ANKA Seminar Ultra-low emittance for the CLIC damping rings using super-conducting wigglers Yannis PAPAPHILIPPOU October 8th, 2007

ANKA Seminar, Y. Papaphilippou Outline Overview of the CLIC Project CLIC damping rings design Design goals and challenges Energy Lattice choice and optics optimisation Circumference Wiggler design and parameter scan Final emittances including Intra-beam scattering Chromaticity correction and dynamic aperture Low emittance tuning in the presence of coupling Summary and open issues M. Korostelev (PhD thesis, EPFL 2006) 08/10/2007 ANKA Seminar, Y. Papaphilippou

ANKA Seminar, Y. Papaphilippou The CLIC Project Compact Linear Collider : multi-TeV electron-positron collider for high energy physics beyond today's particle accelerators Center-of-mass energy from 0.5 to 3 TeV RF gradient and frequencies are very high 100 MV/m in room temperature accelerating structures at 12 GHz Two-beam-acceleration concept High current “drive” beam, decelerated in special power extraction structures (PETS) , generates RF power for main beam. Challenges: Efficient generation of drive beam PETS generating the required power 12 GHz RF structures for the required gradient Generation/preservation of small emittance beam Focusing to nanometer beam size Precise alignment of the different components 08/10/2007 ANKA Seminar, Y. Papaphilippou

Base line configuration Injector complex DC gun Unpolarized e- 3 TeV Base line configuration Laser Polarized e- Pre-injector Linac for e- 200 MeV e-/e+ Target Linac for e+ Primary beam 2 GeV Injector Linac 2.2 GeV e+ DR e+ PDR 2.424 GeV 360 m Booster Linac 6.6 GeV 3 GHz e+ BC1 e- BC1 e+ BC2 e- BC2 e+ Main Linac e- Main Linac 12 GHz, 100 MV/m, 21 km 1.5 GHz e- DR e- PDR 162 MV 12 GHz 2.3 GV 9 GeV 48 km maximum  30 m  10 m  360 m  150 m  15 m L. Rinolfi

Damping ring design goals Ultra-low emittance and high beam polarisation impossible to be produced by conventional particle source: Ring to damp the beam size to desired values through synchrotron radiation Intra-beam scattering due to high bunch current blows-up the beam Equilibrium “IBS dominated” emittance should be reached fast to match collider high repetition rate Other collective effects (e.g. e--cloud) may increase beam losses Starting parameter dictated by design criteria of the collider (e.g. luminosity), injected beam characteristics or compatibility with the downstream system parameters (e.g. bunch compressors) PARAMETER NLC CLIC bunch population (109) 7.5 4.1 bunch spacing [ns] 1.4 0.5 number of bunches/train 192 316 number of trains 3 1 Repetition rate [Hz] 120 50 Extracted hor. normalized emittance [nm] 2370 <680 Extracted ver. normalized emittance [nm] <30 < 20 Extracted long. normalized emittance [eV m] 10890 <5000 Injected hor. normalized emittance [μm] 150 63 Injected ver. normalized emittance [μm] 1.5 Injected long. normalized emittance [keV m] 13.18 1240

ANKA Seminar, Y. Papaphilippou Ring energy Choice dictated by spin tune (half integer) for maintaining high-spin polarisation Frozen on early design stage Advantage of lower energies: For same equilibrium emittance i.e. smaller circumference and radiated power (cost), high momentum compaction (longitudinal stability). Advantages of higher energy For fixed damping fraction due to wigglers and wiggler peak field, i.e. easier magnetic design (lower main field) and smaller total wiggler length 08/10/2007 ANKA Seminar, Y. Papaphilippou

Lattice choice Usually racetrack configuration with Theoretical Minimum Emittance (TME) arcs and damping wigglers in the straights NLC DR was based on lattice with 32 TME arc cells and wigglers of 62m total length (A. Wolski et al. 2003) ILC has a large ring of more than 6km for accepting large number of bunches with reduced e-cloud effect TME and FODO lattice considered (A. Wolski et al. 2007) 08/10/2007 7

CLIC damping ring layout 08/10/2007 ANKA Seminar, Y. Papaphilippou

TME arc cell TME cell chosen for compactness and efficient emittance minimisation over Multiple Bend Structures (or achromats) used in light sources Large phase advance necessary to achieve optimum equilibrium emittance Very low dispersion Strong sextupoles needed to correct chromaticity Impact in dynamic aperture

ANKA Seminar, Y. Papaphilippou Phase advance choice Optimum horizontal phase advance of cells for minimising zero current emittance is fixed (284o for TME cells) Vertical phase advance is almost a free parameter First iteration based on lattice considerations, i.e. comfortable beta functions and relaxed quadrupole strengths and chromaticity Low horizontal phase advance gives increased momentum compaction factor (high dispersion) but also chromaticity 08/10/2007 ANKA Seminar, Y. Papaphilippou

ANKA Seminar, Y. Papaphilippou Phase advance with IBS Horizontal phase advance for minimum horizontal emittance with IBS, is found in an area of small horizontal beta and moderate dispersion functions (between 1.2-1.3π, for CLIC damping rings) Optimal vertical phase advance quite low (0.2π) The lowest longitudinal emittance is achieved for high horizontal and low vertical phase advances The optimal point has to be compromised due to chromaticity considerations and dynamic aperture optimisation 08/10/2007 ANKA Seminar, Y. Papaphilippou 11

ANKA Seminar, Y. Papaphilippou Circumference Usually chosen big enough to accommodate number of bunches Drift space increase essential for establishing realistic lattice, reserving enough space for instrumentation and other equipment For constant number of dipoles (TME cells), zero equilibrium emittance is independent of circumference Normalised emittance with IBS increases with circumference (no wigglers) When dipole lengths increase with drifts, emittance grows due to increase of damping time (inversely proportional to radiation integral I2 which decreases with length) When only drifts increase, smaller emittance growth due to increase of optics functions Impact on chromaticity + dynamic aperture Compensation may be achieved due to increase of bunch length with circumference (momentum compaction) Drifts + dipoles Only Drifts Drifts + dipoles Only Drifts 08/10/2007 ANKA Seminar, Y. Papaphilippou

ANKA Seminar, Y. Papaphilippou Damping wigglers Damping wigglers are used to increase radiation damping and reduce the effect of IBS in order to reach target emittances The total length of wigglers is chosen by its dependence with the peak wiggler field and relative damping factor Damping factor increases for higher fields and longer wiggler occupied straight section Relative momentum spread is independent of total length but increases with wiggler field 08/10/2007 ANKA Seminar, Y. Papaphilippou

Wigglers effect in emittance For fixed value of wiggler period, equilibrium emittance minimum for particular value of wiggler field By reducing total length, optimal values necessitate higher fields and lower wiggler periods Optimum values change when IBS included, necessitating higher fields Damping rings cannot reach 450nm with normal conducting wigglers 08/10/2007 ANKA Seminar, Y. Papaphilippou

Wigglers’ effect with IBS For higher wiggler field and smaller period the transverse emittance computed with IBS gets smaller The longitudinal emittance has a different optimum but it can be controlled with the RF voltage The choice of the wiggler parameters is finally dictated by their technological feasibility ANKA SC wiggler BINP SC BINP PM The choice of the wiggler parameters is finally dictated by their technological feasibility. Normal conducting wiggler of 1.7T can be extrapolated by existing designs Super-conducting options have to designed, built and tested

ANKA Seminar, Y. Papaphilippou Wiggler prototypes Parameters BINP ANKA Bpeak [T] 2.5 2.7 λW [mm] 50 21 Beam aperture full height [mm] 12 5 Conductor type NbTi NbSn3 Operating temperature [K] 4.2 Two wiggler prototypes 2.5Τ, 5cm period, built by BINP 2.7Τ, 2.1cm period, built by ANKA Aperture reduced for the more challenging design Current density can be increased by using different conductor type Short version to be installed and tested at ANKA (energy of 2.5GeV) Lifetime of 8-10h for lower gap, enough for the beam tests 08/10/2007 ANKA Seminar, Y. Papaphilippou

RF voltage and frequency The smallest transverse emittance is achieved for the lowest RF frequency and higher voltage, while keeping the longitudinal emittance below 5000 eV.m Reversely the longitudinal emittance is increased for small RF frequency 08/10/2007 ANKA Seminar, Y. Papaphilippou

Wiggler FODO cell Average horizontal β function should be small enough for the wiggler period not to exceed the value producing efficient damping FODO cell structure chosen with phase advances close to 90o giving average β’s of around 4m and reasonable chromaticity Quad strength adjusted to cancel wiggler induced tune-shift 18

Non-linear dynamics Two sextupole schemes considered Two families / 9 families of sextupoles Dynamic aperture is 9σx in the horizontal and 14σy in the vertical plane (comfortable for injection) Wiggler effect should be included and optimised during the design phase

Coupling correction Coupling effect of wigglers should be included in simulations Correction with dispersion free steering (orbit and dispersion correction) Skew quadrupole correctors for correcting dispersion in the arc and emittance minimisation Iteration of dynamic aperture evaluation and optimisation after correction In CLIC damping rings, the effect of vertical dispersion is dominant (0.1% of coupling and 0.25μm of dispersion invariant)

ANKA Seminar, Y. Papaphilippou Bunch charge Approximate scaling laws can be derived for a given damping ring design For example, for the CLIC damping rings, the horizontal normalized emittance scales approximately as The above relationship is even more exact when the longitudinal emittance is kept constant (around 5000 eV.m, in the case of the CLIC damping rings) Vertical and longitudinal emittance are weakly dependent on bunch charge, and almost linear with each other 08/10/2007 ANKA Seminar, Y. Papaphilippou 21 21

Damping rings’ parameters 2005: original ring 2006a: super-conducting wiggler considered 2006b: vertical dispersion included 2007a: 12GHz structure 2007b: reduced bunch population 2007c: CLIC_G structure 08/10/2007 CLIC PWG Y. Papaphilippou ANKA Seminar, Y. Papaphilippou 22 22

ANKA Seminar, Y. Papaphilippou Concluding remarks Robust design of the CLIC damping rings, delivering target emittance with the help of super-conducting wigglers Prototype to be built and tested in ANKA Areas needing further optimisation Pre-damping ring optics design Collective effects including electron cloud Realistic cell length and magnet design Sextupole optimisation and non-linear dynamics including wiggler field errors 08/10/2007 ANKA Seminar, Y. Papaphilippou