COORminutes021217

Progress since last meeting

Higher intensity positron operation in December failed because of vacuum problems of the superconducting cavities. The cavities were warmed up to room temperature during the Christmas break to release gas.
Warming up the GO/GG magnets in the north and south interaction regions during the break did not give any significant improvement of the vacuum.
The startup was promising. 36mA of positron beam could be injected after 2 days of operation. There were several luminosity fills with Ie > 30mA.
Progress was stopped by a non-understood intensity and energy dependent positron lifetime problem. Attempts to locate the lifetime problem by beam steering caused vacuum leaks NR11 and NR14. The vacuum system of the interaction region was vented to 1mbar. The pressure is now at 5 10**-8mbar instead of 2 10**-9 before the incident.

Proposed running

It was proposed to warm up the GO/GG beam pipes to room temperature and to do one week of high intensity beam bake-out at 12GeV. During the bake-out proton beam should be kept in the machine.
Continue with high positron intensity luminosity operation after the bake-out.
Machine studies last two weeks of February.

High priority items before the shutdown are:

The detector is in good conditions. At the present beam currents background conditions are usually acceptable. The aim is to collect at least 2pb-1 with CTD and MVD turned on. So far got 30%.
No improvement of the vacuum and CTD chamber currents was observed after the Christmas break
The following questions should be answered before the shutdown:

Continue running until 27.01.2003 with increasing positron currents. If Ie=50mA is reached, continue with 12GeV running for 2-3 days for HOM studies, one long positron only fill, rotator studies. If 50mA is not achieved until 27.01., postpone high intensity running until end of running period.

The detector should tolerate background conditions at a luminosity of 4 10**31 cm-2 s-1 mA-2 (CTD HV off).
The CTD trip limits were set very conservatively. No significant loss in gain has been observed so far. The plan is to increase the thresholds.

The total delivered integrated luminosity is 8.0 pb-1. The collected luminosity is 6.3pb-1 (H1 active), 2.0pb-1 (HV on), which is limited by high background conditions and background spikes.
The chamber currents improved by a factor of 20 over 100 days. In December, the vacuum pressure at the IP was a factor of 2 higher than in 2000.
The IP vacuum system was vented on 12.01.2003 due to a leak at the main absorber. The proton beam gas background is still too high to turn on the CJC high voltage . Recovering from the previous leak took 20-30 days.
Proposed schedule:

Installation of the VFPS. The VFPS bypass cold test started at DESY yesterday.
Modification of C5b collimator. The proposal is to have vertically asymmetric jaws to compensate for the H1 offset with respect to HERA.
Repair of CIP electronics. Mounting SMD components failed. A new technique is used. First results are expected next week.
Improve background monitoring (add Si PIN diodes).

The trip and operational limits of the CJC are not conservative. Some degradation of the gain was already observed before the luminosity upgrade. The HV was reduced in 2000. The single wire efficiency is 95%.

The detector is in good shape. Data taking is not limited by the background conditions. About 250k DIS events were taken in November. The delivered luminosity was much lower in December and January.
The positron current was not increased in January.
The total data sample is 684k DIS events. The projected data sample until the end of the running period is 0.85M events. 1.4M events are needed for a meaningful physics program.
Requests:

Non-colliding beam operation between now and 17.02.2003. The effective gain for HERMES would be 20-40%
Dump positron beam at 10mA, gain 20-30%

The tungsten wire was damaged again (3rd time). The damage was associated with beam movement. Another W wire (so-called emergency wire) is presently used for data taking. It has poorer rate stability. It is planned to replace the broken wire during the access in February. This will take 16 hours plus time for pumping.
HERA B did not get much running time in January. The total J/Psi data sample is 150k events. The proposal was 2M events. The physics program is highly statistics limited. A Psi' measurement does not seem to be possible.
Request:

6 week proton only run before the shutdown This would increase the data sample by a factor of 8-9.

Some collaborating institutes committed to HERA B only for this running period. It would be difficult to turn on after the shutdown. The possibility of stable proton beam running during 12GeV positron beam bake-out after the shutdown was discussed. It is not clear whether this is realistic.

Interlock tests in March are mandatory, which will take 3 weeks.
The synchrotron radiation background is well understood. The understanding of the proton beam gas background is reasonable, although not complete. No major new ideas have come up so far. It is not yet clear whether the proposed modifications will be ready on time.
A review committee will be set up to review the engineering aspects of the proposed changes to ensure that they are technically sound. The directorate will be in close contact with the committee. Resources in the mechanical shops will be allocated.
Although there are certain risks, the directorate decided to begin the shutdown 03.03.2003. It is clear that is a major set-back for the HERA B and HERMES physics programs. The hope is that HERA B will be able to take data after the shut-down (stable proton beam during 12GeV positron beam bake-out).
The readiness of the modified collimators, H1 CIP electronics repair, GA magnet electron vacuum chamber and coating of the main absorber was discussed. It is not yet clear whether the HERA tram will be needed for opening of the GA magnet.

Starting today, one week of 12GeV positron beam bake-out. Proton beam will be kept in the machine for HERA B. (HERA B has to turn off for about 15 minutes for positron re-injection.) The GO/GG magnet beam pipes will be warmed up to 170K during the bake-out.
Tuesday next week continue with luminosity operation.
The further schedule will be discussed in the weekly HERA meetings.
Machine studies 17.02.2003 to 03.03.2003
Shutdown start: 03.03.2003, length 18 weeks.

Uwe Schneekloth