Care of Miguel Jimenez

Beams were back without any problem as expected since no signal was detected by the BLMs. We now have a better understanding of the sequence of events:

1. The pressure interlock was given to the sector valve left to the arc of 7-8. By setting, the valve n-1 (left to the first NEG sector in 7R) and n+1 (right of the arc 7-8).

2. The interlock for closure of this valve are given by 2 Penning gauges, one on each side of the valve and an ion pump.

    It happened that:

        i) A Penning gauge on the left side of the 7-8 arc (cold side) had problems of ignition

        ii) Due to the cold coming from the continuous cryostat, this gauge is "cold", e.g. 10-15 °C so this gauge, when no ignited, traps gas

3. The gauge got ignited (? not understood, dust arcing could be one explaination, ionisation could be another one - that waas happening in LEP) and induced a huge gas load,

     above 10-4 mbar.

4. Due to the huge conductance, the gas load was seen also by the second Penning gauge which created the interlock and closed the valve.

5. The others pressure rises were induced by the closure of the sector valves

6. The pressure signal coming from the arc and seen by the gauge on the NEG sector (which was misinterpreted as helium coming from the arc) was in fact induced by the

     failing gauge which was inducing a huge gas load. This has been confirmed by switching off that gauge, background pressure was recovered...

In conclusion: NO LEAK, just a problem of vacuum instrumentation, we will follow this type of issues.