Electron Cloud Studies for Tevatron and Main Injector Xiaolong Zhang AD/Tevatron, Fermilab

Electron Cloud Studies for Tevatron and Main Injector

Xiaolong Zhang AD/Tevatron, Fermilab

13-15 March 2007 IU ep-Miniworkshop, Xiaolong Zhang-FNAL, AD/Tevatron

In Memory of A Great Physicist and an Excellent Mentor

Francesco Ruggiero 1957–2007


In This Report …

Electron Cloud Observations at Fermilab;

The impact on Main Injector Upgrades and the research activities at Accelerator Division;

Simulation methods, programs and results;

Future study plans.


Mechanism of Electron Cloud Buildup

Short bunch: Initial electron produced by photos, beam loss, ionization, etc. Density of the electron increased by generating secondary electrons. Exponential growth of electron density happens with appropriate

beam conditions. Electron cloud saturated by its space charge effect.


Electrons Trapped in the Beam

Long bunch or coasting beam Initial electron generated Electrons are trapped by beam potential Trailing edge multipacting (long bunch case)


Effects of Electron Cloud

Vacuum instabilities: Fast vacuum jumps of several order of magnitude

Beam instabilitiesBeam lossesHeat loadingNoise on beam instruments


Activities at Accelerator Division

Initial observation of pressure rise at Tevatron with high intensity uncoalesced beam in Dec. 2002.

Initiated by Weiren Chou and Francois Ostiguy for Proton Driver Study in April 2005.

More beam studies at Tevatron and some observations at Main Injector

Obtained simulation codes POSINST, ECLOUD, PEI, etc.

Collaborations with LBNL, CERN, APS, BNL, SLAC, etc.

Got 2 RFA electron detector as gifts from APS, thanks for Kathy Harkay and Richard Rosenberg


SEY of S.S. Chamber Material

SEY of new S/S chamber material for Fermi MI, after 1hr 150°C baking and cooling back to room temperature, prepared by W. Chou and measured by R. Kirby (SLAC)


RFA Testing Beam Pipe in Tevatron and MI

RFA ION GAUGEION PUMP


Beam Studies at Tevatron (1)

Bunch intensity threshold around 4e10/bunch for 30 bunches, vacuum worsen @warm section A0, D0, C0, E0, not @B0 and F0



Beam lifetime 24.4hrs Emittance growth 34.8/hr

Tevatron 150GeV, 116e10/30bunches



Some beam Schottky power rise observed when the vacuum pressure rising


Vacuum Pressure @Tev

Squeeze,Sep. ON

Ramp


Beam Threshold of Vacuum Pressure Jump at Tevatron

~1.2E12/30 bunches


Simulations for MI Upgrades

Basic beam parameters

Beam energy 8.9 GeV

Ring circumference 3319.419 m

Maximum bunch intensity 30e10/bunch

Bunch number 6 batch of 84 bunches

Bunch spacing 5.645 m

Maximum bunch length 0.75 m Gaussian

Beam size rms 5 mm round

Residual gas pressure 20 nTorr, room tempeture

Beam pipe 6.15 cm x 2.15 cm elliptical

Electrons/proton loss 1.27e-7 (e/p)/m


Elliptical Beam Pipe

Gröbner multipacting parameterHorz 1.28 Vert: 8.04

@30e10/bunch Energy required for electrons to

traverse beam chamber in one RF period

Horz: 120 eV Vert: 19 eV Electron energy gain at the

extremities of the ellipse (impulse aproximation)

Horz: 2.3 eV Vert: 7.3 KeV Maximum beam kick (finite

bunch length) 772 eVLarmor radius: 0.47 mm @ 2 KGs

(From Report of Miguel Furman)

2/ bebbm rrLNG


Elliptical Beam Pipe

Electron Cloud in Bend

1.0E+00

1.0E+02

1.0E+04

1.0E+06

1.0E+08

0.2 0.4 0.6 0.8

Full Bunch length (m)

Ave

rag

e D

ensi

ty N

e (e

/cm

**3)

6e10, SEY 1.3

3e11, SEY 1.2

With normal MI elliptical vacuum chamber and within bend magnets, at proton bunch intensity of 6e10, the electron cloud threshold for the bunch length of 0.54m, which means electron cloud happens during ramping and transition crossing where bunch length becomes shorter


Elliptical Beam PipeWith Clearing Electrode

Above electron cloud can be suppressed by the 500V clearing electrode in the beam pipe.


6” Beam Pipe

For the 6” beam pipe, the electron cloud happens even at bunch intensity of 10e10 proton/bunch at low SEY=1.3

The electron cloud can be suppressed by 50Gs solenoid or over 500V clearing electrode


Electrons Collected @MI


Electron Signal @MI


Mitigations (1)

Beam Scrubbing


Mitigations (2)

Bunched Beam Injection Pattern Solenoid


Mitigations (3)

Surface Coating with TiN or TiZrV (NEG)Surface Grooving

1 mm

Measured SEY reduction < 0.8. More reduction depending

geometry.

Measured SEY reduction < 0.8. More reduction depending

geometry.

Special surface profile design, Cu OFHC. EDM wire cutting. Groove: 0.8mm depth, 0.35mm step, 0.05mm thickness.


Mitigations (4)

Clearing Electrode

E

Feedthrough

E


Mitigation Method (5)

Alternate beam parameters, such as bunch length; intensities; etc.

Change filling patterns, such as bunch spacing, satellite clearing bunches; etc.


Simulations

Small section of the beam pipe. Macro particles and discrete beam kick Space charge, electron and beam image

charge included. Gaussian bunches (longitudinal bunch profile

available for long bunches) Realistic secondary electron yield model. Electron longitudinal motion neglected Theoretical primary electron distributions


Secondary Electron Yield

CERNSLAC


Future Plan

Continue the detailed simulation for various beam and surface conditions

Beam studies at Tevatron and MI: Electron density vs. beam intensity. Electron energy spectrum. Bunch by bunch tunes, loss, emittances. Vacuum changes.

● Comparing and benchmarking the simulation codes● Test of mitigation methods with the test beam pipe:

Solenoid, clearing electrodes, coating, grooving, etc.● Does it exists in Booster?


Summary

Electron cloud effect is a limiting factor to the high energy, high intensity accelerator performance.

It might have some impacts on magnet design. The simulations and initial observations show

the electron cloud will be a problem for SNuMi and future Fermi neutrino programs.

More studies and investments should be put into this researches.


Backup slides


History of Electron Cloud Studies

1967 Novosibirk proton rings with coasting beam: Two Stream Beam Instabilities; Cure: various beam intensity and clearing electrode

1970 CERN ISR coasting beam Cure: clearing electrode

1977 ISR bunched beam. Vacuum pressure jumps

End of 80s: KEK PF Beam instabilities when switched from electron to positron PSR Beam instabilities

1995 Two B-Factories: KEKB: Simulation code PEI; Beam studies KEK-BEPC; PEPII: Simulation code POSINST; LBNL-SLAC; TiN coating

1997 ECLOUD for LHC and SPS. CESR, APS, SNS, RHIC, etc. New simulation codes and methods keep appearing. Extensive SEY measurements; material and surface treatments.


Electron Cloud for Various Accelerators

MI


Effects of the Electron CloudBeam Instabilities

KEKBBEPC

PSR, 1988

Sideband Peak Height

Betatron Oscillation Sidebands


Effects of the Electron CloudBeam Emittance Growth

0

1

2

3

4

5

0 200 400 600 800 1000 1200 1400 1600

Ver

tica

l b

eam

siz

e@IP

(m

icro

n)

LER beam current (mA)

1 train, 1153 bunches, 4 rf bucket spacing

2001 July : on

2001 Dec. : on

2002 Feb. : on

2001 July : off

KEKB SPS


Effects of the Electron CloudVacuum Pressure Bump

0

2

4

6

8

10

12

14

0.E+00 1.E+10 2.E+10 3.E+10 4.E+10 5.E+10 6.E+10 7.E+10 8.E+10 9.E+10 1.E+11

Proton bunch intensity

P/P

Duty cycle ~ 45 %after 17h integratedtime of LHC beam

0

2

4

6

8

10

0.0E+00 2.0E+10 4.0E+10 6.0E+10 8.0E+10 1.0E+11Protons bunch intensity

P/P

Duty cycle ~ 45 %after 17h integratedtime of LHC beam

dipole field

no field

RHIC SPS


Effects of the Electron CloudNoise on BPM Pickup

SPS


Effects of the Electron CloudBeam Loss

SPS

RHIC


Effects of the Electron CloudEstimation of the Heat Load for LHC (Frank Zimmermann)

high luminosity

arc heat load vs. intensity, 25 ns spacing, ‘best’ model

calculation for 1 train

R=0.5

max=1.7

max=1.5

max=1.3max=1.1

max=1.3-1.4 suffices

BS cooling capacityinjection

low luminosity

ECLOUDsimulation


RFA ion signal

Ions Collected by RFA

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0.00 0.20 0.40 0.60 0.80 1.00 1.20

time (sec)

Vo

lts

Documents

Electron Cloud Studies for Tevatron and Main Injector Xiaolong Zhang AD/Tevatron, Fermilab