11-06-07 1

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

11-06-07 2

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.

11-06-07 3

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)

11-06-07 4

Sigma_z=0.5mm

Min/Max = -2.205E+01/ 5.099E+00 V/pCLoss Factor = -1.617E+01 V/pC

F0=650MHz

ABCI

Numerical modeling of devices:SC cavities

DR Cavity (scaled Cornell): sigma_z=0.5mm

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07s (m)

W_

L,

Q

Long. Wake

Charge

DR Cavity (scaled Cornell): sigma_z=0.5mm

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.00 0.01s (m)

W_

L, Q

Long. Wake

Charge

Calculations by C. Ng, Z.Li

11-06-07 5

10 mm button

25 mm radius

= 2 mm

3D codeT3P used

Numerical modeling of devices:BPMs

Snapshots of beam transit

Calculationsby (C. Ng, Z. Li)

PEP-II model

11-06-07 6

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 Longitudinal wake potential

including rf-cavities and RW

PA

C07

FR

PM

S06

1

Wake potential from BPMsGrowth rate vs. NGrowth 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

12-19-07 7

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.

12-19-07 8

Almost all people who have expressed an interest in WP5 were present or represented at this workshop

12-19-07 9

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 agothought to be adequate to do the job

Planning future work: Main achievement

We agreed on assignment of specific tasks concerning impedance

modeling

12-19-07 10

SLAC, LBLSLACSLAC

SLAC

LBLLBL

ANL

ANL

C.I

ANL

SLACSLAC

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

12-19-07 11

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?

12-19-07 12

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.

12-19-07 13

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 12-19-07 14

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 DANE

– 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 12-19-07 15

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

12-19-07 16

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)