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Minutes of the ABP-RLC team meeting of 28.10.2005
present: EB, UD, SF, AG, WH, EM, TP, FR, GR, RT, DS
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) Minutes of last meeting, pending actions, announcements
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The minutes of the RLC meeting of 21.10.2005 are approved
with some changes concerning the presentation by EM on the
TMCI instability (effect of chromaticity on the traveling
wave pattern).
The revised list of pending actions is posted at
http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm.
(2) Flat beams for nominal LHC and LHC upgrade (SF)
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Following the LHC upgrade scenarios discussed in Arcidosso,
SF presented his study on luminosity gain and best use of the
triplet aperture achievable using flat beams.
Contrary to the symmetric doublet solution explored by J. Johnstone,
requiring two separate magnetic channels for the two beams, SF has
obtained an anti-symmetric optics using the nominal LHC harware.
Definition of flat beam in a machine like LHC: the emittances
in the two transverse planes are equal, the aspect ratio of the beta*'s
(which we define as 'r') is different from unity. Using flat beams
allows a reduction of the crossing angle, which translates into a
reduction of the geometric loss factor, and hence into a luminosity gain.
As the crossing plane is always the plane where the beam is larger at the
IP, the beam will also be smaller in the crossing plane at the triplet,
which means that the triplet aperture can be decreased. The flat beams
have an impact on the beam screen orientation, which should be opposite
compared to the case of round colliding beams.
For the nominal LHC with round colliding beams, the beam separation
in the triplets ranges from 9.5 to about 7 sigma_max, where sigma_max is
the maximum beam size: indeed the beams are not round in the triplet.
WH has provided plots of the relative beam separation which are posted
on the web. This should help clarifying the aperture requirements for the
IR magnets of the LHC luminosity upgrade. The most stringent requirement
is for Q2, where the separation is about 7.5 sigma_max.
SF has studied several aspect ratios for the flat beam option:
r=1: case of round beams, the crossing angle is 285 murad,
luminosity is nominal.
r=2: the optics is fine, beta_max is doubled in one plane (y for ex., IR1),
halved in the other plane (x for ex., IR1) . The crossing angle,
which scales like 1/sqrt(r) will be 285*sqrt(0.5)=201 murad.
From the aperture plots we see that the beam screen is orientated
in the plane in which we separate. The luminosity gain is 1.13.
r=1.6: the crossing angle is 225 murad, the luminosity gain is 1.10 while
leaving an aperture margin deltan1=0.5 in the triplet (better
matching between the beam and the beam-screen aspect ratio) and
therefore allowing for a further reduction of beta_y to gain luminosity
(see next case).
r=1.7: achieved by only reducing beta*_y by 15% and keeping beta*_x constant
like in the previous case (we are considering IR1 as an example),
crossing angle obviously does not change (225 murad) but the
luminosity gain is 1.18, therefore close to 20% just due to the
reduction of beta*_y.
All the schemes considered are viable for nominal LHC hardware.
With respect to crab-cavities or decreased bunch length, the solution
of flat beams does not require hardware modifications and leads to gains
in luminosity and aperture.
Open issues or subjects for future studies are:
- chromatic correction of Q' and Q''
- parasitic tune shift: Delta Q_par is proportional to r-1/r and
compensation is only partially granted by the H/V scheme for flat beams
- for H/V crossings the head-on tune shift is independent of r
FR asks about the possible effect of the experimental solenoids,
leading to a rotation of the betatron planes and to a possible luminosity
loss with flat beams. SF thinks this effect is negligible.
The MAD-X file of the IR optics for flat beams will be posted soon on the
CARE-HHH web site. SF will prepare thin lens versions for H/H, V/V and H/V
scheme within a few weeks, for further robustness beam-beam studies.
(3) Longitudinal and transverse impedance of the TCDD (AG)
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The TCDD is a kind of collimator-absorber. The structure is rich of sharp
transitions, expected to be bad in terms of impedance (both contribution
to the Broad Band component and to narrow band resonances associated
with trapped modes).
There are two geometries for the TCDD, open and closed.
Tapering of the movable structure is not straightforward,
for fixed structure it can be easily done.
The impedance calculated with GdFdl shows:
- longitudinal, low frequency (to give an estimation of the contribution
of this object to the BB impedance), Z/n=13.5 mOhm (closed)
and Z/n=10.1 mOhm (open). These values seem large, this is probably due
to two sharp steps that need therefore to be tapered.
- longitudinal, full spectrum (identifies resonance structures),
there are two main trapped modes:
(#1) f=38.5 MHz, Q=6320, R/Q=157.5 LinacOhm k=19.4 V/nC.
The corresponding power loss is very high for LHC: ~400kW!
(#2) f=648 MHz, Q=18400, R/Q=21 LinacOhm, k=21.6 V/nC.
The corresponding power loss is ~44 kW.
- transverse, low frequency (contribution to BB impedance),
it is evaluated to be 12 kOhm/m, which is very small with respect
to the foreseen LHC budget ~2 MOhm/m.
- transverse, full spectrum, there is one main trapped mode:
(#1) f=54.8 MHz, Q=4870, R_t=49.2 MOhm/m.
The tune shift induced by this impedance has been evaluated
(both from single and multi-bunch formulae), as well as multi-bunch
instability growth times. Recommendations:
- To avoid the transverse mode (#1), rf contacts are needed
between upper and lower parts of the absorber.
- To avoid longitudinal mode (#1), rf contacts between pipe and absorber
are needed on either side.
- To avoid high contribution to the BB impedance, tapering with 15 deg
is strongly recommended.
(4) Impedance of the tertiary collimator "sausage" chamber (EM)
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The "sausage" is a transition chamber round->round->elliptical with
the following diameter (axis) changes (80mmx80mm)->(212mmx212mm)->(180mmx70mm).
The central cylindrical piece is 1500 mm long.
Alternative: hand-made elliptical tube of 180mmx70mm.
The "sausage" certainly creates a resonator, the frequency is given by
(for first quick estimation): f [GHz]=10/b [cm].
The alternative does not trap modes.
HFSS has been used to estimate trapped modes:
f_r1=1.0857 GHz, Q~7 104, strength to be evaluated.
FR says it is advisable to avoid the "sausage" because we do not want
more trapped modes, the argument "short in time" is not very strong.
(5) Impressions/comments on the recent CAS school (TP, UD, AG)
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The CAS took place close to Trieste in the first two weeks of October.
Some lectures were very tough, especially those on collective effects
and Landau damping, there were some organizational problems, the visit
of the lab (ELETTRA) was not very exciting because the machine was
in operation and things were only shown through posters. Nevertheless,
it was fun to see an ELETTRA rf cavity that can be fine-tuned by heating.
The afternoon courses of diagnostics & instrumentation was very interesting,
WH said that his course on beam optics and use of MAD-X (which could not be
possibly commented by UD, as WH was present in the meeting) suffered from
the short time given to it (10h, FR says that should be enough though).
AOB
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EM showed the results of measurements of the impedance of PS kickers
(with 1 wire in the center for the longitudinal, with 1 wire displaced
in 5 positions for the transverse). The horizontal impedance shows
a negative real part.
A. Burov has said that this could be due to the technique of measurement:
when the measurement is done with 1 wire, one measures the difference
between dipole and quadrupole modes. People from the team expressed
some doubts on this statement.
GR is presently doing the study of TMCI threshold for increased
injection energy in the SPS. Simulations are running, but he had
to face last week several problems in setting up the simulation tools
(adaptations to the update of the operating system) and getting the right
to run jobs on the NAP LSF cluster. Even if he seems to have been
incorporated now among the users whose jobs are by default submitted to
these privileged queues, most of the jobs from the list submitted yesterday
are still pending in the batch queues. The slap-support team hasn't replied
to this issue.
FR reported shortly about beam-beam simulations by Ji Quiang at Berkeley,
in the framework of US-LARP (http://larp.fnal.gov/IR2005/talks/17/Qiang.ppt).
The estimated emittance growth of about 10^-5 from long range collisions is
probably dominated by numeric noise. FR and WH recommended to study
emittance increase effects with small head-on beam-beam offsets, say 10-30%
of a sigma.
Posted on the web: slides by SF, WH, AG, and EM.
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/