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Minutes of the ABP-RLC team meeting of 03.03.2006
present: EB, RM, FR, RT, FZ
web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/
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(1) LHC HEADTAIL simulations with stripes and dipole field (EB)
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EB presented results of HEADTAIL emittance growth simulations
for the LHC as a function of electron density. She considered
both field-free regions and dipole fields, in the latter
cases taking a stripe-like electron distribution with parameters
obtained by fitting to SPS strip detector data.
Chromaticity Q' was set to 2, and the beam was round.
The simulation results for both field-free regions and dipoles
are similar, with a similar threshold for the vertical fast
instability. Only with a dipole the horizontal instability
is somewhat suppressed above the threshold, and the
incoherent growth below the threshold is a little lower
in both planes. The latter can be explained by the fact that
in a dipole the pinch occurs only in the vertical direction.
The instability threshold is at about 4e11 e- per cubic meter.
Growth rates above are of order 10000% per second, the
growth rates just below the threshold about 10% per second.
At ten times smaller density, the growth has a magnitude
of less than 1% per second. The chamber size used in these
simulations was 20 times the rms beam size.
FZ pointed out that the numbers for the incoherent
emittance growth likely represent an upper bound, as only
10 IPs are taken per turn. More precise numbers are expected
from Giuliano Franchetti's code. However, Giuliano first needs
to implement the stripes and the dipole field. Also, EB
observes little effect of chromaticity, whereas, for a field-
free case without stripes, GF saw that larger chromaticity
increases the incoherent emittance growth.
FR asked about the dependence on the synchrotron frequency.
The incoherent growth is expected to become smaller for lower
synchrotron tune. Without synchrotron motion the growth
stops. FR suggested studying the incoherent growth for
a barrier rf bucket.
(2) Rating and magnet aperture for the LHC IR upgrade (RT)
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RT summarized the consensus on the aperture calculation for
LHC IR upgrades, which has emerged from a meeting with
FR, RT, FZ, Jean-Pierre Koutchouk, and Walter Scandale.
There are a few open questions, such as the scaling of
the closed orbit error with beta*, the value of the
momentum range to be considered (at the moment 8 times
the longitudinal sigma, which is used for chromaticity
measurements), and the impact of spurious dispersion.
FZ emphasized that the spurious dispersion from the
other IP or from the arcs does not compromise the
local chromaticity correction, as long as the
geometric sextupole aberration of the chromaticity
sextupoles are properly corrected by other compensating
sextupoles.
FZ also suggested to check whether in the present
LHC optics the outgoing beam is closer to the wall.
If this were the case, one could envision to invert
the polarity of the final triplets for the IP(s)
with horizontal crossing.
FR mentioned that the tolerance on the vertex
position is +/- 3 mm in total, and that we can move
the IP by about a mm without running into problems.
RT summarized the ongoing work, which includes
the implementation of JPK's notebook, the need of
full ring lattice descriptions, the role of the
spurious dispersion for the local chromatic
correction, and the aperture model based on
a python script external to MAD.
=> ACTION => Develop a lattice for crab cavity option [RT, FZ]
(3) Two-beam resistive-wall wake field (FZ)
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FZ has studied the resistive-wall wake field for two
beams propagating in opposite direction. He found that
the Panofsky-Wenzel theorem no longer holds.
Unlike for the 1-beam wake field the magnetic field
and the electric field contribute to the transverse
wake field. Considering only the long-range electro-
magnetic fields computed by Chao yields a result
which diverges at short distances. FZ then considered
two different approximations for the complex Fourier
transform of the Lorentz force, obtained by looking
at the limiting cases above and below cutoff.
Above pipe cut off, he found two poles and zero
contributions from integration over the branch cut
of the k plane. this gives an exponentially
decaying real wake, which does not have the
correct long-range behavior. Below the cutoff
he found that there was a single pole only
on the second Riemann sheet (hence not
contributing) and the wake field followed from
the nonzero integral over the branch cut.
The long-range wake is consistent with the one
originally expected from Chao's long-range solution,
but without any singularity at short distances.
The long-range wake turns out to be of exactly
the same form as the conventional, only with
opposite sign and the need for integrating
over z.
(4) Impedance localization for the SPS - new fitting results (FZ)
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FZ followed up two outstanding actions from the APC
related to the localization of the SPS transverse
impedance. He fitted to local impedance sources located
either at all quadrupoles, a selected subset thereof,
or at all kickers and rf elements. The fit quality
was similar in all cases, with a total impedance of
about 30 MOhm/m. The solution is not clearly determined,
but 5 or 6 conspicuous locations were identified,
including the MKE and MKP kicker areas.
FZ stressed that the exact and unambiguous identification
of the impedance sources is only possible with an
accurate SPS optics model.
FZ obtained from ES various literature and data on
the lowest transverse mode of the 200- and 800-MHz
cavities. The broadband impedance he computed from these
single mode data alone is negligible compared with the
total SPS broadband impedance. More information
on the cavity broadband impedance would be needed
before it can have an effect in the fit.
In addition, there is an uncertainty in the definition
of the transverse impedance.
(5) Pros and cons of 75 and 25 ns spacing (FZ)
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FZ and GA have prepared a number of slides comparing advantages
and drawbacks of 75-ns compared with 25-ns spacing, for the
next LHC commissioner meeting. The advantages include
less long-range beam-beam effects, less electron cloud,
and higher luminosity for the same current, the disadvantages
a larger event pile up, more commissioning steps, and a delay
in discovering possible electron cloud limitations.
(6) AOB: definition of transverse resonator impedance (FZ)
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FZ discussed an apparent inconsistency in the definition
of a transverse resonator impedance, with the rf
group, the old ZBase code, and possibly Bruno Zotter
using one definition, while Chao and Ruggiero
use another one, with a different dependence on
frequency.
=> ACTION => FZ will check the definition with
J. Tuckmantel and perhaps B. Zotter.
Posted on the web: Slides by EB, RT (soon), and FZ
Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/