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Minutes of the ABP-LCE team meeting of 19.09.03
present: GA, TdA, EB, EM, FR (secretary), EV, DS, LV, FZ
excused: AK, WH
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(1) Minutes of last meeting approved.
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(2) LHC stability at injection: octupoles + space charge (EM)
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New analytic results were presented by EM (see enclosed slides)
for the stability diagram with space charge and octupoles.
The space charge detuning with x-y betatron oscillation amplitude
for pseudo-parabolic phase space distribution and elliptic beam
can be treated (FR thinks approximated) as an octupolar term.
The resulting stability diagram is dominated by the tune spread
of the octupoles for imaginary tune shifts and by the space charge
detuning for real tune shifts. It coincides with the Berg-Ruggiero
stability diagram in the absence of space charge and shrinks to
zero (i.e. no stability region) when the octupolar strength goes
to zero, in agreement with earlier predictions by D. Mohl.
ACTION -> EM will further check the space charge detuning.
(3) Decoherence and emittance growth at injection in the LHC (FR)
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FR has added a section on emittance growth caused by decoherence
and resistive wall instability to the LCE chapter for the new LHC
design report (the latest version can be found in the public NICE
directory of frgo). The question whether space charge can cause
decoherence in addition to the lattice nonlinearities seems to
be a direct consequence of EM's results: i.e. the decoherence
should be dominated by the octupolar nonlinearities. However LV
reports that in the SPS p-pbar collider about 3/4 of the space
charge tune spread gave rise to decoherence, even for a very linear
lattice. Discussion to be followed-up.
(4) SPS kicker impedance measurements by orbit bumps (GA)
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GA proposes to measure the SPS kicker impedance by the non-closure
of local orbit bumps. Using Seidel's formula for the beam deflection
by a collimator and inserting the kicker resistivity rho~10^5 Ohm.m,
he gets huge deflection angles and beam displacements of about 1 cm.
LV says that Seidel's formula can not be applied since the complex
permeability of the ferrite plays an important role: this issue is
discussed in a forthcoming report on the SPS inductive impedance.
LV suggests to measure the kicker impedance by the coherent tune shift
or possibly by the local phase advance versus beam intensity.
(5) Other possible SPS machine studies
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RA proposes to install an LHC collimator prototype in the SPS in 2004
to measure impedance, heating, robustness, effect of coatings, etc.
EM suggests to measure stability of single LHC bunches with reduced
longitudinal emittance (0.18 instead of 0.35 eV.s). Assuming a broad
band transverse impedance with R_shunt=20 MOhm/m, Q~1, and f_r=1.3 GHz,
the TMCI threshold is 8.8x10^10 i.e. below nominal LHC intensity.
LV points out that the present value of the shunt impedance is rather
10 MOhm/m and thus I_th~1.7x10^11 is close to ultimate LHC intensity.
According to LV, the SPS impedance will roughly double when all the
foreseen extraction kickers will be installed in 2005.
(6) Future work on ZBASE (FR)
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The LCE team will have to share the workload to set-up an updated
LHC impedance data base using ZBASE. The impedance of each major item
should be (and in most cases has been) computed analytically,
numerically and measured in the lab. The existing data has to be i)
collected,
ii) documented when appropriate, and iii) included in ZBASE with a
suitable interpolation in case of narrow-band impedance. In particular
we should be able to plot the TOTAL LHC impedance as a function of
frequency.
FR suggests to ask Deepa Angal to send us her recent ZAP input files
with LHC data.
ACTION -> FR will suggest a possible LCE team sharing of the workload.
(7) Incoherent vs coherent tune shifts due to collimators
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FR raises the question whether the tune shift induced by the collimator
impedance in the non-collimation plane plays any role in the stability
diagram. The real part of this tune shift is compensated by the
incoherent (quadrupole like) tune shift as in the case of space charge
and a possible conclusion is that only the imaginary tune shift in the
non-collimation plane plays a role: we need to decide a consistent
approach for space charge and collimator impedance.
Discussion to be followed-up.
--
E-mail: Francesco.Ruggiero@cern.ch, Location: Building 9/1-008
Address: CERN, A&B Division, CH-1211 Geneva 23, Switzerland
Telephone: +41 (22) 767 3726 or 767 5272, TeleFax: +41 (22) 783 0552
WWW: http://wwwslap.cern.ch/~rgo