From: "Frank Zimmermann" To: "Frank Zimmermann" ; ; "Francesco Ruggiero" ; "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Gilbert Guignard" ; "Jacques Gareyte" ; "Werner Herr" ; "Luc Vos" ; "Alex Koschik" ; "Bruno Muratori" ; "Tommaso D'Amico" ; "Rita Paparella" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Maxim Korostelev" ; Cc: "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Roberto Cappi" ; "Charles Hill" ; "Gianluigi Arduini" ; "Helmut Burkhardt" ; "Daniel Brandt" ; ; "Ralph Wolfgang Assmann" Subject: Minutes of LHC Collective Effects meeting 11/04/2003 Date: Monday, April 14, 2003 4:58 PM ------------------------------------------------------------------------ minutes ABP-LCE meeting 11.04.03 present: EB, JJ, AK, EM, DS, FR, LV, FZ special guests: G. Bellodi, R. Gluckstern, G. Rumolo excused: WH ------------------------------------------------------------------------ (1) comments on the minutes from the last meeting ------------------------------------------------------------------------ B. Zotter and A. Koschik clarified some remarks concerning Gerry Dugan's paper. AK could indeed reproduce the simulation results by G. Dugan for CESR, where a single bunch can oscillate at large amplitudes without coupling to other bunches in the presence of a tune shift with amplitude. This decoupling, however, is a simple consequence of the fact that the bunch oscillation frequencies are different. It is not thought to be a manifestation of a soliton, as suggested in the paper. For the LHC, with only 9 bunches, neither multi-bunch modes not (pseudo-)solitons were seen in the simulation. ACTION AK -> continue simulation studies for LHC (2) luminosity scaling with collimator aperture (FR) ------------------------------------------------------------------------- Two effects contribute to the aperture in the final triplet, the beam size and the orbit offset from the center, which are related to the beta function at the IP and to the crossing angle, respectively. If the normalized separation at the parasitic collision points is kept constant, they scale in the same way. The aperture at the triplet, in units of sigma, must be 1.4 times the aperture at the primary collimators. Combining the various factor Francesco Ruggiero found that an increase in the collimator aperture from 6 to 10 sigma results in a luminosity loss by a factor 1.8. In other words, a realistic beta* would be more like 1 m instead of 0.5 m, for the present triplet. The argument above contained some consideration of beta-beating and residual spurious dispersion, whose changes largely cancel each other. (3) impedance challenge by Fritz Caspers (FR) ------------------------------------------------------------------------- Fritz suggested that a proper assessment of the collimator impedance should proceed in 3D and not in 2D. He expects that the low-frequency impedance of a collimator jaw will be small if the jaw is electrically isolated from the rest of the vacuum chamber. An example of a similar system with small impedance is a lambda/4 strip line. In addition, to suppress the impedance at higher frequency Fritz conceived the idea of using rotated or skewed jaws (e.g., X-shaped with about 5 degree angle). FR asked the LCE team, why, if Fritz is right, this scheme is not used in any storage ring to reduce the resistive impedance, or, for example, in light sources, to reduce the impedance of insertion devices. His next question is whether such a scheme could create additional problems (longitudinal impedance, resonances?), and what conclusions can be drawn from existing measurements. ACTION LV -> estimate the impedance around different revolution harmonics, e.g., at 20 MHz and higher, and confirm suspicion that the reduction of the overall impedance by the electrical isolation is small, even if the impedance is reduced at 0 frequency. ACTION EM -> learn HFSS and run it to check Fritz' idea. (4) analytical approaches (R. Gluckstern) ------------------------------------------------------------------------- After some historical remarks on N. Christofilos, a Greek elevator engineer who invented strong focusing two years before Courant, Livingston, and Snyder, flooded BNL with (too) many interesting ideas, and was later "forced" to learn some maths to check his own ideas (see http://www.bnl.gov/bnlweb/history/Blewett_talk.htm), Bob presented some analytical calculations for the collimator impedance problem. He considers a somewhat simplified model, where a gap with a resistive medium is embedded in an otherwise perfectly conducting pipe. He described the various steps of the impedance calculation, for example the distinction between pipe kernel and kernel of the resistive medium, the matching of boundary conditions for the magnetic fields of even and odd modes, and the resulting integral equation. He first computed the longitudinal impedance, and outlined the analogous steps for the transverse case. FR pointed out that terms propagating at much less than the speed of light may be important, and he also asked whether the use of impedance boundary conditions for E/H could simplify the problem. ACTION RG -> continue calculations for transverse impedance (possibly during the vacation period!) (5) frequency maps for electron-cloud simulations (FZ) ------------------------------------------------------------------------ Frank Zimmermann presented two frequency maps computed by Yannis Papaphilippou (ESRF) for 1000 particle trajectories obtained in electron-cloud HEADTAIL simulations by E. Benedetto. A large tune spread was found extending over several low-order resonance lines, but not revealing any influence by these resonances. Yannis had pointed out that the frequency maps for this example are not very accurate, because of the presence of synchrotron motion and since the trajectories are the result of a multi-particle simulation, not governed by a weak-strong Hamiltonian for which KAM tori are known to exist. Nevertheless it surprises that any resonance structure is conspicuously absent. DS interpreted this result as a confirmation that the emittance blow-up is caused by a coherent phenomenon, and not by incoherent single-particle motion. JJ suggested to look at the wider frequency spectrum following his earlier work with R. Burgess. (6) emittance growth due to quadrupole wakes (FZ) ------------------------------------------------------------------------ Responding to a question by AK, Frank first reviewed the definition of quadrupole wake fields. A. Chao and R. Cooper defined quadrupole wakes as wakes driven by a (normal or skew) quadrupole moment of the beam. In addition, the wake force on a test particle resembles the force by a quadrupole magnet with a linear dependence on the offset of the test particle. By extension, later any wake force which is linear in the offset of the test particle has been called a quadrupole wake. If the surrounding structure is not cylindrically symmetric, the beam does not need to contain a quadrupole moment to drive such a wake. In particular such generalized quadrupole wakes exist for the geometry of a flat collimator. The term quadrupole wake in this second sense was used by J. Irwin (NLC ZDR, 1996) and S. Heifets, A. Wagner, and B. Zotter (SLAC-AP note, 1998). Using the resistive-wall wake field for a flat parallel-plate collimator, computed by A. Piwinski (DESY HERA 1992) or K. Yokoya (Part. Acc. 1993), one can compute the deflection of a particle at longitudinal position z and transverse position y in the bunch. Computing the projected emittance before and after the kick by calculating the second moments y^2, yy', y'^2 before and after the collimator, Frank estimated the growth in projected emittance for a single traversal through the collimator. It scales with the fourth power of the beam size and the inverse sixth power of the aperture. Insering numbers for a single 1-m long Al collimator jaw with 1 mm half gap, Frank estimated an emittance doubling time of 1 billion turns for the LHC at 7 TeV. This estimate assumes that the emittance growth is additive from turn to turn, which may not fully be the case, due to synchrotron motion. A simulation study by E. Benedetto is underway to explore the accumulated growth over many turns. (7) report from SNS ASAC and KEKB (FZ) ------------------------------------------------------------------------ The SNS Accelerator Systems Advisory Commmittee meeting was held March 10-12. Highlights included the numerous counter-measures and remedies taken against the electron cloud (TiN coating of all components, solenoids at the collimators, electric fields and electron collector at stripping foils, electron-catcher viewing system), actions to stabilize the beam against any potentially occurring electron-cloud instability (such as enhanced momentum spread, chromaticity, and wide-band feedback), and the development of the ORBIT code, which is rapidily expanding, and will be the work horse of the SNS commissioning. The program ORBIT was written by a collaboration involving ORNL, FNAL, BNL, IUCF, TRIUMF, etc. The code combines all or most features of MAD with an accurate representation of transverse and longitudinal impedance and space charge in 3D. Comparison of simulated transverse and longitudinal beam profiles with measurements at the Los Alamos PSR showed an impressive agreement. The modelling of scattering in a stripping foil has been greatly improved, and the resulting tail distribution has changed compared with earlier models. In the future, it is planned to include an electron cloud module in ORBIT. FZ suggested to install this code at CERN. Recent electron-cloud observations at KEK include a substantial change in the measured blow-up threshold from one day to the next due to unknown origin, a non-saturation of the threshold increase for maximm available solenoid strengths (about 50 G), which is different from earlier findings when only part of the ring was covered by solenoids, and saturation (of the lower threshold) was observed below 30 G. BPM and PMT data showing that above the blow-up threshold individual bunches of the long train suddenly become unstable in an apparent random fashion. The beam size of individual bunches spontaneously increases and damp down to the original level within about 1500 turns (compared with a transverse damping time of 9000 turns). Simultaneously, above the threshold the tune line, seen on both BPMs and PMT, splits into two, which separate for increasing beam current. Presentations by WH (Crossing angle at IP2) and EM (Highlights from Oxford workshop) will take after Easter, at the next LCE team meeting of 25 April. Slides from presentations by Francesco and Frank Z. are attached.