International Technology Recommendation Panel. X-Band Linear Collider Path to the Future. RF System Overview. Chris Adolphsen

Transcription

1 International Technology Recommendation Panel X-Band Linear Collider Path to the Future RF System Overview Chris Adolphsen Stanford Linear Accelerator Center April 26-27, 2004

2 Delivering the Beam Energy Approach - Choice of X-Band RF Technology - Evolution of Component Designs from SLAC S-Band Technology Validation - RF System Test Facilities - Demonstration of Key Performance Requirements Readiness to Begin Project Phase - Complete Design, Major Components Ready to Industrialize Outlook - Manageable Technical Risks - Higher Energy Capability

3 Why X-Band (11.4 GHz) RF Technology Warm approach builds on 40 year experience with the S-Band (2.86 GHz) technology of the SLAC Linac. - It is evolutionary, not revolutionary. In general want higher rf frequency because: - Less rf energy per pulse is required, so fewer rf components. - Reasonable efficiencies are achievable at higher gradients, so linac shorter. Offsetting these advantages are: - Tighter alignment constraints due to stronger wakefields. - Increasing cost of rf energy.

4 Why X-Band (11.4 GHz) RF Technology The choice of 11.4 GHz (4 times the SLAC Linac frequency) provides the cost benefits with manageable wakefields. This choice has a profound influence on the collider design, for example, RF Attenuation Time in Copper (~ 100 ns) Structure Fill Time (~ 100 ns) Beam Train Length (~ 300 ns) Circumference of Damping Rings (~ 300 m)

5 Simplified Layout of NLC/GLC RF System (Efficiencies & Powers in Parentheses)

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8 GLC Two Tunnel Layout

9 RF System Test Facilities Next Linear Collider Test Accelerator (NLCTA) at SLAC - First Generation RF Components - Beam Acceleration Eight-Pack Project at NLCTA - Second Generation RF Components - Demonstrate NLC/GLC RF Unit Operation Global Linear Collider Test Accelerator (GLCTA) at KEK - Facility for Initial Stage Operation Recently Completed - Status Will Be Presented During KEK Visit.

10 Next Linear Collider Test Accelerator (NLCTA) In 1993, construction began using first generation X-band components. In 1997, demonstrated 17% beam loading compensation in four, 1.8 m structures at ~ 40 MV/m. In , added second klystron to each linac rf station. In , used for high gradient studies.

11 Eight-Pack Project: Second Generation RF Unit Test Phase I: Generate RF Power and Transport to Loads (Instead of Accelerator Structures) Eight-Pack Modulator Dual-Moded SLED II Pulse Compression Klystrons Load Tree

12 RF Component Performance

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14 Eight-Pack Modulator 76 Cores Three-Turn Secondary > 500 Hours of Operation Waveforms When Driving Four 50 MW Klystrons at 400 kv, 300 A Each

15 Features Next Generation Induction Modulator: The Two-Pack kv IGBTs with in-line multi-turn 1:10 transformer. - Industrialized cast casings. - Improved oil cooling. - Improved HV feed through. 2-Pack Layout Bechtel-LLNL-SLAC 20 kv Test Stack A Hybrid 2-Pack Modulator is currently under test at SLAC Power Conversion Department Lab.

16 Modulator Performance (1.6 µs Pulse Width) Config Load Voltage (kv) Current (A) Rate (Hz) Efficiency (%) 8-pack Water Achieved 8-Pack Four XL4 Klystrons Pack Hybrid Water NLC/GLC Baseline 2-Pack Two PPM Klystrons Prototype modulators operate at voltages and currents exceeding NLC/GLC requirements. 2-Pack efficiency is lower than goal due to hybrid transformer expect > 70% in next version with integrated transformer.

17 RF Component Performance

18 X-Band Klystrons

19 PPM Klystron Overview PPM Klystrons being developed at SLAC, with some industry involvement, and at KEK in collaboration with Toshiba. 50 MW and 75 MW Tubes tested during past six years: Five at KEK/Toshiba. Six at SLAC. Two industrial (EEV and Toshiba). Two tubes to date have met NLC/GLC requirements (all key parameters concurrently). TRC R2 requirement of 120 Hz operation has been met.

20 PPM Klystron Performance (75 MW, 1.6 µs, 120/150 Hz, 55% Efficiency Required) KEK/Toshiba Two tubes tested at 75 MW with 1.6 µs pulses at 50 Hz (modulator limited). Efficiency = 53-56%. SLAC Two tubes tested at 75 MW with 1.6 µs pulses at 120 Hz. Efficiency = 53-54%.

21 For RF Unit Test, Four 50 MW Solenoid-Focused Klystrons Installed in the Eight-Pack Modulator (In Place of Two 75 MW PPM Klystrons)

22 RF Component Performance

23 First Generation RF Pulse Compression (SLED II) at NLCTA

24 TE 02 TE 01 TE 01 TE 02 For NLC/GLC, Use Dual Moded Delay Line to Reduce Delay Line Length in Half

25 Also Use Over-Height Planar Waveguide to Lower Surface Fields (< 50 MV/m) Example: Power Splitter

26 Dual-Moded SLED-II Performance Operated 300 Hours at 500 MW (475 MW Required) with Required Reliability (Pulse Rate = 30 & 60 Hz) Output Power (Gain = 3.1, Goal = 3.25) Combined Klystron Power

27 RF Component Performance

28 Accelerator Structure Requirements Convert rf energy to beam energy efficiently. Short-range (intra-bunch) transverse wakefields small so bunch emittance growth in the linacs is manageable. Determined by average iris radius - 17% of rf wavelength chosen as optimal. Long-range (bunch-to-bunch) wakefields suppressed so bunch train effectively acts as a single bunch. Dipole mode power coupled out for use as guide for centering the structure on the beam. Operate reliably at the design gradient and pulse length. The structure development program is joint SLAC, KEK and FNAL effort with the KEK/SLAC work spanning more than a decade.

29 Structure Cells and Coupler Assembly Cells with Slots for Dipole Mode Damping High Power Output Coupler Port for Terminating and Extracting Dipole Mode Power

30 Wakefield Damping and Detuning Dipole Mode Density Ohmic Loss Only Wakefield Amplitude (V/pC/m/mm) Time of Next Bunch Measurements Detuning Only Frequency (GHz) Damping and Detuning Time After Bunch (ns)

31 High Gradient Structure Development In 1999, discovered gradient limitations in original 1.8 m structures have since: Traveling-Wave Structure - Tested 34 structures with over 20,000 hrs of high power operation at NLCTA. - Improved structure preparation procedures - includes various heat treatments and avoidance of high rf surface currents. - Found lower input power structures to be more robust against rf breakdown induced damage. - Developed NLC/GLC Ready design with required wakefield suppression features it is 33% as long (60 cm) and requires 40% of the power of the 1.8 m design.

32 Making the Gradient: Achieved 90 MV/m (65 MV/m Required) with Shorter, Lower Power Structures Unloaded Gradient (MV/m) Original Experimental 1.8m Design Low Power Structures 1500 hrs 1700 hrs 500 hrs 1200 hrs 1400 hrs 700 hrs Operation Period of Each Structure Set

33 High Gradient Performance of Five Recent NLC/GLC Structures Breakdown Rate at 60 Hz (#/hr) Average Rate Limit for 99% Availability (2% Overhead and 5 sec Recovery) Design Average Rate Limit (~100% Availability) Unloaded Gradient (MV/m)

34 Expect Lower Rates During Beam Operation 75 Gradient Profiles Along Structure Gradient (MV/m) During Structure Testing Average = 65 MV/m During NLC/GLC Beam Operation Average = 52 MV/m Cell Number

35 RF Unit Test: Phase II (In Progress) Power Eight Accelerator Structures in NLCTA (TRC R2 Requirement) From Station 1 From Station 2 From Eight-Pack 3 db 3 db 3 db 3 db 3 db Beam Operate Eight, 60 cm Long Structures at 65 MV/m, 400 ns Pulses

36 Results from First Week of Eight Structure Operation (> 100 hr) with Beam Structure Manufacturer Gradient (MV/m) Trip Rate (#/hr) H60vg4R17-1 SLAC H60vg4R17-2 SLAC H60vg3S17-FXC4 FNAL H60vg3S17-FXC3 FNAL H60vg3-FXB6 FNAL H60vg3-FXB7 FNAL H60vg4S17-1 KEK/SLAC H60vg3R17 SLAC Average

37 Cell Fabrication Need to: - Control fundamental frequency by tuning or feed-forward machining. - Maintain smooth dipole frequency profile. Deviations of Cell Dipole Mode Frequencies: Require < 3 MHz RMS Single-Crystal Diamond Turning Poly-Crystal Diamond Turning RMS = 0.4 MHz RMS = 1.0 MHz Frequency (MHz) Frequency (MHz)

38 Structure Fabrication at FNAL (See Poster Presentation) Inspecting Parts Structure Assembly Clean Room (Class 1000)

39 Structure Fabrication at SLAC Ready for Coupler Braze After Braze

40 Structure Fabrication at KEK Hydrogen Furnace with Short Structure Chemical Etching of Cells

41 RF System Readiness All Key Feasibility Demonstrations Complete - TRC R1 requirements satisfied. Solid State Induction Modulator - Starting construction of more compact, efficient 2-Pack. - Off-the-shelf components and modular design lends itself to mass production. PPM Klystrons - Tubes already being built by industry. Dual-Moded SLED II - Standard machining tolerances may be built by tube companies. - Engineering required to improve manufacturability.

42 RF System Readiness Accelerator Structures - Continue efforts to improve high gradient performance. - Well developed fabrication procedures to achieve wakefield and energy performance. - Three production groups churning out structures. System Integration - Accelerating beam with eight structures at NLCTA. Summary - Ready for industrialization (John Cornuelle s talk) - Plan to expand NLCTA and GLCTA to test industrially-built components.

43 Outlook: Technical Risks SLAC Linac operation of 40 years with excellent reliability is proof a large-scale warm rf system can be made robust. NLC/GLC linacs for 500 GeV cms operation are only twice the SLAC Linac length. Technical risks identified in US Cold-Warm Options Study considered low as problems would be discovered early. Warm approach has advantage of quick repair/recovery times from beam line vacuum leaks (one shift in SLAC Linac).

44 Outlook: Higher Energy Warm Approach Affords Higher Energy Reach Structures qualified at unloaded gradient allows 1.3 TeV operation at reduced current in the 1 TeV configuration. Can exploit flexibility of pulse compression system and the improved high gradient performance at shorter pulse lengths to achieve even higher energies. Natural first step toward multi-tev linear colliders such as the CLIC two-beam accelerator. X-Band Technology is Indeed the Path to the Future

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46 Bead-Pull Phase Advance Measurements of H60VG3 (FXB2) Before and After Processing to 70 MV/m (7000 Breakdowns, 300 Hours RF On) Integrated Phase Advance (degrees) Before After1 After Cell Number

47 NLC Total Project Cost (TPC) Versus Gradient Cost of the linac is a balance between cost of the power sources (which increases with gradient), and cost of accelerator length (which decreases with gradient). Minimum occurs when these are roughly equal, and is rather shallow. The linac is about half the total cost of the collider. Relative TPC If desire more margin, a collider optimized at 60 MV/m would be 10% longer, and cost 3% more than with the present design at 65 MV/m. Unloaded Gradient (MV/m)

48 Site Power Site Power (MW)* 0.5 TeV 1.0 TeV Warm Baseline Warm with DLDS and 3.2 µs Klystron Pulse Length Cold US Design * See Answer to Question 28

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50 RF Component Efficiencies Efficiency (%) Design Achieved Comment Two-Pack Modulator Use Integrated Transformer (> 70% Expected) PPM Klystron ~ 60% Possible SLED II Improve Flanges/Fab/Assembly Wave Guide Transport Use Design Layout / Reduce High- Loss WG Structures Use Round Cell Shape