Nick Walker DESYITRP Meeting - RAL TESLA Linear Collider Luminosity Related Issues Nick Walker (DESY) Nick Walker DESYITRP Meeting - RAL Content Luminosity Issues oft quoted advantages of s.c. RF in a nutshell Main linac dynamics emittance tuning Bunch Compressor Undulator-Based Positron Source Damping Ring many critical issues Beam Delivery System head-on collision scheme machine (collimator) protection philosophy Luminosity Stabilisation & Feedback Nick Walker DESYITRP Meeting - RAL The Luminosity Issue Nick Walker DESYITRP Meeting - RAL The Luminosity Issue Low repetition rate: 5 Hz limited by cryogenics power impact on ground motion stabilisation (feedback) Nick Walker DESYITRP Meeting - RAL The Luminosity Issue Compensated by long bunch train: n b = 2800 fast intra-train orbit stabilisation (feedback) High bunch charge: N = 210 10 Nick Walker DESYITRP Meeting - RAL The Luminosity Issue Emittance Preservation: low wakefields (low frequency) relatively loose tolerances Nick Walker DESYITRP Meeting - RAL 6 10 5 10 4 10 3 Ratio of deflecting wakefield to accelerating field for 1mm offset Wakefields (alignment tolerances) Transverse Wakefield Kick f 3 TESLA C-band X-band CLIC Nick Walker DESYITRP Meeting - RAL The Luminosity Issue High Beam-Beam Disruption (Enhancement) factor ~2 for luminosity collision is unstable (kink instability) tighter tolerance on emittance dilution banana effect Nick Walker DESYITRP Meeting - RAL GeV C.M. Parameters Nick Walker DESYITRP Meeting - RAL the TESLA TDR linear collider ML dynamics Damping Ring Sources (e + ) Beam Delivery & IR Luminosity Issues: Nick Walker DESYITRP Meeting - RAL TESLA Linac Beam Dynamics Emittance Preservation Alignment tolerances Beam based alignment Nick Walker DESYITRP Meeting - RAL TESLA Long Range Wakes All modes damped below 1 cavity average, 0.1% frequency spread 337 ns bunch spacing Random detuning HOM absorbers Nick Walker DESYITRP Meeting - RAL TESLA Long Range Wakes bunch number vertical offset ( m) Effect of 1 y oscillation along linac Pattern remains the same (difference at nm level) Result of loose tolerances (cavity offsets) Static part (almost all) can be fixed with feed forward Nick Walker DESYITRP Meeting - RAL Single Bunch Wakefields z ( m) V/pC/m Accurate calculation of single-bunch transverse wakefield 30% less transverse kick than previous TDR estimate. I. Zagorodnov, T. Weiland (2003) Nick Walker DESYITRP Meeting - RAL Assumed Alignment Errors quad offsets:300 m cavity offsets:300 m cavity tilts:300 rad BPM offsets:200 m CM offsets:200 m BPM resolution:10 m wrt CM axis single-shot these values have been used in simulations of linac tuning Nick Walker DESYITRP Meeting - RAL Dispersion Free Steering The effect of upstream beam jitter on DFS simulations for the TESLA linac. 1 y initial jitter 10 m BPM noise average over 100 random machines TDR budget uncorrected cavity tilts cause problems for TESLA Nick Walker DESYITRP Meeting - RAL Ballistic Alignment Less sensitive to model errors beam jitter average over 100 seeds systematically turn off sections of linac Use ballistic beam to define (straight) reference line. Nick Walker DESYITRP Meeting - RAL Ballistic Alignment Less sensitive to model errors beam jitter average over 100 seeds systematically turn off sections of linac Use ballistic beam to define (straight) reference line. 3% Energy Spread from Bunch Compressor Nick Walker DESYITRP Meeting - RAL Ballistic Alignment We can tune out linear and correlation using bumps or dispersion correction in BDS average over 100 seeds Nick Walker DESYITRP Meeting - RAL Random Machines dispersion corrected 94% 85% Nick Walker DESYITRP Meeting - RAL Ballistic Alignment TO DO Control of Ballistic Beam Show that fat ballistic beam can be safely transported through linac Large cavity irises (70mm) a benefit Study additional potential problems stray magnetic fields etc. Confident we can achieve desired budget Nick Walker DESYITRP Meeting - RAL Other Sub-Systems Spin Rotation / Bunch Compression Source dog bone damping ring undulator-driven positron source Beam Delivery System critical TESLA systems Nick Walker DESYITRP Meeting - RAL TDR Bunch Compressor Compression factor of 20 in single stage z = 6 mm 300 m rms = 1.3 3% RF (4 cryomodules) with V pk ~1 GeV, = -155 V = -423 MV Wiggler section (~100 m) to generate required R 56 Problems: Cavity Tilts in Module (see later) Large 3% P/P Tuning! Nick Walker DESYITRP Meeting - RAL TDR Ring-to-Linac (RTL) Spin Rotator Bunch Compressor Diag- nostics (emittance measurement) RFwiggler Nick Walker DESYITRP Meeting - RAL BC Cavity Tilts Slope of tilted RF results in correlated z-y kick along long bunch ( z = 6 mm) 300 rad RMS tilts gives average of y ~140% for TDR design! Nick Walker DESYITRP Meeting - RAL BC Cavity Tilts cavity tilt kick acceleration (along bunch) Resulting correlation (dispersion) Nick Walker DESYITRP Meeting - RAL BC Cavity Tilts Results of tracking simulations. Emittance estimated at exit of RF section Emittance after removing correlation [best you can achieve] mean: 138% mean: 2% different scale! 1000 random seeds Nick Walker DESYITRP Meeting - RAL RTL Emittance Dilution Tuning dispersion (bunch tilt) out downstream requires tuning knobs (bumps) emphasis on emittance measurement final achievable emittance set by resolution (10% ?) Re-think of design stronger focusing in RF section (smaller ) possible two-stage compression system No budget in TDR assumed = 0 Nick Walker DESYITRP Meeting - RAL TESLA TDR Positron Source Photons (~20 MeV ) produced by high energy electron beam in undulator placed at exit of e linac (upstream of BDS and IR) Thin target (0.4X 0 ) converts the to e + e pairs Nick Walker DESYITRP Meeting - RAL TDR e + Source Parameters SLCTESLA e+/pulse(3-5) 10 13 bunches/pulse12820 pulse duration3 ps0.95 ms bunch spacing8.3 ms337 ns rep. frequency120 Hz5 Hz Nick Walker DESYITRP Meeting - RAL TDR e + Source Parameters undulator length135 m (TDR: 100m) av. photon power135 kW av. deposited target power5 kW photon beam size on target0.7 mm capture efficiency16% e + / e ~2 see damping ring Nick Walker DESYITRP Meeting - RAL Advantages significantly reduced power deposition in thin target (~5 kW) smaller emittance beam produced less multiple coulomb scattering reduced acceptance requirements for DR no pre-DR foreseen much cheaper / less complex than equivalent conventional source for TESLA if conventional source is even possible! Naturally allows upgrade to polarised e + source Nick Walker DESYITRP Meeting - RAL Disadvantages Requires e-linac with 150 GeV TDR solution to use main e- linac coupling e- to e+ production raises questions of operability reliability commissioning strategy Never been done before although physics is well understood! E166 experiment at SLAC can be mitigated through R&D no real show stoppers Nick Walker DESYITRP Meeting - RAL TESLA Damping Ring TESLA bunch train 2820 337 ns = 950 s 285 km long Extract every bunch separately, bunch spacing given by shortest kicker rise/fall time 20 ns 2820 56 s 17 km long Save tunnel cost: DR in main linac tunnel and short return arcs dogbone Nick Walker DESYITRP Meeting - RAL Dogbone DR Concept B 2 dl= 605 T 2 m Permanent Magnet Wiggler with B max = 1.6 T, =0.4 m Radiated Power (160 mA) over 450 m : 3 MW Need ~450m of wiggler to achieve required damping time (28 msec) Nick Walker DESYITRP Meeting - RAL TESLA DR Parameters e+e+ ee Injected RMS Emittance 0.01 m410 -5 m Ejected Emittance hor / ver810 -6 m / 210 -8 m Injected Energy Spread0.5 % Ejected Bunch Length6 mm Damping Time28 msec44 msec Number of Bunches2820 Ejected Bunch Spacing337 ns Particles per Bunch210 10 Nick Walker DESYITRP Meeting - RAL Wiggler Nick Walker DESYITRP Meeting - RAL Space Charge Tune Shift Unusually large circumference / energy ratio final emittance is space-charge limited! Quantitive effect on steady-state y unknown, but probably >factor 2 increase. Solutions: Increase energy difficult lattice in arc more RF needed cost optimum turns out to be at 4-5 GeV increase transverse beam size in long straight sections through local x-y coupling radical! multiple ring designs (cost!) Nick Walker DESYITRP Meeting - RAL Kicker Requirements 337 ns 40 ns 0.6 mrad 0.05% 0.01 Tm Ripple: 0.05% 2820 pulses with 3 MHz repetition rate 5 Hz repetition rate of macro-pulse Nick Walker DESYITRP Meeting - RAL RF Kicker RF kicker system Delahaye 93 Koshkarev 95 Gollin et al INFN-LNF 2003 With enough harmonics very sharp pulse possible No flexibiliy for different bunch distances Nick Walker DESYITRP Meeting - RAL Stripline Kicker Stripline Kicker (1996) C-Yoke Kicker ( ) Kicker technology available Main Challenge: Pulser IGBT Transformer Switch MOSFET Stacks ongoing R&D (XFEL needs fast kickers too!) Nick Walker DESYITRP Meeting - RAL TTF Measurements Frank Obier (DESY), Guido Blokesch (IPP) averaged over 50 pulses / point Nick Walker DESYITRP Meeting - RAL Dynamic Aperture Large average injected beam power 224 kW Wiggler dominated dynamics leads to too small (dynamic) aperture for e+ ring: acceptance approx. factor 2 too small culprit: wiggler non-linearities Needs additional study R&D on wiggler to reduce non-linearities introduction of octupoles into lattice Do have factor 2 safety margin in e+ production requires careful collimation in DR transfer line to reduce losses in ring Nick Walker DESYITRP Meeting - RAL Emittance Control BPM and H+V steerer at each quadrupole (800) Skew windings on every sextupole (300) Combined orbit and dispersion correction with steerer Skew correction linear response approach Nick Walker DESYITRP Meeting - RAL Emittance Control horizontalvertical Quadrupole00.1 mm Sextupole00.1 mm BPM resolution0 1 m BPM (relative to quadrupole)00.1 mm Simulated alignment errors BPM resolution critical for required level of dispersion control for all LC DR Nick Walker DESYITRP Meeting - RAL Emittance Control (simulation) goal Simulation of vertical emittance after application of orbit tuning algorithm 100 random alignment seeds 88% of machines below achieved 1 T Effect checked by simulating 5 T m at each klystron position (every 48 m) Nick Walker DESYITRP Meeting - RAL Stray Field Problems Leads to variation of closed orbit dispersion at extraction Blow-up of projected emittance Fast correction needed: dispersion correction (difficult) fast turn-by-turn distributed orbit feedback [accuracy 75 mm RMS] Nick Walker DESYITRP Meeting - RAL Beam Delivery System Issues To large extent linac techology independent Two possible areas of difference: possibility of a head-on collision spoiler protection philosophy Nick Walker DESYITRP Meeting - RAL Head-On Collision Large bunch spacing (337ns) puts first parasitic crossing at 50 m from IP outside of physics detector allows for a head-on collision arrangement Head-on scheme does not require compact final quadrupole relatively large aperture s.c. magnet based on LHC can be used Some potential benefit for physics small angle tagging etc. (still under discussion) or: too cross, or not too cross: that is the question Nick Walker DESYITRP Meeting - RAL Head-on collision pros no crab-crossing cavities required no tilted solenoid field no need for compact final quad solution compact s.c. design from BNL looks very promising! low angle physics contentious comparatively cheap single tunnel no separation shafts Nick Walker DESYITRP Meeting - RAL Head-on collision cons extraction system complex requires electrostatic separators and a septum magnet (reliability/operability?) masking and collimation difficult beamstrahlung stay-clear difficult current TDR solution does not work! difficult to optimise extraction line for diagnostic use All said: TESLA does have the choice! solutions under consideration Nick Walker DESYITRP Meeting - RAL Machine Protection long bunch train + large bunch spacing allows to abort pulse within the train fast kickers can extract beam to the dump in the case of a fault beam can be turned off at the DR from BDS approx. 200/2820 bunch delay Nick Walker DESYITRP Meeting - RAL TDR BDS Layout Nick Walker DESYITRP Meeting - RAL Fast Extraction can achieve single bunch delay Nick Walker DESYITRP Meeting - RAL Luminosity Stability Ground motion vibration; slow drifts Fast Intra-Train Feedback beam-beam collision feedback Effect of slow drifts Importance of orbit control (BDS: critical) High-Disruption Regime beam-beam kink instability makes TESLA sensitive Nick Walker DESYITRP Meeting - RAL IP Fast (orbit) Feedback Beam-beam kick Long bunch train: 2820 bunches t b = 337 ns Nick Walker DESYITRP Meeting - RAL IP Fast (orbit) Feedback Systems successfully tested at TTF Simulation of system with realistic errors Nick Walker DESYITRP Meeting - RAL Long Term Stability 10 days 1 hour 1 day 1 second Nick Walker DESYITRP Meeting - RAL Long Term Stability 10 days 1 hour 1 day 1 second No Feedback Nick Walker DESYITRP Meeting - RAL Long Term Stability 10 days 1 hour 1 day 1 second With Fast Beam-Beam Feedback Nick Walker DESYITRP Meeting - RAL Long Term Stability 10 days 1 hour 1 day 1 second With FBBF and (slower) BDS orbit correction Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues: Bananas TESLA: high disruption regime: long. correlated emittance growth causes excessive luminosity loss (banana effect) Brinkmann, Napoly, Schulte, TESLA-01-16 Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues TESLA luminosity as a function of linac emittance growth D. Schulte. PAC03, RPAB004 Note: y will contain a correlated component due to wakefields Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues Rigid bunch approximation D. Schulte. PAC03, RPAB004 Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues GUINEAPIG result banana effect Now optimise (scan) collision offset and angle (collision feedback) D. Schulte. PAC03, RPAB004 Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues optimise beam-beam offset D. Schulte. PAC03, RPAB004 Nick Walker DESYITRP Meeting - RAL Beam-Beam Issues optimise beam-beam offset and angle OK for static effect dynamic effects still a problem D. Schulte. PAC03, RPAB004 Nick Walker DESYITRP Meeting - RAL Simulating the Dynamic Effect Realistic simulated bunches at IP linac (PLACET, D.Schulte) BDS (MERLIN, N. Walker) IP (GUINEAPIG, D. Schulte) FFBK (SIMULINK, G. White) bunch trains simulated with realistic errors, including ground motion and vibration Luminosity assumed measured by fast lumi (e+e- pair) monitor LINACBDSIRBDSIR IP FFBK Nick Walker DESYITRP Meeting - RAL Simulating the Dynamic Effect IP beam angleIP beam offset intra-train fast feedback scans angle/offset at IP to optimise luminosity Nick Walker DESYITRP Meeting - RAL Simulating the Dynamic Effect 2 cm 2 s 1 (one seed) currently studying cause of 30% reduction room for improvement (additional key feedbacks) Nick Walker DESYITRP Meeting - RAL Last Word LINAC technology is mature and [we believe] ready to go (cf. talk by RB) TESLAs ? lie mostly in the other critical sub-systems: dogbone DR e+ source bunch compressor No show stoppers International Design Team effort