Yesterday we prepared the SPRB minitower for pump down. The electronic tank was closed and the bench suspended (see the other report on this activity).

The minitower was closed this morning (cupola, door, flanges on the flat door).

A first evacuation started at 8:17 UTC. A big leak was observed on the air-boxes of B4_PD1 and B4_cam1. The minitower was vented when the pressure was 800 mbar, not low enough to be visible in the monitoring channel VAC_TOWERPR_MG2_CH6. The photodiode (SN 21) was replaced by a spare one (SN 28), but designed for 50 mW instead of 12 mW. The B4_cam1 camera was swapped with the one of B6.

A second evacuation started at 9:18 UTC.  The first figure is showing the SPRB minitower pressure from there. While no obvious leak was observed on the air boxes, the minitower vacuum level reached a floor around 40mbar. After wondering about the viewport of the phase camera (see the photo showing this viewport and its clean O-ring seen from the minitower) and a short pump down at 12:53 UTC, it turns out that the leak was due to the two small CF40 flanges that were not properly closed. This has been fixed with new gaskets.

The last evacuation started at 14:25 UTC. The pump was stopped at 17h37. The vacuum level reached 0.1 mbar. No obvious leak is observed on the minitower or on the air-boxes of SPRB (see the last figure), the pressures being driven by the usual temperature increase after going under vacuum.