What does mean free path measure in a vacuum system?

In the context of vacuum technology, the mean free path measures the average distance a particle, such as a molecule, travels between successive collisions with other particles in a system.12

In a vacuum system, this concept is critical because the mean free path is inversely proportional to the pressure of the gas. As the pressure is decreased, the mean free path increases. When the pressure is sufficiently low, the mean free path becomes large enough that collisions between gas molecules and the surfaces of the vacuum chamber or internal components become more frequent than collisions between the molecules themselves.3

Gases in this state are referred to as free-molecule gases or Knudsen gases. Understanding the mean free path is essential for vacuum applications, as it dictates how particles interact with their environment, influencing phenomena such as gas diffusion and the performance of devices like radiometers.3