Cause of post-test Beam-1 BCM background excess

Identify the physical cause of the large Beam 1 BCM background excess observed in the first several physics fills following the final pressure bump test, and determine whether it arises from desorption of gas from the beam screen or from another mechanism.

Background

After the last pressure bump test (fill 7212), the BCM recorded unusually high Beam 1 backgrounds—up to ten times normal levels—at the start of subsequent physics fills, with a gradual decay during each fill and across fills. This excess did not have a corresponding anomaly in the nearby vacuum gauge readings at 19 m and 22 m.

The authors suggest a possible explanation (e.g., gas condensed on the beam screen and later desorbed by collision debris) but cannot confirm the mechanism. Determining the cause is essential for understanding and mitigating unintended consequences of local gas injections and for ensuring reliable background conditions in future fills.