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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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Presentation on theme: "International Linear Collider (ILC) Linac Basics Part I: General Design Constraints Part II: ILC Design Choices Chris Adolphsen SLAC."— Presentation transcript:

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

2 Part I: General Design Constraints
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

3 1.3 GHz TESLA Cavities 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 (~ a3 ), small wakes (~ a-3.5 ).


5 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 !

6 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/w) ~ 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). 1.3 1.8 2.3 1.5 3.1 4.6 Qo ~ 2e10 at T = 2K Temperature (K) Niobium Surface Resistance

7 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 Best 9 Cell Cavity Result to Date: CW Performance of a Cavity Electro-Polished at DESY Qo Gradient (MV/m)

8 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

9 Bunch Charge and Length
Nominal Bunch Charge (N = 2e10) and Length (sz = 300 mm) 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 sz / a 3.5 ) Short-Range Loading (N / sz / a 2 ) Wakefield in a PETRA cavity Bunch a Iris Radius

10 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.

11 Damping Ring Constraints
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. Dog Bone Damping Ring

12 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 ms Train Length = 950 ms

13 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: w/Qo very high (much different than for a warm machine) Strong linac constraints Power source costs ~ 1 / Bunch Spacing Cryogenic costs ~ Bunch Spacing

14 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 msec 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.

15 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.

16 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. Input Power Coaxial Power Coupler

17 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 ms Train Length = 950 ms

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

19 Cost Optimization Major cost components that depend on Gradient (G) and Bunch Train Length (Tb). Cooling Power Length ~ A ´ Tb ´ G B ´ Tb 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

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

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

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

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

24 Part II: ILC Design Choices

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

26 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.

27 Parameter Plane N nb gex,y bx,y sx,y tb dBS sz Pbeam L 2 1 2820 5640
nominal low Q large Y low P N 1010 2 1 nb 2820 5640 1330 gex,y mm, nm 9.6, 40 10,30 12,80 10,35 bx,y cm, mm 2, 0.4 1.2, 0.2 1, 0.4 1, 0.2 sx,y nm 543, 5.7 495, 3.5 495, 8 452, 3.8 tb ns 308 154 462 dBS % 2.2 1.8 2.4 5.7 sz mm 300 150 500 200 Pbeam MW 11 5.3 L 1034

28 Main Linac Design Baseline Configuration Document (BCD) distilled from Snowmass Working Group recommendations in August 2005. 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.

29 1.3 GHz TESLA Cavities 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).

30 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 ms Train Length = 1000 ms (8 Cavities per Cryomodule)

31 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

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

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

34 High Gradient R&D: Low Loss (LL) and Re-Entrant (RE) Cells with a Lower Bpeak/Eacc Ratio

35 Single Cell Results: Eacc = 47 - 52 MV/m
Fabricated at Cornell /Ichiro

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

37 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.

38 RF Fill Dynamics For ILC, Qext = 4e6 so cavity BW = 325 Hz (DL = 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

39 Low Level RF Feedback Control

40 RF Distribution Math (for 35 MV/m Max Operation)
10 MW Klystron 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

41 Pulse Transformer Style
Modulators (115 kV, 135 A, 1.5 ms, 5 Hz) (~ 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 Pulse Transformer Style

42 ILC Baseline Pulse Transformer Modulator IGBT’s

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

44 Modulators Baseline: Pulse Transformer Alternative: Marx Generator
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.

45 SNS High Voltage Converter Modulator (Unit installed at SLAC)

46 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

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

48 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).

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

50 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.

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

52 Need more compact design (Each Cavity Fed 350 kW, 1
Need more compact design (Each Cavity Fed 350 kW, 1.5 msec Pulses at 5 Hz) Two of ~ 16,000 Feeds

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

54 Adjustable Tap-Offs Using Mode Rotation
rotatable joints load feed 1 2 3 4 mode rotator a 2a Rotatable section with central elliptical region, matched for both polarizations of circular TE11 mode with differing phase lengths. C. Nantista

55 Proposed RF Distribution Layout
Adjustable power to pairs of cavities No circulators Pairs feed by 3 dB hybrids (requires nl +/- 90 degree cavity spacing – only 7 mm longer than TDR/BCD spec) loads cavity couplers flexible waveguide three-stub tuners diagnostic directional couplers 1.326 m beam direction 3-dB hybrids C. Nantista

56 Cryomodules CryoCap Oct 96 50 M1 Mar 97 5 M1 rep. Jan 98 12 M2 Sep 98
TTF Module Installation date Cold time [months] CryoCap Oct 96 50 M1 Mar 97 5 M1 rep. Jan 98 12 M2 Sep 98 44 M3 Jun 99 35 M1* MSS Jun 02 30 8 M3* M4 M5 Apr 03 19 M2* Feb 04 16

57 Cryomodule Cross Section

58 Multipacting Simulation of TTF3 Coupler
“Cold Side” Bellows Primaries -Green, Secondaries- Red These are the first results from the simulation with the eigensolver Omeag3P. They are only possible with new advances in nonlinear eigensolvers which I will touch on in the next slide, Because the couplers are extracting power from the cavity, the egienvalues are complex from which we get what they call the external Q of the modes. We have calculated the external Qs of the HOMs in both the DESY and KEK design and the design challenge now is to optimize the end cells of the cavity to lower the external Qs of the modes to below a threshold so that the beam will be stable. There is a slide on shape optimization later

59 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.

60 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%).

61 Quad / Corrector / BPM Package
887 mm 666 mm 66 77 78 Quad and Correctors TDR BPM ILC Proposal

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

63 Vertical Quad Vibration at TTF (ILC Goal: < 100 nm uncorrelated rms motion > ~ 0.2 Hz)
1000 0.1 Frequency (Hz) Integrated RMS Motion (nm) Noise

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

65 Alternative Quad Location
TTF Cavity Quad Cavity Quad Alternative

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

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

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

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

70 TESLA Tunnel (TDR)

71 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.

72 Lifetime Improvements
Tom Himel

73 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.

74 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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