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Linear Colliders Lecture 4 The real designs

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1 Linear Colliders Lecture 4 The real designs
The designs Parameter Overview NC/SC driven differences Damping rings CLIC two beam scheme

2 Generic Linear Collider
Electron Gun Deliver stable beam current Damping Ring Reduce transverse phase space (emittance) so smaller transverse IP size achievable Bunch Compressor Reduce σz to eliminate hourglass effect at IP Positron Target Use electrons to pair-produce positrons Main Linac Accelerate beam to IP energy without spoiling DR emittance Final Focus Demagnify and collide beams C.Pagani We have seen the different sub-systems in the previous lectures Now let’s look at the real designs in a bit more detail…

3 Linear Collider Designs
TEV Superconducting Linear Accelerator (TESLA) Based on 1.3 GHz superconducting cavities developed by DESY Japanese Linear Collider / Next Linear Collider (JLC/NLC) Based on 11.4 GHz normal conducting RF system promoted by SLAC/KEK Compact Linear Collider (CLIC) Multi-TeV study based on 12 GHz (30 GHz previously) normal conducting RF using a Two-Beam RF generation concept developed at CERN International Linear Collider (ILC) International technology choice after review and evaluation of different designs  Based on SC cavities, 500 GeV → 1 TeV International groups are reviewing the design Reference Design Report (like Technical DR) released

4 First LC: SLC T.Raubenheimer

5 Parameter comparison SLC TESLA ILC J/NLC CLIC
Technology NC Supercond. Gradient [MeV/m] 20 25 31.5 50 100 CMS Energy E [GeV] 92 RF frequency f [GHz] 2.8 1.3 11.4 12.0 Luminosity L [1033 cm-2s-1] 0.003 34 21 Beam power Pbeam [MW] 0.035 11.3 10.8 6.9 5 Grid power PAC [MW] 140 230 195 130 Bunch length sz* [mm] ~1 0.3 0.11 0.03 Vert. emittance gey [10-8m] 300 3 4 2.5 Vert. beta function by* [mm] ~1.5 0.4 0.1 Vert. beam size sy* [nm] 650 5.7 2.3 Parameters (except SLC) at 500 GeV

6 Accelerating gradient
Normal conducting cavities have higher gradient with shorter RF pulse length Superconducting cavities have lower gradient (fundamental limit) with long RF pulse WARM SC

7 SC Technology In the past, SC gradient typically 5 MV/m and expensive cryogenic equipment TESLA development: new material specs, new cleaning and fabrication techniques, new processing techniques Significant cost reduction Gradient substantially increased Electropolishing technique has reached ~35 MV/m in 9-cell cavities Still requires essential work 31.5 MV/m ILC baseline Chemical polish Electropolishing

8 Achieved accelerating gradients
Recent progress by R&D programme to systematically understand and set procedures for the production process goal to reach a 50% yield at 35 MV/m by the end of 2010 already approaching that goal 90% yield foreseen later ILC design 2009 2007

9 Damaged CLIC structure iris longitudinal cut
Warm structures Need conditioning to reach ultimate gradient RF processing can melt field emission points  higher fields More energy: electrons generate plasma and melt surface Molten surface splatters and generates new field emission points! Excessive fields can also damage the structures Design structures with low Esurf/Eacc Study new materials (Mo, W) Damaged CLIC structure iris longitudinal cut

10 Warm structure conditioning
Increase input power gradually NLC structure processing CLIC structure conditioning

11 Bunch structure SC allows long pulse, NC needs short pulse with smaller bunch charge 0.370 2625 ILC 970 ILC 20000 0.0005 312 0.37 12 0.156

12 Warm vs Cold RF Collider
Normal Conducting High gradient  short linac  High rep. rate  GM suppression  Small structures  strong wakefields  Generation of high peak RF power  Superconducting long pulse  low peak power  large structure dimensions  low WF  very long pulse train  feedback within train  SC structures  high efficiency  Gradient limited <40 MV/m  longer linac  (SC material limit ~ 55 MV/m) low rep. rate  bad GM suppression (ey dilution)  Large number of e+ per pulse  very large (unconventional) DR 

13 Damping rings TME (theoretical minimum emittance) lattice
NLC: similar to existing synchroton light sources 300 m Main Damping Ring 3 Trains of 192 bunches 1.4 ns bunch spacing 231 m Predamping Ring 2 Trains of 192 bunches 30 m Wiggler Injection and RF Circumference Correction and Extraction 110 m Injection Line Transfer Line 90 m Extraction Line Spin Rotation High field in dipole vertical focusing Sextupoles at high dispersion points, with separated betas Low dispersion and horizontal beta function in the dipole

14 TESLA/ILC damping ring
Long pulse: 950ms  c = 285 km!! Compress bunch train into 17 km (or less) “ring” kick individual bunches Min. circumference by ejection/injection kicker speed (20 ns) “Dog bone” ring with ≈ 400m of 1.67 T wigglers 6.7 km circular rings in the baseline ILC design Very demanding kicker rise+fall time < 6 ns

15 DR test at ATF ATF at KEK in Japan tests DR issues
ATF, 3rd gen. SRS, SLC “Laser Wire”

16 Damping Ring emittance

17 CLIC scheme Very high gradients (>100 MV/m) possible with NC accelerating structures at high RF frequencies (30 GHz → 12 GHz) Extract required high RF power from an intense e- “drive beam” Generate efficiently long pulse and compress it (in power + frequency) Long RF Pulses P0 , n0 , t0 Short RF Pulses PA = P0  N1 tA = t0 / N2 nA = n0  N3 Electron beam manipulation Power compression Frequency multiplication ‘few’ Klystrons Low frequency High efficiency Accelerating Structures High Frequency – High field Power stored in electron beam Power extracted from beam in resonant structures

18 CLIC two beam scheme High charge Drive Beam (low energy)
Low charge Main Beam (high collision energy) => Simple tunnel, no active elements => Modular, easy energy upgrade in stages Transfer lines Main Beam Drive Beam CLIC TUNNEL CROSS-SECTION Drive beam A, 240 ns from 2.4 GeV to 240 MeV Main beam – 1 A, 156 ns from 9 GeV to 1.5 TeV 4.5 m diameter

19 CLIC – overall layout – 3 TeV
Drive Beam Generation Complex Drive beam Main beam Main Beam Generation Complex

20 Drive Beam Decelerator Section (24 in total) RF Transverse Deflectors
CLIC Drive Beam generation CLIC RF POWER SOURCE LAYOUT Drive Beam Accelerator efficient acceleration in fully loaded linac Power Extraction Drive Beam Decelerator Section (24 in total) Combiner Ring  3 Combiner Ring  4 pulse compression & frequency multiplication Delay Loop  2 gap creation, pulse compression & frequency multiplication RF Transverse Deflectors 140 ms train length - 24  24 sub-pulses 4.2 A GeV - 60 cm between bunches 240 ns 24 pulses – 100 A – 2.5 cm between bunches 5.8 ms Drive beam time structure - initial Drive beam time structure - final

21 Lemmings Drive Beam Alexandra Andersson

22 Fully loaded operation
efficient power transfer from RF to the beam needed “Standard” situation: small beam loading power at structure exit lost in load “Efficient” situation: high beam current high beam loading no power flows into load VACC ≈ 1/2 Vunloaded

23 Fully loaded operation
Disadvantage: any current variation changes energy gain at full loading, 1% current variation = 1% voltage variation at 20% loading, 1% current variation = 0.2% voltage variation Requires high current stability Stable beam successfully demonstrated in CTF3 > 95% efficiency Beam on ~ 0 MW Output power from accelerating structure 1.6 ms compressed RF pulse Beam off ~ 24 MW

24 Frequency multiplication
basic principle of drive beam generation transform very long pulses into short pulses with higher power and higher frequency use RF deflectors to interleave bunches  double power  double frequency

25 Delay Loop Principle double repetition frequency and current
parts of bunch train delayed in loop RF deflector combines the bunches (fdefl=bunch rep. frequency) Path length corresponds to beam pulse length

26 RF injection in combiner ring
combination factors up to 5 reachable in a ring Cring = (n + ¼) l injection line septum local inner orbits 1st deflector 2nd deflector 1st turn lo RF deflector field 2nd Cring has to correspond to the distance of pulses from the previous combination stage! 3rd lo/4 4rd

27 Demonstration of frequency multiplication

28 CTF 3 demonstrate remaining CLIC feasibility issues, in particular:
Drive Beam generation (fully loaded acceleration, bunch frequency multiplication) CLIC accelerating structures CLIC power production structures (PETS) Bunch length chicane 30 GHz “PETS Line” Delay Loop – 42m Combiner Ring – 84m RF deflector TL1 Injector Linac 4A – 1.2µs 150 MeV Laser 32A – 140ns 150 MeV 30 GHz test area CLEX TL2

29 Drive beam generation achieved
combined operation of Delay Loop and Combiner Ring (factor 8 combination) ~26 A combination reached, nominal 140 ns pulse length => Full drive beam generation, main goal of 2009, achieved Current from Linac Current after Delay Loop Current in the ring 30A DL CR

30 Power extraction structure PETS
must extract efficiently >100 MW power from high current drive beam passive microwave device in which bunches of the drive beam interact with the impedance of the periodically loaded waveguide and generate RF power periodically corrugated structure with low impedance (big a/λ) ON/OFF mechanism The power produced by the bunched (0) beam in a constant impedance structure: Beam eye view Design input parameters PETS design P – RF power, determined by the accelerating structure needs and the module layout. I – Drive beam current L – Active length of the PETS Fb – single bunch form factor (≈ 1)

31 Present best structure performance
Frequency: GHz Cells: 18+2 matching cells Filling Time: 36 ns Length: active acceleration 18 cm Iris Dia. a/λ 0.155~0.10 Group Velocity: vg/c % Phase Advace Per Cell 2π/3 Power for <Ea>=100MV/m 55.5 MW Unloaded Ea(out)/Ea(in) 1.55 Es/Ea 2 2 structures T18_VG2.4_disk (no damping) tested at SLAC and KEK Exceeded 100 MV/m at nominal CLIC breakdown rate Improvement by conditioning CLIC nominal

32 Summary Linear e+/e- Collider the only realistic approach to higher energy Many challenges!!! Efficient acceleration RF system High gradient Extremely small beam sizes Damping ring performance is crucial Emittance preservation Alignment and stabilisation Much interesting work left to do!!! Much more detailed lectures at last ILC school

33 Documentation about CLIC
General documentation about the CLIC study: CLIC scheme description: CLIC Physics CLIC Test Facility: CTF CLIC technological challenges (CERN Academic Training) CLIC Workshop 2009 (most actual information)


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