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 • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 2
Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 3
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
Facility Layout
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
Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 7
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
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
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
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
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
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
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
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
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
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
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
Pico-second Multi-bunch UV Laser ~ 10 ps, To User Room ~ 20 ps, (Pump-Prove) 1064 nm 266 nm 19
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
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
Beam Current FEL 22
Outline • Introduction • Present Status of MIR-FEL • Present Status of THz-FEL • Perspective 23
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
Present Condition Dipole RF-gun CTR Monitor Chicane Solenoid Triplet Carbon Laser port Quadrupole Faraday cups Undulator has not been installed yet. 25
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
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
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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