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Bob Palmer 1990

International Linear Collider (ILC) Linac Basics Part I: General Design Constraints Part II: ILC Design Choices Chris Adolphsen SLAC. Bob Palmer 1990. Part I: General Design Constraints.

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Bob Palmer 1990

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  1. International Linear Collider (ILC) Linac BasicsPart I: General Design ConstraintsPart II: ILC Design ChoicesChris AdolphsenSLAC

  2. Bob Palmer 1990 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.

  3. 1.3 GHz TESLACavities • 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 ).

  4. 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 !

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

  6. Operating Gradient • Qo varies with gradient due to a number of mechanisms. • In recent years, gradients are approaching fundamental limit: Bc * (Grad / Bsurface) ~ 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)

  7. 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 = 25-50 MV/m * 1m * Beam Current • Next look at beam structure • Beam Current = Bunch Charge / Bunch Spacing • Bunch Train Length = Number of Bunches * Bunch Spacing

  8. Iris Radius a Bunch 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

  9. 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.

  10. 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

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

  12. 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

  13. 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.

  14. 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.

  15. Other Bunch Spacing Considerations Input Power • 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. Coaxial Power Coupler

  16. 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

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

  18. 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

  19. 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.

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

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

  22. 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

  23. Part II: ILC Design Choices

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

  25. 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.

  26. Parameter Plane

  27. 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.

  28. 1.3 GHz TESLACavities • 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).

  29. 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)

  30. Achieved Gradients in Single and 9-Cell Cavities • In recent years, single-cell cavity gradients approached fundamental limit: Bc * (Grad / Bsurface) ~ 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

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

  32. 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

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

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

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

  36. 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. Tuning the Cavities

  37. 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

  38. Low Level RF Feedback Control

  39. RF Distribution Math(for 35 MV/m Max Operation) 35 MV/m * 9.5 mA * 1.038 m = 345 kW (Cavity Input Power) • 24 Cavities • 1/.93 (Distribution Losses) • 1/.89 (Tuning Overhead) • 10.0 MW 10 MW Klystron

  40. 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

  41. ILC Baseline Pulse Transformer Modulator IGBT’s

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

  43. 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.

  44. Other Alternative Modulators 13.8KV 3Ø CØ BØ -HV -HV -HV 10ohm 20mH 4mH 400A .03uF 50mH 3Ø (ON/OFF) .05uF VMON 6 EACH .03uF AØ BØ CØ RTN AØ INPUT LINE CHOKE 5th HARMONIC TRAP 7th HARMONIC TRAP 6 EACH HV OUTPUT 4mH 400A SNS High Voltage Converter Modulator (Unit installed at SLAC) RECTIFIER TRANSFORMER AND FILTERS BOOST TRANS-FORMER HV RECTIFIER AND FILTER NETWORK SCR REGULATOR ENERGY STORAGE SWITCHING RECTIFIER TRANSFORMER AND FILTERS SCR REGULATOR EQUIPMENT CONTROL RACK HVCM

  45. 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

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

  47. 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).

  48. 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

  49. 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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