1. Polarized Electron Source for ILC in Korea 김귀년(경북대학교 CHEP), 박성주(포항가속기연구소)

2. Polarized photoemission
1.76eV
4.0eV
• Circularly polarized light excites electron
from valence band to conduction band
• Electrons drift to surface
L < 100 nm to avoid depolarization
• Electron emission to vacuum from
Negative-Electron-Affinity (NEA) surface
NEA Surface – Cathode “Activation”
• Ultra-High-Vacuum < Torr
• Heat treatment at 600° C
• Application of Cesium and NF3/O2

3. (Nakanishi’s summary )
Polarized Electron Source　
(Nakanishi’s summary )
- DC gun with NEA–GaAs photocathode
Goal is not so far -----
☺ Photocathode
------GaAs–GaAsP strained superlattice-----
Pol. ∼ 90%, QE ∼ (0.5∼1.0)%
(Nagoya/KEK, SLAC, St. Petersburg,----)
☺ High gradient gun
120 keV (SLAC, worked well for SLC)
200 keV (Nagoya---under test, SLAC---planned)
500 keV (JLAB/Cornell, Nagoya/KEK---planned)

4. Photocathode R&D in the last 13 years
1978 First GaAs polarized electron source (E122) SLAC – 37% polarization
1991 Strained InGaAs/GaAs (MBE) SLAC/Wisc AlGaAs/GaAs superlattice (MBE) KEK/Nagoya
Strained GaAs/GaAsP (MOCVD) Nagoya
Strained GaAs/GaAsP (MOCVD) SLAC/Wisconsin
1992 High gradient doping technique applied to AlGaAs/GaAs KEK/Nagoya
1993 Surface photovoltaic effect observed at SLC
Strained GaAs/GaAsP used for SLC
1994 InGaAs/GaAs strained-superlattice (MBE) KEK/Nagoya
1995 InGaAs/AlGaAs strained-superlattice (MBE) St. Petersburg
1998 No Charge limit in high gradient doped superlattice Nagoya/KEK
2000 GaAs/GaAsP strained-superlattice (MOCVD) Nagoya/KEK
2001 No charge limit in high gradient doped strained GaAs SLAC/Wisconsin
2003 GaAs/GaAsP strained-superlattice (GSMBE) SLAC/Wisconsin
2004 InAlGaAs/AlGaAs strained-superlattice (MBE) St. Petersburg

5. GaAs/GaAsP Superlattice
Nagoya (MOCVD)
SLAC (GSMBE)
Polarization 85 – 90%
QE 0.5 – 1%
 Clear HH and LH transitions.
 The step-like behavior of
2D structure is observed.

6. No Surface Charge Limit
SLAC
Nagoya
p-p : 2.8ns,
bunch-width : 0.7ns
Charge: 1nC/bunch
11012 e- in 60 ns
→ 4.51012 e- in 270 ns
(×3 NLC train charge)

7. Parameter NLC ILC ILC SLC at Source NCRF SCRF NCRF-Inj/ Design
Clendenin (SPIN2004)
Parameter NLC ILC ILC SLC
at Source NCRF SCRF NCRF-Inj/ Design
S-band L-bandSCRF-Linac (2-cm)
ne nC
Dz ns
Impulse, avg A
Impulse, peak A (SCL)
Conclusion: Space charge limit a problem for ILC source only if try
to operate with NCRF injector S-band linac

8. 3rd generation polarized gun
Inverted gun (SLAC)
Nagoya
JLAB
3 chambers:
HV Gun chamber
Inverted or Double insulator
Prep chamber
Load-lock
Atomic hydrogen cleaning

9. Next generation guns Polarized RF gun
Holy grail of polarized electron source
UHV requirement precludes current photocathodes
Two photon excitation?
Large band gap materials like strained InGaN
> 500 kV DC gun
Proposal to build 500 kV gun (Nagoya)
Higher voltage and smaller emittance
vs.
Higher leakage current and shorter cathode lifetime

10. ☺ Important gun performances
☻ Laser system
No complete system exists, considerations are needed.
(Homework; Solutions must be proposed before the next WS ?)
Bunch–structure depends on the DR scheme (by Urakawa)
1) 2.8ns100bunches (300Hz) may be no problem
2) 337ns2820bunches (5Hz) may be not easy
☺ Buncher system (beam–width: 1ns  5ps)
depends on bunch structure may be no problem
☺ Important gun performances
○ NEA lifetime---- o.k. by recesiation and reactivation
○ Surface charge limit effect---- may be negligible
○ Gun emittance ( ≤ 10πmm-mrad) may be o.k.

11. Laser
Laser for the ILC polarized electron source requires considerable R&D
Pulse energy: > 5 J
Pulse length: 2 ns
# pulses/train: 2820
Intensity jitter: < 5%
Pulse spacing: 337 ns
Rep rate: 5 Hz
Wavelength: 750 ~ 850 nm (tunable)
Photoinjector laser at DESY-Zeuthen

12. Towards ILC Polarized Electron Source
Photocathode R&D
JLAB
Nagoya/KEK
SLAC
St. Petersburg Technical University
Gun R&D
FNAL
Nagoya
Laser R&D
DESY-Zeuthen

13. Specifications for ILC polarized electron source
Parameters units TESLA-TDR NLC/GLC
US-COLD
Gun bunch charge nC (#e-) 4.5 (2.8×1010)
Polarization % > 80
Bunch length ns
Cathode bias voltage kV -120
Beam radius mm 12
# bunches / pulse
Bunch spacing ns
Pulse length µs
Repetition rate Hz

14. Korea’s Capabilities Relevant to ILC Injcetors
PES Test Stand
GaAs-NEA Photocathode Production
Compact Mott Polarimeter
Electrostatic Bend
PEGGY Source (provided by the SLAC)
PAL XFEL Injector
GTS (Gun Test Stand)
BNL Gun-IV-type 1.6-Cell RF Gun
Ti:Sapphire Laser
Dedicated RF Source
Beam Diagnostics
PPI (Pohang Photo-Injector)

15. 1. Polarized Electron Source Test Stand
Layout of Test-Stand
O2 leak Valve
Gun Chamber
Faraday cup
Mott Chamber
RGA
Laser
e beam
Mott Chamber
Gun Chamber
Faraday Cup
(120l/s)
RGA

16. Mini-Mott Chamber

17. Polarization Measurement at Test Stand
J. Korean Phys. Soc. 44, (2004) 1303

18. GTS (Gun Test Stand) with
2. PPI - PAL XFEL Injector Gun
PPI (Pohang Photo-Injector)
Experiences from
GTS (Gun Test Stand) with
modified BNL Gun-IV

19. Layout of PAL - GTS Laser System

20. Specification of PAL - GTS Laser System
Pulse Frequency
(Repetition Rate)
Oscillator: MHz (= 2856 MHz / 36), Synchronized to the accelerator RF
Amplifier: 1020 Hz
Final Output: 30 or 60 Hz
Lasing Material
Ti:Sapphire
Pulse Energy
> 2.5 mJ at 800 nm, > 250 μJ at 267 nm
Wavelength
800 +/- 10 nm, Third Harmonic at 267 nm
Pulse Duration
Minimum: < 100 fs at 800 nm, < 120 fs at 267 nm, FWHM
Maximum: 15 ps, FWHM
Pulse Shape
Gaussian, nominal
Timing Jitter
< 0.25 ps rms, < 1 ps pk-pk
Accessories
Pulse Compressors
Pulse Picker (Chopper)
THG optimized at ps pulses
THG optimized at fs pulses
Diagnostics

21. Layout of GTS (Gun Test Stand) for PAL XFEL & FIR-FED Facilities

22. 1.6-Cell RF Gun

23. Fabrication of Aluminum Model Cavity

24. Emittance Compensating Solenoid

25. Korea’s Capabilities Relevant to ILC Injcetors
- Continued -
Special facilities for klystron fabrication
XHV Baking Station
Various Furnaces
HP Microwave Test-Lab
Infra-Structures
Chemical Cleaning Shop, Plating Facility, Welding Shop, 3D CMM,…
Magnetic Field Measurement Facilities
Full-line of Microwave Equipments
High-Quality Manpowers
Beam-Dynamics Experts
Mechanical Engineers
High-Power Electrical Engineers
RF Engineers (LL & HL)
XHV Experts
were involved in the PLS construction, now in the PAL XFEL project

26. Polarized Positron Source for ILC

27. Conventional vs. Gamma Based Positron Source
Primary Beam
Target
Capture Optics
Photons MeV
thin target: 0.4 X0
Electrons GeV
thick target: 4-6 X0

28. Gamma Based Positron Source
For the production of polarized positrons circularly photons are required.
Methods to produce circularly polarized photons of MeV are:
radiation from a helical undulator
Compton backscattering of laser light off an electron beam

29. 1. Undulator Based Positron Source
Undulator length depends on the integration into the system, i.e. the distance between undulator exit and target which is required for the beam separation:
~ m

30. 2. Polarized Positron based on Laser Compton Gamma

31. Laser Compton Scattering Beam Line using Pohang Linac
Pohang Accelerator Lab.

32. Summary Based on R&D work Polarized Electron :
- 500 keV Gun Development
- Gun Test
2. Polarized Positron :
- Laser Compton Beam Line
- Test Facility for Positron Target