Summary of Impedance Session  Where do we stand? A summary of progress since the CS and RDR (see also C. Ng’s and G. Stupakov’s talks on Day 1)  Review of deliverables for WP5  What we have accomplished during this Workshop: –Brief summary of talks contributed to the parallel session on impedance and instability issues –Report on the (quite productive) discussion we had in the second part of yesterday parallel session

Work on impedance modeling initiated with lattice Configuration Studies  A first pass at estimating impedance-driven instabilities was made during the baseline Configuration Studies – the OCS lattice considered at the time not too far from the current version of the OCSn lattice.  Configurations Studies were limited in many ways: –Goal to provide relative ranking of various lattices being considered rather than accurate characterization of effects. –Estimates of instabilities based on rough (and in some cases admittedly unreliable) analytical models for impedances. –Parameters in analytical models determined from measurements on existing machines and/or scaled to design specifications of DR lattices.  Results from Configurations Studies helped in shaping priorities for further studies. For example: –Current thresholds for single-bunch longitudinal instabilities turned out a bit low. –Current thresholds for TMCI were well above design current.

Work after Configuration Studies, Cornell Workshop  Started work toward a more accurate characterization of the machine impedance and estimate of the current thresholds for instability.  Following up on the outcome of the Configuration Studies the initial focus has been placed on the study of longitudinal single-bunch (‘microwave’) instability.  Began a program (defined at DR Workshop at Cornell, 2006) to generate numerical modeling of the short-range wake potentials for the main components. –Design of components scaled from design of devices in existing machines  Conducted bench-mark studies to test a few alternate methods to detect instability in the beam dynamics for a given model of wake potential: –Macroparticle simulations –Vlasov solver –Linearized Vlasov equation (mode analysis; direct solver)

Sigma_z=0.5mm Min/Max = E+01/ 5.099E+00 V/pC Loss Factor = E+01 V/pC F0=650MHz ABCI Numerical modeling of devices: SC cavities Calculations by C. Ng, Z.Li

mm button 25 mm radius  = 2 mm 3D code T3P used Numerical modeling of devices: BPMs Snapshots of beam transit Calculations by (C. Ng, Z. Li) PEP-II model

Estimate of current threshold from latest available model of wake potential  Longitudinal dynamics simulated with Vlasov solver  Incomplete wake-potential including only rf-cavity, RW, and BPMs  Resulting critical current for instability well above design value Longitudinal wake potential including rf-cavities and RW PAC07 FRPMS061 Wake potential from BPMs Growth rate vs. N

Multibunch instabilities  Work by K. Hock and A. Wolski on coupled-bunch instabilities driven by resistive wall (PRST-AB paper this year)  Growth times comparable to shortest damping times achievable with present technology  Simulations show the importance of including beta-function breathing in model for determining growth rates

Organizational matters, etc.  The WP5 managers are C. Ng (impedance modeling) and G. Stupakov (characterization of instabilities), SLAC  Three deliverables proposed toward EDR: 1. Calculation of broadband impedance –Identify major components that contribute to the impedance budget and calculate short-range wakefields of vacuum chamber components for single-bunch instability studies 2. Calculation of narrowband impedance –Identify trapped modes in rf cavity and vacuum chamber components, calculate their beam impedances for coupled-bunch instability studies, and determine their effects on heating 3. Calculation of impedance-driven instabilities thresholds –Calculation of single bunch microwave and TMCI instabilities; calculation of thresholds and growth rates for the longitudinal and transverse multibunch instabilities

Almost all people who have expressed an interest in WP5 were present or represented at this workshop  SLAC: –C.Ng, Z. Li –G. Stupakov, K. Bane, S. Heifets + ‘volunteers’  Cockcroft I. –K. Hock, M. Korostelev, A. Wolski  ANL –Y-C Chae, X. Dong  LBNL –D. Li, M. Venturini  IHEP ? O.2 FTE? FTEs expected as of two weeks ago thought to be adequate to do the job

Planning future work: Main achievement  We agreed on assignment of specific tasks concerning impedance modeling SLAC, LBL SLAC LBL ANL C.I ANL SLAC ANL,SLAC Flag chambers, gap in flanges, septum ANL Can wait

Summary of discussion I  For now assume the level of funding we were expecting before Dec 07  The engineering design for many components will be available late (at best) or may not become available at all before the stated EDR timeline  For almost all the impedance sources identified as significant we re- iterated (and agreed on) the proposal that we should produce a first estimate of the impedances/wakes based on designs of components in existing machines scaled to meet basic DR requirements. –In most cases this first estimate is also likely to be the only estimate to be produced  The inj/extr. kickers are the exception. We will wait until a mature design becomes available

Summary of discussion II  In the first pass we decided that generally we should avoid duplications (e.g. the same device modeled starting from two different existing designs)  However, for the cavities it may be useful consider the KEKB SC cavity model in addition to the Cornell SC cavity model used so far –In particular if this helps strengthen a collaboration with IHEP toward engineering the ILC DR cavities  As they become available designs used for the components and estimates of their impact on impedance budget to be shared with people working on other WPs for feedback – proper communication channels to be set up.  Some iterations likely to be needed to converge on mutually acceptable designs.  Similarly, input needed from other WPs should be defined and agreed upon between WP’s managers. Table of dependencies in place by Feb?

Summary of discussion III  Once the mechanical lay-out of the machine is in place survay location of devices to evaluate the need to do a more integrated analysis of adjacent devices.  Make an effort to have designs of devices in CAD files (except for the SC cavities, at least until somebody will start to work on the engineering design).  There are a variety of mature codes for modeling impedances.  We believe that we already have or can obtain the codes we need for evaluating/simulating the instabilities.  There seems to be room for a need to revisit the TMCI theory (you’ve got to let theory guys have some fun …). Some refinement of analytical model used for estimating RW also needed (non-circular cross-sections, account of coating etc.)  We need a database (possibly with Web interface) to share info within the WP5 people, document progress, and help communications with other WPs. Wait for adoption of EDMS across the ILC community.  An ‘open’ WP5 mailing list to be created as main vehicle for communications

Summary of talks in parall. session I  D. Li (LBL) Work on SC modelling at LBL –Continue the scaled 650-MHz RF cavity design in collaboration with SLAC –Re-visit the scaled 650-MHz design, check if there is a need for geometry modifications (Beam pipe, RF power coupler, HOM damping): –Lunch collaboration with BEPC-II team at IHEP, Beijing to scale the KEK cavity to 650-MHz  Takaaki Furuya (KEK): Experience with design and operation of KEKB SC cavities  Yong-Chul Chae (ANL): Impedance Database and Its Application to the APS Storage Ring –Extensive experience with detailed numerical modeling of machine components in light sources and excellent success in predicting measured instabilities

Summary of talks in parall. session. II  Maxim Korostolev (CI) Preliminary Estimates of Impedance for the ILC DR –Started to become familiar with impedance calculation (HFSS, GDFIDL) codes. –Preliminary evaluations of BPMs, tapers  Kai Hock (CI): The Effect of Beta Function Variation on Wakefield Coupled Bunches –Effect is relatively modest (25% over model with smooth betas) but expected growth time (~30 turns) is close to damp time achievable w/ current technology to deserve careful look. –First estimates of inj/extr transient effects  Fabio Marcellini: (ANL) Impedances in DA  NE –Many machine components have been fully characterized using simulations, bench measurements and effects on beam dynamics –Confirms the generally good reliability of simulations  Junji Urakawa (KEK): Instability measurement plans at ATF

On the choice of momentum comp.  Based on simulations of microwave instability so far we can make a case reducing the momentum compaction  Present  set to 4*10 -4 down from 1.6*10 -4 as a reaction to overly conservative estimates of instability during CS  For fixed  _z, N_crit for instability scales linearly with mom comp.  N_crit ~ 100*(design bunch population) for  =4*10 -4  We may be able to cut back to 1.6*10 -4  Substantial savings of rf $$ (also, smaller no. of rf cavities to further reduce microwave instability)