High Gradient Studies at the NLC Test Accelerator (NLCTA)

Transcription

1 Chris Adolphsen

2 High Gradient Studies at the NLC Test Accelerator (NLCTA) NLCTA Linac RF Unit (One of Two) Contributors C. Adolphsen, G. Bowden, D. Burke, J. Cornuelle, S. Dobert, V. Dolgashev, J. Frisch, E. Garwin, R. Kirby, K. Jobe, R. Jones, F. LePimpec, Z. Li, G. Loew, D. McCormick, R. Miller, C. Nantista, J. Nelson, C.K. Ng, M. Ross, R. Ruth, T. Smith, S. Tantawi, J. Wang and P. Wilson Arbitrary Function Generator GHz RF Reference RF Amplitude Control 1 kw TWT Relative Phase Control Klystrons (50 MW, 1.5 µs Pulses) SLED II Pulse Compression 4 3 db Hybrid 40 m Resonant Delay Lines Beam NLCTA Linac Accelerator Structures

3 Structure Development Overview After improvements to the rf processing capabilities at NLCTA in 2000, realized nominal NLC/JLC 1.8 m long structures were being damaged during processing and would not meet performance requirements at 65 MV/m. Launched aggressive R&D program Build/Test low group velocity traveling wave structures and standing wave structures. Improve structure handling, cleaning and baking methods. Study characteristics of rf breakdown in structures, cavities and waveguides. Thus far have tested 20 structures (over 10 khr operation at 60 Hz). T-Series Structures Essentially the downstream (low group velocity end) portions of the 1.8 m structures. Chosen because this region of the 1.8 m structures showed little damage. Produced structure with acceptable trip rate at gradients up to 90 MV/m. H-Series Structures Developing low group structures with acceptable iris sizes to limit short-range wakefields and slots in cells to damp long-range wakefields.

4 T-Series Structures Tested Group Velocity (% c) Cell Machining Etch Depth for Cell Cleaning Input Coupler T20VG5 T105VG5 Diamond Turned 0.3 microns Standard T53VG5R Conventionally Machined 3 microns Standard T53VG3R T53VG3RA Conventionally Machined 1.5 microns Low Field/ Low Impedance T53VG3F Diamond Turned No Etch Standard Cell Number

5 Example of Low Group Velocity Structure Performance at 70 MV/m (120 Hours of Operation at 60 Hz with 400 ns Pulse Widths) Fractional Missing RF Energy Breakdowns: Missing Energy -vs- Location Breakdown Location (Cell Number) Breakdown rate in structure body (blue events) = 0.2 per hour or about one in a million pulses. NLC goal is < 0.1 per hour: measure from < 0.1 to 0.3 per hour in five structures. Breakdown rate in the two coupler cells (green and red events) = 5.5 per hour Rates in other structure couplers vary from 0.1 to 5 per hour suspect pulse heating at the coupler waveguide openings as the root cause.

6 Breakdown Damage on Coupler Horns Input Coupler with Upstream Cover Removed RF RF T53VG3 Structure

7 Coupler Improvements to Lower Pulse Temperature Rise from > 130 C to < 40 C Mode Converter Coupler (¼ Cutaway View) Fat Lip Coupler (Round Horn Edges) RF Increase Radii from 76 m to 3 mm TM01 Build T53VG3MC Structure to Test Low Pulsed Temperature Rise Designs - T53VG3 Body Design - Mode Converter Input Coupler - Fat Lip Output Coupler

8 T53VG3MC Processing History (Total Number of Trips = 1600) T53VG3MC Structure Gradient (MV/m) 1Trip per 25 Hrs 400 ns Pulse Width 1 Trip per 25 Hrs NLC/JLC Trip Spec: < 1 per 10 Hrs at 65 MV/m No Phase Change (< 0.5 ) Time with RF On (hr)

9 H-Series Structures Although the structure with the improved couplers (T53VG3MC) performed very well, it cannot be used in the NLC/JLC. The average iris radius, <a/ >, is smaller (0.13) than desired (0.18), yielding a transverse wakefield three times larger than considered acceptable. As the next step toward an NLC/JLC-ready structure, 150 degree phase advance designs with <a/ > = 0.18 & 0.17 (called H-Series structures) are being developed. Pay twice in loss of shunt impedance input power/length 50% larger than T-Series Five <a/ > = 0.18 structures have been tested so far: H90VG5: Sharp-edge couplers prevented full processing. H60VG3: Sharp-edge couplers slowed processing body breakdown rate OK at 65 MV/m. H60VG3(FXB2): FNAL first full length structure would not process above 70 MV/m. H90VG3: Meets trip rate specs at 60 MV/m. H60VG3(6C): Contains six slotted cells close to acceptable trip rate at 65 MV/m.

10 Designing an a/λ = 0.18 Structure with Low Group Velocity 120 Parameters of Upstream, Middle and Downstream Cells -vs- Phase Advance and Iris Thickness Shunt Impedance (MOhm/m) mm 5 mm 4 mm 3 mm Iris Thickness 2 mm 1 mm Iris Radius (mm) Phase Advance (deg) Upstream Cell Middle Cell Downstream Cell Group Velocity (% c) Zenghai Li

11 Prototype Cells to Damp the Long-Range Wakefield - Modified earlier designs to lower pulse heating. - Expect ~ 50 C temperature rise at 70 MV/m, 400 ns. Rounded Corners in Damping Slots Damping Manifolds Rounded Corners at Openings to Cell - High Gradient Tests of Slotted Cells: Currently Testing H60VG3(6C) that Includes 6 Slotted Cells. First Test of Fully Slotted H60VG3 Structure in June 2003

12 Breakdown Statistics for H60VG3(6C) at 65 MV/m, 400 ns Trips per Hour Goal Mean Number of Trips (80 Total) Days Time Between Trips (Minutes) (Times > 30 Plotted at 30)

13 H60VG3(6C) Breakdown Locations at 65 MV/m, 400 ns Pulse Width Inferred from Reflected and Transmitted RF (Slotted Cells Between Dashed Green Lines) Each Cross-Hatch = Cell 60 Position of Breakdown (ns) Cell Number of Breakdown (1-83) Phase of Reflected RF (degrees) Time with RF On (hr)

14 H90VG3 Breakdown Rate -vs- Gradient for 3 Pulse Lengths H90vg3N ns 240 ns 100 ns Breakdown Rate (#/hr) Breakdown rate per hour 10 0 Slope ~ 8 MV/m / decade Average gradient Data Near End of Run Structure Gradient (MV/m) S. Doebert

15 H90VG3 Breakdown Locations at MV/m, 400 ns Pulse Width Inferred from Reflected and Transmitted RF 120 Each Cross-Hatch = Cell 80 Position of Breakdown (ns) Cell Number of Breakdown (1-83) Phase of Reflected RF (degrees) Time with RF On (hr)

16 NLC - The Next Linear Collider Project Breakdown Analysis Using Acoustic Sensors Cross Sectional View of Structure with Sensors Attached Signal -vs- Cell Number for Four Events with Clearly Discernable Azimuthal Asymmetry Event 1 Event 2 Event 3 Event 4 wall H 2 O X top H 2 O H 2 O bottom H 2 O aisle J. Nelson, NLCTA Ops, et. al.

17 Bead-Pull Phase Advance Measurements of H60VG3 (FXB2) Before Processing and After 300 Hours of Processing to 70 MV/m During Which It Incurred About 7000 Breakdowns Integrated Phase Advance (degrees) Before After1 After Cell Number R. Romanov et al

18 H90VG5 Cell 13 Iris After RF Processing (Worse Than Neigboring Cells Due to Al Inclusion)

19 Development and Schedule Overview The current H60VG3(6C) has the essential features of an NLC/JLC structure and basically meets performance requirements. To improve efficiency and perhaps provide more operating overhead, an a/ =.17 version of this structure (H60VG3S17) has been adopted for the NLC baseline design. The main goal for the next year is to have eight of these structures operating for > 2000 hr at 65 MV/m in the NLCTA Linac, powered by the 8-Pack Source. FNAL and KEK will fabricate these structures. Structure development will also continue, including tests of: Several H60VG3 s (a/ =.18) built by FNAL. A fully slotted version of H60VG3 (H60VG3S18). A longer H-type structure (H75VG4S18). An a/ =.17 version of H60VG3 without slots (H60VG3A18). A pair of standing wave structures with low pulse temperature rise couplers. CERN test structures with Mo and/or W irises (SLAC studying Mo tipped irises)

20 H60VG3 Structure Parameters -vs- a/λ v g /c (%) R (M /m) Distance Along Structure (m) Temperature Rise at 70 MV/m (deg C) E S,E acc at 70MV/m (MV/m) Distance Along Structure (m) Input Power (MW) for 70 MV/m a/λ = a/λ = a/λ = a/λ = Z Li

21 Possible Scheme for Powering Eight H60VG3 Structures in NLCTA Using the Eight-Pack Power Source ~ 500 MW, 400 ns from SLED II 3 db WC db height taper WR90 TE01-TE20 circ.-rect. tapers load diameter step tapers WC db db height taper TE01-TE20 WC293 pump circ.-rect. T tapers diameter step tapers TE20-TE10 bend converter load 3 db WC293 pump T 3 db E-plane 3 db E-plane 3 db bend bend WR90 WR90 WR90 pump T H-plane pump T H-plane bend bend TE20-TE10 bend converter 0.6m structures quad 0.6m structures quad 60 cm Structure Beam C. Nantista

22 Standing-Wave Structures In NLC, standing-wave structures would operate at the loaded gradient of 52 MV/m. 15 Cell, 20 cm Standing-Wave Structure Of three pairs tested, one pair had breakdown rates of < 1 per 8 million pulses at 55 MV/m and no discernable frequency change after 600 hrs of operation. Pulse heating in coupler likely limiting higher gradient operation will be reduced for next test in June, 2003.

23 Structure Testing Schedule 2003 Dec. Jan. Feb. Mar. April May June July Aug. Sept. Oct. Nov. Dec. Jan. Feb. H90VG3N (0.18, 150, no slots) KEK/SLAC H60VG3(FXB2) (0.18, 150, no slots) FNAL H60VG3(6C) (0.18, 150, 6 slotted) KEK/SLAC H60VG3(FXB3) (0.18, 150, no slots) FNAL SW20a375 (Standing Wave Structure Pair) KEK/SLAC H60VG3S18 (0.18, 150, slots) R1 DEMONSTRATION KEK/SLAC CERN Test Structures (W and/or Mo irises) CERN H75VG4S18 (0.18, 150, slots) KEK/SLAC H60VG3(FXB4) (0.18, 150, no slots) FNAL H60VG3A17 (0.17, 150, no slots) SLAC H60VG3(FXB5) (0.18, 150, no slots) FNAL Up to Four H60VG3S17 (0.17, 150, slots, HOM output) starting in November KEK Up to Five H60VG3S17(FXC) (0.17, 150, slots, HOM output) starting in November FNAL

24 FNAL 2003 Structure Fabrication Plan Oct. Nov. Dec. Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. Parts Procurement 6 FXC-Style Duplicate #2 Cells Arrive (Medco) Structure Fabrication FWG Couplers for FXB-004 thru -006 (3 Sets---Medco) 6 FXC-Style Duplicate #2 Cells Arrive (Lavezzi) 20 FXC Brazing Test Cells (Med) 15 FXC Brazing Cells (Med &Lav) Diffusion Bonding and Brazing Tests FXC-Prototype and FXC-001 thru -006 Cells: Start procurement NLT 4/10/03 FXC HOM Cells, Loads, etc. (7 Sets) UMC and FWG Couplers (4/7 Sets) for FXCs FXC Prototype FXC-001 FXB-002 FXB-003 Bake/Ship FXB-004 FXB-005 FXB-006 Bake/Ship Assemble 7 Sets of FWG In/Out Couplers Furnace Rework Opportunity FXC-002 FXC-003 FXC-004 FXC-005 H. Carter

25 High Gradient Summary Making Steady Process Toward an NLC/JLC Ready Structure Produced a T-Series structure that reliably operated at 90 MV/m. Developing structures with acceptable average iris radii: H60VG3_6C has essential features of an NLC/JLC structure and basically meets performance requirements. Have adopted a lower a/ design to improve efficiency and performance at the cost of somewhat larger wakefields. Pulse heating in slotted cells does not appear to be a problem will test fully slotted structure in June Will operate 4.8 m of structures with Eight-Pack power source for system test and to improve performance statistics.