1 High Polarization and Low Emittance Electron Source for ILC Nagoya University Dept. of Physics (SP-Lab) Masahiro Yamamoto

2 Nagoya University T.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 University M.Kuriki Osaka Prefecture University H. Horinaka, T. Matsuyama KEK F.Furuta, T.Omori, H.Mastumoto, M.Yoshioka, Y.Kurihara, J.Urakawa Collaborators of PES development in Japan

3 International Linear Collider  High luminosity Low emittance beam Multi-bunch beam structure  polarized electron (positron) Detail studies of Higgs, SUSY search, … INTERACTIONS.ORG—Particle Physics and Resources Polarized electron source L ~ cm -2 s -1 !!

4 SUSY Search Unpolarized Scalar muon production Background signal Polarized (90% right-handed e - ) Suppressed !!

5 Photoemission from GaAs D. T. Pierce and F. Meier, Phys. Rev. Lett. B 13, 5484 (1976) Polarization = N↑－N↓N↑－N↓ 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 Historically, PES using GaAs was developed by Dr. Pierce in 1970s

6 How to generate polarized electron NEA surface is extremely delicate 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

7 Beam Structure for ILC 5 nC × 2625 bunch × 5 Hz ~ 65  A Muti-bunch Structure Average current peak current ~ 5A, current density ~ 20mA/mm 2

8 PES Technologies & Challenges Photocathode High Voltage Gun Ultrahigh Vacuum High Field gradient Polarized Electron Source Laser High polarization Pol.> 80%, QE>0.5% High beam current I peak > 5A, (>20mA/mm 2 ) Long lifetime ≥100 h, ≥ 65  A 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) ≤ Pa vacuum Load-Lock Field emission study Electrode development

9 1.Photocathode development

10 Polarization = N↑－N↓N↑－N↓ N↑＋N↓N↑＋N↓ > 50 % How to generate “High” polarized electron 

11 T. Omori et al., Phys. Rev. Lett. 67 (1991) 3294 T. Nakanishi et al., Phys. Letters A158 (1991) 345 How to generate “High” polarized electron m* > m* |Jz|=3/2 |Jz|=1/2 Strained GaAs Superlattice GaAsP (substrate) GaAs (compressed)  R : residual strain, C ij : elastic stiffness b: deformation potential GaAs (well) AlGaAs (barrier)

12 GaAs-GaAsP strained Superlattice Faculty of Engineering, Nagoya University Structure of GaAs-GaAsP superlattice Well layer (GaAs)4nm Barrier layer (GaAsP) 4nm E th =1610 meV 81meV 3.5meV Splitting of HH and LH ~80meV

13 Performance of GaAs-GaAsP superlattice GaAs-GaAsP strained SL Polarization ≥ 85% Quantum Efficiency ≥ 0.5% T.Nishitani et al., J. Appl. Phys. 97, (2005) T.Maruyama et al., Appl. Phys. Lett. 85, 2640 (2004) Nagoya SLAC

keV Gun development

15 Ceramic 200 keV Polarized Electron Gun Ultra high vacuum ≤ Pa High field gradient ≥ MV/m Photocathode preparation with Load-Lock (cleaning, NEA activation) Photocathode puck (  23mm)

16 Insulation tank Preparation Chamber -200kV power supplyGun Chamber

17 Photocathode Lifetime QE(t) = QE 0 ・ exp t  1  = 1  gas 1  DC 1  ion ++  is determined by Adsorption molecules 2. Field emission dark current 3. Ion bombardment Measured by 70keV Gun

18 Gap 0.5mm results Reducing field emission dark current Electrode shape F.Furuta et al., NIM-A 538 (2005) Nagoya & KEK Test sample

19 Mo CathodeTi Anode Electrode Design & Fabrication Mo cathode Material : pure Mo (>99.96%) Size :  162mm Space Charge Limit: 30A Maximum field gradient: 7.8 Ti anode Material : pure Ti (JIS-grade 2) Gap:22mm

20 After 80 breakdowns, the break- down voltage up to 212kV, and the 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.,

21 Photocathode Lifetime Preliminary Base Pressure Gun:2.7x10 -9 Pa 2NEG:2.0x10 -9 Pa UHV & Dark current conditions seem no problem. Lifetime measurement of ~100mA operation is under way.

22 Laserano second pulse generation Laser system for nano second pulse generation ~1ns pulse is produced by a combination of fast pockels cell and polarized beam splitter. Repetition : 10Hz Transmitted energy : ~10  J/pulse 2 ,532nm 700~900nm Driver& Pockels cell filter Nd:YAG Ti:Sa PBS PD

23 ILC:5nC/bunch ano second bunch generation from superlattice Nano second bunch generation from superlattice Photocathode : GaAs-GaAsP strained SL Bunch charge : 8nC Laser 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/mm 2 No Charge Limit M.Yamamoto et al.,

24 Solenoid 4.8nC,  16mm [m] anode Solenoid 200kV,1.0ns,4.8nC Norm.  x.rms ~ 8.8 .mm.mrad with optimization emittance compensation by solenoid. Emittance simulation

25 Emittance Measurement System Pepper Pot Mask (  40  m holes) Plastic sintillator

26 N. Yamamoto et al., J. Appl. Phys. 102, (2007) Measurement of ~1ns, 5nC bunch is progressing.

27 Summary(1) Photocathode High Voltage Gun Ultrahigh 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 operation for > 200 hours without breakdown, dark current ≤ 1nA

28  High polarization Pol.> 80% with high QE  High beam current I peak > 5A, (>20mA/mm 2 )  Long lifetime ≥100 h, ≥ 65  A operation  Low emittance Norm.  x.rms ≤ 10 .mm.mrad  Multi bunch structure≥5  J/micropulse, (drive laser development)2860micropulse/train Summary(2)