As a toy picture, imagine the case of our atoms floating among particles in the air. Any time we try to slow or cool our atoms, it can collide with a rogue molecule provided by the air. This process of ambient molecules colliding with our atoms will add energy to the system, leading to what is known as collisional broadening.

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.

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.