Wednesday
May 3rd, 14:30
CCC
conference room 874/1-011
Thijs had commented by email on several points in the minutes of the previous meeting. Concerning the requirement for the luminosity measurement, he remarked that even the radiation monitoring system will easily give a 10 % resolution from day 1, but he wondered if the objective of 0.25% for full optimization is realistic. Mike explained that the 0.25% should be obtained at nominal luminosity by pure statistics. Concerning the need of developing a scenario for how the two beams are brought into collision, Thijs added another reason for doing this in addition to Oliver’s, namely that a transverse offset of the impact of the neutral particles at the TAN will affect (and could possibly damage) much more LHC equipment than just LHCf alone (the Hydrostatic Leveling System of the ATLAS detector for example, will have serious difficulties to measure at high precision). Concerning the TOTEM request, Thijs noticed that measuring the total cross section has always been used as an argument for Roman Pots in various machines but with limited success. He considered a detailed plan for TOTEM as very welcome, and suggested that it could be presented at the same time as the request to the LHCC for an early 90-m run.
Roger and Massimiliano reported from a meeting with the experiments, where an LHCf scenario with zero impact on the LHC machine commissioning has been agreed upon. Various options for installation and removal of the detector exist.
Roger reported some news, through Steve, from the LHC
Team meeting of Tuesday May2nd. Lyn said categorically that there will be no sector
test with beam in 2006. The project goal is now to achieve circulating beam in
2007. If there is a slippage which means no beam before 2008, a sector test in
2007 will come back into the picture. A definite schedule should be presented
to council in June. With this in mind, Steve is organizing a meeting on Tuesday
May 9th to look at the planning and inter-relationship between installation,
hardware commissioning, cold checkout and beam commissioning.
Ralph Assmann
recommended consulting chapter 18 of
the LHC Design Report, Chamonix’06 and Conference
papers for a detailed description of the collimation system. Many
additional details can also be found in his recent talk to the LTC.
He first presented Mike’s empty table which was
to be filled in. The collimation system is complex, and it will be installed in
various phases before the nominal intensity can be reached. Only the zeroth and first phases have been defined completely. A
total of 30 potential collimator locations are reserved for phase 2. Ralph
explained the meaning of collimator acronyms.
Concerning the collimators in the transfer line,
Verena commented that the momentum collimator in TI8
likely will not be installed, since it does not work properly and would be at a
bad location endangering a downstream magnet. Therefore, only 6 (betatron) collimators may be installed in TI8. All 7 collimators,
including the momentum one, will be installed in TI2.
Ralph next emphasized that the prerequisites for any collimator operation
are (1) a stable orbit and (2) stable beta functions. In addition, working BLMs
are needed at all collimators and at a
few critical locations. In this phase the BLMs
are required not for machine protection but for adjusting the collimator
positions.
The commissioning of the collimation system
follows the setting up injection and injection protection and setting up the
beam dump and dump protection. Only then will the beam-based calibration of all
ring collimators be performed. Quench thresholds are not established during
this process. The result of the collimator set up is a “ring safe” condition.
Protection and cleaning may be verified with beam measurements.
In the following, Ralph described the various
classes of collimators, characterized by different functions and different
degrees of robustness. A critical parameter is the cold aperture of the ring,
which is expected to be 7.5 sigma including specified errors. If the real
aperture is much smaller than this, this will necessitate a much tighter
collimation. Each type of collimator has a design setting, the less robust ones
being farther away from the beam. The protection against off-momentum injection
may need to be further looked at. For the nominal LHC parameters, in some cases
there is only a 0.1 sigma difference in the collimator position, e.g., between
secondary betatron collimators and injection
protection collimators. Ralph emphasized
that all collimator settings depend on
the cold aperture and on the worst-case beam lifetime. He proposed an
extension of Mike’s table which also includes the minimum beam lifetime as well
as anticipated static and dynamic orbit errors and beta beating. The dynamic
part would consist of two components, characterizing the variation on the
injection plateau and the reproducibility from cycle to cycle, respectively.
Frank suggested that coupling and dispersion may be added as well.
After lively discussion, it was agreed that in
about six weeks we should have a global view of imperfections, including static
and dynamic beta beating and estimates of the resulting aperture. ACTION:
Jean-Pierre and Massimo
Ralph (Assmann)
distinguished two possible approaches: either assuming the design values from
the very beginning or anticipating a learning curve with larger initial errors.
The expected quench level corresponds to continuous losses of 7e8 p/m/s. At a
lifetime of 0.1 hour, this translates into a total intensity of 5e11 protons.
The collimators are closed in phases. The initial pilot bunch requires no
collimation. With 12 bunches in the
machine, some of the collimators need to be moved in, namely the primary
collimators to 6 sigma, and the triplet protecting tertiary collimators to 30
sigma. The TCDQ is set at 10 sigma. This setting
could remain in place throughout the 450 GeV commissioning
with 43x43 bunches. A more ambitious programme would,
however, aim at testing the tighter scheme needed at higher energy and/or
intensity. In 43 bunch operation, the primary collimators would then be set to 5.7 sigma. the secondaries to
6.7 sigma, absorbers to 10 sigma, the tertiaries left at 30 sigma, the TCDQ closed to 9 sigma,
and the TDI set to 6.8 sigma. These numbers complete Mike’s original table for
450 GeV, assuming a 7.5 sigma aperture in the arcs.
Ralph next sketched the collimator set-up
procedure, including beam-based calibration of the jaw positions. In case of
problems (e.g., frequent quenches), he recommends the following sequence of
actions: (1) increase aperture, (2) improve machine stability, (3) improve
cleaning efficiency, (4) decrease intensity (or increase beta*). Ralph stressed that the input for a realistic
scenario must be specified by the LHCCWG, and that the goals of the
commissioning also have to be clearly defined.
Roger asked whether the beam-based collimator
set up would be done manually or with automation. Ralph replied that data are
provided at 1 Hz, and that the initial setting up will be manual. The Tevatron has an automated system. The Tevatron
collimators are re-adjusted twice per fill (with a typical fill length of 10
hours). Stefano and Michel Jonker are following up
this complex of questions. Ralph emphasized that the beam-based setting of the
collimators cannot be done with high intensity beam at 7 TeV. Therefore, the
planned procedure requires that the machine (orbit and optics) is reasonably
reproducible from fill to fill. Another possibility would be to use their
effect on the secondary halo for adjusting the secondary collimators. Frank
suggested to also look at the possible use of impedance-related signals like
orbit deflection or tune shift. Rudiger proposed to
include a few bunches with larger emittance for the
adjustment of the secondary collimators. Ralph mentioned that the so-called
phase-2 collimators will be equipped with BPMs
integrated into the jaws. Thijs highlighted that at
the Tevatron the detector regions are the aperture
bottleneck. Ralph responded that at top energy in the squeezed optics the
situation at the LHC will be similar to the Tevatron,
with the triplets representing the limiting aperture.
Jan’s review was based on his, Jorg’s and Verena’s presentations at Chamonix’06 (Chamonix slides by Jan, Jorg, Verena ). The goal of the machine protection commissioning is to prevent damage and to prepare the next phases. Commissioning procedures need to be well defined. An example is the note by Jorg on the beam interlock for CNGS, whose completion took about 8 months. All machine protection systems should be active from the start. Masking interlocks should be avoided. The system will be tested without beam first.
Indeed the machine protection comprises a huge number of systems, including beam dumping system, beam interlock system, collimation, BLMs, safe beam parameters, beam presence flag, software, sequencer, etc. Completeness of the MPS layout diagram needs to be checked, as well as the interplay between the different systems involved. 43 bunches are above the safe limit and require prior commissioning of the MPS. Partial checks of the MPS need to be repeated every time a step in intensity is made or beam conditions change (optics change, emittance change, transition to ion operation), as, e.g., EMC noise may increase.
Mike
pointed out that, starting from Verena’s table in
The MPS needs to be commissioned at the 1st step of intensity increase. 43 bunches with 3e10 are 30% above the safe beam limit at 450 GeV. Jan proposed the following commissioning order of systems: injection – dump – others (BPMs, FMCM, noise) – BLMs – collimation, which is consistent with Ralph’s presentation.
The commissioning of the injection system involves checking the setting and readback of active elements, and the position of protection elements. Jan stressed that almost every system related to injection-MPS is needed at an early stage. The beam dump system poses similar requirements (active and protection elements are to be checked), but in addition its commissioning comprises the post mortem and a verification of re-triggering with beam. For the BIC (beam interlock controller) additional tests with beam and study of EMC noise effects are required. BPM readings likely depend on intensity. The acceptable number of bad BPMs is another question. BPMs in the dump and collimation areas are of critical importance. The BLM readings could depend nonlinearly on intensity. Procedures for testing the BLM-triggered beam dump and adjusting the BLM thresholds will need to be established. Collimation has numerous interdependencies with most other systems. A global coordination appears required. The functioning of the safe beam flag has to be verified too.
A
follow up of this presentation is mandatory. A starting point could be the
tables presented by Jorg and Verena
in
It was commented that Jan’s talk raised a lot of questions. Now the detailed answers must be worked out.
A
team consisting of Verena and the EIC’s
has taken on the task of defining commissioning procedures, in an interactive,
iterative process. The strategy envisioned is to extract information from
LHCCWG, LHC-OP, and
Rudiger suggested using EDMS for the approval. Mike thought
that approval was needed only in certain cases. Roger commented that the next
Ralph S. proposed a wiki system for facilitating contributions and corrections. Mike considered this a little bit too flexible. He rather preferred the simpler solution of granting author rights to anybody wanting to expand or correct a web page.
Roger
encouraged that an example procedure, e.g., for the 1st turn or collimation,
be presented in the next meeting. ACTION:
EICs & Verena
Wednesday May 17th, 14:30
CCC conference room 874/1-011
Provisional
agenda
Minutes of
previous meeting
Matters arising
450GeV
commissioning: increasing the beam intensity
Example
commissioning procedure (EICs & Verena)
AOB
Reported by Frank