Beam dynamics on damping rings and beam-beam interaction

Dec. 28 2004

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Introduction Studies on beam dynamics for damping rings are imp

ortant to provide stable and high quality beam into the main linac.

: Careful studies of lattice design and beam instabilities should be required.

Studies on beam-beam interaction in IR region are key issue to optimize high luminosity.

: related to crossing angles, wakefield, background

Studies for damping ring design and beam- beam interaction have been suggested.

Designs of damping rings are determined by upstream and downstream systems

• source

pre-linac

Damping ring

Bunch compressor

linac

Beam delivery

Interaction region

Design choice are based on

- injection/extraction scheme

- beam dynamics

- reliability and flexibility for operation

Beam has a high bunch charge (2*1010) and low emittance

- collective instabilities is important

Damping ring requires sufficient acceptance in transverse and

longitudinal directions.

- dynamic aperture is a key issue.

Damping ring must have large circumference and long damping wiggler.

Requirements on damping rings

Large dynamic aperture is needed to accept high-emittance positron beam.

Large circumference ( > 3km) and rapid damping (27ms) are needed to meet specifications for train length and repetition rate.

Bunch length is set by energy bandwidth of bunch compressor.

- Bunch length < 6 mm

- Energy spread < 0.15 %

Highly stable beam in damping ring - low phase jitter - low charge jitter - small halo

Major parameters of the ILC damping rings

Beam energy ~ 5 GeVCircumference 3km - 17kmBunch charge 2x1010

Normal x-emittance 5-6umNormal y-emittance 0.02umRms bunch length 6 mm

Rms energy spread 1.2-1.5x10-3

Momentum compaction factor 1.2-2.9x10-4

Mean vertical beta function 26-121 mLongitudinal damping time 10- 14 msBetatron tunes horizontal/vertical (integer) 51-76/31-41 Synchrotron tune 0.027-0.071Bunch spacing 3.1-20 ns Number of bunches per train 47 - 2820

TESLA dogbone damping ring

5 GeV, 17 km long ( Arcs 2 km, straights 15 km )

Bunch spacing 20ns

440 m wiggler in e+ damping ring for 27 ms damping time

Merit : relatively small distance for extra tunnel

large circumference helps in instabilities.

Weakness : coupling bumps for large space-charge tune shift

FNAL damping ring

5 GeV, 6 km long (six-fold symmetry)

Merit : less space-charge effects due to relatively small circumference

Weakness : strong electron cloud and ion effects due to higher average beam current

LBNL damping ring

16 km dogbone lattice with FODO arcs

dynamic aperture > 10

Instabilities and collective effects in damping rings

Longitudinal single bunch instabilites driven by impedance Coherent synchrotron radiation (CSR) Space charge tune shift Transverse and longitudinal multi-bunch instabilities driven by i

mpedance Intrabeam scattering Resistive wall instability Ion effects and fast beam-ion instability : tight pressure toleranc

e of 0.1 nT Electron cloud instability

Experimental studies for Beam instabilities

Experimental studies are necessary to examine theory and simulation.

Electron cloud instabilities

: In TESLA damping ring, instability occurs at average cloud density around 1011/m3

Fast-ion instabilities

: Threshold for FII in ILC is estimated to be ~100 turns.

Recent experiments on fast-Ion instability at PAL(PAL-KEK-IHEP collaboration)

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40

PLS_size_P

Ver

tica

l si

ze(m

icro

n)

Pressure(nTorr)

400 bunchesBeam current : 100 mA

Ion pump off, He injected.

2004.07.22

2004.12.19

Vertical beam oscillation by turn-by-turn BPM

Must be first measurement of growth rate.

KEK-ATF is the world’s largest LC test facility.

Experiences on damping ring

• Bunch-lengthening in the ATF damping ring, Eun-San Kim, KEK-Preprint 98-21

• Longitudinal impedance in ATF damping ring, Eun-San Kim, KEK-Report 98-6• Transverse instability in ATF damping ring, Eun-San Kim, KEK-Report 98-7• Impedance measurement of ATF DR, Eun-San Kim et al., EPAC98 p.481 • Observation on the longitudinal beam oscillation at ATF-DR.

Eun-San Kim et al., KEK-PREPRINT-2003-73 • Impedance measurement utilizing bunch lengthening damping ring . Eu

n-San Kim et al. 11th Symposium on Accelerator Technology and Science, 476-478 1997

• Extremely low vertical emittance beam in accelerator test facility at KEK Eun-San Kim et al, Phys. Rev. Let. 88:194801,2002

• Recent results on KEK / ATF damping ring.Eun-San Kim et al., SLAC-PUB-7952, KEK-PREPRINT-98-154

• Beam based measurement of focusing errors in quadrupole magnets by a local bump orbit in ATF damping ring. Eun-San Kim, KEK-PREPRINT-97-148,

-5.0 -2.5 0.0 2.5 5.00.0

0.4

0.8

1.2

(d)

(c)

(b)

(a)

(d) N=1x1010

(c) N=3x1010

(b) N=5x1010

(a) N=7x1010

z

Inte

nsi

ty

Beam distributions due to PWD in ATF damping ring

Eun-San Kim, KEK Preprint 98-21

Cited by US Particle Accelerator School, 2003

Possible things for

damping ring design of ILC-ASIA Optics

* Lattice design ( suggested dogbone and small DR ) * Effects of edge field of wigglers on dynamic aperture * Tune survey * Optics/dispersion correction with space charge * Tolerances for the emittance

Tracking simulations * Emittance growth and particle loss due to space charge.

* Effects of wakes in damping rings * Multi-bunch instabilities due to superconducting RF cavities

* Electron cloud instabilities and ion instability.

* Wiggler effects on beam parameters

Possible things for

beam-beam interactions of ILC-ASIA

• Optimize IR optics design

• Beam-beam simulations for head-on and crossing angles -> Choice of crossing angles

• Development of model of beam halo

• Impact of background on collimation design