TESLA Linear Collider Luminosity Related Issues Nick Walker

TESLA Linear Collider Luminosity Related Issues Nick Walker (DESY) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

The Luminosity Issue Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

The Luminosity Issue Low repetition rate: 5 Hz • limited by cryogenics power • impact on ground motion stabilisation (feedback) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

The Luminosity Issue Compensated by • long bunch train: – fast intra-train orbit stabilisation (feedback) nb = 2800 • High bunch charge: N = 2× 1010 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

The Luminosity Issue Emittance Preservation: • low wakefields (low frequency) • relatively loose tolerances Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Wakefields (alignment tolerances) Transverse Wakefield Kick f 3 Ratio of deflecting wakefield to accelerating field for 1 mm offset 10 -3 10 -4 10 -5 10 -6 TESLA Nick Walker DESY C-band X-band ITRP Meeting - RAL - 28. 02. 04 CLIC

The Luminosity Issue High Beam-Beam Disruption (Enhancement) banana effect • factor ~2 for luminosity • collision is unstable (kink instability) • tighter tolerance on emittance dilution Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

500 Ge. V C. M. Parameters Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

the TESLA TDR linear collider Luminosity Issues: • ML dynamics • Damping Ring • Sources (e+) • Beam Delivery & IR Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA Linac Beam Dynamics • Emittance Preservation • Alignment tolerances • Beam based alignment Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA Long Range Wakes 337 ns bunch spacing • Random detuning • HOM absorbers 36 cavity average, 0. 1% frequency spread All modes damped below 1 105 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA Long Range Wakes vertical offset (mm) Effect of 1 sy oscillation along linac bunch number • 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 DESY ITRP Meeting - RAL - 28. 02. 04

Single Bunch Wakefields V/p. C/m Accurate calculation of single-bunch transverse wakefield 30% less transverse kick than previous TDR estimate. z (mm) I. Zagorodnov, T. Weiland (2003) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Assumed Alignment Errors • • • quad offsets: 300 mm cavity tilts: 300 mrad BPM offsets: 200 mm CM offsets: 200 mm BPM resolution: 10 mm wrt CM axis single-shot these values have been used in simulations of linac tuning Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

norm. vertical emittance (nm) Dispersion Free Steering 45. 0 The effect of upstream beam jitter on DFS simulations for the TESLA linac. with incoming jitter fitted out 40. 0 no jitter 35. 0 1 sy initial jitter TDR budget 30. 0 10 mm BPM noise 25. 0 20. 0 0 50 100 150 200 250 Quadrupole # 300 average over 100 random machines Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 350 uncorrected cavity tilts cause problems for TESLA

Ballistic Alignment systematically turn off sections of linac Use ‘ballistic beam’ to define (straight) reference line. Less sensitive to • model errors • beam jitter average over 100 seeds Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Ballistic Alignment systematically turn off sections of linac Use ‘ballistic beam’ to define (straight) reference line. 3% Energy Spread from Bunch Compressor average over 100 seeds Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 Less sensitive to • model errors • beam jitter

Ballistic Alignment We can tune out linear and correlation using bumps or dispersion correction in BDS average over 100 seeds Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

100 Random Machines 94% 85% dispersion corrected Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Ballistic Alignment TO DO • Control of Ballistic Beam – Show that ‘fat’ ballistic beam can be safely transported through linac • Large cavity irises (Ø 70 mm) a benefit • Study additional potential problems – stray magnetic fields etc. • Confident we can achieve desired budget Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Other Sub-Systems • Spin Rotation / Bunch Compression • Source – ‘dog bone’ damping ring – undulator-driven positron source • Beam Delivery System Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 critical TESLA systems

TDR Bunch Compressor • Compression factor of 20 in single stage sz = 6 mm 300 mm drms = 1. 3‰ 3% • RF (4 cryomodules) with Vpk ~1 Ge. V, f = -155° DV = -423 MV • Wiggler section (~100 m) to generate required R 56 • Problems: – Cavity Tilts in Module (see later) – Large 3% DP/P – Tuning! Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TDR Ring-to-Linac (RTL) RF Spin Rotator wiggler Bunch Compressor Diagnostics (emittance measurement) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

BC Cavity Tilts • Slope of tilted RF results in correlated z-y kick along bunch (sz = 6 mm) • 300 mrad RMS tilts gives average of Dey~140% for TDR design! Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

BC Cavity Tilts acceleration (along bunch) Nick Walker DESY cavity tilt kick ITRP Meeting - RAL - 28. 02. 04 Resulting correlation (dispersion)

BC Cavity Tilts mean: 138% Results of tracking simulations. Emittance estimated at exit of RF section mean: 2% different scale! Emittance after removing d correlation [best you can achieve] 1000 random seeds Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 b) – possible two-stage compression system • No De budget in TDR – assumed De = 0 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA TDR Positron Source • Photons (~20 Me. V g) produced by high energy electron beam in undulator placed at exit of e- linac (upstream of BDS and IR) • Thin target (0. 4 X 0) converts the g to e+e- pairs Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

+ Source Parameters TDR e SLC TESLA e+/pulse (3 -5)× 1010 5. 6× 1013 bunches/pulse 1 2820 pulse duration 3 ps 0. 95 ms bunch spacing 8. 3 ms 337 ns rep. frequency 120 Hz 5 Hz Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

+ Source Parameters TDR e undulator length 135 m (TDR: 100 m) av. photon power 135 k. W av. deposited target power 5 k. W photon beam size on target 0. 7 mm capture efficiency 16% e+ / e- ~2 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 see damping ring

Advantages • significantly reduced power deposition in thin target (~5 k. W) • 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 DESY ITRP Meeting - RAL - 28. 02. 04

Disadvantages • Requires e-linac with 150 Ge. V – TDR solution to use main e- linac – coupling e- to e+ production raises questions of • operability • reliability • commissioning strategy • Never been done before can be mitigated through R&D no real show stoppers – although physics is well understood! – E 166 experiment at SLAC Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA Damping Ring • TESLA bunch train 2820 × 337 ns = 950 ms 285 km long • Extract every bunch separately, bunch spacing given by shortest kicker rise/fall time 20 ns × 2820 56 ms 17 km long • Save tunnel cost: DR in main linac tunnel and short return arcs dogbone Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Dogbone DR Concept Need ~450 m of wiggler to achieve required damping time (28 msec) B 2 dl= 605 T 2 m • Permanent Magnet Wiggler with Bmax = 1. 6 T, l=0. 4 m • Radiated Power (160 m. A) over 450 m : 3 MW • Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

TESLA DR Parameters e+ 0. 01 m Injected RMS Emittance Ejected Emittance hor / ver Injected Energy Spread e 4× 10 -5 m 8× 10 -6 m / 2× 10 -8 m 0. 5 % Ejected Bunch Length Damping Time 0. 5 % 6 mm 28 msec Number of Bunches 44 msec 2820 Ejected Bunch Spacing 337 ns Particles per Bunch 2× 1010 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Wiggler Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Space Charge Tune Shift • Unusually large circumference / energy ratio – final emittance is space-charge limited! – Quantitive effect on steady-state ey 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 Ge. V – increase transverse beam size in long straight sections through local x-y coupling • radical! – multiple ring designs (cost!) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Kicker Requirements 337 ns 0. 6 mrad ± 0. 05% 0. 01 Tm Ripple: 0. 05% 40 ns • 2820 pulses with 3 MHz repetition rate • 5 Hz repetition rate of macro-pulse Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

RF Kicker • RF kicker system – – Delahaye 93 Koshkarev 95 Gollin et al. 2002 INFN-LNF 2003 • With enough harmonics very sharp pulse possible • No flexibiliy for different bunch distances Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Stripline Kicker • • Stripline Kicker (1996) C-Yoke Kicker (2000 -. . . ) Kicker technology available Main Challenge: Pulser – IGBT Transformer Switch – MOSFET Stacks Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 ongoing R&D (XFEL needs fast kickers too!)

TTF Measurements Frank Obier (DESY), Guido Blokesch (IPP) averaged over 50 pulses / point Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Dynamic Aperture • Large average injected beam power – 224 k. W • 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 DESY ITRP Meeting - RAL - 28. 02. 04

Emittance Control • BPM and H+V steerer at each quadrupole (800) • Skew windings on every sextupole (300) • Combined orbit and dispersion correction with steerer linear response approach • Skew correction Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Emittance Control Simulated alignment errors horizontal vertical Quadrupole 0 0. 1 mm Sextupole 0 0. 1 mm BPM resolution 0 1 mm BPM (relative to quadrupole) 0 0. 1 mm BPM resolution critical for required level of dispersion control for all LC DR Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Emittance Control (simulation) Simulation of vertical emittance after application of orbit tuning algorithm 100 random alignment seeds 88% of machines below achieved <14 nm (goal) goal Nick Walker DESY space-charge coupling bumps not included (vertical correction only). ITRP Meeting - RAL - 28. 02. 04

Emittance Stability • Quadrupole vibration of 350 nm (RMS) gives 10% increase in emittance • Slow drifts [based on ATL model] indicate the following corrections will be needed: – closed-orbit correction every 2 minutes – dispersion correction every 11 hours Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Collective Effects • IBS no issue because of high energy • Coupled Bunch e+/e- e+/e– HOM‘s suppresed by SC cavities e+/e– resistive wall damped with feedback e– ion trapping requires P 1× 10 -10 mbar in straight sections (nb: no synch. rad. ) – more studies needed for fast beam ion instability e- (common problem) – e-cloud seems OK because of bunch distance e+ [input from LHC] Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Stray Field Problems • Time varying stray fields at beginning of linac beam pulse from Klystron turn-on • Measured to be > 1 m. T • Effect checked by simulating 5 m. T m at each klystron position (every 48 m) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

Head-On Collision or: “too cross, or not too cross: that is the question” • Large bunch spacing (337 ns) 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) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

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! solutions under consideration – difficult to optimise extraction line for diagnostic use All said: TESLA does have the choice! Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

TDR BDS Layout Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Fast Extraction can achieve ‘single bunch delay’ Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

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 DESY ITRP Meeting - RAL - 28. 02. 04

IP Fast (orbit) Feedback Long bunch train: 2820 bunches Beam-beam kick tb = 337 ns Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

IP Fast (orbit) Feedback Simulation of system with realistic errors Systems successfully tested at TTF Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Long Term Stability 1 second Nick Walker DESY 1 hour ITRP Meeting - RAL - 28. 02. 04 1 day 10 days

Long Term Stability 1 second 1 hour No Feedback Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 1 day 10 days

Long Term Stability 1 second 1 hour With Fast Beam -Beam Feedback Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 1 day 10 days

Long Term Stability 1 second 1 hour 1 day With FBBF and (slower) BDS orbit correction Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 10 days

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 DESY ITRP Meeting - RAL - 28. 02. 04

Beam-Beam Issues D. Schulte. PAC 03, RPAB 004 TESLA luminosity as a function of linac emittance growth Note: Dey will contain a correlated component due to wakefields Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Beam-Beam Issues D. Schulte. PAC 03, RPAB 004 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 Rigid bunch approximation

Beam-Beam Issues D. Schulte. PAC 03, RPAB 004 GUINEAPIG result ‘banana effect’ Now optimise (scan) collision offset and angle (collision feedback) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Beam-Beam Issues D. Schulte. PAC 03, RPAB 004 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04 optimise beam-beam offset

Beam-Beam Issues D. Schulte. PAC 03, RPAB 004 optimise beam-beam offset and angle OK for ‘static’ effect dynamic effects still a problem Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Simulating the Dynamic Effect LINAC BDS IR IR BDS IP FFBK • 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 Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Simulating the Dynamic Effect intra-train fast feedback scans angle/offset at IP to optimise luminosity IP beam angle Nick Walker DESY IP beam offset ITRP Meeting - RAL - 28. 02. 04

Simulating the Dynamic Effect 2 1034 cm-2 s-1 (one seed) currently studying cause of 30% reduction room for improvement (additional key feedbacks) Nick Walker DESY ITRP Meeting - RAL - 28. 02. 04

Last Word • LINAC technology is mature and [we believe] ready to go (cf. talk by RB) • TESLA’s ‘? ’ lie mostly in the other critical sub-systems: – dogbone DR – e+ source – bunch compressor Nick Walker DESY No show stoppers International Design Team effort ITRP Meeting - RAL - 28. 02. 04