High Gradient Tests of Dielectric Wakefield Accelerating Structures

Transcription

1 High Gradient Tests of Dielectric Wakefield Accelerating Structures John G. Power, Sergey Antipov, Manoel Conde, Felipe Franchini, Wei Gai, Feng Gao, Chunguang Jing, Richard Konecny, Wanming Liu, Jidong Long, Haitao Wang, Zikri Yusof Argonne National Laboratory US High Gradient Research Collaboration Workshop, SLAC May 24, 2007

2 Outline Introduction 1

3 High Gradient Studies at Argonne Dielectric Loaded Accelerating (DLA) Structures Dielectric Wakefield Acceleration Beam-Driven DLA Structures Using the Argonne Wakefield Accelerator Facility RF-Driven DLA Structures (C. Jing) Collaboration between ANL, NRL, & SLAC 2

4 Why use dielectrics?? Geometry Slow wave accelerator Dielectric (instead of irises) is used to reduce v ph to c. ε a b ε L Advantages of DLA Simple geometry No field enhancements on irises High gradient potential Comparable shunt impedance Easy to damp HOM Electric Field Vectors Open Questions Breakdown? Joule Heating? Multipactor? 3

5 Why use e-beam Driven? GW s of Power transferred by the electron beam. Short pulse operation can increase the breakdown threshold. More material options: Low and high Q structures produce the same wakefield. Applications Collinear wakefield acceleration schemes Single Mode Multimode Enhanced Transformer Ratio Two-beam acceleration Dielectric Dielectric TBA Vacuum Metal Witness Beam Drive Beam Power Source 4

6 Wakefield Scaling in DLA Structures (a short Gaussian beam) 2b 2a ε Q Key to Large W z DRIVE BEAM Charge Bunch length Emittance Energy Beam quality Beam energy (= $$) High Charge (photocathode) W z Q a cos( k z) 2 n small tube means small bunch ε N 1/2 σ r = [ β] γ Short Bunch (killer!) e πσ 2 z λn 2 5

7 Wakefield Scaling in DLA Structures (a short Gaussian beam) 2b 2a ε Q W z Q a cos( k z) 2 n e πσ 2 z λn 2 Key to Large W z DRIVE BEAM Charge Bunch length Emittance Energy Ez(MV/m/10nC) Wakefield Amplitude Dependence on Inner Radius Inner Radius a (mm) 6

8 AWA Drive Beamline 1.3 GHz RF Photocathode Gun 1.3 GHz Linac & Steering Coils Quads Wakefield Structure Spectrometer 4.5 m Experimental Chambers ICT1 YAG1 GV YAG2 GV YAG3 ICT2 YAG4 Slits BPM YAG5 Dump/ Faraday Cup Single bunch operation Q=1-150 nc (World s High Q Photoinjector!) Energy=15 MeV (30 MeV upgrade after adding new 30 MW Klystron) High Current = 10 kamp Bunch train operation 4 bunches x 10 nc 3 ns long (present) 64 bunches x 50 nc 50 ns long (future, after Cs2Te photocathode) 7

9 AWA Photocathode Laser System 532 nm pump 532 nm pump Ti:Sapphire Amplifier (Spitfire regenerative amplifier + 2 linear amps) x2 --- Quanta-Ray 170 x2 --- Quanta-Ray nm seed 744 nm 15 mj x3 Ti: Sapphire Oscillator 248 nm 1.5 mj Millennia V KrF Amplifier 248 nm 248 nm mj 8 ps FWHM 10 Hz 15 mj Multisplitter 8

10 Outline High Gradient Breakdown Tests using Short Standing-Wave Dielectric Structures Goal Test breakdown strength of dielectrics in the microwave band Method Pass maximum charge through a small I.D. structure 9

11 Experimental Setup for High Gradient Tests WF signal Monitor for breakdown nc High Charge e - bunch RF field probe (- 60 db) time (ns) Short SW DLA Structures Infer Gradients from MAFIA SW Structure #1 Corderite- ID10-Length102 #2 Corderite- ID10-Length23 #3 Corderite- ID5.5-Length28 #4 Quartz- ID3.8-Length25.4 Material Cordierite Cordierite Cordierite Quartz Dielectric constant Freq. of TM01n modes 14.1 GHz 14.1 GHz 9.4 GHz 8.6 GHz Inner radius 5 mm 5 mm 2.75 mm 1.9 mm Outer radius 7.49 mm 7.49 mm 7.49 mm 7.49 mm Length 102 mm 23 mm 28 mm 25.4 mm Wakefield Gradient 0.5 MV/m/nC 0.5 MV/m/nC 0.91 MV/m/nC 1.33 MV/m/nC 10

12 #1 Corderite-ID10-Length102 (Summer 2005) TM GHz structure 23 MV/m gradient. No signs of breakdown. 102 mm 43 nc mixer output (mv) time (ns) Integrating Current Transformer (nc) time (μs) Mafia simulation 11

13 #2 Corderite-ID10-Length23 (Winter ) MAFIA simulation of wakefield of the TM GHz structure 23mm 7.5mm 2.5mm E-field pattern (1nC, z=1.5mm) Wz = ~ 1nC for 14 GHz Structure 12

14 #2 Corderite-ID10-Length23 (Winter ) Er probe signals of TM GHz structure 43 MV/m gradient. No signs of breakdown. Measurement 86nC HEM 111 (12.3GHz) Frequency Spectrum of measured signal TM 013 (14.3GHz) TM 014 (16GHz) Simulation TM 012 (13GHz) Frequency Spectrum of simulated signal 13

15 #3 Corderite-ID5.5-Length28 (Summer 2006) MAFIA simulation of wakefield of the short 10GHz structure 28mm 7.5mm 2.5mm E-field pattern Wz (V/m) Wz > 1nC for 10GHz Structure 14

16 #3 Corderite-ID5.5-Length28 (Summer 2006) Measured wakefield signal (Er) of the 10GHz short structure 78 MV/m gradient. No signs of breakdown. 86nC 15

17 #4 Quartz-ID3.8-Length25.4 (Spring 2007) Measured wakefield signal (Er) of the short Quartz structure 100 MV/m gradient. No signs of breakdown. 75 nc TM 012 TM 013 HEM 111 TM

18 Short SW DLA Test Summary The 90 s ~10 MV/m Structure 1 (Summer 2005) 23 MV/m Structure 2 (Winter 05/06): 43 MV/m Structure 3 (Summer 2006) 78 MV/m Structure 4 (Spring 2007) 100 MV/m Next Structures (Near Term Future) Final assembly in beamline 17

19 Progress Towards Dielectric Two-Beam Acceleration 18

20 Previous AWA Beamlines: Two Beam Accelerator Configuration After successfully upgrading the Drive Gun, the AWA Group now aims at restoring the TBA capability. New Witness Gun being fabricated 19

21 An Example of Two-Beam Accelerator (Future Goal) Witness Beam Drive Beam Drive beam: 64 bunches of 50 nc, each separated by 0.77 ns, total 50 ns long. Witness beam: High quality beam: Q=1nC; ε n = 2 μm; σ z = 1mm; δ = 0.5% Total energy gain = ΔE=95 MeV over length ~1m 20

22 Development of a 7.8 GHz Power Extractor = Deceleration tube + Coupler DLA deceleration tube RF power e - L TM 01 -TE 10 coupler f RF GHz ID mm OD mm L mm ε r β g t d ns δ d 10-4 Q w Q [r/q] KΩ/m r sh MΩ/m dielectric =cordierite 21

23 Generated gradient and power by Gaussian bunch (σ z = 2mm) Step 1. Single Bunch Excitation dielectric beamhole Single Bunch ( q = 100 nc per bunch f b = 1.3GHz ) G s (MV/m/nC) ~ q P s (MW) ~ q Step 2. Bunch Train Excitation dielectric beamhole Bunch Train ( q = 30nC per bunch f b = 1.3GHz ) G (MV/m) ~ q P (MW) ~ q

24 Simulated and measured S 21 and S 11 of the TM 01 -TE 10 coupler DLA deceleration tube TM 01 -TE 10 coupler RF port 23

25 Power Extractor Installed in AWA beamline 24

26 First Beam Test of 7.8 GHz Pwr Ext Measured voltage signal (q = 66nC, σ z =2mm) Amplitude Spectrum Generated power v.s. charge (single bunch test) Next Steps Increase Single Bunch Charge Q = 100 nc P = 79 MW 4 x10 nc Pulse Train P = 11 MW RF pulse = 10 nsec 25

27 Outline A Few other activities at the AWA facility 26

28 A Dedicated RF Photocathode Gun Test Stand for the Study of Breakdown Related Phenomena Photocathode Gun ICT1 ~1m Ce:YAG Use solenoids to make an image of the dark current at YAG screen dark current image streaks emitter site Key Features: Field up to μsec on cathode; Removable cathode (different materials, different surface preparation); Lasers available to trigger breakdown First Experiment: β Enhancement Studies with a 1.3 GHz RF Photocathode Gun Motivation 1. Relationship between β and the surface features is not known 2. Number and spatial distribution of the emitters is not known. 27

29 A Dedicated RF Photocathode Gun Test Stand for the Study of Breakdown Related Phenomena* Gun ICT1 ~1m Ce:YAG YAG screen Surface Analysis (geometry, impurities, bulk properies ) Correlate Surface Emission Features Sites emitter site streaks *(collaboration with V. Dolgashev, SLAC) 28

30 High Gradient Test of an GHz, iris-loaded PBG Accelerator Hmax Pin=1W Temperature Rise due to Pulsed Heating e - ΔT ( t) / o K Pin=100MW Fabricated by Tsinghua Univ. ΔT ( t) = 2 1 πρ t 0 dt t t R S H ( t) ck 2 t /ns MAFIA T3 Simulation Wakefield Test Planned: 3-cell SW PBG structure 100 nc 77 MV/m & 100 MW 29

31 Conclusions and Future Directions Beam-driven test of Short SW DLA Structures 100 MV/m! (No breakdown observed) Continue parametric studies with different materials and geometries Dielectric Two-Beam Acceleration 7.8 GHz Power Extractor Charge transmitted 66 nc Power Generated & Extracted >30 MW of RF Next: Higher Charge and longer beam pulse train Dielectric-based TBA demonstration experiment High Quality Beam; 100 MeV in 1 meter AWA Facility Single Bunch Q = 160 nc (present) Short Pulse Train = 4 x 10 nc (present) Upgrade: Ultrahigh Charge Bunch train = 64 x 50 nc; 50 nsec RF pulse (Cs2Te photocathode development) Upgrade: 15 MeV 30 MeV (with second klystron) 30

32 A Dedicated RF Photocathode Gun Test Stand for the Study of Breakdown Related Phenomena Photocathode Gun ICT1 ~1m Ce:YAG Use solenoids to make an image of the dark current at YAG screen dark current image streaks emitter site Key Features: Field up to μsec on cathode; Removable cathode (different materials, different surface preparation); Lasers available to trigger breakdown First Experiment: β Enhancement Studies with a 1.3 GHz RF Photocathode Gun (collaboration with V. Dolgashev, SLAC) Motivation 1. Relationship between β and the surface features is not known 2. Number and spatial distribution of the emitters is not known. 31

33 Experimental Setup Upstream ~14MeV Oscillascope ~0.36m e - 100nC Power Extractor P s =79MW G s =18MV/m t drain =2.2ns RF CH1 Attenuator 1 Isolation CH2 Attenuator 2 Forward (coupling) Reflected Back >10ns Later Bidirectional Coupler Shorted Waveguide e - 100nC spectrometer ~0.35m ~1.4m Downstream ~10MeV (Downstream beam hole is cutoff for 7.8GHz RF Signal ) 32