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High Polarization and Low Emittance Electron Source

for ILC

Nagoya UniversityDept. of Physics (SP-Lab)

Masahiro Yamamoto

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Nagoya UniversityT.Nakanishi, S.Okumi, M.Yamamoto, N.Yamamoto,A.Mano, Y.Nakagawa, T.Konomi

T.Ujihara, T.Katoh, X.G.Jin, M.Tanioku, Y.Takeda

Hiroshima UniversityM.Kuriki

Osaka Prefecture UniversityH. Horinaka, T. Matsuyama

KEKF.Furuta, T.Omori, H.Mastumoto, M.Yoshioka, Y.Kurihara, J.Urakawa

Collaborators of PES development in Japan

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International Linear Collider

High luminosity Low emittance beamMulti-bunch beam structure

polarized electron (positron)Detail studies of Higgs, SUSY search, …

INTERACTIONS.ORG—Particle Physics and Resources

Polarized electron source

L ~ 1034 cm-2s-1 !!

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SUSY Search

Unpolarized

Scalar muon production

Background signalPolarized (90% right-handed e-)

Suppressed !!

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Photoemission from GaAs

D. T. Pierce and F. Meier, Phys. Rev. Lett. B 13, 5484 (1976)

Polarization 　 =N↑ － N↓

N↑ ＋ N↓

< 50 %

Because point of GaAs is degenerated.

How to generate polarized electron

Using a selection role of photo-excitation at point

Supplying high intensity polarized electrons

Quantum Efficiency

][][

][24.1

nmWP

nAI

N

N

photon

electron

Historically, PES using GaAs was developed by Dr. Pierce in 1970s

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How to generate polarized electron

NEA surface is extremelydelicate in vacuum conditions(harmful residual molecules, surface cleanliness)

Weak point…

Negative Electron Affinity

1.Making a band bending by p+ doping to the surface

2.Electric dipole layer by vacuum evaporation of Cs and oxygen

≤ -100meV

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Beam Structure for ILC

5 nC × 2625 bunch × 5 Hz ~ 65A

• Muti-bunch StructureAveragecurrent

peak current ~ 5A, current density ~ 20mA/mm2

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PES Technologies & Challenges

Photocathode

High Voltage GunUltrahigh Vacuum

High Field gradient

Polarized Electron Source

Laser

• High polarization Pol.> 80%, QE>0.5%

• High beam current Ipeak > 5A, (>20mA/mm2)

• Long lifetime ≥100 h, ≥ 65A operation

• Low emittance Norm. x.rms ≤ 10 .mm.mrad

• Multi bunch structure ≥5nC/micropulse (for laser), 2860micropulse/train

Technical Challenges

Crystal structure design(strained, superlattice structure)

≤ 10-9 Pa vacuumLoad-Lock

Field emission studyElectrode development

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1.Photocathode development

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Polarization 　 =N↑ － N↓

N↑ ＋ N↓

> 50 %

How to generate “High” polarized electron

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T. Omori et al., Phys. Rev. Lett. 67 (1991) 3294T. Nakanishi et al., Phys. Letters A158 (1991) 345

How to generate “High” polarized electron

m* > m*|Jz|=3/2 |Jz|=1/2

RC

CCb

11

1211 22

Strained GaAs Superlattice

GaAsP (substrate)

GaAs (compressed)

ExV

xm)(

2 2

2

*

2

R: residual strain, Cij: elastic stiffnessb: deformation potential

GaAs(well)

AlGaAs(barrier)

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GaAs-GaAsP strained Superlattice

Faculty of Engineering, Nagoya University

• Structure of GaAs-GaAsP superlattice

Well layer (GaAs) 4nm

Barrier layer (GaAsP) 4nm

Eth=1610 meV

81meV 3.5meV

Splitting of HH and LH~80meV

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Performance of GaAs-GaAsP superlattice

GaAs-GaAsP strained SL

Polarization ≥ 85%Quantum Efficiency ≥ 0.5%

T.Nishitani et al., J. Appl. Phys. 97, 094907 (2005)T.Maruyama et al., Appl. Phys. Lett. 85, 2640 (2004)

Nagoya SLAC

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2. 200 keV Gun development

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Ceramic

200 keV Polarized Electron Gun

• Ultra high vacuum ≤ 10-9Pa

• High field gradient ≥ MV/m

• Photocathode preparation　 with Load-Lock (cleaning, NEA activation)

Photocathode puck(23mm)

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Insulation tank

PreparationChamber

-200kV power supply

Gun Chamber

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Photocathode Lifetime

QE(t) = QE0・expt

1 =

1gas

1DC

1ion

+ +

is determined by ...

1. Adsorption molecules2. Field emission dark current3. Ion bombardment

Measured by 70keV Gun

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R 24

R 15

cathode

anode

flat top

:mm

gap

0

200

400

600

800

1000

1200

1400

0 50 100 150

DA

RK

CU

RR

EE

NT

[pA

]

FI ELD GRADI ENT [MV/ m]

SUS42MV/m

Cu50MV/m

Ti103MV/m

Mo115MV/m

Ti-Mo130MV/m

Gap 0.5mm results

Reducing field emission dark current

Electrode shape

F.Furuta et al., NIM-A 538 (2005) 33-44

Nagoya & KEK

Test sampleTest sample

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Mo Cathode Ti Anode

Electrode Design & Fabrication

Mo cathode

Material : pure Mo (>99.96%)

Size : 162mmSpace Charge Limit: 30AMaximum field gradient: 7.8 MV/m @electrode

Ti anode

Material : pure Ti (JIS-grade 2)Gap:22mm

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After 80 breakdowns, the break-down voltage up to 212kV, andthe state of 200kV was main-tained more than 200 hours. (dark current ≤1nA)

Breakdown voltage rises about 0.4kV per one breakdown.

Stable operation> 200 hours

Electrode Conditioning

Dark current characteristic isn’t degraded even if many breakdowns were occurred.

Advantages of Ti-Mo electrode

M.Yamamoto et al., PST2007@BNL

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0

50

100

150

200

250

300

350

0 5 10 15 20 25

Beam Current (nA)F.C.(nA)

Cu

rren

t (n

A)

Time(hour)

Bulk GaAs, He-Ne 633nm, 200kVVacuum Pressure:

4.0x10 -9Pa @Gun

6.1x10 -9Pa @2-NEG.Cham.

Trans. 85~88%

Photocathode Lifetime

Preliminary

Base PressureGun:2.7x10-9Pa2NEG:2.0x10-9Pa

UHV & Dark current conditions seem no problem.

Lifetime measurement of ~100mA operation is under way.

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LaserLaser system for nano second pulse generationano second pulse generation

~1ns pulse is produced by a combination of fast pockels cell and polarized beam splitter.

Repetition : 10HzTransmitted energy : ~10J/pulse

2

,53

2nm

700

~900n

m

Driver&Pockels

cell

filter

Nd:

YAG

Ti:S

aPBSPD

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ILC:5nC/bunch

Nano second bunch generation from superlatticeano second bunch generation from superlattice

Photocathode : GaAs-GaAsP strained SL

Bunch charge : 8nCLaser spot size :~20mm,Bunch width(FWHM): 1.6ns

space-charge-limit was appeared clearly over 6nC/bunch under under condition with -70kV.

Peak current density ~18 mA/mm2

　 No Charge Limit

M.Yamamoto et al., SPIN2006 @Kyoto

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Solenoid

4.8nC,16mm

0 0.15 0.5[m]

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

z [m]

-15

-10

-5

0

5

10

15

x [m

m]

anode

Solenoid

200kV,1.0ns,4.8nC

Norm. x.rms ~ 8.8 .mm.mrad with optimization emittance compensation by solenoid.

Emittance simulation

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Emittance Measurement System

Pepper Pot Mask (40m holes)

Plastic sintillator

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N. Yamamoto et al., J. Appl. Phys. 102, 024904 (2007)

Measurement of ~1ns, 5nC bunch is progressing.

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Summary(1)

Photocathode

High Voltage GunUltrahigh Vacuum

High Field gradient

Polarized Electron Source

Laser

GaAs-GaAsP strained superlattice

UHV (2.2x10-9 Pa) was achieved.Lifetime measurement under progressing…

under developing…(Ti-Sapphire laser system is planed by SLAC)

Pol ≥ 85%, Q.E ≥ 0.5% are demonstrated.

200kV (3MV/m@photocathode) operation for> 200 hours without breakdown, dark current ≤ 1nA

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High polarization Pol.> 80% with high QE

High beam current Ipeak > 5A, (>20mA/mm2)

Long lifetime ≥100 h, ≥ 65A operation

Low emittance Norm. x.rms ≤ 10 .mm.mrad

Multi bunch structure ≥5J/micropulse, (drive laser development) 2860micropulse/train

Summary(2)