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International Linear Collider (ILC) Linac Basics Part I: General Design Constraints Part II: ILC Design Choices Chris Adolphsen SLAC

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Linear collider design is complex due to the interrelationships among the various parameters and the soft constraints on their values. I will give one or many possible descriptions of the rationale behind the ILC linac design. Bob Palmer 1990 Part I: General Design Constraints

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‣Made with solid, pure niobium – it has the highest Critical Temperature (Tc = 9.2 K) and Thermodynamic Critical Field (Bc ~ 1800 Gauss) of all metals. ‣Nb sheets are deep-drawn to make cups, which are e-beam welded to form cavities. ‣Cavity limited to ~ 9 cells (~ 1 m Long) to reduce trapped modes, input coupler power and sensitivity to frequency errors. ‣Iris radius (a) of 35 mm chosen in tradeoff for low surface fields, low rf losses (~ a), large mode spacing (~ a 3 ), small wakes (~ a -3.5 ). 1.3 GHz TESLA Cavities

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The Basics The low rf surface losses in superconducting cavities allow essentially 100% RF-to-beam transfer efficiency in steady state: RF Input Power = Cavity Voltage * Beam Current First look at what limits cavity voltage (gradient * cavity length) –Note: highest gradient not always cost optimal !

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Cavity Operating Parameters Operate at 2K (in super-fluid He) to reduce resistivity: 1.3 GHz frequency (f) chosen to reduce power loss, sensitivity to thermal instabilities, wakes, and cavity size and to match available sources. At Qo = 1e10, cavity time constant (Qo/ ) ~ 1 sec, and at 35 MV/m and 0.1% duty factor, average power loss ~ 1 W/m (but it takes ~ 1 kW/m of AC power to remove heat) Qo ~ 2e10 at T = 2K Temperature (K) Niobium Surface Resistance

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Q o Gradient (MV/m) Best 9 Cell Cavity Result to Date: CW Performance of a Cavity Electro- Polished at DESY Operating Gradient Qo varies with gradient due to a number of mechanisms. In recent years, gradients are approaching fundamental limit: Bc * (Grad / B surface ) ~ 1800/41.5 ~ 43 MV/m

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The Basics (2) The low rf wall losses in superconducting cavities allow essentially 100% RF-to-beam transfer efficiency in steady state: 1.3 GHz Input Power = MV/m * 1m * Beam Current Next look at beam structure –Beam Current = Bunch Charge / Bunch Spacing –Bunch Train Length = Number of Bunches * Bunch Spacing

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Bunch Charge and Length Nominal Bunch Charge (N = 2e10) and Length ( z = 300 m) –Mainly determined by damping ring, linac energy spread and IP considerations. Bunch length reduced from 6 mm to 300 microns prior to linac injection –Also constrained by Short-Range Transverse Wake Kicks (N z / a 3.5 ) Short-Range Loading (N / z / a 2 ) Bunch a Iris Radius Wakefield in a PETRA cavity

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Number of Bunches per Pulse, Repetition Rate, Luminosity and AC Power Luminosity ~ Rep Rate Number of Bunches per Pulse –Repetition rate (5 Hz) constrained by damping ring store time (see next slide). –Number of Bunches per Pulse constrained by Train Length > Cavity Fill Time: for > 50% rf-to-beam efficiency, minimum number of bunches is1870 (at 35 MV/m and 2e10 e/bunch) independent of bunch spacing ! Length and cost of damping rings (newer, smaller designs assume smaller bunch separations). Train Length < Max klystron pulse length (limited by pulse heating). –Product constrained by practical site power (few hundred MW). Would limit rep rate even damping rings did not.

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Optimal bunch train length very long (>> linac length), so –Minimize bunch spacing in the damping ring – limited by separation (3-20 ns) required to extract individual bunches with a kicker magnet. –Still, damping ring is long (17 km circumference in TDR, 6 km in current design), which makes the required store time long (200 ms), even with a few hundred meters of wigglers. –RF pulse length (1.4 ms) << store time so ring needs to hold full bunch train. –Store time limits machine repetition rate (5 Hz). Could increase wiggler length, but already about 20% of damping ring cost. Damping Ring Constraints Dog Bone Damping Ring

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TESLA Design Choices (2001 TDR) Gradient = 23.4 MV/m Bunch Charge = 2e10 e Rep Rate = 5 Hz # of Bunches = 2820 Bunch Spacing = 337 ns Beam Current = 9.5 mA Input Power = 230 kW Fill Time = 420 s Train Length = 950 s

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Bunch Spacing Non-linac constraints on minimum spacing –Peak positron target heating –IP bunch separation Weak linac constraints –Bunch coupling from long-range transverse wakefields –Steady state RF-to-Beam efficiency: /Qo very high (much different than for a warm machine) Strong linac constraints –Power source costs ~ 1 / Bunch Spacing –Cryogenic costs ~ Bunch Spacing

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Klystron Economics Cost of 1.3 GHz Klystron + Modulator (crude approximation) –Similar for similar average power –Independent of peak power. However, rf energy per pulse differs greatly, for example, –20 MW peak, 10 sec pulses (300 Hz, 60 kW) –5 MW peak, 2 msec pulses (10 Hz, 100 kW) –0.1 MW peak, CW Number of klystrons ~ Cavity Input Power / Peak Klystron Power ~ 1 / Bunch Spacing (with fixed peak power) With smaller bunch spacing, would not use full average klystron average power capability.

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Bunch Spacing and Fill Time Adjust Qext to match cavity impedance (R/Qo * Qext) to the beam impedance (Gradient / Current). So Fill Time ~ Bunch Spacing For TDR parameters, Qext = 3e6 so cavity BW = 430 Hz. Need to achieve < 0.1% energy gain uniformity.

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Coaxial Power Coupler Input Power Other Bunch Spacing Considerations Input coupler power limitations –Power ~ 1 / Bunch Spacing –Baseline TTF3 design processed to 1 MW and tested up to 600 kW for 35 MV/m operation (1000 hours): long term reliability for required operation at 350 kW not known.

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TESLA Design Choices (2001 TDR) Gradient = 23.4 MV/m Bunch Charge = 2e10 e Rep Rate = 5 Hz # of Bunches = 2820 Bunch Spacing = 337 ns Beam Current = 9.5 mA Input Power = 230 kW Fill Time = 420 s Train Length = 950 s

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Where Does the Power Go (NLC/GLC vs TESLA TDR Efficiencies and Average Power)

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Cost Optimization Major cost components that depend on Gradient (G) and Bunch Train Length (Tb). Cooling + Power + Length ~ A Tb G + B Tb -1 + C G -1 Cost Study: Compute Cost vs G and Tb for fixed Luminosity (L) –Assume charge per bunch and number of bunches constant –Cavity fill time (Tf) scales as G * Tb –RF pulse length (Trf) = Tb + Tfill

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* TPC is for 500 GeV machine in US Options Study but does not include additional unpowered tunnel sections. Relative Total Project Cost* (TPC) -vs- Linac Gradient Gradient ( MV/m) Relative Cost

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Gradient ( MV/m) Cryo Plant Cost (B$) Gradient ( MV/m) Cryomodules Contributions to TPC (One Linac)

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Relative Cost Relative TPC -vs- Bunch Train Length Bunch Train Length (us) G = 35 MV/m 23.4 MV/m

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Reduce Train Length Relative Cost Relative TPC -vs- Luminosity (35 MV/m, Fixed Bunch Charge, Linac Changes Only) Reduce Beam Current Reduce Rep Rate Relative Luminosity

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Part II: ILC Design Choices

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ILC Layout (not to scale) 1000 GeV CMS Initial Future Upgrade 500 GeV CMS

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Operational Parameter Plane Rather than a specific machine design, the baseline configuration is for an operating ‘plane’. Four parameters sets proposed to achieve design peak luminosity: –Nominal. –Low bunch charge (Q). –Large vertical IP beam size (Large Y). –Low Beam Power (Low P). Not strictly fixed sets -> used to define necessary operational flexibility.

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Parameter Plane nominallow Qlarge Ylow P N nbnb x,y m, nm 9.6, 4010,3012,8010,35 x,y cm, mm 2, , 0.21, 0.41, 0.2 x,y nm 543, , , 8452, 3.8 tbtb ns BS % zz mm P beam MW L

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Main Linac Design Baseline Configuration Document (BCD) distilled from Snowmass Working Group recommendations in August Major differences from 2001 Tesla TDR 500 GeV Design. –Higher gradient (31.5 MV/m instead of 23.4 MV/m) for cost savings. –Two tunnels (service and beam) instead of one for improved availability. The Linac Area Group of the Global Design Effort (GDE) is continuing to evolve design.

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‣ For ILC, would accept only ‘vertically’ tested cavities (using CW rf without high power couplers) that achieve gradients > 35 MV/m and Q > 8e9 (discard or reprocess rejects). ‣ When installed in 8 cavity cryomodules, expect stable operation at an average gradient of 31.5 MV/m and Q = 1e10 (rf system designed for 35 MV/m). ‣ Derating due to desire for overhead from quench limit, lower installed performance and limitations from using a common rf source. ‣ For a 1 TeV upgrade, expect average gradient = 36 MV/m, Q = 1e10 for new cavities (the TDR 800 GeV design assumed 35 MV/m and Q > 5e9). 1.3 GHz TESLA Cavities

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ILC Linac RF Unit (1 of ~ 600) Gradient = 31.5 MV/m Bunch Charge = 2e10 e Rep Rate = 5 Hz # of Bunches = 2967 Bunch Spacing = 337 ns Beam Current = 9.5 mA Input Power = 311 kW Fill Time = 565 s Train Length = 1000 s (8 Cavities per Cryomodule)

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Achieved Gradients in Single and 9-Cell Cavities In recent years, single-cell cavity gradients approached fundamental limit: Bc * (Grad / B surface ) ~ 1800/41.5 ~ 43 MV/m for Tesla-shape cavities. During past 2.5 years, DESY has produced 6 fully-dressed cavities with Gradients > 35 MV/m and Q > 8e9. Yield for such cavities < 30%. Test Results for Dressed-Cavities that will be used in a ’35 MV/m’ Cryomodule

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Main Production Problem Has Been Poor Reproducibility Gradients achieved over time in DESY cavities ILC Goal

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Achieved Gradients in Tesla Test Facility (TTF) 8-Cavity Cryomodules (Cavities not Electro-Polished) Cryomodule Number Gradient (MV/m) Diamonds and Error Bars = Range of Gradients Achieved in Individual CW Cavity Tests. = Average Gradient Achieved in Cryomodule

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High Gradient R&D: Low Loss (LL) and Re- Entrant (RE) Cells with a Lower B peak /E acc Ratio

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Fabricated at Cornell Single Cell Results: Eacc = MV/m /Ichiro

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BCP + 120C Baking Studies also underway using single or large grain Nb – could eliminate need for Electro-Polishing (EP)

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Tuning the Cavities Both slow (500 kHz over minutes) and fast (2.5 kHz during the 1.6 ms pulse) tuning required – achieve by compressing the cavity (~ 1 micron per 300 Hz). Want tuners located away from cavity ends to minimize cavity spacing. ‘Blade Tuner’ shown below. To date, have not achieved more than ~1kHz range of fast tuning. Final design for BCD not yet chosen.

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RF Fill Dynamics For ILC, Qext = 4e6 so cavity BW = 325 Hz ( L = 1 micron). Need to achieve < 0.1% energy gain uniformity with Low Level RF (LLRF) system –Feedback to maintain constant ‘sum of fields’ in 24 cavities

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Low Level RF Feedback Control

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RF Distribution Math (for 35 MV/m Max Operation) 35 MV/m * 9.5 mA * m = 345 kW (Cavity Input Power) ×24 Cavities ×1/.93 (Distribution Losses) ×1/.89 (Tuning Overhead) ═10.0 MW 10 MW Klystron

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Modulators (115 kV, 135 A, 1.5 ms, 5 Hz) Pulse Transformer Style (~ 2 m Long) To generate pulse, an array of capacitors is slowly charged in parallel and then discharged in series using IGBT switches. Will test full prototype in 2006

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ILC Baseline Pulse Transformer Modulator IGBT’s

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Marx Generator Modulator 12 kV Marx Cell (1 of 16) IGBT switched No magnetic core Air cooled (no oil)

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Modulators Baseline: Pulse Transformer –10 units have been built over 10 years, 3 by FNAL and 7 by industry. –8 modulators in operation – no major reliability problems (DESY continuing to work with industry on improvements). –FNAL working on a more cost efficient and compact design, SLAC building new dual IGBT switch. Alternative: Marx Generator –Solid state, 1/n redundant modular design for inherent high availability, reliability. –Highly repetitive IGBT modules (90,000) cheap to manufacture. –Eliminating transformer saves size, weight and cost, improves energy efficiency.

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SNS High Voltage Converter Modulator (Unit installed at SLAC) RECTIFIER TRANSFORMER AND FILTERS SCR REGULATOR SWITCHING BOOST TRANS- FORMER HV RECTIFIER AND FILTER NETWORK 13.8KV 3Ø INPUT LINE CHOKE 5 th HARMONIC TRAP 7 th HARMONIC TRAP 50mH AØAØ BØBØCØCØ RECTIFIER TRANSFORMER AND FILTERS SCR REGULATOR HVCM EQUIPMENT CONTROL RACK ENERGY STORAGE Other Alternative Modulators

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Series Switch Modulator (Diversified Technologies, Inc. ) IGBT Series Switch 140kV, 500A switch shown at left in use at CPI As a Phase II SBIR, DTI is building a 120 kV, 130 A version with a bouncer to be delivered to SLAC at the end of 2006

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Klystrons Baseline: 10 MW Multi-Beam Klystrons (MBKs) with ~ 65% Efficiency: Being Developed by Three Tube Companies in Collaboration with DESY ThalesCPIToshiba

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Status of the 10 MW MBKs Thales: Four tubes produced, gun arcing problem occured and seemed to be corrected in last two tubes after fixes applied (met spec). However, tubes recently developed other arcing problems above 8 MW. Thales to build two more without changes and two with changes after problem is better diagnosed. CPI: One tube built and factory tested to 10 MW at short pulse. At DESY with full pulse testing, it developed vacuum leak after 8.3 MW achieved – has been repaired and will be tested again. Toshiba: One tube built and achieved operation spec but developed arcing problems above 8 MW – being shipped to DESY for further evaluation. These are vertically mounted tubes – DESY will soon ask for bids on horizontally mounted tubes for XFEL (also needed for ILC).

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Alternative Tube Designs 10 MW Sheet Beam Klystron (SBK) Parameters similar to 10 MW MBK Low Voltage 10 MW MBK Voltage 65 kV Current 238A More beams Perhaps use a Direct Switch Modulator 5 MW Inductive Output Tube (IOT) Drive Output IOT Klystron SLACCPIKEK

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Klystron Summary The 10 MW MBK is the baseline choice – continue to support tube companies to make them robust (DESY needs 35 for XFEL although will run at 5 MW). SLAC funding design of a 10 MW sheet-beam klystron (will take several years to develop). Backup 1: Thales 2104C 5 MW tube used at DESY and FNAL for testing – it appear reliable (in service for 30 years) but has lower effiency compared to MBKs (42% vs 65%). Backup 2: With increased DOE funding next year, propose to contract tube companies to develop high efficiency, single-beam, 5 MW klystron.

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RF Distribution Baseline choice is the waveguide system used at TTF, which includes off- the-shelf couplers, circulators and 3-stub tuners (phase control).

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Need more compact design (Each Cavity Fed 350 kW, 1.5 msec Pulses at 5 Hz) Two of ~ 16,000 Feeds

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Baseline Alternative Design with No Circulators And should consider simplifications (circulators are ~ 1/3 of cost)

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C. Nantista Adjustable Tap-Offs Using Mode Rotation Rotatable section with central elliptical region, matched for both polarizations of circular TE 11 mode with differing phase lengths. rotatable joints load feed mode rotator

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cavity couplers diagnostic directional couplers three-stub tuners flexible waveguide 3-dB hybridsbeam direction loads C. Nantista m Proposed RF Distribution Layout Adjustable power to pairs of cavities No circulators Pairs feed by 3 dB hybrids (requires n +/- 90 degree cavity spacing – only 7 mm longer than TDR/BCD spec)

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TTF ModuleInstallation date Cold time [months] CryoCapOct 9650 M1Mar 975 M1 rep.Jan 9812 M2Sep 9844 M3Jun 9935 M1* MSS Jun M3* M4 M5 Apr M2*Feb 0416 Cryomodules

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Cryomodule Cross Section

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Multipacting Simulation of TTF3 Coupler Bellows Primaries -Green, Secondaries- Red “Cold Side”

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Cryomodule Design Relative to the TTF cryomodules –Continue with 8 cavities per cryomodule based on experience and minimal cost savings if number increased (12 in TDR). –Move quad / corrector / bpm package to center (from end) to improve stability. –Increase some of cryogenic pipe sizes (similar to that proposed for the XFEL). –Decrease cavity separation from 344 mm to 283 mm as proposed in the TDR.

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Beam-Related Design Issues Optics / Tolerances / Operation similar to that in TDR: –One quad per rf unit (three, 8-cavity cryomodules). –Few hundred micron installation tolerances for cavity, quad and BPM (demonstrated with TTF cryomodules). –Cavity BPM resolution of a few μm (should be achievable). –Use quad shunting and DFS tuning algorithms for dispersion control (need to better understanding systematic effects). –Assume beamline will follow Earth’s curvature. –XFEL will serve as a benchmark although emittance much larger. Alternatives for cost savings. –Larger quad spacing at high energy end of the linac where wake and dispersion effects smaller. –Halve quad and bpm aperture to allow superferric quad and higher resolution BPMs (increases wakes by 10%).

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Quad / Corrector / BPM Package TDR ILC Proposal 887 mm mm 78 BPM Quad and Correctors

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Quad / Corrector / BPM Package Al Cylinder Iron Yoke Block SC Coils S-Band BPM Design (36 mm ID, 126 mm OD) Dipole Design: Flux density and Flux Lines SC ‘Cos(2 )’ Quadrupole Magnet

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Frequency (Hz) Integrated RMS Motion (nm) Vertical Quad Vibration at TTF (ILC Goal: ~ 0.2 Hz) Noise

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Vertical Displacements of Cryomodule 4 and 5 after Cooldown Relative to Positions Just Prior to Cooldown EPAC04

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Alternative Quad Location Cavity Quad Cavity Quad Alternative TTF

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S-Band BPM Triplet Results (0.8 micron resolution, 1.4e10 electrons, Q of 500 for clean bunch separation)

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TESLA cryogenic unit Assume static heat leaks based on TTF measurements instead of the smaller values assumed in the TDR Cryogenic System To Cryoplant

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Cryoplant Layout For ILC 500, require 57 MW of AC power for Cryoplants

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For baseline, developing deep underground (~100 m) layout with 4-5 m diameter tunnels spaced by 5 m. ILC Tunnel Layout

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TESLA Tunnel (TDR)

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ILC Availability Challenge The ILC will be an order of magnitude more complex than any accelerator ever built. If it is built like present HEP accelerators, it will be down an order of magnitude more (essentially always down). For reasonable uptime, component availability must be much better than ever before. Must do R&D and budget for it up-front.

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Lifetime Improvements Tom Himel

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SC Linac Summary A superconducting linac allows for efficient acceleration – for the ILC with a 35 MV/m gradient, –60% RF-to-Beam Efficiency –15% AC-to-Beam Efficiency A low beam current is required to make this approach cost competitive (matches rf source capabilities). The resulting long bunch trains require large damping rings with untested designs.

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ILC Design Summary Basic linac design complete: converging on details –Tradeoffs of operability, availability and cost. Major cost and technical risks –Producing cryomodules that meet design gradient at a reasonable cost (cost model still in development, XFEL will provide a reference, and will get new industry-based estimates). –Producing a robust 10 MW klystron. Potential Cost Savings –Adopt Marx Modulator –Use simpler rf distribution scheme –Have one tunnel although ‘the additional cost is marginal when considering the necessary overhead and equipment improvements to comply with reliability and safety issues.’ –Reduce cavity aperture to 60 mm for 21% reduction in dynamic cryo-loading and 16% reduction in cavity fill time.

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