Summary notes of the LCU meeting on 03/12/2007

Present: IA, MA, CB, HB, UD, MG, AG, JJ, TK, PL, MM, EM, TP, TR, FeRo, FS, RT, TW, SW, SY, FZ
Excused: FC, WH

Report from meetings

• SLM -> MG
  Concerning EPAC attendance, students could attend provided they get supported from their home institute. People not attending can still submit abstracts which will be accepted if co-authors are going. Attendance might be re-discussed for papers selected for oral contribution.
  There is a request for volunteer guides for the open days 5-6 April 2008.

Update on resistive wall transverse impedance measurements at low frequency (graphite samples and PIMs) -> FeRo  (pdf)

A series of laboratory-based impedance measurements started to benchmark analytical and numerical predictions of low-frequency impedances of relatively poor conducting materials like graphite collimator jaws and of the impedance of the PIMs.
Standard wire methods are not sensitive enough at low frequency. Coils are used, instead. First measurements were performed on parallel plate set-ups. It is not always possible to obtain the required impedance information directly. In some cases, the information is rather obtained using the difference between measurements with graphite and copper plates together with the predictions for copper. As far as the real part of the transverse impedance is concerned, good agreement with the expectations was found down to about 10 kHz. Extension to lower frequencies down to 1 kHz looks possible but would require more work. As far as the imaginary part of the transverse impedance is concerned, the agreement between theory and measurements is excellent well-below the 1 kHz range.
A resonance is introduced by the measurement coils at about 1 MHz.
FZ suggest to perform measurements by displacing the coils in the device gap.
Measurements will now be extended to stand alone collimators jaws and in January 2008 to a collimator assembly.
PIM impedances were measured for different elongations. The transverse impedance peaks at about 2.5 kOhm/m. The original estimate was 3 kOhm/m, which confirms that there is some safety margin. The next step will consist of measuring the transverse impedance vs. elongation and to quantify the impact of a certain number of non-touching finger on the transverse impedance (i.e. transverse impedance vs. resistance of the PIMS). MG suggests to measure both nominal and out-of-spec PIMS to evaluate the impact of the non-conformity on the PIMs performance.
These nice measurements give good confidence in our understanding of the LHC impedance and confirm the assumptions on the inductive bypass.

Lower frequency impedance simulation -> TK (pdf)
TK reported about studies which have only started recently aimed at simulating the transverse impedance at low frequency. So far the simulation tools could not provide any useful result below the 1 MHz range. He used analytical physical arguments to describe the main features : the linear rise in Re Zt(f) with frequency f for low f and the fall off with sqrt(f) for high frequencies. Using the Ansoft Maxwell program and mesh sizes of the order of the skin depth, he obtained numerical predictions for the geometries measured and tested theoretical predictions.
Interestingly enough, in the case of the collimators the transverse impedance showed characteristics of both the metallic support structure (low frequencies) and the graphite jaws (high frequencies).
In general, a good agreement between measurements and theory is found. This gives further confidence in our understanding of the LHC impedance as now theory, simulations and measurements converge nicely.

Some changes in the LHC sequence -> TR
TR reports that a new LHC sequence might be made available by the layout database experts before the end of the year, which will be called V6.503.
V6.502 was planned to be a sequence with only some of the collimators in place. Due to the cancellation of the 450 GeV engineering run in 2007, it is now expected that all Phase 1 collimators will be installed in the LHC ring for the startup, which makes V6.502 obsolete.
TR recently completed the implementation of solenoids for V6.501.
The new V6.503 LHC sequence will include minimum and maximum strength values known to us (via the layout database or the MEB activity - non-conforming MQTLs) as additional magnet properties. The same will be for the polarity flag and, possibly, the cryostat name. This latter piece of information will be required to generate automatically the PTC on-line model. A sub-class of elements will be added to allow switching on closed orbit correctors without enabling also physical elements declared as kicker in the sequence, which will not be used for closed orbit correction.
More work is needed on the spool pieces and, in general, for the nonlinear, nested correctors. For the latter a change in the naming might be useful, requiring to update the script written by SF for the setting of the nonlinear correctors in the triplets. The proposal by Thys is to have zero-length magnet in the sequence, possibly located at the same longitudinal position (a potential issue with the aperture/plotting module should be considered in details). This would have the draw back of increasing the overall number of elements in the MAD-X sequence. JJ proposed to allow for separate min/max values for all multipole components and to combine zero-length elements into a single multipolar element. Thys will verify also the efforts required at the level of the layout database to implement these changes and the final proposal from us will be discussed at the next meeting.
FS reported that several issues with the aperture calculation and plotting in Mad-X have recently been found by IA and mostly fixed.

Influence of missing closed orbit correctors and monitors on LHC orbit -> WH
Postponed to next meeting.

AOB
None

Last update: 04-December-07

MG & HB

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