Summary notes of the LCU meeting on 02/12/2008

Present: IA, MA, CB, RC, MG, WH, YL, SP, TR, FeRo, FS, RT, TW, SW, SY, FZ

Synchrobetatron optics: the grazing function g -> Steve Peggs (pdf)

The work triggered by the analysis of the effect of crystal on beam halo.
The grazing function "g", which turns out to be proportional to the normalized dispersion derivative, gives a condition to be satisfied such that the grazing angle does not depend on the synchrotron amplitude.
The grazing angle should be within the crystal angular acceptance for all synchrotron amplitudes, otherwise some particle would be channeled by the crystal. This condition is easily met for volume reflection, while for channeling (in particular at low energy) this is not always the case. 
The evolution of the grazing function is analyzed for matched FODO structures (thin as well as thick lens version). The grazing function features kicks at thin dipoles and it is always smaller than a quarter of the deflection angle of the cell.
The analysis of the variation of the grazing angle with Dp/p for various machines (SPS, RHIC, Tevatron, LHC) show the larger variation for RHIC, which might explain why the tests with crystal there were not so encouraging.

Continuous LHC beta-beating measurement -> R. Steinhagen (pdf)

This presentation is a follow up of the one recently made at the APC.
The system was successfully tested in the SPS (achieved performance: Db/b < 1 % at 1 Hz).
Its usefulness for monitoring the beta-beating in the LHC is clear. This could be a nice complement of the b-beating measurements developed by RT et al.
A test installation is proposed: a splitter should be installed, thus reducing the signal level and preventing the measurement of low intensity beams (< 2 × 10⁹ p). Three (plus two) BPMs could be modified with the dedicated electronics to allow continuous beta-beating measurements. Such a solution could be easily implemented, as there will be to cables to be pulled and the costs are very limited. The proposed location is IR7. The proposal will be presented to the LHC Performance Committee for approval.

Update on the MADX 'thintrack' module   -> YS (pdf)
Two topics are presented:
Inclusion of "racetrack" aperture type in the tracking module. Such a shape was not available for tracking, while it can be specified for aperture computation.
The computation of the 6D closed orbit has been included in the tracking module. This is a long-standing issue. So far, the closed orbit was inherited from the Twiss module. However, this is not correct, as the physics in Twiss and tracking is not exactly the same (which is a very unfortunate situation, but difficult, if not impossible, to fix). With the new implementation the coherence is restored and the tracking is performed with respect to a correctly computed 6D closed orbit.

Recent MAD-X activities -> FS (pdf)
The key activities are:
BV kill initiative (SF, JLN, TR, FS): triggered by detailed cross-checks. SF prepared specifications for the correct behaviour of the BV flag. Implementation is made by FS. TR provided debugging and testing support. The specifications will be used to develop an automatic generator of Beam 4 sequence and strength files (JLN).
Restore coherence between aperture types active in various MAD-X modules (YP for tracking module and FS for PTC).
Fix a number of bugs in the tracking module (FS) and introduce the computation of the 6D closed orbit (YS)
General work on aperture module (IA, HG, JBJ, TR).
Catastrophic Failures of Twiss for CLIC (SF, HG, JBJ, FS): JBJ found a number of bugs in Twiss in the framework of CLIC activities. In particular it was found that the time variable was always forced to be periodic! The derivatives of Twiss parameters vs. The source of this behaviour is not completely understood, yet.
PTC_TWISS upgrade (JLN): a new version is available with new features such as Twiss parameters inside a magnet, Delta-dependence of Twiss parameters and full summary table.
Sector map output is not in TFS format (JLN).

PTC standalone model of LHC (JLN, FS): such a model should be ready by end of February to allow JLN to move to ZBASE activities.
Beam-Beam element in PTC (almost finished with help from various sources).
A recent issue with a new name for a element attribute ("beam_sep" has been changed to "mech_sep" to avoid any confusion). led to the decision to assign a fatal error to avoid misleading or wrong results from MAD-X! The user can always decide to supersede such a behaviour.
A development version of MAD-X has been defined: it will contain all new features and it will be debugged via the automatic example testing developed by JLN. Once considered bug-free is will become the production version. It is planned to have a release of a new version only once a month (thus reducing the current pace). It is also stressed that the module keeper should provide their contribution to the cross-check of the development version by analysing the differences found by the automatic testing and explicitly accepting the results.

AOB

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Last update: 06-January-09

MG

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