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Present Status and Perspectives of Long Wavelength Free Electron Lasers at Kyoto University Heishun ZEN, Sikharin SUPHAKUL, Toshiteru KII, Kai MASUDA and Hideaki OHGAKI Institute of Advanced Energy, Kyoto University 1 Outline Introduction


  1. Present Status and Perspectives of Long Wavelength Free Electron Lasers at Kyoto University Heishun ZEN, Sikharin SUPHAKUL, Toshiteru KII, Kai MASUDA and Hideaki OHGAKI Institute of Advanced Energy, Kyoto University 1

  2. Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 2

  3. Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 3

  4. Long Wavelength FEL at Kyoto Univ. • MIR-FEL (named as KU-FEL, 3.6 – 23 µ m) • FEL related research started in 1995. • First Lasing in 2008. • Opened for user experiments (2009 ~). • Routinely operated. • First Lasing with Photocathode operation (2014). • THz-FEL (under construction) • Project started in 2008. • Sharing an RF source with MIR-FEL. • First e-Beam in 2015. • CTR observation in 2016. • First light from an undulator will be in this summer. 4

  5. Facility Layout

  6. Operation Time and User Proposals Operation Time of MIR-FEL User Proposals in this year 1000 • NIR/ MIR -Pump, MIR -Probe experiment of Operation Time [Hour] 900 Maximum Operation Time per Year = 960 h polymer film. (1 internal user) 800 700 • MIR -Pump, Visible-Probe experiment of solid Total 600 samples. (1 internal and 2 external user ) 500 400 • Investigation of MIR sensitivity of crayfish 300 User eyes. (1 external user) 200 Exp. 100 • System development for Photoacoustic 0 2010 2012 2014 2016 spectroscopy using MIR-FEL . (1 internal user) Fiscal Year Since 2014, high voltage capacitors in PFN start • Investigation of scintillation properties of to break due to aging. Therefore, the total various crystals by high energy single operation time can not be long in 2014 & 2015. electron irradiation . (1 external user) We got used capacitors from other institutes and purchased new capacitors.  Now the trouble was solved. 6

  7. Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 7

  8. MIR-FEL in Kyoto Univ. – KU-FEL – Unique point : • 4.5-cell thermionic RF Gun  8.4 MeV e-Beam 8.4 MeV • Alpha-magnet is unavailable.  Dog-leg for energy filter • Seriously strong back-bombardment effect!  10-year continuous fight!  Countermeasures developed. 19 – 40 MeV • Photocathode operation is also available.  Higher peak power than thermionic operation 8

  9. Thermionic RF Gun Main Parameters Resonant Frequency ~ 2856 MHz Coupling β 2.8 Q value 12500 Structure 4.5-cell side couple π mode Accelerating Mode Cathode Material LaB 6 (100) Cathode Radius 1 mm E-field on cathode ~ 27 MV/m • Very compact; just 30 cm for 8.4 MeV beam. • Cost effective • Relatively low emittance ( ε n < 10 π mm-mrad) • Serious Back-bombardment Effect  Countermeasures have been developed. 9

  10. Accelerator Tube Main Parameters Resonant Frequency 2856 MHz Structure Constant Gradient Traveling Wave 2/3 π mode Accelerating Mode Effective Length 2.9 m 10

  11. Undulator Main Parameters Period Length 33 mm Number of Periods 53 Total Length 1.8 m Maximum K-value 1.35 Structure Planer Hybrid • This undulator had been used for ERL- FEL in JAEA until 2009. • Transported from JAEA to KU in 2010. • Installed to KU-FEL in 2012. 11

  12. Optical Resonator Main Parameters Upstream Mirror Chamber Up: 2.946 m Mirror Curvature Down: 2.456 m Cavity Length 5.038 m Out-coupling way Hole couple Hole Diameter on 1 mm Upstream Mirror Mirror Substrate Copper Mirror Coating Gold There is no in-vacuum mirror changer which is commonly used in rich FEL facilities. 12

  13. MIR Beam Transport Line Coupling Hole 0.78 m d = 1 mm Red Laser Diode f = 0.75 m for Alignment Resonator Mirror To User KRS-5 Window R:T = 30 : 70 Stations KRS-5 Vacuum Window • KRS-5 (T~70 % : 0.7 – 30 µ m) is used. R:T = 30 : 70 • Only one focus mirror Step Variable Attenuator • Transport line is covered by plastic tubes. N 2 Gas filling  Remove H 2 O and CO 2 . Power Evolution Monitor Fast TE-cooled MCZT Detector MCZT : HgCdZnTe • Fast detector (< 1 ns) to monitor the FEL Power Evolution. 13

  14. Beam Size in Beam Transport Line 1” 24 20 µ m 4 σ Beam Size [mm] 20 15 µ m 16 User Station#2 10 µ m 12 5 µ m 8 User Station#1 Calculated 4 by ZEMAX 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Distance from 1st Focus Mirror [m] 14

  15. e-Beam Current Profile and FEL Power Evolution 200 1.0 FEL Power [Arb. Units] Beam Current [mA] 0.8 Beam 150 Current 0.6 FEL 100 Power 0.4 50 0.2 0 0.0 0 2 4 6 8 10 12 Time [ µ s] • Beam current ramping due to the back-bombardment effect. • Beam energy was kept constant  FEL can be lased. 15

  16. Time Structure of KU-FEL Pulse Macro-pulse Repetition rate : 1 or 2 Hz Duration : ~ 2 µ s-FWHM Micro-pulse Repetition rate : 2856 MHz  Interval : ~350 ps Minimum Length : < 1 ps-FWHM 16

  17. KU-FEL Performance with Thermionic Cathode Macro-pulse Energy Wavelength Spectrum H 2 O CO 2 CO 2 H 2 O 100 Norm. Intensity [Arb. Units] Macro-pulse Energy [mJ] 1.0 0.8 10 24 0.6 40 31 0.4 1 19 MeV 0.2 0.0 0.1 4 6 8 10 12 14 16 18 20 22 4 6 8 10 12 14 16 18 20 22 24 Wavelength [ µ m] Wavelength [ µ m] • Measurement was done @user station #1 w/o N 2 filling. • Tunable Range was from 3.6 – 23 µ m. • Maximum Macro-pulse energy was 30 mJ/pulse @5 µ m. • Typical FEL bandwidth ~3% @Max. power optical cavity length. 17

  18. Photocathode Operation of KU-FEL LaB 6 thermionic cathode can also be used as a photocathode and Mark-III FEL succeeded in Lasing with LaB 6 photocathode. Reference : M. Curtin, et al., NIM A296 (1990) 127-133. The photocathode operation of LaB 6 cathode was one of a possible upgrade of KU-FEL. Get free from back-bombardment effect. Electron bunch charge can be higher. Need expensive mode-locked laser  We got budget!! A picosecond multi-bunch UV laser was developed. 18

  19. Pico-second Multi-bunch UV Laser ~ 10 ps, To User Room ~ 20 ps, (Pump-Prove) 1064 nm 266 nm 19

  20. Result of Demonstration Experiment Cathode Temperature Thermionic : 1900 K Photocathode : 1400 K Beam Current Profile @Gun Exit Beam Current Profile @Undulator 600 150 Thermionic Averaged Current [mA] Averaged Current [mA] 500 Photocathode 400 100 Thermionic 300 Photocathode 200 50 100 0 0 -2 0 2 4 6 8 10 0 2 4 6 8 10 Time [ µ s] Time [ µ s] No back-bombardment effect in photocathode operation. e-bunch repetition rate : 2856 MHz (Thermionic)  29.75 MHz (Photocathode) Bunch charge @Undulator : 40 (Thermionic)  150 pC (Photocathode) Macro-pulse duration @Undulator : 7 (Thermionic)  4 µ s (Photocathode) 20

  21. Result of Demonstration Experiment Normalized FEL Power [Arb. Units] 1.2 e-Beam Energy : 23.8 MeV Thermionic Photocathode 1.0 Undulator Gap : 19.5 mm (13 mJ) (0.8 mJ) 0.8 FEL Wavelength : ~11.7 µ m 0.6 0.4 • 6.5 times higher micro-pulse energy 0.2 • 1/16 macro-pulse energy 0.0 -0.2 -1 0 1 2 3 4 5 6 7 8 9 10 Good for nonlinear experiments!! Time [ µ s] Thermionic Photocathode Ratio (Ph / Th) Repetition Rate 2856 MHz 29.75 MHz 1 / 96 ~ 2 µ s 2 µ s FEL Macro-pulse Duration ~1 Max. Macro-pulse Energy 13 mJ 0.8 mJ 1 / 16 ~2 µ J 13 µ J Max. Micro-pulse Energy 6.5 FOM (Micro E / Macro E) 1.5E-4 1.6E-2 ~ 100 21

  22. Beam Current FEL 22

  23. Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 23

  24. Schematic Layout Triplet Dipole 1.6-cell Photocathode Quadrupole RF Gun Chicane CTR Beam UV-laser Monitoring Dump Injection Solenoid Undulator Chamber • One of the smallest configuration of THz-FEL. • Short e-bunch is generated by RF gun and chicane bunch compressor. • Compressed e-bunch is injected to undulator and generate THz radiation. • Phase 1 : Measure e-beam properties. • Phase 2 : Measure coherent undulator radiation. Under Preparation • Phase 3 target will be determined based on phase-2 results. 24

  25. Present Condition Dipole RF-gun CTR Monitor Chicane Solenoid Triplet Carbon Laser port Quadrupole Faraday cups Undulator has not been installed yet. 25

  26. 1.6-cell RF Gun Cu Cathode • No laser injection port  Injection at 0-degree • Demountable cathode  Cu photocathode in use • Push-pull tuner • π -mode at 2856 MHz • Q : ~12000, β : ~ 1.1 • Power probe in pumping port • ps multi-bunch UV laser for photocathode excitation 26

  27. Typical RF Waveforms UV laser injection at the end of RF macro-pulse 12 10 Reflected RF Power (MW) 8 Forwarded 6 Probe (x10) 4 2 0 6 7 8 9 10 11 12 Time (µs) 27

  28. Summary of Phase 1 Experiments • Max. Beam Energy : 4.6 MeV • Bunch Charge : up to 1.4 nC • Normalized Emittance : < 10 π mm-mrad @50 pC • CTR dependence on operation condition checked • Observed CTR frequency up to 0.25 THz 28

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