Summary notes of the LCU meeting on 4/11/2008
Present: IA, MA, RA, GB, HB, RC, SF, MG, NH, JJ, JM, JN, TO, FR, FS, RT, TW, SW, SY
Report from meetings
Cryogenic collimator optics for LHC IR7 -> JJ (pdf)
As discussed in the meeting on the 15/07/08, cryogenic collimators
are expected to improve significantly the collimation efficiency, both for proton and ion operation. Installation of cryogenic collimators is proposed close to
Q8 and Q10 in IR7. Several elements in the dispersion suppressor would have to move.
Due to the displacement of the dipoles to allow installing the two collimators,
the ring geometry is changed and the machine is no more closed. The sequence was
adapted to restore closure (with a slightly shorter circumference). Furthermore,
it was possible to rematch beta functions and dispersion also in this case.
A complete set of files was generated that should be used for new numerical
simulations with ICOSIM (the previous campaign of simulations was performed with
a mismatched optics, about 20% beta-beating).
RA stated these studies are ongoing and elements may still move a bit. It is planned to arrive at a definite proposal by January 2009 before a collimation upgrade workshop planned for mid February.
Uniqueness of the aperture description by RECTELLIPSEs -> JJ (pdf)
JJ pointed out that the current definition and use of RECTELLIPSE apertures in MAD-X is not unique:
the same physical aperture corresponds to infinitely many sets of four numbers
[a, b, c, d] describing the RECTELLIPSE. Using Mathematica he proposed a way to
make the definition unique.
JJ pointed out that in the new file with the vacuum markers a number of different descriptions correspond to the same physical shape. MG asked to provide a list of elements with aperture corresponding to the same shape: such information will be transmitted to the database experts to minimize the number of different aperture types used.
Optimisation of collapsing time of separation bumps -> SW (pdf)
Colliding beams with small offsets can result in emittance growth. SW showed that the time needed to collapse the separation bumps to get beams into collision for the nominal physics optics can be as long as 40s.
This is due to the performance of the power converters of the dipolar correctors
as well as the QPS system. During the hardware commissioning and the cold check
out stages measurements were carried out to quantify the actual speed.
Furthermore, potential improvements were also quantified.
The time for collapsing the separation bump can be reduced by redistribution of the strengths among the correctors used to produce the separation.
It is clear that the actual time will depend not only on the performance of the
hardware, but also on the value of beta*. Even with such optimization, the
collapsing time will be of several tens of seconds. Hence, it seems necessary to
ask for the best possible performance for the power converters.
Following the discussion, it is agreed that the the separation
bumps at injection will not be changed with respect to the current configuration
(apart from the modification in IR8 to close the bump between Q5, which will
have the beneficial effect of decoupling the injection process from the setting
of the separation bump). At top energy (or during the ramp) the configuration of
the bump will be changed to the optimized one for collision.
The next step will consist of simulating the emittance growth for the optimized
separation bump.
A by-product of this study is a faster stepping in beam separation scans as planned for luminosity optimization and absolute luminosity calibration.
Coupling correction for the machine "as-built" -> FR (pdf)
FR studied linear machine coupling compensation for the LHC as-built, i.e. using measured a2 components
for all classes of magnets. He uses the correction routines provided by SF. The first step is to correct locally each arc, using two couples of MQS. As a second step, a global correction is applied. The remaining beta and dispersion beat are very small.
The impact on c± was also evaluated. The study was done at 450 GeV
and several seeds were considered. The effect of roll angle on the alignment of
quadrupoles was found small and additional cross checks should be performed to
ensure the correctness of the results. Feed down from sextupoles will be also
included in a second stage. Two interesting points:
An improved (faster) methods to assign the magnetic errors was tried (suggested
by WH).
KQS should be reversed for Beam 2: a problem linked with the bv
flag? This point is under investigation.
The impact of the missing skew quadrupole correctors sector 3-4 due to the LHC incident was analyzed. The study should be pursued by:
- using peak quantities in the analysis
- try using all possible correctors (i.e. MQSX) to correct the
coupling
All this will be implemented in the on-line model (PTC part).
Update for the new optics for V6.503 -> MA (pdf)
MA reported about studies to check and finalize the LHC optics V6.503, which features magic pi/2 phase advances between IP5 to IP1 in H and IP1-5 in V. The phase advance is now the same as
for V6.4 after the drifts in the V6.5 (and following) versions.
SF commented that the choice of the phase advance between the
experimental insertions 1 and 5 is dictated by the minimization of the
off-momentum beta-beating in the betatron collimation insertion (as it was the
conclusion when the situation for the Phase 1 upgrade
studies was considered).
With the new phase advance the beta-beating will be minimized in half of the
machine and maximized in the remaining part. SF
suggests to change the phase advance per cell to minimize the off-momentum
beta-beating in both planes between IP5 and IP1. MA will follow
this up.
It is worth stressing that the proposed solution was in any case optimized in
terms of aperture and the phase split between Beam 1 and Beam 2 was removed.
Some work remains to be done on the pre-squeeze and squeeze for IR2 (Beam2) and
IR8 (Beam 1). This is a very good starting point for a further optimization.
AOB
None
Last update: 04-November-08
MG & HB
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