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Summary of Session E: Electron Cloud an Ion Desorption

A. Kraemer and S.Y. Zhang

Three Workshops in one year:• Beam Induced Pressure Rise in Rings, BNL, Dec.

9-12. 2003• ECLOUD´04, Napa, California, April 19-23. 2004• HB-2004, Bensheim, Oct. 18-22. 2004

Major Issues of Pressure Rise Workshop• Ion desorption: Intensity limit of low energy heavy ion

accelerators, possibly relevant for high energy hadron accelerators

• Surface treatment for electron cloud and for electron and ion desorption induced pressure rise. NEG and TiN coatings.

• Electron cloud for short and long bunches. EC induced pressure rise.

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Major Issues of ECLOUD`04•Summary of Pressure Rise Workshop•Simulations of EC for short and long bunches. Electrons below 10eV, Quadrupole effect, simulation of heat load.•EC related beam instability and emittance growth•Supress the EC effect: solenoid fields in lepton machines, beam scrubbing at SPS and RHIC. Why the scrubbing is less effective at cold? Surface of NEG and TiN coatings. Grooved surface for beam tests.

Some Issues in HB-2004•Summary of ECLOUD`04•Progress in cold scrubbing at SPS, for LHC•Progress in ion beam induced desorption and others for GSI upgrade•Progress in ion beam induced desorption on cold walls for LHC heavy ion beam•RHIC pressure rise and electron cloud•Heavy ion fusion studies•Progress in code development

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Summary of ECLOUD`04 WorkshopRobert Macek, LANL

Progress in Cures• Weak solenoids were very effective in reducing e-cloud and ECI at B-factories

(KEKB and PEP-II)

• Tests of NEG coatings for reducing SEY are very encouraging (e.g. see talk by A. Rossi, also M. Pivi summary of session C)

– NEG coatings planned for warm sections of LHC

• Test of grooved metal surface showed 30% reduction in effective SEY (see talk by G. Stupakov)

• Beam scrubbing/conditioning to reduce SEY shown to be effective for LHC beams at SPS, also effective at PSR

– Tests at CERN SPS also suggest scrubbing maybe slower on a cold surface

• Damping of ECI by feedback effective at SPS for coupled-bunch instabilities in the horizontal plane (see talk by G. Arduini)

• Landau damping of e-p by increasing tune spread in various ways effective at PSR as is coupled Landau damping

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Pressure rise mechanisms considered so far• Electron cloud probably dominating for operational problems

– Coherent tune shift in bunch train– Electron detectors– Comparison with simulations

• Ion desorption tolerable for operation– Rest gas ionization, acceleration through beam– Ion energies ~15 eV for Au, ~60 eV for p– Visible pressure rise, may lead to instability

in conjunction with electron clouds (Au only)

• Beam loss induced desorption tolerable for operation

– Need large beam loss for significant pressure rise– New desorption measurements in 2004

(H. Huang, S.Y. Zhang, U. Iriso, and others)

Electron clouds and pressure rise in RHICW. Fischer, BNL

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Measurements on Beam Induced Desorption at GSIH. Kollmus, GSI

September 2004: First test-experiment to measure ion beam induced desorption yields of U73+ at energies from 15 to 1000 MeV/u bombarding stainless steel 316LN, stainless steel P506, Al, Cu and Inconel625

Experiment by: M.C. Bellachioma, M. Bender, H. Kollmus, A. Krämer (GSI), E. Mahner (CERN), O. Malyshev (ASTeC; UK), L. Westerberg, E. Hedlund (TSL; Sweden)

Results:• preliminary energy dependence for U73+ measured – dE/dx scaling

Todo:• charge state dependency? • angle dependency of desorption yield – depth of energy loss test of dE/dx• desorption yields of cold surfaces (with condensed gases)• ERDA-Measurements – understanding of the physics behind the ion induced desorption

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LHC and SPS electron cloud studiesJ.M. Jimenez, CERN

Physisorbed water identified as a potential problem:– Conditioning has been observed in the SPS if the cold detector is protected against

water back streaming from the unbaked parts– In the LHC, low water coverage is expected:

• Pumping down to 10-4 torr of the cold parts prior to the cooling• Controlled cool down sequence where the cold bore is cooled while the beam

screen is kept as warm as possible

1.0E-04

1.0E-03

1.0E-02

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Dose (A.h)

Flu

x to

th

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all

(A/m

)Dipole field (30-50 K), 25ns bunch spacing

3.7 W/m

0.35 W/m

LHC extrapolation (calculation)

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Ion Desorption Issues at RHICS.Y. Zhang , BNL

Ion Desorption and RHIC concerns• Normal incident, yield 1 to 10. Scraping incident, yield 105 observed at low energy heavy ion accelerators

• Ion desorption may cause pressure rise at RHIC. More concerned is the positive ion production, which may explain the electron multipacting in RHIC warm sections, with large bunch spacing.

RHIC observations and studies• Many cases show large desorption rate, but the contributions of electron multipacting or non-beam ions are not clear.• Cases of collimator scraper and other indicate desorption rate 107 or higher.• Similar desorption rate in beam studies, but only in irregular cases. More beam study is needed.

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A new cold-bore experiment for heavy-ion induced desorption studies at low temperatures: first results obtained at 300K, 77K, and 15K

E. Mahner, CERN

ePbPbPbPb 81828282

0

100

200

300

400

sm

0.02

0

0.02

xm 0.03

0.02

0.01

0

ym 0.02

0

0.02

xm

John Jowett

Motivation: Electron Capture by Pair Production

Secondary Pb81+ beam out of IP. Energy deposition by ion flux onto a Cu beam screen in a dispersion suppressor dipolePotential Consequences

Quench limit exceededHeavy-ion induced desorption of cold surfaces? Unknown!

New cold-bore setup for heavy-ion inducedmolecular desorption experiments, in collaborationwith GSI, at LINAC 3.

Pb53+ ions (4.2 MeV/u) bombarded under grazingincidence (14 mrad) onto Cu.Desorption studied with single shots and scrubbing at 300 K, 77 K, and 15 K.Partial and total pressure rise measurements at alltemperatures.

Results (all preliminary)Single shots: Yields decrease with temperatureScrubbing runs (short):Smaller pressure rises at lower temperatures.CO and H2 dominate at 300 K, H2 at 77 K and 15 K.Very low P measured directly on the 15 K coldBore.

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Modeling of beam loss induced vacuum breakdownE. Mustafin, GSI

• The use of the diffusion type equation to simulate the vacuum pressure evolution has been proven to be a fruitful approach in theoretical consideration of the vacuum breakdown description in the heavy-ion machines.

• The proposed method allows to describe the vacuum instability development, steady-state vacuum pressure profiles and the other phenomena related to the beam-loss induced pressure rise.

• Further work is necessary to determine experimentally the phenomenological parameters of the theory

0.1

1

10

0 500 1000 1500

time, msec

cu

rren

t, m

A P0, 0

P > P0,

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U28+ at constant energy

RHIC

GSI SIS18

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Intense Ion Beam transport in Magnetic Quadrupoles: Experiments on Electron and Gas Effects, P. A. Seidl (LBNL)

Electron physics & beam dynamics

1. Rough surface reduces desorption, e- coefficient (from primary ion).

2. 2nd generation electron cloud diagnostics deployed.

3. Testing a self-consistent model of e-, K+ in magnetic quadrupoles; vs experiment with large source of e-

X

X'

3D PIC simulation with e-

3 s

409220100 - 409220138

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Experiment: clearing electrodes & e-supressor off

2 < t < 3 s

1 MeV, 0.18 A, t ≈ 5 s, 6x1012 K+/pulse, beam potential ~2 kV

e- (color)

K+ (black)

K+, e- dist. In quadrupole.

magnet bore

X

Y

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The CMEE library for computer modeling of ion-material interactions

P. Stoltz, presented by D. Bruhwiler, Tech-X Corp.

CMEE = Computational Modules of Electron Effects• Latest version of CMEE now provides routines for modeling– secondary electron yield– Ion stopping, range, and ion-induced electron yield– neutral gas ionization by electrons and protons• The approach is– use tested routines– make them available on any platform or language

• CMEE secondary electron model based on POSINST routines• CMEE ion-material routines are based on CRANGE code• CMEE impact ionization routines are based on fits from Reiser• The next release of CMEE will include some heavy-ion cross sections (ionization, stripping, capture, excitation)• The next release of CMEE will also include ion scattering

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Estimations of beam life time in the SIS18G. Rumolo, GSI

• Projectile and target ionization can both be responsible for beam loss in the SIS18.

• Beam losses from few tenths of percent to about 20% (U28+) can be expected to occur during the ramp, even if the ring operates in a stable regime.

• More accurate information on the energy and angle dependence of the desorption yield is necessary for the prediction of a safe region of operation for the SIS18 under tolarable losses.

Losses for U28+, U73+ and Ar10+ over acceleration ramp

Initial currents (N0) were chosen between 0.1 and 0.9 Nthr . Currents very close to the threshold value can lose up to almost 20% over the ramp.

For U73+ and for Ar10+ (currents N0= 0.9 Nthr), losses are much smaller over the ramping time.

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Test of Anti-grazing Ridges at RHICS.Y. Zhang and P. Thieberger, BNL

IV. Summary and remarks1. RHIC warm EC• RHIC warm section electron cloud is currently a luminosity limit.• It is not a normal electron cloud, beam halo scraping may have played a role in electron's lifetime.2. Ion desorption in scrapings• A new model of ion desorption with scraping angles is proposed.• Anti-grazing ridges have been installed to study the effect on RHIC warm electron cloud.3. An addition to surface cures• For normal incident, rough surface reduces secondary electron yield.• Grooved surface may reduce SEY, with similar mechanism.• RHIC anti-grazing ridge is designed to reduce beam scraping generated ions.

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Working Session Discussion I

1. E-Cloud Issues• Missing physics for hadron machines:

• Contributions from residual gas at PSR is significantly larger than from simulations. • Electrons (sometimes) survive long time, observed at SPS (for fixed target after LHC beam) and RHIC warm straight sections. Are ions playing a role?

• Quadrupole effect • SPS observed strong electron multipacting at quadrupole in a pattern predicted by simulation.

• Effect of electrons below 10eV2. Ion Desorption• GSI, CERN, BNL test stands• RHIC concerns and studies• Among many aspects, incident angle effect

• RHIC study showed that mrad, no dramatic increase of desorption rate. Even smaller angle? • GSI plans test of angular effect.

• Ions from other sources, ionization, low energy electrons

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Working Session Discussion II

3. Cryogenic Problem• SPS COLDEX and cold scrubbing

• Scrubbing inefficiency at cold walls is identified due to water, opened door to LHC scrubbing• Some discrepancies between electron signal and heat load.

• RHIC cold pressure study• Cold pressure rise observed at 2x1011 protons/bunch with 108ns bunch spacing. Study is prepared.• eRHIC, with 1011 charges/bunch, 35ns bunch spacing will have same problem as LHC.

4. Surface• NEG, large scale installation at RHIC

• Activation reactivation, saturation, and possible poissoning.• RHIC activate at 250°C, 1hour per CERN´s recipe; but CERN is doing 230°C, 24hours

• TiN • Serrated and grooved surfaces• Anti-grazing ridges