1 Outline Introduction Present Status of MIR-FEL Present Status - - PowerPoint PPT Presentation

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 µm)
• 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

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2010 2012 2014 2016 100 200 300 400 500 600 700 800 900 1000 Maximum Operation Time per Year = 960 h

User Exp.

Operation Time [Hour] Fiscal Year

Total

Operation Time of MIR-FEL User Proposals in this year

• NIR/MIR-Pump, MIR-Probe experiment of

polymer film. (1 internal user)

• MIR-Pump, Visible-Probe experiment of solid
• samples. (1 internal and 2 external user )
• Investigation of MIR sensitivity of crayfish
• eyes. (1 external user)
• System development for Photoacoustic

spectroscopy using MIR-FEL. (1 internal user)

• Investigation of scintillation properties of

various crystals by high energy single electron irradiation. (1 external user) Since 2014, high voltage capacitors in PFN start to break due to aging. Therefore, the total

• peration time can not be long in 2014 & 2015.

We got used capacitors from other institutes and purchased new capacitors.  Now the trouble was solved.

Outline

• Introduction
• Present Status of MIR-FEL
• Present Status of THz-FEL
• Perspective

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MIR-FEL in Kyoto Univ. – KU-FEL –

8.4 MeV 19 – 40 MeV

Unique point :

• 4.5-cell thermionic RF Gun

 8.4 MeV e-Beam

• Alpha-magnet is unavailable.

 Dog-leg for energy filter

• Seriously strong

back-bombardment effect!  10-year continuous fight!  Countermeasures developed.

• Photocathode operation is

also available.  Higher peak power than thermionic operation 8

Thermionic RF Gun

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Resonant Frequency ~ 2856 MHz Coupling β 2.8 Q value 12500 Structure 4.5-cell side couple Accelerating Mode π mode Cathode Material LaB6 (100) Cathode Radius 1 mm E-field on cathode ~ 27 MV/m

Main Parameters

• Very compact; just 30 cm for 8.4 MeV beam.
• Cost effective
• Relatively low emittance (εn< 10 πmm-mrad)
• Serious Back-bombardment Effect

Countermeasures have been developed.

Accelerator Tube

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Resonant Frequency 2856 MHz Structure Constant Gradient Traveling Wave Accelerating Mode 2/3 π mode Effective Length 2.9 m

Main Parameters

Undulator

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Period Length 33 mm Number of Periods 53 Total Length 1.8 m Maximum K-value 1.35 Structure Planer Hybrid

Main Parameters

• 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.

Optical Resonator

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Upstream Mirror Chamber Mirror Curvature Up: 2.946 m Down: 2.456 m Cavity Length 5.038 m Out-coupling way Hole couple Hole Diameter on Upstream Mirror 1 mm Mirror Substrate Copper Mirror Coating Gold

Main Parameters There is no in-vacuum mirror changer which is commonly used in rich FEL facilities.

MIR Beam Transport Line

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Coupling Hole d = 1 mm

0.78 m f = 0.75 m

Resonator Mirror KRS-5 Vacuum Window R:T = 30 : 70 KRS-5 Window R:T = 30 : 70

Power Evolution Monitor Fast TE-cooled MCZT Detector MCZT : HgCdZnTe

Step Variable Attenuator

Red Laser Diode for Alignment

To User Stations

• KRS-5 (T~70 % : 0.7 – 30 µm) is used.
• Only one focus mirror
• Transport line is covered by plastic tubes.

N2 Gas filling  Remove H2O and CO2.

• Fast detector (< 1 ns) to monitor

the FEL Power Evolution.

Beam Size in Beam Transport Line

Distance from 1st Focus Mirror [m]

4σ Beam Size [mm]

2 4 6 8 10 12 14 20 22 24 16 18 4 8 12 16 20 24

5 µm 10 µm 15 µm 20 µm User Station#1

User Station#2 1”

Calculated by ZEMAX

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e-Beam Current Profile and FEL Power Evolution

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2 4 6 8 10 12 50 100 150 200

Beam Current [mA] Time [µs]

0.0 0.2 0.4 0.6 0.8 1.0

FEL Power [Arb. Units] Beam Current FEL Power

• Beam current ramping due to the back-bombardment effect.
• Beam energy was kept constant  FEL can be lased.

Time Structure of KU-FEL Pulse

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Macro-pulse Repetition rate : 1 or 2 Hz Duration : ~ 2 µs-FWHM Micro-pulse Repetition rate : 2856 MHz Interval : ~350 ps Minimum Length : < 1 ps-FWHM

4 6 8 10 12 14 16 18 20 22 24 0.1 1 10 100

19 MeV 24 31

Macro-pulse Energy [mJ] Wavelength [µm]

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KU-FEL Performance with Thermionic Cathode

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Macro-pulse Energy Wavelength Spectrum

• Measurement was done @user station #1 w/o N2 filling.
• Tunable Range was from 3.6 – 23 µm.
• Maximum Macro-pulse energy was 30 mJ/pulse @5 µm.
• Typical FEL bandwidth ~3% @Max. power optical cavity length.

4 6 8 10 12 14 16 18 20 22 0.0 0.2 0.4 0.6 0.8 1.0

• Norm. Intensity [Arb. Units]

Wavelength [µm]

H2O CO2 CO2 H2O

Photocathode Operation of KU-FEL

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LaB6 thermionic cathode can also be used as a photocathode and Mark-III FEL succeeded in Lasing with LaB6 photocathode.

Reference : M. Curtin, et al., NIM A296 (1990) 127-133.

The photocathode operation of LaB6 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.

Pico-second Multi-bunch UV Laser

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To User Room (Pump-Prove)

~ 20 ps, 266 nm ~ 10 ps, 1064 nm

Result of Demonstration Experiment

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2 4 6 8 10 50 100 150

Averaged Current [mA] Time [µs]

• 2

2 4 6 8 10 100 200 300 400 500 600

Averaged Current [mA] Time [µs] Thermionic Photocathode Photocathode Thermionic

Beam Current Profile @Gun Exit Beam Current Profile @Undulator Cathode Temperature Thermionic : 1900 K Photocathode : 1400 K

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 µs (Photocathode)

Result of Demonstration Experiment

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Thermionic Photocathode Ratio (Ph / Th) Repetition Rate 2856 MHz 29.75 MHz 1 / 96 FEL Macro-pulse Duration ~ 2 µs 2 µs ~1

• Max. Macro-pulse Energy

13 mJ 0.8 mJ 1 / 16

• Max. Micro-pulse Energy

~2 µJ 13 µJ 6.5 FOM (Micro E / Macro E) 1.5E-4 1.6E-2 ~ 100

e-Beam Energy : 23.8 MeV Undulator Gap : 19.5 mm FEL Wavelength : ~11.7 µm

• 1

1 2 3 4 5 6 7 8 9 10

• 0.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Photocathode (0.8 mJ)

Normalized FEL Power [Arb. Units] Time [µs]

Thermionic (13 mJ)

• 6.5 times higher micro-pulse energy
• 1/16 macro-pulse energy

Good for nonlinear experiments!!

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FEL Beam Current

Outline

• Introduction
• Present Status of MIR-FEL
• Present Status of THz-FEL
• Perspective

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Schematic Layout

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1.6-cell Photocathode RF Gun Solenoid Chicane Triplet Quadrupole UV-laser Injection Chamber Undulator Dipole Beam Dump

• 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.
• Phase 3 target will be determined based on phase-2 results.

CTR Monitoring

Under Preparation

Present Condition

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Chicane Dipole Solenoid RF-gun Triplet Quadrupole CTR Monitor Carbon Faraday cups Laser port

Undulator has not been installed yet.

1.6-cell RF Gun

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• 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

Cu Cathode

2 4 6 8 10 12 6 7 8 9 10 11 12

RF Power (MW)

Time (µs)

Reflected Forwarded Probe (x10)

Typical RF Waveforms

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UV laser injection at the end of RF macro-pulse

Summary of Phase 1 Experiments

• Max. Beam Energy : 4.6 MeV
• Bunch Charge : up to 1.4 nC
• Normalized Emittance : < 10 πmm-mrad @50 pC
• CTR dependence on operation condition checked
• Observed CTR frequency up to 0.25 THz

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Representative Results

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1 2 3 4 5 6 7 8 0 10 20 30 40 50 60 70 80 90 100 CTR Intensity (a.u.) Laser Injection Phase (deg) Uncompression Compression

CTR Intensity dependence

• n laser injection timing

1 2 3 4 5 0,00 0,10 0,20 0,30 0,40 0,50 Spectral Power Density (a.u)

Frequency (THz)

4.80 5.60 6.00 6.8 Chicane Current [A]

CTR Spectrum dependence

• n Chicane Excitation Current
• Measurement was done with 4 bunch condition to increase S/N ratio.
• A Michelson Interferometer was used for spectrum measurement.
• High sensitivity pyroelectric detector was used for this experiment.

Expectation of Phase 2 Experiment

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Undulator

Number of periods : 10 Period length : 7 cm

• Max. K-value : 2.7 @Gap 30 mm

Beam Energy : 4.6 MeV  γ = 10

λ0 = (λu/2γ2) (1+K2/2)

Longest Wavelength = 1.6 mm Lowest Frequency = 184 GHz

Coherent undulator radiation at 200 GHz will be measured in Phase 2 experiment.

Outline

• Introduction
• Present Status of MIR-FEL
• Present Status of THz-FEL
• Perspective

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Possible Future Upgrade of Facility

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• Complete the compact THz-FEL machine
• 2nd oscillator FEL in longer wavelength
• New beamlines for non-FEL application

 e-beam irradiation & THz generation.

Feasibility Study of THz Generation

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 10

• 4

10

• 3

10

• 2

10

• 1

10

Intensity [Arb. Units] Frequency [THz]

Edge Radiation from final bending magnet was measured @40 MeV CER up to 2 THz was observed.

• Pre-bunched FEL in 2nd branch?
• THz generation branch is feasible

Summary

• MIR-FEL

– Tunable range from 3.6 to 23 µm – Routinely operated for user experiments – Photocathode operation for nonlinear experiments

• THz-FEL

– Compact configuration – e-beam property measurement was finished.

• Perspective

– Facility will be extended to longer wavelength.

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Thank you for your attention!!

Influence of Back-bombardment Effect

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• 1

1 2 3 4 5 6 200 400 600

Beam Current [mA] Time [µs]

> x 2

• 700 keV

Thermionic cathode supplies electron continuously. Some electron cannot be in acceleration phase decelerated back to the cathode. The energetic electron hit and heat up the cathode. Cathode temperature increase and beam current increase during a macro-pulse. Decrease of beam energy is caused by increase of beam loading.

Beam Current Profile Beam Energy Evolution

Countermeasures against Back-bombardment

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Oscillator FEL

Beam current increase in macro-pulse : Acceptable Beam energy decrease in macro-pulse : Unacceptable We accept the beam current increase and compensated the beam energy decrease. RF Power Ramping

• 1

1 2 3 4 5 6 2 4 6 8

RF Power [MW] Time [µs]

Increase of beam loading can be compensated.

RF Cavity Detuning

Detune the resonant frequency of RF cavity to slightly lower frequency of the feeding RF frequency. Cavity voltage sensitivity against the change of beam current get smaller.

Ref.: H. Zen, et al., IEEE transaction on nuclear science, Vol. 56, No. 3, Pages 1487-1491 (2009).

Unexpected Increase of Optical Cavity Loss Observed in Long Wavelength Region of KU-FEL

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5 10 15 20 2 4 6 8 10 Simulation (20 mm Gap) Simulation (15 mm Gap) 19.1 MeV 23.9 MeV 30.9 MeV 40.0 MeV

Optical Cavity Loss [%] Wavelength [µm] Reflection Loss (1% x 2) Outcoupling Loss

Diffraction Loss

A fast pyroelectric detector ELTEC 420-0 was used.

Machine Trouble in 2014 & 2015

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• High voltage capacitors in the klystron

modulator were broken due to aging.

• Successively more than 10 capacitors

were already dead.

• We got some used capacitors from

Waseda Univ. and AIST.

• We purchased 10 capacitors in last year

and ordered another 10 in this year.

• Some of new capacitors have been

already installed and they have no problem for real operation. Broken capacitors New capacitor with attachment

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CER Measurement Setup

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Vacuum Window : Crystal Quartz, φ32 mm Beam Splitter : Inconel coated pellicle

e-Beam Energy = ~40 MeV Critical Freq. fc = ~100 THz

R= 0.34 m

• Measurement was performed at the

downstream of the Undulator.

• Radiation emitted on the 0-deg. line of

bending magnet was reflected by a in- vacuum mirror and transported to the Michelson interferometer.

• A beam splitter made by OHP film was

used to obtain the total power for normalization.

• Because the radiation intensity was

enough high, low sensitivity pyroelectric detectors can be used.

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Signal Detector Reference Detector OAP OAP Movable Mirror Fixed Mirror Spherical Mirror Inconel Coated Pellicle BS OHP Film BS

CER

Interferogram

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• 2.0 -1.5 -1.0 -0.5 0.0

0.5 1.0 1.5 2.0

• 0.4
• 0.2

0.0 0.2 0.4 0.6 0.8 1.0

• Norm. Intensity [Arb. Units]

Path Difference [cm]

• 0.2
• 0.1

0.0 0.1 0.2

• 0.4
• 0.2

0.0 0.2 0.4 0.6 0.8 1.0

• Norm. Intensity [Arb. Units]

Path Difference [cm]

Detector Window

• Beam Splitter of Michelson Interferometer : Inconel Coated Pellicle
• Pyroelectric Detector : QE8SP-I-BL-BNC (Gentec-EO)
• Vacuum Window : Z-cut Crystal Quartz Window (t = 4 mm)

Around the center burst Long Range Interferogram