From: Francesco Ruggiero
Sent: Thursday, May 06, 2004 8:03 PM
To: Bruno Zotter; Daniel Schulte; Elias Metral; Frank Schmidt; Frank
Zimmermann; Gianluigi Arduini; Helmut Burkhardt; Hiroshi Tsutsui; John
Jowett; rlg@physics.umd.edu; Stephane Fartoukh; Werner Herr; Maxim
Korostelev; Alexander Koschik; Eric D'Amico; Walter Wittmer; Gregory
Penn; lronen@techunix.technion.ac.il; Elena Benedetto; Elmar Vogel;
Javier Resta Lopez; Peder Eliasson; fwj@triumf.ca
Cc: Jean-Pierre Riunaud; Karlheinz Schindl; Louis Rinolfi; Michel
Martini; Oliver Bruning; Charles Hill
Subject: Minutes of LHC Collective Effects team meeting 23/04/2004
Minutes of the ABP-LCE team meeting of 23.04.04
present: EM, FR, EV
excused: EB, WH, AK, DS, FZ
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(1) Follow-up of pending actions
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=> perform full simulation for the SPS (AK) -> ongoing
ACTION => compare with analytic estimates for uniform bunch filling (AK)
A coasting beam model for 270 GeV gives 12000 turns growth time, while a simulation with 924 bunches and the same current yields 416 turns.
=> compare resistive wall impedance results and Mathematica notebooks
(FZ, EM, AK)
L. Vos and Burov-Lebedev model for the LHC collimators are now roughly consistent. It remains to clarify why B. Zotter's results are an order of magnitude larger.
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.
EM attended a meeting with W. Wetering et al about the impedance of the TCDS (Target Collimators Dump Septum) + TCDQ diluter blocks and the
need to copper coat them.
ACTION => EM will follow up TCDS+TCDQ impedance aspects.
(2) SPS loss factor vs effective impedance (EM)
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Following a discussion with E. Shaposhnikova about the correct way to measure the longitudinal loss factor at injection in the SPS by a measurement of the synchronous phase shift, EM confirmed a discrepancy in the definition of synchronous phase shift and incoherent synchrotron frequency shift given by several well known authors (see enclosed slides by EM). L. Laclare (CAS CERN 87-03) defines synchronous phase shift and incoherent synchrotron frequency shift in terms of the bunch line density Fourier transform. On the other hand, A. Hofmann (CAS CERN
95-06) defines these quantities in terms of the bunch power spectrum, i.e. the absolute value squared of the line density. B. Zotter (Handbook Chao-Tigner, 2nd printing, p. 118) starts using Laclare's definition and ends-up with Hofmann's result. Finally, Alex Chao (“Physics of Collective Beam Instabilities”, p. 291, Eq. (6.59)) agrees with Laclare and also with L. Palumbo (CAS CERN 95-06).
This issue is not academic and should be clarified with high priority to understand beam losses in the SPS. FR thinks that Hofmann's is the only physically sound result.
(3) Results for CNGS beam extraction and SPS feedback (EV)
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With the new measured MKE kicker pulse shape, the first few bunches of the second bunch train will receive a smaller kick, but the remaining pulse amplitude has a bump giving rise to a 12% residual kick (~70
micro-rad) for bunches further inside the second train. In the absence of feedback nearly half of the bunches would miss the CNGS target at the second extraction, while by properly adjusting the feedback all the bunches are sufficiently damped and stay within the target acceptance corresponding to 1% of the maximum kick. The optimal damper phase is insensitive at the ~1% level.
In case of longer gaps corresponding to bunch trains with a reduced number of bunches (2070 instead of 2100 bunches, i.e. a 1.4% reduction of the beam intensity) only 500 turns are needed to dump the residual oscillations of the second bunch train, corresponding to a ~tenfold increase of the emittance margin and to a ~tenfold reduction of the beam losses at the septum, assuming no particles in the gaps (see enclosed slides by EV).
(4) Dynamic heat load for a "warm" LHC dipole beam screen (FR)
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R. Saban has asked a question concerning the (quick-and-dirty) possibility of dis-connecting the cooling capillaries of a few dipole beam screens, in case of helium leaks detected during LHC magnet installation. Neglecting heat conduction through the beam screen supports or at the dipole ends, the dynamic heat load on the beam screen would then lead to an increase of the screen temperature until all the power deposited by the beam is radiated towards the cold bore. The beam induced heat load depends on the screen temperature via the resistivity of the copper coating, also affected by magneto-resistance and by the anomalous skin effect at sufficiently low temperatures and high frequencies. Therefore the equilibrium screen temperature has to be computed self-consistently. Preliminary estimates for nominal LHC beam intensity indicate a screen temperature of about 200 K at injection and 290 K at top energy, corresponding to ohmic losses between 0.22 W/m and 0.95 W/m, respectively. Such high screen temperatures depend on the small SS emissivity eps~0.12 sqrt(T) extrapolated using Drude's law from the measured emissivities at 80 K and at 300 K. The corresponding increase of the transverse resistive wall impedance is large.
Meanwhile FR has discussed with L. Tavian and discovered that the heat conduction through the beam screen supports dominates over thermal radiation (this information is contained in LHC Project Note 300, published in November 2003 and never distributed to the ABP group!).
Therefore the equilibrium beam screen temperature will be considerably reduced and probably stay below 100 K. FR will perform new self-consistent calculations of heat load and resistive wall impedance.
Attached: slides by EM and EV.
--
E-mail: Francesco.Ruggiero@cern.ch, Location: Building 9/1-008
Address: CERN, A&B Department, CH-1211 Geneva 23, Switzerland
Telephone: +41 (22) 767 3726 or 767 5272, TeleFax: +41 (22) 783 0552
WWW: http://wwwslap.cern.ch/~rgo/