Minutes of the ABP-LCE team meeting of 07.05.04
present: AK, WH, FJ, EM, FR, EV, WW (partially), FZ
excused: EB, DS
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(1) Minutes & Follow-up of pending actions
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FZ asked for the conclusion on the correct formula
of the synchronous phase shift from the last meeting,
discussed by EM. FR responded that he thinks only the
formula from A. Hofmann is correct.
Concerning the 370% margin for the damper with a slightly
reduced number of bunches in the CNGS beam, EV clarified that
this number refers to the inverse ratio of the damping time
to the time interval between the 1st and second extraction.
ACTION => EB will extrapolate HEADTAIL results for 30 min operation
and extend the simulations to SPS conditions
(with feedback and dipole field) -> partly done.
ACTION => EM will update the LHC collimators impedance according to
the latest layout provided by R. Assmann. -> still active?
Other actions are mentioned in the individual reports below.
(2) Beam-Beam Studies & Visit by Fred Jones (FJ)
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FJ is visiting AB/ABP until 20 May. He is working with
WH on almost-3D self-consistent strong-strong beam-beam
simulations. Currently the program runs on 10 processors
and turn-around-time is 1-2 days. The code is only almost
3D since longitudinally a slicing is applied.
The longitudinal field is not fully self-consistent yet.
Additional computing resources have become available
at UBC and Alberta.
In addition FJ has a long-standing interest in space-charge
simulations for the PS booster, where his contact is
M. Martini.
FR pointed out that Lihui Jin in collaboration with
Jack Shi recently published a paper on beam-beam
simulations in Phys. Rev. E 69, 036503 (2004), and that
there is a paper on the Tevatron beam-beam effects
in this month's PRST-AB issue. FZ mentioned a similar
program development by Andreas Kabel at SLAC, which
could serve for comparisons and cross checks.
He referred to A. Kabel's and other talks on the web site
of the FNAL beam-beam meeting in March 2004:
http://www-ap.fnal.gov/~tsen/TEV/beambeam_march04.html
(3) Update on Collimator Res.-Wall Simulations (AK)
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OLD ACTION: perform full simulation for the SPS
AK used his program to compute tune shifts and rise
times from resistive wall, for the bare SPS machine
and with an additional collimator closed to a half
gap of 2 mm. He considered parameters for the SPS
collimator test (288 bunches in 4 batches at 270 GeV/c),
and compared results for with and without inductive
bypass.
Without inductive bypass the collimator reduces the
growth time from about 410 turns to 116 turns, with
inductive bypass the growth rate is hardly unchanged.
With inductive bypass the collimator creates a
tune shift of 5e-4, without the bypass the tune shift
is 1e-3. Especially the growth rate is sensitive
to the impedance model, and the difference in the
tune shift could be visible.
FZ asked how the growth rate for 288 bunches would
be measured and whether this has ever been tried?
FR suggested to use the feedback for this type
of measurement, and possible input from EV.
ACTION => Clarify why B. Zotter's results are an order of
magnitude larger than the results from Vos,
Burov&Lebedev
BZ will give a presentation in one of the next LCE meetings.
(4) Update on LHC Impedance (EM)
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EM discussed impedance calculations for 3 items:
(1) beam screen, (2) warm pipe, (3) TCDS
For the beam screen a 3 layer model is used consisting
of 50 micron Cu, 1 mm Stainless Steel, and vacuum.
The magnetoresistance of Cu is taken into account.
The computed effective impedances agree with the results
of Luc Vos in the LHC design report to within 10 or 20%,
the only exception being the imaginary part at top energy,
where EM's value is almost twice as large. Also the
heat load at top energy agrees with LV's number.
FR suggested to consider 75 micron Cu layer (and drop
the RRR to the correct value) and to include the cold
bore on the outside of the pipe.
Also for the warm pipe, LV's values were confirmed.
The TCDS is 6 m long. Its first part is made from
carbon and carbon composites, which is followed
by the isolator AlN, and then by blocks of Ti
and Stainless Steel etc. EM assumed that only the
AlN will be coated with a Cu layer of 5 micron.
He finds that the impedance is about 1% of the total
LHC impedance and the heating 185 W, whereas LV had
calculated 260 W and 1 kW cooling is foreseen.
FZ asked if the resistivity of the carbon changes
with temperature. (This question would also apply
to the collimators.)
The impedance values that EM calculated differ
from the usually defined impedances by a factor
sb/(2 sqrt(pi) sigmaZ) which is 16 at injection and
28 at top energy.
(5) Update on RF AM and Transverse Feedback (EV)
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EV discussed the bunch length variation as a
result of AM modulation observed in the SPS
experiments. The bunch length change measured
from the start to the end of a batch was
6% in good agreement with theoretical
expectation. After reaching top energy the
difference dropped to 3-4%. This drop occurred
at about the time when the rf voltage was
raised to match the beam for (future) extraction
into the LHC.
Also, the absolute value of the bunch length
(without modulation) appears to increase near
top energy compared with the expectation,
presumably as a result of collective instabilities.
EV added 12-bit feedback quantization to his
modeling code. The quantization noise has a
small impact on the short-term emittance
growth when damping an oscillation.
FR and FZ suggested that the quantization
noise could lead to long-term emittance
growth. EV, FR and FZ mentioned literature
by Daniel Boussard and Luc Vos on this
topic. Possibly a model for the long-term orbit
motion should be part of the long-term assessment.
(6) BBLR Impedance (FZ)
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The BBLR assembly is nearing completion. The first
new device will be installed in LSS5 on May 26.
An impedance estimate had earlier been suggested
by G. Arduini. DS agreed to such calculation, but
no result is available yet.
FR informed the team that following an agreement
with the CLIC Steering Committee, in the future all
Gdfidl simulations required for LHC will be done
by E. Jensen and his coworkers in the RF group.
(7) Follow up from ECLOUD04 and Future Plans (FZ)
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FZ presented some highlights and inspirations from
the ECLOUD'04 workshops. There are lots of new
ideas related to upgrades and better understanding
of the codes ECLOUD and HEADTAIL. See the attached
power point file for details. Two highlight
presentations were the calculation of the electron
cloud build up using fitted 3rd order polynominal
maps and the search for phase transitions by U.
Iriso, and the theoretical and experimental study
of grooved surfaces with reduced secondary emission
yield by G. Stupakov and M. Pivi at SLAC.
(8) Report from PEP-II MAC (FZ)
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FZ summarized presentations by S. Ecklund,
M. Sullivan, A. Novoktahsky and A. Kulikov
on HOM heating and electron cloud. HOM heating
is somewhat higher than expected and a strong
limit to present operation. Bellows and pumps
are particularly affected. NEG pumps were found
to be heated up to almost 1000 degree F, by
HOM power entering through the pumping screens.
The HOM power increases linearly with rf voltage.
One of the suscpected sources are collimators.
Crosstalk between the two beams was observed.
Electron cloud is not a limitation at the
moment, after doubling the strength of the
solenoids and changing the working point.
When swtiching off solenoids locally there
is still a large multipacting and a high electron
flux. The flux does not decrease with time
(no conditioning?). A field of a few Gauss
is sufficient to suppress the multipacting.
Again, see attached slides for details.
PEP-II luminosity per month was increased
by about 50% in 6 months. PEP-II is thus
on track for achieving the director's goal
of increasing the luminosity by a factor 3
every 18 months.
Attached: slides by AK, EM, and FZ