ILC Accelerator 2/2 Remaining of Acceleration Damping ring Some other parts of LC (if we have time) Test facilities Kiyoshi Kubo (KEK) Collision High luminosity requires small vertical emittance beam-beam force Final focus and Collimation Acceleration Beam parameter for high power efficiency High gradient Damping Ring Bunch compressor (if possible) Test facilities Damping ring Main Linac Particle source Injector linac Ring to Main Linac Transport Bunch compressor Collimation and Final focus Dump Collision Saturday Today Noting about upstream of damping ring. ILC Why higher gradient? Based on present status, conservative accelerating field of superconducting cavity, for large scale stable operation ~ 25 MV/m (?) ILC design 31.5 MV/m Studying for even higher gradient Higher Gradient reduce site length L(km)=1000/G(MV/m)/ (Old parameters) This is very clear. But importance depends on where the machine is built. For ECM 1 TeV 1/3 Higher Gradient reduce Cost ? Construction cost Tunnel a/G reduce length Acc. cavities b/G reduce number of cavities RF source c + dG * Cryogenics eG + f /G ** Operation cost (power) RF g(c + dG) * Cryogenics h(eG + f /G) ** Rough dependence of cost on gradient G There must be cost minimum gradient. 2/5 Cost minimum is 35~45MV/m (?) (example of calculations) Present ILC design is 31.5 MV/m, still lower than cost minimum? 1/7 SUMMARY of Acceleration Performance of cavity: Max. field gradient and Q0 Surface quality (practical limit) Cavity shape (fundamental limit) Beam parameter RF power efficiency Cavity wall loss cryogenics power Limit from damping ring Compromise Lorentz detuning Cost vs. accelerating gradient ILC design gradient is still lower than cost minimum. (?) 1/8 Damping Ring Creation of low emittance beam Limit of number of bunches Etc. Important conclusions of Saturdays story We need small vertical emittance beam for high luminosity. Main Linacs pawer efficiency requires large number of bunches per pulse. 1/9 What is emittance, again Emittance ( denotes average over particles in the beam) ~ (beam size) x (angular divergence) if ignore the correlation Expressing beam quality. Because: Determines beam size limit at collision point. Invariant during beam transport (in ideal case). (Beam size is varying along a beam line.) 2/11 Emittance is based on mean squares of particle distribution Good measure of beam quality, for Gaussian or Gaussian like distribution, without long tail. without particles far from the core of the beam But, not always: 2E10 particles y=20 m, y=1 rad 2E10 2E4 particles y=10 m y=1 rad 17 mm 2E4 particles Beam A: Beam B: Which is better? A and B have the same emittance. B is better looking at luminosity but the small part, far away from the core, may cause problems (e.g. background) Be careful. Emittance is not always representing beam quality well. 2/13 But, Emittance is a very good, useful, simple and well defined parameter representing beam quality in most cases. (See also the 20th slide from here.) 1/14 Normalized emittance Transverse momentum does not change in acceleration. Normalized Emittance does not change. Angle is changed in acceleration. Emittance is reduced in acceleration. (Lorentz factor) x emittance 1/15 Why damping rings are necessary? Emittance of beam from particle sources is much larger than required. Much larger than achievable in damping rings. electronpositron Before Damping ring x y 7E-5 ~1E-3 After Damping ring x y 8E-6 2E-8 8E-6 2E-8 Normalized emittance (m) of ILC design 1/16 Why emittance is damped in damping rings Beam energy is constant Energy is lost due to synchrotron radiation. Compensated in RF cavities. RF cavities By radiation, angle pf particle is (almost) not changed: Emittance is not changed. Normalized emittance is reduced. By acceleration, transverse momentum is not changed: Normalized emittance is not changed. Emittance is reduced 2/18 Basic dynamics in circular accelerators (1) Design orbit direction of design orbit, longitudinal coordinate system usually horizontal usually vertical 1/19 Basic dynamics in circular accelerators (2) Assume particle energy is constant Closed orbit One-turn closed line, which can be a orbit of a certain particle. The particle is on this line in any turns. Depend on particle energy. 1/20 Basic dynamics in circular accelerators (3) Orbit of each particle is not necessarily closed Transverse oscillation around the closed orbit (x and y directions) [betatron oscillation] 1/21 Emittance is average of J beta-function : average over all particles Basic dynamics in circular accelerators (4) Dispersion Energy dependence of closed orbit (same for vertical) 1/22 Basic dynamics in circular accelerators (5) Synchrotron oscillation Oscillation in energy - longitudinal position RF cavities: advanced particle is accelerated, delayed particle is decelerated. t Vacc High energy particle goes around long pass Low energy particle goes short pass in one turn. Harmonic oscillation 1/23 (for small z ) Basic dynamics in circular accelerators (Summary) Coordinate system Closed orbit Dispersion Betatron (transverse) oscillation Synchrotron (longitudinal) oscillation 1/24 Beam energy is constant Energy is lost due to synchrotron radiation. Compensated in RF cavities. RF cavities By radiation, angle pf particle is (almost) not changed: Emittance is not changed. Normalized emittance is reduced. By acceleration, transverse momentum is not changed: Normalized emittance is not changed. Emittance is reduced Duplicated Why emittance is damped in damping rings beam direction Radiation loss acceleration Radiation damping (transverse motion) Radiation loss/time Design energy Roughly, 2/26 A B C Radiation damping (longitudinal motion) Higher energy particles loose more energy in unit time. Damping Roughly, 2/28 Radiation excitation 2, Photon angle is uncertain. 1, Number of photons and photon energy are uncertain. e-/e+ energy and angle of electron (positron) becomes uncertain Effect of angle fluctuation is usually very small, compare with the effect of energy fluctuation. radiation in magnetic field 2/30 Radiation excitation The energy fluctuation is source of Energy spread, directory Bunch length, through synchrotron oscillation Transverse emittance through dispersions (through coupling between energy and transverse motion) see next two slides 2/30 In classical theory non-zero dispersions zero dispersion Beam line with 4 bending magnets Look orbits of two particles with different energy. Orbit does not depend on energy Radiation is exactly predictable. beam line can be designed to make: The final orbits are the same if the initial orbits are the same 1/31 In quantum theory non-zero dispersions zero dispersion Beam line with 4 bending magnets Average orbits of two different initial energy particles Possible orbit of one particle Average over many particles is predictable. Beam line is designed for average (center of mass of the beam). But, The final orbit of each particle is uncertain. 2/33 Equilibrium emittance Radiation excitation and radiation damping are balanced at some point. 1/34 Equilibrium Transverse emittance Horizontal: Design almost determines equilibrium emittance. Because designed horizontal dispersion is large. Usually, ring is constructed in a horizontal plane. Dispersions cannot be avoided in bending region. Radiation in bending magnets contribute excitation. Vertical: Errors determine equilibrium emittance. In perfect machine, there is no coupling between vertical motion and other degrees of freedom. Errors (alignment of magnets, etc.) create vertical dispersions (E-y coupling) and x-y coupling These couplings are dominant source of vertical emittance. 2/36 For low vertical emittance Accurate alignment of magnets Mechanical stability and electric stability of magnets Accurate beam position monitors Low emittance beam creation has been studied using ATF at KEK (see later), and is being studied using CesrTA at Cornell. Normalized vertical emittance ( y ) Design of ILC Damping Ring 2.0 x m Achieved in ATF Damping Ring < 2.1x10 -8 m ATF Damping ring is much smaller than ILCs and beam energy is low. (140 m vs. 6 km, 1.3 GeV vs. 5.0 GeV) 2/38 By the way, Particle distribution in e-/e+ circular accelerators The equilibrium distribution is determined from large number of random processes. From central limit theorem, the distribution is (close to) Gaussian (normal distribution). This is one of the main reasons why emittance represents beam quality very well. 1/39 Emittance should be damped between beam pulses. ILC: 200 ms/pulse Damping rate determines equilibrium emittance too. (equilibrium between damping and excitation) Radiation in bending is relatively weak in large rings. Damping wigglers in straight sections. SKIP On Damping Time (inverse of damping rate) SKIP Appendix: Damping time Scattering of particles in a bunch: Momentum exchange between three directions Particles in a bunch have various momentum. Some times, two particles collide each other. exchange momentum. Intra-beam scattering - another excitation 2/41 Intra-beam scattering : Momentum exchange between three directions Typical momentum spread of three directions in ILC damping ring design. Laboratory frameBeam CM frame px 45 keV/c py 2 keV/c pz 7 MeV/c0.7 keV/c x y z From statistical consideration, scattering is regarded as heat transfer between three directions. Dominantly, Heat transfer from horizontal direction to longitudinal direction 2/43 Dominant result of intra-beam scatterings is Heat transfer from horizontal direction to longitudinal direction. (Causing Energy fluctuation of particles.) Very similar to radiation excitation. Radiation excitation: only in magnetic field. (mainly in bending magnets) Intra-beam scattering: Everywhere Intra-beam scatterings increase energy spread and transverse emittance The energy fluctuation is source of Energy spread, directory Bunch length, through synchrotron osccillatin Transverse emittance through dispersions (through coupling between energy and transverse motion) Copied from radiation excitation 2/45 Effect of intra-beam scattering is significant in ILC damping rings, because of high density (large bunch charge and small beam size), and relatively low energy. Now, equilibrium condition is Tail formation due to hard scatterings Hard scattering: large angle change. Rarely occurred. Jump damping core Central limit theorem: Large number of random processes Normal distribution Hard scattering is no longer large number. Making long tails. 2/48 Beam loss due to hard scatterings Typical momentum spread of three directions in ILC damping ring design. Laboratory frameBeam CM frame px 45 keV/c py 2 keV/c pz 7 MeV/c0.7 keV/c Energy acceptance of the ring Laboratory frameBeam CM frame EE ~ 50 MeV5 keV Smaller than horizontal momentum spread large angle scattering can cause beam loss. Reduction of beam life time. This effect is important for circular colliders and light source rings (beam is in the rings for hours), but not for damping rings, where beam is in the ring only for 200 ms. 2/50 Limit of number of bunches and beam current in Damping Rings Injection/extraction kicker speed Instabilities Electron cloud in e+ ring Ions in e- ring 1/51 Injection/ extraction Kicker speed Limit of number of bunches Bunch spacing in linac: ~300 ns Compressed in damping ring as short as possible Circumference/bunch spacing = max. bunch number / pulse Kicker field Extract (inject) bunch by bunch using very fast kicker magnet. Kicker speed determine the bunch spacing circumference/number of bunches 300 ns extraction Main Linac Damping Ring 2/53 Electron cloud instability in positron ring e+ beam pipe e- Formation of electron cloud (Multi bunch effect) Synchrotron radiation photons hit beam pipe wall. photo electrons Some electrons hit the wall again and create secondary electrons. Electrons are attracted to positron beam and accumulated. e+ e- cloud photo electrons secondary electrons Important for positron ring. Electron beam does not attract electrons electron cloud formation is (much?) less significant. 3/56 Electron cloud instability Instability due to electron cloud (Single bunch effect) Electrons move to head part of positron bunch. Following part of bunch is kicked by the gathered electrons. (With small position deviation between head and tail) Electrons move toward that part of the bunch. same for following parts Interaction between the positrons in a bunch and electrons in the cloud causes oscillations of the positrons and electrons. This is instability. Beam current (bunch spacing) is limited. 2/58 Beam pipe surface treatment for low secondary electron emission Beam pipe geometry reducing electron emission Beam pipe geometry reducing electrons travel toward beam Electron absorber (positive electrodes in beam pipe) e+ B In the magnetic field, Electrons cannot reach around the beam. Appling magnetic field reducing electrons travel toward beam Some of these have been already demonstrated (e.g. B-factories). More studies using CesrTA (Cornell U.) starting this year. 2/60 SKIP ? Cures: Preventing/reducing electron accumulation Ion Instability in electron ring electrons are repelled and ions are attracted and remained Electron bunch collides and Ionizing residual gas Following bunches are kicked by ions created by leading bunches. Ions are attracted by the following bunches. Following bunches also create ions. interactions between bunches and ions can cause big oscillation after many bunch passing through the ions. residual gas ions electron bunch 2/62 Introducing gaps in bunch train mitigates Ion Instability Number of bunches in mini-trains is limited. mini-bunch train Ions are disappeared in the train gap no interaction between different mini-trains still instability may be significant in each min-train (Fast Ion Instability) If there is ions remained between gaps (or there is no gap) And ions remained multi-turn. conventional ion instability. 1/63 SUMMARY of Damping Ring Radiation damping Radiation excitation (quantum effect) Intra-beam scattering Limit of bunch numbers, beam current Injection/Extraction kicker speed Instabilities 1/64 SKIP ? Bunch length in damping ring (6~9 mm) is much longer than suitable at IP (0.3 mm). Long bunch in damping ring Power of RF cavities Instabilities (peak current) Short at IP luminosity (e.g. hourglass effect) Bunch is compressed after damping ring. Bunch Compression Wiggler or chicane Make z E correlation Make E z correlation Put a bunch near zero-cross of RF field. accelerate decelerate head Lower energy particles take longer pass Delayed Higher energy particles take shorter pass Advanced SKIP ? Principle of Bunch compressor RF cavities Wiggler or chicane E Z Area in the phase space (longitudinal emittance) is preserved. Decrease bunch length, increase energy spread. SKIP ? Principle of Bunch compressor in phase space Why two stage? Reduce relative energy spread 1-stage compression will make energy spread too large. Large chromatic, dispersive effect In 2-stage system, beam is accelerated after 1st stage, then relative energy spread is reduced. Reduce effect of longitudinal bunch position jitter from damping ring 1-stage: rotate about 90 degree in phase space. position energy, energy position 2-stage: rotate about 180 degree, position position, energy energy Beam energy in damping ring is very stable, but longitudinal position is not. Longitudinal position jitter in the main linac and at IP is important. Energy jitter at main linac entrance is not important, negligible compared with final beam energy. SKIP 2-stage bunch compressor system. 1-stage compression will make momentum spread too large. Large chromatic, dispersive effect 1 RF unit, 3 cryomudules 8 cavities and 1 quad/module 14+1 spare RF units, each is identical to ML RF unit 3 cryomudules/unit, 26 cavities and 1 quad/unit wiggler stage-1, 9 mm 1 mm 5 4.88 GeV stage-2, 1 mm 0.3 mm 4.88 GeV 15 GeV SKIP 2-stage bunch compressor system. ILC Bunch Compressor Longitudinal Phase Space distribution Before BC1 Middle of BC1After BC1 Middle of BC1After BC2 By Peter TenenbaumRF Wiggler SKIP particle source damping ring IP or spin rotators or turn around Main linac - BDS SKIP ? Spin Rotator electron and positron sources produce longitudinally polarized beam. In the damping ring: polarization is in vertical direction. At collision, polarization is longitudinal direction. spin should be rotated Polarization direction can be set from the vertical to any arbitrary angle. SKIP ? solenoid +I in horizontal -I in vertical +I in horizontal -I in vertical Arc 7.9deg Rotation around z-axis Rotation around y-axis (Pair of solenoid for removing x-y coupling) Spin rotates around magnetic field direction.. Spin Rotator Test facilities Advertisement Major test facilities for ILC Accelerating system (cavities and cryomodules) TTF FLASH (DESY) STF (KEK) see later slides ILCTA(?) (FNAL) Damping rings ATF (KEK) see later slides CesrTA (Cornell) Final focus ATF2 (KEK) see later slides ??? (This is old version) STF phase-1 Construction of accelerating system. (in small scale) Two cavities from DESY Two cavities from FNAL Four cavities by KEK Before STF phase 2 Demonstration of 1 Cryo-module (8 cavities) operation Average acc. field >= 31.5 MV/m S1 Global project at STF (KEK) cryostat by KEK cryostat by INFN Demonstration of 1 RF unit of ILC, 3 cryomodules, 26 cavities, Average acc. field >31.5 MV/m Phase 2 of STF (KEK) figure from ILC RDR ATF (Accelerator Test Facility) at KEK ATF - prototype of damping ring of (warm) linear colliders In operation for more than 10 years. Producing very small emittance beam and extract it. Techniques of production of very small emittance. Development of various instrumentations. ATF2 - prototype of final focus system of ILC Test and demonstrate the ILC final focus scheme and stabilization of beam. Operation of newly constructed beam line will start in November, 2008. ATF including ATF2 at KEK Final focus system Newly constructed ~ 100 m 1.3 GeV Linac 140 m ring Extraction line ATF injector Linac ATF Damping Ring Parameters of ATF2 final focus horizontal beam size ~2 um ~700 nm vertical beam size ~40 nm ~ 6 nm Energy is much lower (x1/200) Normalized emittance is comparable emittance is larger (x200) Beam size at IP is larger (x10) Difficulty, sensitivity to various errors are comparable. First Goal of ATF2 project Demonstration of the focusing method of ILC ~40 nm RMS vertical beam size Will be confirmed using Shintake-monitor Beam size monitor for ATF2-IP (Tokyo Univ., KEK) FFTB result Result in FFTB at SLAC Installation at ATF2-IP (2008/5) by Terunuma e- detector laser bend Second Goal of ATF2 project Demonstration of beam stabilization ~2 nm vertical jitter Feedback using nano-BPM (beam position monitor with nano-meter resolution) ATF2 - international collaboration ATF started as a project of KEK. But big contributions in operation, beam studies and improvement of the machine from over seas. Operation/study plans are controlled by international committees. ATF2 started as an international project Design, construction and operation are all international collaboration. Construction cost is shared by three regions. (except for infra- structures) Model of ILC, though the scale is much smaller. Major members from France and Asia LAL, LAPP, IHEP, KNU, PAL, KEK, U.Tokyo, (CERN) Beam position monitors for ATF2, made by PAL (Korea) photo by Toge Construction of ATF2 beam line, Quad magnets are made by IHEP(China) photo by Toge Installation of final quad magnet system photo by Toge Daily ATF Operation meeting photo by Araki END SKIP Preservation of Low Emittance (in main linac) SKIP Need very low emittance beam at IP, especially in vertical. Emittance becomes very low in the damping ring. Transport and accelerate the beam from the damping ring preserving the low emittance is a big issue. (Vertical projected emittance is the most important, and difficult to preserve.) Preservation of Low Emittance y y y SKIP If area in phase space is preserved, it is not necessarily emittance preservation. z y z y y y y Coupling between vertical motion to other degree of freedom should be avoided or should be well controlled. SKIP Even if 6-dimensional volume is preserved, Projected emittance can be increased. (Coupling between y and z) (Coupling between y and E) SKIP Two major sources of emittance dilution Wakefield Transverse wakefield of accelerating cavities Short range: in a bunch Long range: inter-bunch Dispersive effect Particles with different energy have different orbit Different angle change in magnetic field Different angle change in tilted accelerating field Leading particle induce EM field Following particle feel the filed In rotationally symmetric structure, Transverse field strength is proportional to offset of leading particle. (If the offset is small.) : called dipole mode field Transverse momentum change of particle of longitudinal position z: wakefunction Charge and transverse position of i-th leading particle SKIP Transverse Wakefield current and charge are induced at discontinuities Induced fields catch up following particles SKIP Short range wakefield SKIP Short range - Single bunch effect Wakefunction is monotonic function of distance Not seriously important for ILC Transverse Wakefield of Accelerating cavities 1 2 3 n 2nd bunch is kicked by field induced by 1st bunch 3rd bunch is kicked by field induced by 1st and 2nd bunches nth bunch is kicked by field induced by n-1 precedent bunches If so, kicks in all cavities have the same phase. SKIP Wakefunction is sum of some resonance modes Each mode has its own frequency and damping constant Damping is important It is important to make some spread of frequencies, cavity by cavity. (All cavities should not be precisely identical.) Avoid resonant growth of oscillation. Transverse Long range Wakefield of Accelerating cavities - Multi-bunch effect Special shapes: Accelerating mode should be stopped. HOM should go through. - trapped mode may cause problem. TESLA-TDR SKIP Damping of Wakefield Two HOM (Higher Order Mode) Couplers at both sides of a cavity Random spread f /f=0.1% SKIP Wakefunction envelope from HOMs (from TESLA-TDR) with/without random frequency spread (50 cavities) and damping No spread length scale by factor 1/ a frequency change by factor a SKIP Due to low RF frequency of ILC, using superconducting cavity, wakefield is much less serious, compare with warm LC (X-band or higher frequency). SKIP Emittance dilutions due to Dispersive effect Any transverse kick in EM field will cause dispersion (energy correlated orbit difference): Momentum change of a particle in an ultra relativistic beam in any EM fields does not depend on its energy. Then, angle change is inversely proportional to its energy. Beam going through off center of quadrupole filed Misalignment of quadrupole magnet Injection error etc. Tilting of accelerating field Tilting misalignment of accelerating cavities This is a very important for ILC. is called dispersion, or linear dispersion. Higher terms are sometimes called higher order dispersions. This is good in storage rings but is not well defined in transport lines and linear accelerators. Instead of this, we can define as follows. SKIP Appendix: Dispersion Expressing energy dependence of transverse position of the beam: Going through 1 quad off center First order dispersion Going through 2 quads off center 2nd order dispersion..... Going through n quads off center n-th order dispersion First order dispersion can be measured and corrected. But higher order dispersion is difficult to measure and correct. It is desirable to correct (first order) dispersion everywhere locally. SKIP Result of simulation with quad magnet misaligment Each ellipse show monochromatic beam. Ten different energyies. This orbit is no good. This is much better. SKIP One promising method is Dispersion Free Steering Steering beam to minimize dispersion Need to measure dispersion: Need to change beam energy. Need beam position monitors (every quad has) Need many steering magnets (every quad has) Cure of emittance dilutions due to Dispersive effect Quad Offset300 m Quad strength0.25% Quad roll300 rad RF Cavity Offset300 m RF Cavity Pitch200 rad BPM Offset (initial) 300 m Cryomodule Offset 200 m Cryomodule Pitch20 rad SKIP Example of simulations emittance along the linac SKIP SUMMARY of Preservation of Emittance Two major sources of emittance dilution Wakefield Short range: Monotonic with distance Long range: Resonant modes; Detuning Dispersive effect Misalignment quad magnets (offset) and cavities (tilt) Cured by Dispersion Free Steering in ILC main linac