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

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 < 10-11 Torr • Heat treatment at 600° C • Application of Cesium and NF3/O2

(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)

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 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.

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)

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 2.4 6.4 6.4 20 Dz ns 0.5 2 0.5 3 Impulse, avg A 4.8 3.2 12.8 6.7 Impulse, peak A 11 (SCL) Conclusion: Space charge limit a problem for ILC source only if try to operate with NCRF injector S-band linac

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

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

☺ 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.

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

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

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 2 0.7 Cathode bias voltage kV -120 Beam radius mm 12 # bunches / pulse 2820 192 Bunch spacing ns 337 1.4 Pulse length µs 950 0.27 Repetition rate Hz 5 120

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)

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

Mini-Mott Chamber

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

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

Layout of PAL - GTS Laser System

Specification of PAL - GTS Laser System Pulse Frequency (Repetition Rate) Oscillator: 79.33 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

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

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

Polarized Positron Source for ILC

Conventional vs. Gamma Based Positron Source Primary Beam Target Capture Optics Photons 10-20 MeV thin target: 0.4 X0 Electrons 0.1-10 GeV thick target: 4-6 X0

Gamma Based Positron Source 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

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

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

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