Photonic Bandgap Fiber Wakefield Experiment: Focusing and Instrumentation for Dielectric Laser Accelerators

Transcription

1 Photonic Bandgap Fiber Wakefield Experiment: Focusing and Instrumentation for Dielectric Laser Accelerators R. Joel England E. R. Colby, C. McGuinness, J. Ng, R. Noble, T. Plettner, C. M. S. Sears, R. H. Siemann, J. E. Spencer, D. Walz Advanced Accelerator Research Department SLAC National Accelerator Laboratory 1

2 NLCTA E163 Test Beamline E163: A facility for testing laser-driven accelerator structures. Beam energy = 60MeV; t = 1ps to 400 attosec; E = 0.1% 2

3 Dielectric Fiber Accelerator conductor hollow dielectric-lined waveguide aperture ~ 0.26 E z 2.5 GV/m DF ; / 1 2 Rosing & Gai, PRD 42, 1829 (1990) hollow Bragg waveguide aperture ~ 0.3 ; E z ~ 2.5 GV/m DF ; 1 (2 a / )2 2.1 Mizrahi & Schachter, PRE 70, (2004) PBG fiber with central defect aperture ~ 0.68 ; E z ~ 2.5 GV/m X. E. Lin, PRSTAB 4, (2001) conductor lossy at optical wavelengths damage threshold for SiO 2 ~ 1ps E z E pk / DF t (1 v g / c)l fiber 3

4 Optimized PBG Fiber Geometry a = 0.35 ; R = 0.52 a X. E. Lin Photonic bandgap fiber accelerator, PRSTAB 4, (2001) 4

5 The Road to a Fiber-based Accelerator Manufacture/Prototyping Coupling e ± beam: focusing, emittance, microbunching laser: mode-matching, coupling efficiency, phase stability Fiber Characterization Proof-of-Principle Acceleration + Staging 5

6 Manufacturability QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Courtesy Crystal-Fibre, Inc. Custom Fiber Manufacture prohibitively expensive for accel. prototyping SBIR or other funding for collaboration with industry (e.g. Incom, Inc. - Charlton, MA) Pre-made Telecom Commercial Fibers PBG telecom fibers exist (~$500/m) Thorlabs + Crystal-Fibre, Inc. Not designed for accelerator applications µm Polymethylmethacrylate (PMMA) Fibers U. Sydney (A. Argyros, et al) drawing process less expensive technique could be used for geometrical prototyping and tolerance testing A. Argyros, et al., Optics Express 18, 5642 (2008) 6

7 Manufacturability matrix of 10µm holes, 1mm thick Incom, Inc. SBIR proposal Air-core fibers from U. Southampton hole dia = 10µm courtesy J. E. Spencer, B. Noble courtesy Dave Richardson 7

8 Commercial Fibers fibers manufactured by Crystal-Fibre, Inc. (telecom) 2R (defect) (µm) a (pitch) (µm) lattice dia. (µm) / cladding dia. (µm) BANDSOLVE simulation of accelerating mode for HC-1060 fiber maximum gradient ~ 30 MV/m courtesy B. Noble 8

9 Laser Coupling TE mode Bragg reflector TM mode 9

10 Coupler Studies 10

11 Coupler Studies PML decompose excitation into normal modes of the waveguide (including Lin mode): perfh ABC 1/12 section of fiber HFSS Simulation 11

12 Laser Coupling from Free Space S11 (%) Reflected Power Mode 1 = EH11, Mode 2 = TM angle (deg) Mode 1:1 Mode 1:2 Mode 2:2 Total Coupling to fiber tip from free space: shorter term solution HFSS model of simple dielectric waveguide will extend to PBG lattice type fiber max coupling ~ 35% max coupling ~ 10% Cleave angle = 0 deg Cleave angle = 45 deg options: radially polarized laser on flat fiber tip linearly polarized laser on angled fiber tip 12

13 E-Beam Focusing New Halbach Magnet Design Field Gradient ~ 500 T/m Aperture = 6 mm Adjustable z positions of magnets. String encoder readback of magnet positions. On slider stage for insertion/removal of assembly. Magnets aligned on titanium rods. 13

14 Permanent Magnet Quadrupoles PMQ 1: B = T/m PMQ 2: B = T/m PMQ 3: B = T/m PowerTrace Simulation * opt = L fiber /2 0 = 8m 0 = 0 g2 = mm g1 = mm = 0.5 mm 14

15 Microbunch Washout Initial Microbunched Beam IFEL Interaction + Chicane Compression Technique recently demonstrated by C.M.S. Sears -> 400 attosec bunches f 0 sin(k L z 0 ) Dominant washout terms z f z 0 R 56 [ 0 sin(k L z 0 )] T 511 x 0 2 T 533 y 0 2 After PMQ Focus laser I(z) I 0 [1 2 b n cos(nk L z) ] n 1 Primary culprits are the T511 and T533 of the PMQs 15

16 Microbunch Washout Possible Remedies Radially Dependent Amplitude z f z 0 R 56 { 0 (x 0, y 0,z 0 )sin(k L z 0 )} T 511 x 0 2 T 533 y 0 2 (x 0, y 0,z 0 ) T 511x T 533 y 0 R 56 sin(k L z 0 ) this requires the IFEL modulation to increase quadratically with radial distance Collimation NO collimator Q f = Q µm diameter collimator Q f = Q 0 /6 200 µm diameter collimator Q f = Q 0 /

17 Emittance Requirements ELEGANT simulation of focal waist Transmission vs. Normalized Emittance * = 0.5 mm fiber aperture * = 0.5 mm 17

18 Emittance Preservation Measured Emittance Growth in the NLCTA/E163 Beamline = random RMS steering error in quadrupoles N ~ 100 µm! 18

19 Improved Modeling Tools Improved Modeling Tools Matlab-based 19

20 Improved Modeling Tools Improved Modeling Tools ELEGANT-based 20

21 Improved Modeling Tools DIMAD-based: Built into the Control System Alternate definitions of x and y bx ~ 75 m Matlab and ELEGANT models use x (s) 2 x (s)/ x,rms (s) If we instead use x (s) 2 x (s)/ x,initial constant then we (mostly) reproduce the SCP results: 21

22 Experimental Plan 800 nm 800 nm 22

23 Phase 1: Experiment Layout 9.6 Beam µm Charge Normalized Emittance Energy Required Beam Parameters Bunch length 50 pc < 5 mm mrad 60 MeV 1 ps Energy Spread 0.1 % 4 candidate commercial fibers PMQs Fiber Holder fibers exit chamber for spectrographic analysis FIBER HOLDER 4 candidate commercial fibers QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. beam passes through ~1mm of fiber Newport MS 260i Spectrograph 23

24 Summary Issues to be addressed in developing PBG Fibers as Accelerators: Affordable (<$10k) manufacturing of Prototypes For injected test beam: emittance focusing and spot size microbunch washout Laser coupling: optimizing air-to-fiber coupling developing high-efficiency advanced coupler designs Doing proof-of-principle experiments with single and then multiple stages of acceleration. 24

25 Backup Slides 25

26 Coupler Studies code: S3P Advanced coupler design: in/out power couplers analogy to RF tw accelerator S 11 = 0.1: power coupling can be close to 100% how to manufacture? courtesy of Cho Ng, SLAC 26

27 Motivation S-Band RF X-Band RF Gradient P R S L f smaller RF structures: higher gradient machining tolerances transverse wakefields breakdown (E z 100 MV/m) 3D woodpile structure dielectric gratings PBG Fibers Optical to IR laser-driven microstructures lasers offer high rep rates, strong field gradients ( >0.5 GV/m), commercial support dielectrics: high breakdown threshold (~4 GV/m) manufacturability coupling of beam/laser 27

28 Photon Budget & SN Ratio E mode kq 2 e2 c 4 g Z c L 2 1 g 0 E mode E Cherenkov 1 Z C [ ] 120 [nm] 120 HC-1060 fiber: Z C ; 0.12 [nm] k J / C 2 m E mode J / electron N E mode ( 50pC ) photons e h N detector N transmission fiber spectrgraph 1150 photons 50% 98% 38% Cherenkov background cannot couple to the lattice modes at frequencies in the bandgap. Coupling to the defect modes would improve the signal. Coupling to the cladding: can be reduced through bend losses 28

29 Search for Candidate Accel. Modes HC-1060 SEM image RSoft BandSolve Model toward SOL line courtesy of B. Noble 29

30 Schottky vs Cherenkov E mode kq 2 e2 c 4 E mode E Cherenkov [ g Z c L 2 1 g 0 1 E Cherenkov r e Lmc 2 f 0 c g / (1 g ) 4 (1 fl cladding / L fiber ) ] 0 Z c f 3 (1 1 )d v g 0.6; 0.48 nm; L fiber 1 mm; L cladding 164 µm; = 2.13 ; f 10; E mode E Cherenkov 1 Z C [ ] 212 [nm] 212 HC-1550 fiber: Z C 0.2 ; 0.13 [nm] 212 Lin Fiber: Z C 19 ; 0.2 [nm] 30

31 Experimental Layout NLCTA: design parameters Beam Charge Normalized Emittance 50 pc 1-2 mm mrad FIBER HOLDER 4 candidate commercial fibers 9.6 Energy µm Bunch length 60 MeV 1 ps beam passes through ~1mm of fiber Energy Spread 0.1 % fibers exit chamber for spectrographic analysis 31

32 Experimental Layout e-beam image of mounted fiber 32

33 Challenge: Small Spot Sizes PMQ triplet with motorized gap spacing and focal position. 420, 560, 560 T/m field strengths modified Halbach design C.M. Sears, Production, characterization, and acceleration of optical microbunches, PhD dissertation, Stanford U. (2008) 33

34 Challenge: Small Spot Sizes PowerTrace Simulation QuickTime and a TIFF (LZW) decompressor are needed to see this picture. ELEGANT simulation of the final focus transmission ~ 50% (for 1060 fiber with 10 µm defect) QuickTime and a TIFF (LZW) decompressor are needed to see this picture. x, y ~ 0.5 mm x, y ~ 3 µm 34

35 Challenge: Small Spot Sizes July 3, 2008 PMQ Scan QuickTime and a TIFF (LZW) decompressor are needed to see this picture QuickTime and a TIFF (LZW) decompressor are needed to see this picture. QuickTime and a TIFF (LZW) decompressor are needed to see this picture. 35

36 Summary optical to IR accelerating structures: offer high gradients (~ 1GeV/m), high rep rates, high damage threshold of dielectrics require micron-scale focusing, microbunching, and manufacturing PBG fiber accelerators: permit large apertures commercial manufacturing capability; premade fibers are designed for telecom, not acceleration need to develop custom geometries: Lin fiber E163 near-term: focusing of e-beam through fiber cores + spectrally resolving fiber modes from the emitted wakefield radiation long-term: coupling of structure to drive laser and observing net acceleration of microbunched e-beam ---> multiple stages 36

37 Experimental Plan Phase I: Wakefield Excitation (no laser) tightly focus beam through fiber central defect (spot sizes < 10 µm) wakefield excitation of fiber modes resolve accelerator-like modes by spectral analysis Phase II: laser-driven scenario few-100 attosec microbunched beam using IFEL + chicane laser coupled to the fiber accelerator mode measure net microbunch acceleration with magnetic spectrometer 37