X-band Accelerator Structures R&D at SLAC

Transcription

1 X-band Accelerator Structures R&D at SLAC Juwen Wang SLAC/LLNL Discussion March 5, 2011

2 Outline 1. Introduction Brief history Achievements 2. Basics of X-Band Accelerator Structures Design Principle and Description Accelerator Structures Performance 3. Structure Assembly Technology Mechanical QC and Microwave QC Chemical cleaning Accelerator parts joining (diffusion bonding, brazing and welding) Microwave tuning and characterization Vacuum baking Alignment 4. Discussion on Fabrication Technology and Locations

3 Contributors SLAC: C. Adolphsen, N. Baboi, K. Bane, G. Bowden, D.L. Burke, J. Cornuelle, S. Doebert, V. Dolgashev, A. Hasse, H. Hoag, E. Jongewarrd, K. Jobe, R.M. Jones, R. Kirby, k. Ko, Z. Li, G.A. Loew, J. Lewandowski, R.J. Loewen, D. McCormick, R.H. Miller, C. Nantista, C.K. Ng, E. Paterson, C. Pearson, N. Phinny, T. Raubenheimer, M. Ross, R.D. Ruth, S. Tantawi, K. Thompson, J. Van Pelt, F. Wang, J. W. Wang, P.B. Wilson. KEK: Y. Funahashi, Y. Higashi, T. Higo, N. Hitomi, H. Kudo, T. Kume, H. Matsumoto, Y. Morozumi, K. Takata, T. Takatomi, N. Toge, K. Ueno, Y. Watanabe. FNAL: T. Arkan, H. Carter, D. Finley, I. Gonin, T. Khabiboulline, S. Mishra, G. Romanov, N. Solyak. LLNL: J. Klingmann, K. Van Bibber.

4 1. Introduction Brief history Achievements

5 Achievements of X-Band Structures R&D at SLAC Motivation: Main Linac for the Future Linear Colliders (NLC, GLC and CLIC) Brief History: X-Band accelerator structures R&D for the NLC/GLC in collaboration with KEK, FNAL and LLNL. Designed, fabricated and tested 50 X-Band accelerator structure sections. (Among them, 8 were made with KEK collaboration and 12 were fabricated by FNAL) Present X-Band accelerator structures R&D for the CLIC main linac in collaboration with CERN and KEK. Participated the design, fabrication and testing of more than 12 X-Band accelerator structure sections. 3. Ongoing Support Work for LLNL Project of the Compton Scattering Light Source MEGa-Ray.

6 Evolution of Structures New types of couplers Optimized cell shape

7 Contribution to the Accelerator Technology through NLC/GLC X-Band Structures R&D Theoretical analysis for full understanding of HOM suppression in RF accelerator structures. Damped and Detuned Structures can be applied to any low emittance, high beam loading accelerators. Simulation methods for beam-structure interaction: structure wakefield, emittance growth and analysis of structure alignment and dimension tolerances. Optimization of accelerator parameters for highest RF efficiency and dimension determination with sub-micron precision. Manifold damping gives structure position monitor with micron transverse sensitivity and frequency multiplexed longitudinal resolution of the order of several cells. Fabrication technologies for normal conducting accelerator structures such as precision machining, diffusion bonding and long structure alignment. Extensive studies for high gradient RF operation to meet the NLC requirement rate at 65 MV/m: new types of couplers, Procedure for structure treatments.

8 2. Basics of X-Band Accelerator Structures Design Principle and Description Accelerator Structures Performance

9 Requirements of Accelerator Structures for Linear Colliders High Accelerating Gradient to Optimize Length and Cost. Control of Short and Long- Range Wakefields to Ensure the Preservation of Low Emittance for Multi-Bunch Beams.

10 Transverse wakefields - II Single Bunch Emittance Growth (Head-Tail Instability) due to the short range transverse wakefields Computed transverse δ-function wake potential per cell for S-Band SLAC structure. Solid line: Total wake Dashed line: 495 modes Dot-dashed line: lowest frequency dipole mode (λ=7 cm) Multi-bunch Beam Breakup due to the long range transverse wakefields.

11 Early Studies on Two Types of Heavily Damped Structures Example of structure with radial slots in iris. Example of structure with circumferential-slot coupling: crossed-waveguide structure, with two half-cells and one full cell.

12 Test Cells for Damped Structures in 1980s at SLAC

13 CERN CLIC Waveguide Damped (WDS) Structure Minimize E-field Minimize H-field Provide good HOM damping Provide good vacuum pumping

14 Long Range Dipole Mode Suppression - Idea of Detuning of Dipole Modes Cells for a Detuned Structure have profiles with Gaussian dimensional distribution. dn k df 1 Dipole mode distribution for Detuned Structure In the time domain, the excited wakefield by the cells with Gaussian distribution dipole frequencies has Gaussian amplitude profile.

15 F1 (GHz) Long-Range Wakefield Calculation avoided crossing Phase (deg) Treat each cell as periodic. Calculate several sample cells to obtain dispersion curves for studying synchronous kick factor and avoided crossing (coupling). Fit dispersion curves of sample cells to obtain cell parameters for equivalent circuits. Interpolate to obtain parameters of all cells Solve coupled circuit system. Integrate spectrum for wake in order to provide all important design information to optimize cell-manifold parameters

16 Precision Fabrication for Accelerator Discs Profile tolerance 1 μm and Surface finishing better than 50 nm

17 Microwave QC for Single Disk

18 Stack Microwave QC Setup

19 Frequency Deviation [MHz] Super Precision Machining with Single Diamond Cutter Tuning Not Needed 2b offset [micron] Frequency [MHz] Integrated Phase Slip [degree] del_sf00 del_sf0pi del_sf1pi del_sf Single-disk RF-QC 3 2 Accelerating mode frequency Measured Frequency Integrated Phase Slip b_offset Disk number Disk number Single-Crystal Diamond Turning Polycrystalline Diamond Turning

20 Regular Precision Machining with Polycrystalline Diamond Cutter Tuning Needed Microwave QC of Fundamental Modes for H60VG4SL17A/B Regular Cups Temperature and humidity corrected

21 Regular Precision Machining with Polycrystalline Diamond Cutter Tuning Needed (Continued) Microwave QC of Dipole Modes for H60VG4SL17A/B Regular Cups Temperature and humidity corrected

22 Prototype Accelerator Structure for the NLC/GLC Main Linac High Power RF Coupler Port for Terminating and Extracting Dipole Mode Power Cutoff view of a structure end A 60 cm structure with most of final design features

23 Damped Detuned Structures for the NLC/GLC DDS1 (Round Damped Detuned) 2π/3 Mode TW Structure Single diamond turning discs without tuning; Micron level cell-to-cell alignment.

24 High Gradient Test Structures One of four T-type Structures -- T53VG3, 60-Cell 2π/3 Mode TW SW20PIL 15-Cell π Mode SW For the LLNL Campton Scattering Light Source. One of more than 10 High Phase Advance 5π/6 Mode TW Structures, H60VG3S18 with HOM Slots and Manifolds.

25 Theoretical and Experimental Proof of Transverse Wakefield Suppression Comparison of the measurement for a pair of dipole Interleaved 60 cm Damped Detuned X-Band Structures with error bars (red) and calculated wakefield (black) Data from early 2005.

26 Edge Damage on Cavity Side of Coupling Iris due to RF Pulse Heating T53VG3 Distribution of Breakdowns (70 MV/m, 400 ns, 10 hr run) RF Pulse Heating causes: Surface Roughening and Cracks Local Surface Melting Input coupler Rate in cells.1/hr 58 Cells Output coupler Performances of some structures were found to be limited by pulse heating of coupler matching irises. Beam s eye view of input coupler. RF RF SEM picture of input matching iris. Pulse heating was in excess of 100 C.

27 Field Distribution in Coupler Region and RF Pulse Heating Temperature increase due to RF pulse heating T Magnetic field H 2 t T p Pulse width R s c Surface resistivity Specific heat Thermal conductivity Surface temperature distribution in the region of coupler iris for 400 ns pulses, 48 MW. The maximum temperature increase was 127º C.

28 Improved Coupler Design E s max = ~34 48 MW H s max = ~ MW Pulse Heating ~ 3 C By proper choice of matching cell b dimension, matching cell length can be made equal to standard cell length.

29 NLC Prototype Structures Can Stably Operate at 65 MV/m to Meet the Required RF Breakdown Rate Average breakdown rates for a series of NLC test structures as a function of accelerating gradient after 500 hours (upper line) and 1500 hours (lower line) of RF processing.

30 Breakdown Rate Dependence on Pulse Length for Various NLC Structures.

31 Some of KEK/SLAC Made Accelerator Structures for Testing CLIC Main Linac Design T18_VG2.4_DSC with SLAC Flanges TD18_VG2.4_DISC with SLAC Flanges T28_VG2.9 (T26) with SLAC Flanges TD18_VG2.4_DISC with KEK Flanges

32 T18_VG2.4_DISC Structure Test Field Amplitude E acc E _ out acc _ in ~ 1.5 Cumulated Phase Change 120 Microwave Tuning and test High power test set-up

33 CLIC Prototype Structures Can Stably Operate at 100 MV/m to Meet the Required RF Breakdown Rate RF BKD Rate Gradient Dependence for 230ns Pulse at Different Conditioning Time After 250hrs RF Condition RF BKD Rate Pulse Width Dependence at Different Conditioning Time G=108MV/m After 500hrs RF Condition G=108MV/m After 900hrs RF Condition After 1200hrs RF Condition G=110MV/m This performance maybe good enough for 100MV/m structure for a warm collider, however, it does not yet contain all necessary features such as wake field damping. Future traveling wave structure designs will also have better efficiencies

34 3. Structure Fabrication Technology Mechanical QC and Microwave QC Chemical cleaning Accelerator parts joining (diffusion bonding, brazing and welding) Microwave tuning and characterization Vacuum baking Alignment

35 Lathe with Twin Spindles and Twin Turrets Profile tolerance 5 μm and Surface finishing nm

36 ZYGO Surface Flatness Measurement for Typical Cups of T18_VG2.4_DISC Structures Both sides show less than 1 micron concaved 16D-A 14D-C 17D-A 17D-C

37 Stacking for Body Diffusion Bonding of a CLIC Structure

38 Diffusion Bonding of T18_vg2.4_DISC Pressure: 40 PSI Holding for 1 hour at 1020º C

39 Brazing of QUAD with Water Flange Au/Cu Alloy: 25/75 Brazing temperature: º C

40 First Assembly Brazing of T18_vg2.4_DISC Body / Two Coupler Assemblies / Cooling/One Beam Pipe / Tuning Studs Au/Cu Alloy: 35/65 Brazing temperature: º C

41 Final Brazing of T18_vg2.4_DISC Au/Cu Alloy: 50/50 Brazing temperature: º C Adding One Beam Pipe

42 Flange Welding for a Accelerator Structure

43 Microwave Tuning and Characterization

44 Tuning and Structure Characterization

45 Example of Phases and Amplitudes along the Axis of a 77-Cell TW Accelerator Typical modulation due to uncompleted tuning of output coupler Phases and amplitudes plotted in a complex plane (5π/6 mode structure, 2x150º=300º per cell for reflection) Electrical field amplitudes along the structure. There are small amplitude and phase modulation due to slightly imperfections of the couplers, which almost no impact to the power efficiency and beam acceleration.

46 Reflection and Transmission f Wiggles due to the beating of reflections from slightly mismatched input/output couplers. f 1 2 T f Transmitted power (left to the load) = 2 S 21 2 S 12 Reflected S 11 from input coupler as a function of frequencies. Transmission S 12 from input port to output port as a function of frequencies.

47 Vacuum Baking of Two Structures 650 C 10 days

48 Alignment Measurement Using CMM Machine

49 4. Fabrication Technology and Locations

50 Fabrication Locations 1. National Laboratories for the X-Band Structures: SLAC KEK LLNL 2. Private Vendors for the X-Band Structures: US Robertson Precision, Inc. (California) LeVezzi Precision, Inc. (Illinois) Japan IHI Morikawa co. Europe VDL Enabling Technologies Group (Netherland)

51 Manufacturability Case of Small Amount Production (less than few hundreds) Case of Mass Production (10k for future X-Band compact FEL or even 1.8 millions precisely machined parts for Linear Collider, which was studied extensively in late 1990s) Design of Manufacturability (DFM) Studies with Huge Cost Reduction