The CeB6 Electron Gun for the Soft-X-ray FEL Project at SPring-8

Transcription

1 May 25, 2004 DESY, Hamburg, Germany The CeB6 Electron Gun for the Soft-X-ray FEL Project at SPring-8 K. Togawa SPring-8 / RIKEN Harima Institute T. Shintake, H. Baba, T. Inagaki, T. Tanaka SPring-8 / RIKEN Harima Institute H. Matsumoto High Energy Accelerator Research Organization (KEK)

2 Where is SPring-8? Harima Science Garden City SPring-8 SPring-8 KEKK

3 SPring-8 Compact SASE Source ( Project) The project to construct a soft X-ray free electron laser based on SASE (self-amplified spontaneous emission) has started at SPring-8. In-Vacuum Undulator High-Power Short-Pulse Soft X-ray 3.6 nm Electron Gun (500 kev) 476 MHz Booster Cavity (1 MeV) Energy Filter 476 MHz Buncher L-band Pre-Linac (20 MeV) Bunch Compressor C-band Main Linac (1 GeV) Beam Parameters at Undulator ε n,rms < 2 π mm.mrad Charge ~ 1 nc/bunch The gun must produce lower emittance beam!!

4 Requirements of X-FEL Electron Source 1) Low Emittance <1 π mm.mrad 2) High Charge SASE-FEL Saturation ~1 nc/bunch 3) High Beam Quality No Beam Halo, No Dark Current Precise Alignment Protection of Undulator Magnet 4) Stable Small Jitter, Long Lifetime User Experiments Thermionic Gun RF-Gun

5 Low Emittance Injector for SASE-FEL 2002 July HV Pulse Gun Sub-harmonic Buncher Energy Filter 476MHz Booster Cavity L-Band Buncher L-Band Accelerator EZ : Einzel Lens HS, VS : Steering Screen Monitor Troid Current Monitor Collimator Beam Chopper (pulse shaping) 476 MHz, 200 kv Vertical Slit (beam chopper) Horizontal Slit (energy gate) 476 MHz, 700 kv 1428 MHz 10 MV/m x 1 m Traveling Wave 1428 MHz 10 MV/m x 2 m Traveling Wave Energy Analyzer 1:20 To Main Linac Cathode CeB6 single-crystal φ3 mm 1450 deg.c εn 0.4 π µrad Vp 500 kv I p 3 A t FWHM 1.5 µsec fr 60 pps K 0.01 µa/v 1.5 Filter 400~600 kv I p 3 A t FWHM 0.3 nsec Vp 20 MV I p 100 A t FWHM 10 psec Q 1 nc

6 Why do we use the HV thermionic gun? 1) Stable and Long Lifetime Simple. High power pulsed technology and thermionic cathode technology is well established, and used for various electron devices. 2) Low Emittance and Uniform Emission Single crystal cathode has very flat surface. Single crystal cathode provides uniform emission.

7 CeB 6 Cathode Development

8 CeB6 (Cerium Hexaboride) Single-Crystal Cathode CeB6 cathode is widely used in electron microscope!! Properties Very flat surface (surface roughness <1 µm) Low workfunction (~2.4 ev) Long lifetime (>10,000 hours) Rapid recovery from contamination Design parameter of cathode Thermal Emittance r kbt ε n, rms = 2 = 0.4 π mm mrad 2 mec Emission Current Density Cathode Radius r=1.5 mm Temperature T=1450 (1723 K) Richardson-Dashman's Formula (Ideal Case) J = T 2 exp( φ' / kbt) > 42 A/cm 2 Boltzmann's Constant kb = e Effective Workfunction φ = φ 2 ee πε 0 5 ( ev / K ) ~2.3 (ev)

9 Property of CeB6 Cathode Thermal Emittance Emission Current Density Normalized Emittance (RMS) Richardson-Dashman's Formula (Ideal Case) ε kbt 2 mec rc n, rms = 2 = 0.4 π mm.mrad J = T 2 exp( φ' / kbt ) > 42 A/cm 2 = ( ev / K ) Boltzmann's Constant kb Cathode Radius r=1.5 mm e ee Temperature T=1450 (1723 K) Effective Workfunction φ = φ ~2.3 (ev) 2 πε 0 Small Cathode is Important!!

10 CeB6 Cathode Assembly (First Model) CeB6 Single Crystal (FEI Beam Technology Company) Graphite Mount (pyrolytic graphite coated) Molybdenum Pipe 3 mm Graphite Heater (pyrolytic graphite coated) Mechanically and Chemically Stable in Vacuum Stable Electric Resistance High Purity Ceramic Base 50 mm Molybdenum Current Lead Operational Cathode Temperature ~ 1500 (much higher than conventional cathodes) Technical Challenge!!! Dummy Cathode : Graphite Heater :

11 Ceramic Base of Cathode Assembly Si3N4 ceramic base Very strong against thermal stress!! Alumina (Al2O3) ceramic base was broken by thermal heating.

12 Melting of Copper Connector Graphite Heater (inside) Cathode Stem Cu Connector Ta Reflector Viewing Port SiN Base

13 CeB6 Cathode Assembly (New Model) Cathode Assembly Heated Cathode in Stem Cathode Temperature ~1500 C CeB6 Single Crystal 3mm CeB6 Cathode Graphite Sleeve Graphite Heater SiN Ceramic Base Molybdenum Current Lead The gun voltage=500 kv Temperature was measured at the graphite sleeve by a radiation monitor.

14 500 kv Electron Gun

15 500 kv Electron Gun C-band klystron modulator is used as a HV pulsed power supply. Oil-filled Pulse Tank 500 kv Electron Gun Input Pulse Cable Connector CeB6 Cathode Emittance Monitor Magnetic Lens Steering Coil CT Monitor Movable Slit Gate Valve Movable Slit CT Monitor Screen Monitor Steering Coil Movable Slit Movable Slit CT Monitor Screen Monitor Beam Dump Dummy Load Pulse Transformer HV Bushing 1 m

16 500 kv Electron Gun Chamber Corona Dome Ceramics Magnetic Sheld Heater Trans Cathode Stem Cathode Assembly Viewing Port Magnetic Lens Current Lead to Injector Electrodes to Ion Pump 10 cm Conceptual Design (2001)

17 Emittance at Gun Exit (Simulation) R (mm) -500kV 20 EGUN Simulation 0kV 10 Cathode φ3mm Current 3A Z (mm) Non Physical Emittance

18 C-band Klystron Modulator Same model of the C-band klystron modulator is used for the 500 kv electron gun. Gun Pulse Waveform 517 kv 3 µs Dummy Load Current =298 A (peak) Circuit Diagram PFN : 13 section (for 500 kv Gun) PFN Impedance : ~4 Ω Charging Voltage : 50 kv max Max. Repetition Rate : 60 pps

19 500 kv Pulse Transformer Pulse Transformer of 500 kv Electron Gun Conducted by Prof. Baba Winding Circuit Secondary Coil 1 Primary Coil Secondary Coil 2 84 mm 242 mm 268 mm 5 turn 100 turn 4 mm Pulse Shape (@Low Voltage) Fabricated by NIHON DENJI KOGYO Co., Ltd. Turn Ratio 1:21 Input Pulse from Modulator 23.8 kv, 5502 A Output Pulse to Cathode & Dummy Load 500 kv, 262 A Pulse Width 1.6 µs 1.6 µs

20 500 kv Dummy Load Base Plate : Voidless FRP HITACHI Chemical Co., Ltd. Ceramic Resistor : ER65AS-5P TOKAI KONETSU KOGYO Co., Ltd. Pulse High Voltage 500 kv Peak Current 259 A Pulse Width 1.6 µs Dummy Load Impedance 1.93 k Average Power 12.4 kw

21 500 kev Beam Production Beam Waveform I-V Curve Space Charge Limit Schottky Effect Droop of CT Monotor Beam Current (CT Monitor) Gun Voltage (CVD Monitor) ~1 ns part will cut out from the flattop by a beam deflector, and be used for the accelerator. We operate the gun in temperature limited region to reduce emittance growth due to space charge.

22 Emittance Measurement

23 Normalized rms Emittance Definition x' ε n, rms = m c x px x 0 p x 2 = βγ 2 2 x x x x 2 x unit : π mm.mrad x x' x 2 2 x' 2 x i ( x, x' ) dxdx' = i ( x, x' ) dxdx' 2 x' i ( x, x' ) dxdx' = i ( x, x' ) dxdx' x x' i = i ( x, ( x, x' ) dxdx' x' ) dxdx' r c T Cathode ε n, rms r = e - rc 2 m kbt 0 c 2 = 0.4 π mm.mrad T ( c = 1.5 mm, = 1723 K (1450

24 Emittance Measurement by Double-slits Beamlet Profiles (400 kev, 0.9 A) x x 2 Electron Beam Beamlet Sub-beamlet x 1 x 1 x 1 D x 2 z L Drift Space Beamlet Width (FWHM) Upstream Slit CT0 (Current Transformer) CT1 Downstream Slit CT2 Beamlet spread due to space charge is about 15% at 50 µm width.

25 Emittance Monitor Electron Beam CT Monitor Upstream Slits CT Monitor Downstream Slits CT Monitor 50 cm 60 cm

26 Emittance of 500 kev Beam Beam Profile Phase Space Profile φ 5.6 mm (FWHM) 0.24 mrad (σx') φ 5.6 mm (FWHM) Beam Energy : 500 kev Peak Current : 1.0 A Pulse Width : 3 µs Emittance (εn,rms) Requirement : 2π Experiment : 1.1π (preliminary)

27 Emittance Analysis Analysing Method Reproduction Noise signal causes false emiitance. 1) Remove signal data whose amplitude is less than noise level (3σ). Typically, the signal cut level is ~1% of the peak signal. 2) Data is corrected to zero cut level by extrapolation using linear function. We need more studies about noise reduction and analysing method.

28 Emittance Map r ms Emittance Area Emittance including 50% particles

29 Summary 1) We have succeeded in producing a 500 kev, 1 A beam from the CeB6 gun. 2) Measured emittance was ~1π mm mrad. 3) In 2004, we will construct a buncher system and measure the bunched beam emittance.