Beam dynamics on damping rings and beam-beam interaction Dec 포항 가속기 연구소 김 은 산

Introduction  Studies on beam dynamics for damping rings are important 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*10 10 ) 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 GeV Circumference 3km - 17km Bunch charge 2x10 10 Normal x-emittance5-6um Normal y-emittance0.02um Rms bunch length 6 mm Rms energy spread x10 -3 Momentum compaction factor x10 -4 Mean vertical beta function m Longitudinal damping time ms Betatron tunes horizontal/vertical (integer) 51-76/31-41 Synchrotron tune Bunch spacing ns Number of bunches per train

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 impedance  Intrabeam scattering  Resistive wall instability  Ion effects and fast beam-ion instability : tight pressure tolerance 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 /m 3  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 ) 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 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 Impedance measurement utilizing bunch lengthening damping ring. Eun-San Kim et al. 11th Symposium on Accelerator Technology and Science, 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 Beam based measurement of focusing errors in quadrupole magnets by a local bump orbit in ATF damping ring. Eun-San Kim, KEK-PREPRINT ,

Beam distributions due to PWD in ATF damping ring Eun-San Kim, KEK Preprint 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