-----------------------------------------------------------------------
Minutes of the ABP-RLC section meeting of 04.02.05
present: EB, RC, AG, WH, EM, FR, DS, EV, FZ
-----------------------------------------------------------------------
(1) Minutes of last meeting, pending actions
---------------------------------------------
FZ reported that two corrections to the previous minutes had been sent by EV and Karl-Heinz Schindl (on the PSB (not PS) sieve). The minutes on the web are already corrected and augmented.
OLD ACTION: contact Pierre Strubin and Miguel Jimenez for flange impedance (FZ)
DONE: According to M.Jimenez each ring contains about 1000 flanges, overwhelmingly in the long straight sections, and predominantly with a pipe diameter of 80 mm. These numbers translate into a Z/n ~ 6.23 mOhm, about 9% of the total broadband Z/n. Using the Schnell-Sacherer relation, the transverse impedance is estimated as 38 kOhm/m, about 4% of the total. However, the beta functions at the positions of the flanges could be significantly larger than the average beta function.
AG remarked that trapped modes probably are not an issue, as the lowest frequencies would be order 6 GHz.
FR pointed out that the LHC impedance should be small enough to support an upgrade (e.g., higher intensity, half the bunch length).
=> ACTION => Decision if any action for flanges should be taken (FR).
OLD ACTION: inform collimation team of revised heat load (FR)
DONE.
OLD ACTION: WH will provide Yannis Papaphilippou with a first test file.
DONE. MADX input file was sent.
=> ACTION => compare the size of the longitudinal geometric wake with RW longitudinal wake from A. Koschik (FR)
PENDING.
OLD ACTION: FR will contact Stefania Petracca to see whether their theory can be applied to LHC collimator
ABANDONED.
(2) Progress report on Beam-Beam studies (WH)
---------------------------------------------
WH is working, together with TP, on multi-bunch beam-beam simulations. They are presently converting a rigid-bunch model to a multi-particle model. Next week, WH will report on the original motivation and the rigid-bunch scheme. In the following week, TP will present the modifications and new results.
Studies are also being performed for assessing the effect of linear optics imperfections on the dynamic aperture with beam beam. The goal is to derive tolerances on the required orbit and beta beating quality. WH can already say that the effect of the imperfections is significant. A further goal is to define a correction strategy, as, e.g., the orbit with beam-beam can, even in principle, not be corrected to zero.
Dobrin Kaltchev is studying different crossing schemes.
(3) General formula for the transverse resistive-wall impedance (EM)
--------------------------------------------------------------------
EM presented the general formula derived from B. Zotter's theory without making any approximation (i.e., without requiring k b / gamma << 1). For the collimator the same impedance is obtained as presented the last time, and as predicted by Burov-Lebedev. However, for the kicker impedance there is a difference of about 20% between the Zotter's theory and that of Burov. The frequency at which the real part of the impedance is maximum (~1.7 GHz) is the same for the two formalisms.
No Action at the moment. B. Zotter will make a presentation next week.
FZ suggested contacting A. Burov for his opinion.
FR remarked that the experimental data from the SPS suggest missing physics. e.g., the finite length of the elements is not accounted for in conventional theories.
FZ pointed out that a Gluckstern paper was published in PRST-AB last year. -> (see http://link.aps.org/abstract/PRSTAB/v7/e114401) This paper addresses the high frequency contribution of transition radiation arising from the discontinuity in conductivity for a cylindrical metal tube of finite length and finite resistivity attached at each end to perfect conductors of semi-infinite length. The effect of a finite wall thickness and the possible bypass of the image currents through alternative conduction paths does not seem to be included.
EM mentioned a draft paper by Zotter, and two draft papers by Zotter and Gluckstern, some of which describe the general case of finite length resistive elements.
(4) Follow-up of LHC collimator impedance and trapped modes (EM,DS)
-------------------------------------------------------------------
EM selected the 13 strongest modes from the table of AG presented in the last meeting and computed the coherent single-bunch tune shift due to the transverse trapped modes. He obtained a value of 1.1e-6 at 270 GeV in the SPS for the head-tail mode m=0. This is similar to the value computed by AG previously, though the assumed beam energy was 10 times different. In any case, the coherent tune shift is much smaller than that expected from resistive wall and the measured value, which are both of order 1e-4.
FR suggested that multibunch effects in the LHC should be computed next.
Indeed DS already estimated the LHC multibunch effect, by scaling from AG's table under pessimistic assumptions. Even so, he found that the wake effect was an order of magnitude smaller than what he had estimated before.
FR pointed out that the heating of the rf fingers at the collimators from beam loss and from higher-order modes may become comparable (of order 10 W).
FR, AG and DS suggested that damping of the higher-order modes by ferrites would be a good idea. This opinion is shared by Fritz Caspers. AG mentioned that space for the ferrite may be available at the flanges.
FR reminded the team that the impedance of other objects, such as absorbers, should not be forgotten in the impedance estimates. The dielectric constant of the LHC collimator material is not exactly known by the RLC team.
=> ACTION => clarify graphite dielectric constant with collimation team (EM,FR,DS?)
(5) Follow-up from HHH-2004 workshop, AB Seminar, and future HHH
activities/workshops (FR,FZ)
------------------------------------------------------------------
FR described the status of ongoing LHC upgrade studies and CARE-HHH networking activities. These studies consist of events (workshops) and work/results. The highlights of the HHH-2004 workshop were presented by W. Scandale and F. Zimmermann in an AB Seminar on 3 February.
An HHH mini-workshop on crystal collimation will be held 7-8 March at CERN, with a large number of Russian and some US participants. The goal is to explore the application of crystals for monitoring the distance of a collimator jaw from the beam and also their use as collimators themselves. The feasibility of an experiment in the SPS will be discussed specifically.
Two other events are in the planning: one is a workshop on the optimization of the low-energy injectors for an LHC upgrade, also keeping in mind demands by fixed-target and neutrino beam users; the second a workshop on LHC IR upgrade options (e.g., decision between quadrupole first and dipole first options, TAS+TAN for the upgrade, crab cavities, choice of crossing angle).
FR mentioned the idea of employing permanent magnets or iron-free s.c. quadrupoles deep inside the detector.
FZ showed the present status of the simulation code catalogue. Many different codes exist and RLC resources do not allow documenting and maintaining all these codes. In addition, copyright issues will need to be understood. It was suggested to select two or three codes from each category and determine both CERN linkmen and outside collaborators. A table with summary information should be developed and posted. User manuals, input examples, and benchmarking example should be maintained.
=> ACTION => decide on strategy, appoint CERN linkmen (FR)
(6) AOB
-------
EM attended a meeting on gamma jumps. He raised the question why the SPS does not employ a gamma jump. In FR's recollection, this option had been studied by Karel Cornelis who concluded that the gain was rather limited.
Attached: Slides by EM (2x)