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