Summary of TWIICE(Topical Workshop on Instabilities, Impedances and Collective effects)

16-17 January 2014, SOLEIL, Saclay France

H. Bartosik, E. Koukovini Platia, K. Li, Y. Papaphilippou, S. Persichelli, G. Rumolo and B. Salvant

for all CERN participants

CLIC seminar31 January 2014

Workshop facts and figures

• 74 participants:– 16 from CERN– 10 from SOLEIL– Several representatives from US (Argonne, Brookhaven, Fermilab, Berkeley, SLAC, Cornell) and light

sources around the world (Brazil, Switzerland, UK, Sweden, Germany, China, Italy, France)– Large majority of attendants were from the light source community and not so much from damping

rings– 35 talks and 5 summaries/discussion sessions in 2 days very dense program!

Disclaimer:This subjective summary only reflects our point of view!

Main messages• Beam dynamics teams are very small in light sources difficult to find a critical mass for deep beam dynamics

studies.• G. Rehm: The question is not “Do we need a feedback in our machine?” but “For what reason would you not

install a feedback?”. It is off the shelf, not that expensive and could be tremendously helpful in case of issues. • It is important to have feedback systems in our codes, as it changes significantly the beam dynamics: strong

request from the light source community for a code that could predict Headtail instability and TMCI thresholds with damper.

• Experimental observation of damping of single bunch instability by bunch by bunch feedback (ESRF, Spring8, SOLEIL)

• Intricate interplay between impedance heating, outgassing, fast Ion instability and feedback to yield instabilities in SOLEIL

• Still difficulties in IBS modeling. Theories can explain what happens in the core, but different physics in the tails. Need for a strong effort on the codes. Can play on the impact factor to fit measurements, but not satisfactory.

• Many benchmarks between impedance codes. Very dangerous to blindly trust 3D codes in a large range of frequencies, material parameters and geometries!

• Most of our colleagues in low emittance rings use GdfidL and ECHO as their main code for 3D impedance simulations

• Methods to identify optimal bunch length below which there is no additional physics information in the simulated wake potential

• Several labs are building impedance models of their machine or projects and performing comprehensive collective effects assessment (BAPS in China, MAX IV in Sweden, NSLS II in the US)

• CSR instabilities were added into tracking codes. Maybe important effect for TLEP and CLIC.• Always interesting to test negative momentum compaction factor.

Main open questions• Impact of space charge on TMCI threshold? Alexey Burov predicts that TMCI

threshold is relaxed in the presence of strong space charge

• Interest in EM properties of NEG at high frequencies and impact on impedance (CLIC DR and ANL).

• IBS: Scattering in tails is less evident (Touschek-like effect dominant?)• How to disentangle IBS from other collective effects?

• Amor Nadji: “In future ring-based light sources (DLSR), the combination of ultra low emittance, high current and narrow chambers will mean that collective effects will be important!”

Proposed actions for us• Need to gain expertise on CSR instability and include it in tracking codes

(Headtail). Assess if issue for our future lepton colliders

• Light sources look like an ideal testbed for our beam dynamics codes (TMCI with damper in particular, NHT and DELPHI).

• See the applicability of the method of B. Podobedov to find the optimum bunch length for 3D simulations of our devices.

• New possible collaborations: NEG coating RF measurements with ANL, 3D simulations with ESRF.

• Need to gain more expertise in GdfidL and ACE3P and to discuss more with other labs who have similar issues.

Parameter range for various machinesMachine RMS Bunch

lengthTransverse emittance

Intensity/bunch

Number of bunches

Total intensity

Frev Beam freq

SOLEIL ~20 ps (4.7 ps for low alpha operation)

3.9 nm.rad 1.36+5 mA 312+1 0.4 A 846 kHz 350 MHz

LHC ~270 ps ~2000 nm.rad ~0.25 mA ~2000 ~ 0.5 A 11 kHz 20 or 40 MHz

SPS 300 to 1000 ps ~2000 nm.rad 0.25 mA ~200 ~0.05 A 44 kHz 20 or 40 MHz

CLIC damping ring

6 ps 4.8 nm.rad 0.4 mA 312 0.1 A 700 kHz 1 GHz

Impedance and instabilities

Alexej Burov (FNAL)• Very important to add transverse damper to assess beam stability• Stability diagrams are very asymmetric for low emittance rings (due to x >> y ) and can

be shifted due to longitudinal to transverse Landau damping

Na Wang (IHEP Beijing)• Impedance model and associated collective effects for the new Beijing Advanced

Photon Light source project.• Impedance model contains resistive wall, RF cavities, undulator tapers and wiggler

tapers• Comprehensive evaluation of limits from collective effects (e.g. microwave

instability, bunch lengthening, TMCI, transverse coupled bunch instability, fast beam ion instability)

Thomas Perron (ESRF)• Working on ESRF upgrade to reduce horizontal emittance• Lower momentum compaction factor lower Qs more sensitive to instabilities• Asked for help to simulate equipment and build impedance model• Designed HOM-free cavity• Factor 3 to 4 increase of TMCI threshold thanks to using bunch by bunch damper

Eirini (CERN/EPFL)• Measurement of NEG conductivity with waveguide method• Very important input for resistive wall impedance and instability

thresholds, as skin depth is very small in the frequency range excited by the CLIC bunch.

• More at the CLIC workshop next week!

B. Podobedov (BNL)• The hunt for the wake function, as cannot simulate point charge.• Problems of applicability found with scaling law of Stupakov, Bane, and

Zagorodnov (PRSTAB 2011)• Exposed method to find the largest exciting bunch length g for which the wake

potential would contain all the physics needed to reconstruct the short range wake function (PRSTAB 2013). “This g is easy to find for arbitrary geometries.”

• Beware of reducing the bunch length until the wake converges!

Henrique de Oliveira Caiafa Duarte (LNLS, Brazil)

• Benchmark of CST, GdfidL, ACE3P and ECHO for shallow cavities and collimators.

• Comparison between simulations and CST for longitudinal kicker• GdfidL and ACE3P are less prone to dispersion issues than CST

Slide of Henrique from yesterday

Question of Giovanni Rumolo yesterday to Henrique: So there is no way to improve the convergence of CST

for this collimator structure?

New CST simulationperformed this night

We can do better with CST, but I agree one has to be very careful !

T. Gunzel• Heat load of the CLIC-stripline using GdfidL• Computations performed mode by mode and no coherent heating assumed (Ploss

M*Nb2 and not (M*Nb)2 )

• If feedthrough ideal, most of the power leaves the stripline through the ports• “In terms of heat load the design of the CLIC stripline is perfect.” G. Rehm: “

beware of non ideal feedthroughs.” similar studies at Diamond (heating workshop in Oxford in 2013)

O. Frasciello (LNF/INFN)• Impact of geometric impedance of LHC collimators on LHC

transverse impedance model additional ~20% on tune shift

G. Skripka (MAX IV, Sweden)• Short range wake with method of B. Podobedov (computing

optimum exciting bunch length) successful also for periodic cavity

• Impedance model of MAX IV (cavities, BPMs, flanges, tapers)

A. Blednykh (BNL)• Impedance of insertion devices• Available Resources for Impedance Simulations: 42 nodes with 8 Cores per node and 1334GB Total

Memory (RAM) • Benchmark between GdfidL, CST and ECHO, CST performing quite bad and showing unphysical drifts.• Benchmark between GdfidL and Stupakov’s dipolar and quadrupolar impedance equations for rectangular

smooth tapers

C. Belver-Aguilar (IFIC)• Study of kicker for CLIC damping ring: stripline design proposed and

prototype built and will be installed in ALBA for tests• Good agreement between analytical calculations and simulations• Bench measurements with wire under way at CERN

J. Byrd (LBNL)• Playing with higher harmonic cavities to control bunch shape and landau damping• Current study of ALS-II, welcoming collaborations for higher harmonic cavity design

and beam dynamics requirements• Outstanding questions:

– Effect on single bunch instabilities?– Effect of Landau damping on HT instabilities?– Effect of overstretching (multiple longitudinal fixed points)?

Marit Klein (MAX IV)• transverse beam instabilities in MAX IV using the multibunch

code mbtrack• Use of wakes from GdfidL and harmonic cavities

Yong-Chul Chae (ANL)• Off-line request for NEG coated pipe impedance measurements• Optimizing the 3D taper to reduce the vertical impedance (only, and in fact only Ky is

it sufficient to check only the dependence with Ky?)• Simulations with GdfidL• New chamber’s kick factor should be < 60% of old one, despite the smaller aperture.

N. Biancacci (CERN/Sapienza)• Localization of impedance sources using intensity dependent phase

advance measured by the BPM system around the machine• Learnt important measurement constraints: High performance BPM

system, need for high ratio between local machine impedance and measurement noise, High quality optic model.

• Successful application to the PS machine

P. Brunelle (SOLEIL)• Impact of incoherent tune shift on orbit.• Incoherent tune shift due to low vertical aperture (vacuum chamber and insertions)• Since undulator position is changing, impact on tunes feedback implemented to

keep the tunes fixed

Particle scattering

• See also summary of Yannis at the workshop!

Theo Demma (LAL)

Fanouria Hannes Antoniou Bartosik(CERN)

S. Wang (CESR-TA)

• Model and data agree well for “weak IBS” regime

Two stream instabilities

Summary of the two-stream instability session

G. Rumolo, R. CiminoBased on input from the presentations of G.

Iadarola, H. Bartosik, R. Nagaoka, N. Wang, T. Perron

Positron machines

• Primary electrons (mainly photoemission)

• Acceleration and secondary electron production

• Multi-bunch electron cloud build up

• Detrimental effects• Mitigation/suppression

needed

Electron machines

• Ions generation (mainly gas ionization)

• Acceleration and trapping

• Multi-bunch accumulation• Beam instability

• Very good vacuum and vacuum composition needed

C = 427.5 m

Wigglera=40mm, b=6mm

Ltot = 104 m

Dipole a=40mm, b=9mm

Ltot = 58 m

Quadrupolea=9mm, b=9mm

Ltot = 86 m

E-cloud aspects have been investigated in three families of devices

CLIC e+ damping ring

Positron bunch train s

Primary/secondary electron production (PEY, SEY)

PyECLOUDx

y

ELECTRON CLOUD BUILD UP

** This process is only slightly dependent on the beam transverse emittance

⇒ Challenging simulation scenario

• Short bunches Short time step

• Small emittance Beam size 104 smaller

than chamber size

• In the cases of wigglers and dipoles e- in a

narrow stripe close to the beam Fine grid

needed for Poisson solver

1 1.2 1.4 1.6 1.810-3

10-2

10-1

100

101

102

SEY

Hea

t loa

d [W

/m]

wiggler_0p5ns_heatload_vs_SEY_nomint

1.0 ns0.5 ns

1 1.2 1.4 1.6 1.810-2

10-1

100

101

SEY

Hea

t loa

d [W

/m]

300 ns train

1.0 ns0.5 ns

• Thresholds and saturation values lower for 0.5 ns

• Large e- densities (>1e13) at the beam location

• E- in narrow stripe in wigglers/dipoles, around the quadrupole field lines in quads.

• Local low SEY coating or clearing electrode for full e-cloud suppression in all cases possible

Equations of motion of the beam particles

PyECLOUDx

y

ELECTRON CLOUD DRIVEN SINGLE BUNCH INSTABILITY

** This process is strongly dependent on the beam transverse emittance

Positron bunch train s

HEADTAIL

⇒ Beam becomes unstable (few turns rise time) as soon as electron build-up reaches saturation in wigglers

⇒ Chromaticity does not help⇒ Consistent with threshold density found with uniform

electron distributions (1.3 x 1013 m-3)

Possible Solutions

Clearing electrodes installed along the vacuum chambers (only local, impedance)

Applying on the wall thin films with intrinsically low SEY • NEG coating (helps vacuum)• C coating (no activation)

Solenoids (only applicable in field-free regions)

Tolerate e-cloud but damp the instability: feedback system

Machine scrubbing during operation• Limited by reachable

SEY• Depends on e- energy• Relies on surface

graphitization

MITIGATION/SUPPRESSION TECHNIQUES

Surface roughness to stop secondary electrons• Grooves• Rough material coating• Sponges

MITIGATION/SUPPRESSION TECHNIQUES SPONGES

Impedance impact, vacuum behaviour, desorption properties are still under study seems very promising

Electron machinesPositron machines

• Primary electrons (mainly photoemission)

• Acceleration and secondary electron production

• Multi-bunch electron cloud build up

• Detrimental effects• Mitigation/suppression

needed

• Ions generation (mainly gas ionization)

• Acceleration and trapping

• Multi-bunch accumulation• Beam instability

• Very good vacuum and vacuum composition needed

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→ Mainly estimations based on analytical formulae for trapping condition and instability rise time

→ Applied to Beijing Advanced Photon Source (BAPS) and ESRF upgrade→ Detailed simulations foreseen, possibly including a transverse damper

→ Vacuum specifications for CLIC long transfer line, Main Linac and BDS made with strong-strong multi-species FASTION code

→ Different vacuum compositions investigated, NEG & baked vacuum most favorable

The CLIC Main Linac

H2O peak

Observations• Ion instabilities observed in APS (with additional He injection), PLS (with

additional H2 injection), SOLEIL, BESSY II, ELETTRA, ALBA• Fast beam ion instability observed in electron rings

• During commissioning/start up (chamber not yet conditioned, bad vacuum, feedback system not yet operational)

• Because of some local pressure rise (e.g., directly connected to impedance induced heating)

• Artificially induced by injecting gas into the vacuum chamber and raising the pressure by more than one order of magnitude (for studies)

• Usually less severe than predictions, stabilizing effects not included in existing models ?

• Quantitative comparison between theoretical predictions, simulations and measurements yet to be made• Experiment planned at Cesr-TA (April 2014)

Observations (II)• Ions enhanced by local heating (outgassing) seem to trigger some

recently observed high current instabilities @ SSRF and SOLEIL

Observations (III)• mbtrack simulations suggest that SOLEIL instability

results from an intriguing interplay between resistive wall, ion effect and transverse feedback

Wrap up• Two-stream effects often affect the performance of running accelerators

and can be a serious limitation for future low emittance rings→ Electron cloud formation and instabilities in CLIC DRs

• Studies carried out with detailed modeling• Electron cloud in wigglers not acceptable for beam stability→ Promising ongoing research on mitigation or suppression techniques (C and sponge coating, scrubbing mechanism)

→ Ion accumulation and instabilities• Mainly analytical formulae used for future machine design, detailed

simulations needed • Observations in running machines usually made in presence of vacuum

degradation and with high intensity important interplay between several effects (RW, FII, damper) observed

• Beam-induced outgassing enhanced for machines with low-gap chambers and high intensity short bunches, FI effects possibly more serious for future low emittance light sources

Instrumentation and feedback

G. Rehm (Diamond)• Review of available diagnostics for longitudinal and transverse profile and

position• Digital Feedback inside bunch? Yes for Hadrons, probably not for the

bunch lengths in lepton machines...

T. Nakamura (Spring-8)• Longitudinal and transverse feedback systems for SPRING-8• Simulation with home made code (SISR) from Wake potentials

with MAFIA (bellows, weldments, flanges, RF cavities, tapers, BPMs, offset) Resistive wall

• Aiming for an intrabunch feedback? Collaborators welcome.

J. Barros (LAL)• New infrared detector to detect THz radiation and understand

CSR instability

K. Li (CERN)• High bandwidth feedback systems• Headtail simulations of realistic feedbacks

in particular impact of bandwidth

Areas of R&D (A.S. Mueller, G. Rehm)

• TMCI feedback: Is the theory sufficient to quantify how feedbacks on the dipole mode helps? Why does it work to such different degree for various people and chromaticities?

• Better turn-by-turn profile diagnostics, both transversely and longitudinally would be beneficial

• Record whatever measurements synchronously at available at highest possible rate, and stream to storage to compare with simulations

• Better investigation of GHz emissions from ports• Feedback kickers with more bandwidth (if we want to go towards

intrabunch FB), while keeping loss factor manageable• CSR diagnostics are an area as active as CSR theory

CSR instabilities• CSR: Coherent synchrotron radiation

• Synchrotron radiation emitted in a bending magnet affects the preceding electrons

comes on top of the microwave instability as THz bursts

• Very active topic for the LER community!!!!

K. Bane: CSR?

Few ps rms (same order of magnitude as CLIC DR)

Warning: impedance and wakefield terms also used for CSR theory, sometimes in conjunction to usual impedances very disturbing!

P. Kuske (Berlin)• Good agreement between measurements in several storage rings and models• If bunch length is known we can estimate shielding parameter or normalized resonance

frequency and predict the threshold and manifestation of instability prediction for CLIC?• Impact of vertical aperture and RF voltage studied

P. Kuske (Berlin)

M. Ries (MLS)

Conclusion from Y. Cai (SLAC)

• The scaling law found in the parallel-plate mode is extended to the rectangular chamber by adding another parameter: A (aspect ratio of the chamber)

• The threshold of a square chamber is lower by a factor than the one of a rectangular chamber with A > 2

• More effective mean to shorten a bunch to a ps scale is to use superconducting RF at higher frequency

1/17/2014 Yunhai Cai, SLAC 61

R. Bartolini (Diamond)