Minutes of the ABP-LCE team meeting on 29.08.03
present: TdA, EM, FR, EV, LV, FZ
excused: DS
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(1) Stability with Space Charge and Octupoles (EM)
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EM pointed out that in his paper on the PS instability
threshold with octupoles, the space charge contribution
was neglected both in the tune spread and in the impedance
part. If he were to include these two components, the
beam would always be predicted to be stable, in contrast
to observations.
He next reviewed the classical paper by D. Moehl, who for
the 1-dimensional case showed that, depending on the
sign of the octupoles, space charge may change the
instability threshold for a coasting beam by factors
of 2 or 0.67, under certain general assumptions.
For the 2-dimensional case, only simulations were
presented in the same paper.
FR regretted that we have no validated theory for space
charge and octupoles in the LHC.
FZ mentioned that in LHC also the beam-beam interaction
could greatly influence the instability thresholds, as
it is known to be the case for either B factory.
FR asked for a complete assessment of tune shifts
and tune spreads in the LHC at injection.
(2) LCE Contribution to the LHC Design Report (FR)
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The bulk of the design report is finished and FR
presented it to the team. The sections on beam-beam
and electron cloud are complete, and the impedance
section is handled by cut-and-paste from earlier
reports by LV and others.
FR pointed out that the logics of the impedance part
is not yet clear, however. In response, LV summarized
the strategy that was adopted as follows: the high
low-frequency transverse impedance (mainly resistive
wall requires a transverse damper. This damper can
deal with instabilities due to other sources (pumping
holes, shielded bellows, sawtooth, collimators).
Only the emittance preservation was a concern
transversely. In the longitudinal plane there is no
feedback. The loss of Landau damping occurs at
Z/n=0.2 Ohm. Accordingly the goal of the impedance
budget was to stay below 0.1 Ohm.
Responding to a question by FZ on the role of higher
order modes. FR and LV said that most of these had been
treated in a paper by D. Angal-Kalinin and LV.
One of FR's key questions is which nonlinearity is
needed to damp higher-order head-tail or any other
single bunch modes.
LV describes that the collimator impedance is not
additive to the other impedance, since the frequency
dependence is quite different. FR remarks that the
tune shifts are additive.
The further plan is the following. FR will work full
time through the weekend and then send a final draft
to LV on Sunday afternoon. Negotations with John Poole
will likely be needed.
Two impedance tables should be included,
one referring to injection the other to collision.
Also needed is a table of tune shifts for the
different modes, or, best, a plot of their
location on the stability diagram.
The ZBASE cannot be used for this purpose, as the
underlying impedance numbers are out of date.
LV summarizes the key question: we know that LHC is
stable with feedback; the only remaining question
is whether it can be stabilized at top energy without
feedback, but using octupoles.
For most elements the transverse impedance can be
obtained by scaling from the longitudinal plane.
The design report also comprises a section on
parasitic losses. LV states that all contributions
are small compared with the collimators, for example,
the beam screen contributes some 1 MOhm/m.
FR reminds the team that heating could be important
for indivudal elements such as bellows, collimators,
and kickers. To estimate these, the resistive part
of the impedance is needed for the various
elements. This will be provided by LV.
(3) Other Items
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There was a pending action item, whether the ALS
could measure the photon backscattering as part
of the US LHC contributions. FZ has not
yet contacted M. Furman. Recently, Noel Hilleret
recalled that similar results by R. Cimino may
already have been published at this year's Berlin
conference. He is investigating this possibility.
LV shows the first page of his report SPS-87-4 (MAS)
where he found the same factor of 2 as D. Moehl.
Since predictions using this factor gave excellent
agreement with observations both in the PS and in
the SPS collider, he urges a re-assessment.
EM then explains how the factor of 2 came about
in D. Moehl's paper, and why it appears to be wrong
(a confusion between amplitude and rms amplitude).