Wednesday
April 19th, 14:30
CCC
conference room 874/1-011
Jorg had commented by email on two points in the draft minutes of the previous meeting: (1) The calibration procedure for BPMs and CODs was applied for TI8 using on-line analysis. He stressed that for the LHC a "fast on-line analysis" fitting ALL BPM calibrations + corrector calibrations + optics model is neither feasible nor of interest, and that manual interventions by a real human will always be required. (2) Jorg saw no issue with degeneracy in an SVD based orbit-feedback. He emphasized that SVD on the contrary avoids problems due to degeneracy.
After the meeting, Stephane expanded his comment on the orbit correction from the 5th April meeting, namely that the LHC orbit correction algorithm should contain some weight to force the locality of the correction. He stressed that all global corrections depending on phase advances should be as local as possible, such as the coupling or beta-beating correction (if any), which will need to be done at the level of each sector.
The web minutes of the previous meeting had been corrected and slightly modified.
Daniela suggested discussing the operation of the Roman Pots during commissioning. This item was added to the list of LHCCWG actions.
Helmut is drafting two statements, requested by the LHCC before its next meeting on May 10, one concerning the impact on operation by LHCf and the other by the TOTEM 90-m optics.
LHCf
The LHCf detector would be installed in the TAN. It replaces a large 9.4 cm x 9.4 cm Cu converter, which normally is installed in front of the luminosity monitor BRAN, with an active cross section of 8 cm x 8 cm. The concerns are both the effect which LHCf may have on the measurements of luminosity, vertex, and background, as well as the associated downtime for installation, access, and removal.
Helmut started with a number of general considerations on the particles hitting the TAN and their likely transverse position, largely based on LEP experience. He pointed out that neutral particles will be offset by 2 cm transversely from the IP position at the location of the TAN. He found that IP orbit position and angle may vary by up to a few mm’s and several 100 microradians, respectively, due to a number of reasons, such as aperture optimization, crossing angle scans, or a residual common angle that is estimated to be of order 10% of the full crossing angle. Next addressing the requirements on the luminosity measurement, he considered an initial precision of 10% as adequate for collision finding, stressing that full optimization and luminosity tuning are likely to require a much better precision of the order of 0.25%.
Daniela commented that LHCf requests a luminosity of less than a few 10^30/cm^2/s AND 43 bunches, still compatible with commissioning.
The luminosity monitor and the regular Cu converter in front of it cover an area of 8 cm x 8 cm, which corresponds to an angular acceptance of +/- 280 microrad. Installing the smaller LHCf detector, which itself consists of two asymmetrically arranged parts, in front of the luminosity detector will change the effective conversion thickness. More importantly, it will also introduce an asymmetric non-uniformity of the conversion. The consequences were addressed in Monte-Carlo simulations by the LHCf collaboration of the energy flux in the luminosity detector, which indicate a change in the monitor response of +/-20% over the full range of crossing angles, and of 1-2% for the smaller range of IP angle variation expected (~10% of the nominal crossing angle) when operating with zero crossing angle. With zero crossing angle, the impact of LHCf seems marginal during commissioning. However, it would represent a major interference during tests of the crossing angle.
The real data taking of LHCf lasts only a few hours. Mike pointed out that LHCf would only become an issue, if we attempted a precise luminosity optimization at low luminosity.
Helmut proposed to agree on a running scenario which minimizes downtime and interference, and to remove the detector as soon as the maximum acceptable luminosity is approached. He also expressed concern about possible radiation damage to the detector during commissioning even before the start of collisions.
Oliver added two comments: (1) We should develop a scenario how the two beams are brought into collisions. The situation will be different from LEP. If one beam is kept stable (as in HERA) and only the other beam is moved, the IP position and IP angles may not be close to zero. (2) Due to ground settling, the ATLAS detector will move vertically by a total of 10 mm, about 1 mm per year. This large motion cannot be compensated by steering magnets, but it requires an adjustment of the triplet position. In general the IP will not be at the center of the TAN, but it could be off by several mm.
TOTEM
90-m Optics
Helmut
has held discussions with TOTEM,
An intermediate TOTEM optics with beta*=90 m has been studied more recently for the 1st year of LHC operation. ATLAS does not ask for such type of running mode. Two questions to be addressed are: (1) is this 90-m optics easier than the high-beta optics? (2) and is it compatible with the LHC physics run?
The 90-m optics implies a local unsqueezing of beta*. The integer tune is not changed. The commissioning should, therefore, be faster than that of the real high-beta optics.
Oliver pointed out that the commissioning time required for the 90-m optics (“unsqueeze”) should be comparable to the commissioning of the squeeze. The horizontal tune is changed by 0.083 units, which, as stressed by Oliver, is at the limit of what can be corrected globally with the trim quadrupoles MQT. Stephane clarified that the limit for the tune shift induced by the MQT has indeed been set to 0.1. At this level, the induced beta-beat is of the order of +/-5% peak, see Eq. (31) in LPR501, which added linearly to our budget of 21% becomes sizeable. Oliver also commented that, at the beginning of LHC operation, with reduced beam-beam effects, additional space should be available in the tune diagram. Helmut reported that the horizontal tune can be adjusted either globally or by rematching IR4. The latter option would further complicate the commissioning, in particular as IR4 contains most of the instrumentation. Stefano also recalled an aperture problem in IR4, if the latter is used for rematching. Luckily, there appears to be an easy solution: Provided the tune shifts for the two beams have the same sign, as it seems to be the case, instead of using the trim quadrupoles, a global tune adjustment can be performed with the main quadrupoles.
TOTEM intends to submit a request to the LHCC for an early 90-m run.
Helmut’s question how long the LHC will operate with 43 or 156 bunches was answered by Roger as several weeks. TOTEM needs only a few days of beam time for measuring the total cross section, but many more days would be required for diffractive physics or other studies. The exact TOTEM plan was not known.
Oliver commented that the beam sizes at the locations of the Roman Pots will be different for the 90-m optics and that the numbers should be looked at. After the meeting Frank contacted the ABP/RLC team for possible comments on impedance and beam stability with the two different TOTEM optics. Beam stability for the high-beta low-emittance TOTEM run was studied by E. Metral and A. Verdier in LHC Project Note 345.
Further
Steps
It was decided that Helmut would complete a draft statement for both LHCf and TOTEM before Thursday evening. He will present this statement to the LTC prior to the LHCC meeting. Daniela added that a full simulation for the impact of LHCf on the BRAN should be available on May 2.
Background
Summary of TAN Integration workshop, Helmut, ABP/LOC 14.03.2006; TOTEM high-beta optics, Helmut, ABP/LOC 11.04.2006
Roger explained the general idea that the EICs convert the LHCCWG discussions and presentations by the commissioners into a detailed, possibly web-based, documentation containing a clear set of procedures for LHC commissioning. These roles of EICs and commissioners would mimic those during the commissioning. Roger had discussed this idea with Verena, who had already contacted Magali and Stefano. All three expressed their interest in contributing to this endeavour. A first presentation of their plan will be possible only at the next meeting, when Verena will have returned to CERN.
AOB
A draft agenda for the next several meetings will be posted on the web, so that speakers and smaller teams have more time to produce results and to prepare their presentations.
Wednesday May 3rd, 14:30
CCC conference room 874/1-011
Provisional agenda
Minutes of previous meeting
Matters arising
450GeV commissioning: increasing the beam intensity
Discussion of LHCCWG web follow up (Verena, Magali, Stefano)
AOB
Reported by Frank