Polarized Electron Source f or ILC in Korea 김김김 ( 김김김김김 CHEP), 김김김 ( 김김김김김김김김 )
Jan 14, 2016
Polarized Electron Source for ILC in Korea
김귀년 ( 경북대학교 CHEP), 박성주 ( 포항가속기연구소 )
Polarized photoemission
NEA Surface – Cathode “Activation” • Ultra-High-Vacuum < 10-11 Torr • Heat treatment at 600° C • Application of Cesium and NF3/O2
ee e e
holes
G aAs C rystal surface
NEA Surface
C ircularly Polarized Light
Zn p- doped
Polarized E lec trons Tunneling C urrent
C onduc tion Band
Valence Band Vacuum Regions
C s
C s
C s
C s
22
22
P3/ 2
S1/ 2
- 1/ 2- 3/ 2
- 1/ 2 1/ 2
3 1 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
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)
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
GaAs/GaAsP Superlattice
Clear HH and LH transitions.
The step-like behavior of
2D structure is observed.
Nagoya (MOCVD) SLAC (GSMBE)
Polarization 85 – 90%QE 0.5 – 1%
80
70
60
50
40
30
20P
ola
riza
tion
(%)
820800780760740720700680660
Wavelength (nm)
5
4
3
2
1
0
QE
(%)
Strained-superlattice Single strained
No Surface Charge Limit
p-p : 2.8ns, bunch-width : 0.7nsCharge: 1nC/bunch
10
8
6
4
2
0C
ha
rge
(x1
011
pe
r p
uls
e)
50403020100
Laser Energy (uJ)
60 nsec pulse
Before Cesiation
After cesiation
11012 e- in 60 ns → 4.51012 e- in 270 ns (×3 NLC train charge)
Nagoya
SLAC
Clendenin (SPIN2004)
Parameter NLC ILC ILC SLCat Source NCRF SCRF NCRF-Inj/ Design
S-band L-bandSCRF-Linac (2-cm)
ne nC 2.4 6.4 6.4 20z ns 0.5 2 0.5 3Ipulse, avg A 4.8 3.2 12.8 6.7
Ipulse, peak A 11 (SCL)
Conclusion: Space charge limit a problem for ILC source only if tryto operate with NCRF injector S-band linac
3rd generation polarized gun
3 chambers:HV Gun chamber
Inverted or Double insulator
Prep chamberLoad-lock
Atomic hydrogen cleaning
Inverted gun (SLAC) Nagoya
JLAB
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 emittancevs.
Higher leakage current and shorter cathode lifetime
☻ 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) 2.8ns100bunches (300Hz) ---------- may be no problem 2) 337ns2820bunches (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.
Laser• Laser for the ILC polarized electron source requires cons
iderable R&D
Pulse energy: > 5 JPulse length: 2 ns# pulses/train: 2820Intensity jitter: < 5%Pulse spacing: 337 nsRep rate: 5 HzWavelength: 750 ~ 850 nm (tunable)
– Photoinjector laser at DESY-Zeuthen
Towards ILC Polarized Electron Source
• Photocathode R&D– JLAB– Nagoya/KEK– SLAC– St. Petersburg Technical University
• Gun R&D– FNAL– JLAB– Nagoya– SLAC
• Laser R&D– DESY-Zeuthen– SLAC
Specifications for ILC polarized electron source
Parameters units TESLA-TDR NLC/GLC
US-COLDGun bunch charge nC (#e-)4.5 (2.8×1010)Polarization % > 80Bunch length ns 2 0.7Cathode bias voltage kV -120Beam radius mm 12# bunches / pulse 2820 192Bunch spacing ns 337 1.4Pulse length µs 950 0.27Repetition rate Hz 5 120
Korea’s Capabilities Relevant to ILC Injcetors
1. PES Test Stand• GaAs-NEA Photocathode Production• Compact Mott Polarimeter• Electrostatic Bend• PEGGY Source (provided by the SLAC)
2. 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)
O2 leak Valve
Gun Chamber
Faraday cup
Mott Chamber
RGA
Laser
e beam
Layout of Test-Stand
Ion pump
varian TMP
HV distribution-box
CEM HVPS
Ion pump PS
Mott ChamberGun Chamber
Faraday Cup
(120l/s)
RGA
1. Polarized Electron Source Test Stand
Mini-Mott Chamber
Polarization Measurement at Test Stand
J. Korean Phys. Soc. 44, (2004) 1303
2. PPI - PAL XFEL Injector Gun
Experiences fromGTS (Gun Test Stand) with
modified BNL Gun-IV
PPI (Pohang Photo-Injector)
Layout of PAL - GTS Laser System
Pulse Frequency(Repetition Rate)
Oscillator: 79.33 MHz (= 2856 MHz / 36), Synchronized to the accelerator RFAmplifier: 1020 HzFinal 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 DurationMinimum: < 100 fs at 800 nm, < 120 fs at 267 nm, FWHMMaximum: 15 ps, FWHM
Pulse Shape Gaussian, nominal
Timing Jitter < 0.25 ps rms, < 1 ps pk-pk
Accessories
Pulse CompressorsPulse Picker (Chopper)THG optimized at ps pulsesTHG optimized at fs pulsesDiagnostics
Specification of PAL - GTS Laser System
Layout of GTS (Gun Test Stand) for PAL XFEL & FIR-FED Facilities
1.6-Cell RF Gun
Fabrication of Aluminum Model Cavity
Emittance Compensating Solenoid
4. Special facilities for klystron fabrication• XHV Baking Station• Various Furnaces• HP Microwave Test-Lab
5. Infra-Structures• Chemical Cleaning Shop, Plating Facility, Welding Shop, 3D CM
M,…• Magnetic Field Measurement Facilities• Full-line of Microwave Equipments
6. 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 pr
oject
Korea’s Capabilities Relevant to ILC Injcetors- Continued -
Polarized Positron Source for ILC
Conventional vs. Gamma Based Positron Source
Target
Photons 10-20 MeV
Electrons 0.1-10 GeV
Primary Beam Capture Optics
thin target: 0.4 X0
thick target: 4-6 X0
For the production of polarized positrons circularly photons are required.
Methods to produce circularly polarized photons of 10-60 MeV are:
• radiation from a helical undulator
• Compton backscattering of laser light off an electron beam
Gamma Based Positron Source
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:
• ~ 50-150 m
2. Polarized Positron based on Laser Compton Gamma
Pohang Accelerator Lab.
Laser Compton Scattering Beam Line using Pohang Linac
SummarySummary
• Based on R&D work
1. Polarized Electron :
- 500 keV Gun Development
- Gun Test
2. Polarized Positron :
- Laser Compton Beam Line
- Test Facility for Positron Target