What is back streaming and back migration in vacuum technology?

Back streaming and back migration are two phenomena that can occur in vacuum technology.

Back Streaming:

Back streaming refers to the movement of gas molecules from the high-pressure side of a vacuum system to the low-pressure side. This can happen when there is a pressure difference between two regions of the system, causing gas molecules to flow in the opposite direction of the desired vacuum. Back streaming can occur through leaks, permeation through materials, or through the pumping system itself. It can lead to contamination of the vacuum environment and a decrease in the overall vacuum level.

Back Migration:

Back migration, also known as back diffusion, is the migration of gas molecules from the pump exhaust back into the vacuum chamber. This can happen when the pressure in the pump exhaust is higher than the pressure in the vacuum chamber. Back migration can occur due to the design of the pumping system, improper sealing, or inadequate pumping speed. It can result in the reintroduction of contaminants or unwanted gases into the vacuum chamber, affecting the quality of the vacuum.

Both back streaming and back migration are undesirable in vacuum technology as they can compromise the integrity of the vacuum environment and affect the performance of the system. Proper design, sealing, and maintenance of the vacuum system are important to minimize these phenomena and maintain a high-quality vacuum.

Source: Vacuum Technology and Vacuum Design Handbook” by Karl Jousten