Super KEKB Injector Review 19 Feb 2015 Cathode

Super. KEKB Injector Review / 19. Feb. 2015 Cathode for long time stable operation Tokyo Institute of Technology Graduate School of Science and Engineering / D 2 Daisuke Satoh

【OUTLINE】 • Introduction • Cathode Study of Ir 5 Ce compound – Measurement of quantum efficiency – Continuous Photoemission – QE enhancement • Operation of RF gun – Results of commissioning – Operation plan • Additional Cathode Study – R&D of Cs 2 Te photocathode. – Development of load-lock chamber. • Summary

【Introduction 1】 Photocathode RF gun system RF gun structure Photocathode Materials 【QE vs 1/e lifetime】 Multi-alkali, etc… Laser system 【Photoemission Properties】 【 Multi-alkali cathode 】 • High Quantum efficiency • Short lifetime (< 100 h) ・Multi-bunch operation ・High average current (ERL etc…) 【 Metal cathode 】 Metal & Metallic compound • Low Quantum efficiency • Long lifetime (>a few 1000 h) Single bunch operation

【Introduction 2】 Required Beam Parameters： 5 n. C / 10 mm-mrad / 50 Hz / 2 bunches A 1 Photocathode RF gun system in KEK Quasi traveling wave side-coupled RF gun Metal & Metallic compound Photocathode 【QE vs lifetime of photocathode materials】 High power Yb: YAG laser system 【Target value】 Primary electron beam for positron generation QE : >1. 0 × 10 -3 Lifetime : > 4000 h 【 【 Electron beam 】 QE : >1. 0 × 10 -4 Lifetime : > 4000 h 】

【 Candidate Material 】 Candidate : Metallic Compounds La. B 6 / Ir 5 Ce (multicrystal) 【Advantage of Ir. Ce compound[1]】 • • • Stable in air High melting point : > 1900 ℃ Low work function : 2. 57 e. V Long lifetime（Heater cathode）: Ir 5 Ce (× 1000) >> La. B 6 Strong for ion bombardment A good resistance to poisoning 【 Ir 5 Ce compound 】 【 SEM/EDX analysis of the cathode surface 】 Carbon (*Both samples had been stored in air) La. B 6 Carbon Ir 5 Ce Oxygen Impurities : 43% Impurities : 8. 1% [1]. G. I. Kuznetsov, Journal of Physics: Conference Series 2 (2004) 35– 41

Cathode Study of Ir 5 Ce compound • Measurement of quantum efficiency (QE) • Continuous Photoemission • QE enhancement

【Experimental Apparatus】 【Test Chamber】 Turbo pump Photocathode Anode 【Laser System】 Q-switch Nd: YAG Laser Parameters Value Pulse Width 7 ns Repetition Rate 10 Hz Max. Pulse Energy@266 nm 15 m. J/pulse Max. Pulse Energy@213 nm 100 μJ/pulse 【Layout】 HV Laser (incident angle : 55 deg. ) High Voltage : 0 ~ 20 k. V (Pulse) Gap length (anode - cathode) : 10 ~ 20 mm Vacuum Pressure : < 1. 0× 10 -6 Pa (Non-baked)

【 QE Measurement at room temperature 】 【Surface Treatment】 【Measurement Conditions】 Vacuum : 1. 0× 10 -6 Pa Laser : Nd: YAG 4 th Harmonics : λ = 266 nm H. V. : 5 ~ 16 k. V At the start of the experiments, the cathode was treated by laser irradiation (laser cleaning) or heating at high temperature (heat treatment) to clean the surface and provide good uniformity of the surface components. 【Heat treatment】 【 Laser Cleaning 】 La. B 6@266 nm Ir 5 Ce@266 nm 10− 5 Conditions of Laser Cleaning Laser : 266 nm Cleaning area : 0. 55× 0. 55 cm 2 Number of shots : 900 shots H. V. : OFF 10− 6 0 10 20 30 40 50 60 70 80 90 100 Energy density of the cleaning laser (m. J/cm 2) QE as a function of the scanned energy density of the cleaning laser. QE=1. 54× 10 -4 0, 00014 Quantum efficiency 10− 4 Quantum Efficiency 0, 00016 QE=1. 0× 10 -4 0, 00012 0, 0001 8 E-05 6 E-05 4 E-05 969℃ 2 E-05 0 0 200 400 600 800 1000 Treatment Temperature (℃) QE of the Ir 5 Ce photocathode as a function of the heat-treatment temperature. 1200

【Lifetime measurement】 【 After 50 hours 】 【Measurement conditions】 Ir 5 Ce and La. B 6 photocathodes were continuously generating 5 n. C electron beams for 50 h under 5 × 10− 6 Pa, and the dependence of the QE on irradiation time was measured. • 【La. B 6 Photocathode】 • The QE dropped to 1/e of its initial value. (Problem : The oxidization of cathode surface) Laser Power : Fixed • Recovered the QE by the laser cleaning 【 Results】 Charge (n. C/pulse) 6 Instability of laser power 5 【Ir 5 Ce Photocathode】 The QE was hardly decreased → Ir 5 Ce can be used for long time under 5 × 10− 6 Pa. 4 3 2 1/e lifetime of photocathodes depends on the degree of vacuum[*] 1 Ir 5 Ce La. B 6 0 -20 Laser cleaning 30 Time (hour) 80 Ir 5 Ce may have long lifetime [*] Justin Jimenez, doctor thesis (Monterey, California. Naval Postgraduate School)

【 Enhancement of QE 】 The Quantum efficiency depend on ①Wavelength of laser pulse ②Operating Temperature etc…. Quantum Efficiency × 10 -4 25 The Quantum efficiency is increased by ①Irradiation of shorter-wavelength laser ②Operating at high temperature Ir 5 Ce@213 nm Ir 5 Ce@266 nm QE = 2. 70× 10 -3 20 × 15 Irradiation of shorter-wavelength laser QE = 9. 10× 10 -4 (λ=213 nm, room T) 15 QE = 9. 10× 10 -4 QE = 1. 00× 10 -3 10 × 6. 5 × 6 5 1 0 200 400 【 Laser system 】 ・Nd: YAG laser 4 th harmonics (266 nm) ・Nd: YAG laser 5 th harmonics (213 nm) 【 Heater 】－ Ta heater 600 800 1000 Heating QE = 1. 00× 10 -3 (λ=266 nm, 1004℃) Irradiation of shorter-wavelength laser ＋Heating QE = 2. 36× 10 -3 (λ=213 nm, 868℃) Cathode Temperature(℃) Heat treatment ( λ=266 nm, room temperature)

【Summary】 • Ir 5 Ce photocathode has some merits for Super. KEKB electron linac. ・Enough QE at room temperature (QE=1. 54× 10 -4 @266 nm) ・A good resistance to poisoning ・(it may have) Long lifetime. • Ir 5 Ce photocathode operated under high temperature has enough QE to generate primary electron beams for positron production. 【Future Plan】 • Accurate measurement of a lifetime. room temperature / temperature dependence • R&D of Irn. Cem material (n, m : component ration) ・Ir 2 Ce － highest melting point (2250 ℃)[*] • Development of a single-crystalline iridium cerium samples. [*] Thaddeus B. Massalski, et al. , "Binary Alloy Phase Diagrams Second Edition"

Operation of RF gun • Results of RF gun commissioning • Operation Plan in A 1 RF gun

【Results of commissioning & Operation plan】 【 Results of RF gun commissioning in A 1】 Term : 2014/4/16 ~ 2015/2/3 ・ QEMax* = 8. 5× 10 -5 (2014/12/01) ・QE could be kept for 10 month. * Power loss of laser pulse (1, 2) were neglected for the QE calculation. (1. transmission loss, 2. loss from the miss alignment at cathode) 【The cause of low QE】 • Laser cleaning is not sufficient. • Miss alignment at cathode. etc… - Manually operation by a human being. - It is difficult to estimate a laser intensity. etc… 【Operation Plan in A 1 RF gun】 • Activation of cathode surface by Heat treatment. - Development of a new cathode plug equipped with heater. • Consideration of using 5 th Harmonics for photoemission.

【 Development of a new cathode plug 1】 【 New cathode Plug 】 Heating method : CW laser Heating 【 Concept of the CW laser heating 】 cathode UV Laser pulse@266 nm choke structure 【 RF shielding structure 】 【Simulation Results】 IR laser for heating 1 st cell 【Strong Point】 ・ Efficient heating method. ・ Don’t need a heating shielding. ・ Don’t need an electrical circuit in chamber. ・ Easy to repair this system S-parameter (d. B) 0 -20 -40 -60 -80 Reflection Transmission -100 2 2, 5 3 3, 5 Frequency (GHz) 4

【 Development of a new cathode plug 2】 【cathode holder】 【Rough calculation of thermal loss】 100 Ir 5 Ce cathode Loss (W) 80 Ta rod (1. 0φ) 60 Radiation Loss (Ir 5 Ce) Termal Loss (Ta - 0. 2 t× 3) Termal Loss (Ta - 1φ× 4) 40 20 0 500 Ta holder (0. 2 t× 3) 700 900 1100 Temperature(℃) Cathode Temperature (℃) 【Experimental setup】 1200 【Experimental Results】 Ir 5 Ce(8φ3 t)@graphite coating 1000 800 600 400 200 0 0 10 20 30 40 50 60 70 Output power of laser diode (W) Graphite coated photocathode was heated to 1000 ℃

Consideration of using 5 th Harmonics for photoemission 【 】 【 Conversion efficiency from fundamental beams 】 【 QE(Ir 5 Ce)×η】 × 10 4 -6 η = 14. 4%[1] 35 BBO 2, 5 2ω 30 2 25 1, 5 20 1 4ω 15 5 Ir 5 Ce×(CLBO× 2)@213 nm 3 40 10 Ir 5 Ce×CLBO@266 nm 3, 5 QE×η conversion efficiency from ω : η (%) Nd: YAG Laser [1] Pulse width : 3. 5 ns Repetition rate : 10 Hz Max Energy : 400 m. J/pulse - single transverse mode (top-hat) 5ω 0, 5 0 CLBO 0 BBO 0 200 400 wavelength (nm) 200 400 600 800 Temperature (℃) 1000 1200 600 Total efficiency is 4 times higher [1] K. Deki, et al. , “ 193 nm Generation by Optical Frequency Conversion Using Cs. Li. B 6 O 10 Crystal(CLBO

Additional Cathode Study • R&D of Cs 2 Te photocathode. • Development of load-lock chamber. • Pre-test of vacuum deposition.

【Additional Cathode Study】 【Cs 2 Te photocathode】 【QE vs Photon energy[1]】 QE>10%@266 nm 【Previous Study in KEK】 【KEK－ATF[2]】 Gun type : BNL Gun-IV (S-band 1. 5 cell) Laser : Nd: YVO 4 mode-lock laser/ 357 MHz 4 u. J/bunch@266 nm(FHG) Charge : 5 n. C/bunch Vacuum : < 10 -6 Pa Merit : QE = 0. 1 ~ 1. 0 % for 1 month Demerit : Cathode exchange every month. : Weak against a RF breakdown. a ion bombardment. : Ultra-high vacuum (< 10 -6 Pa). : A Load-lock chamber for vacuum deposition. [1]. Cesium-Tellulide and Magnesium for high quality photocathodes Preparation and diagnostics R. A. Loch(Univ of Twente) Master Thesis June 2005 [2]. M. Kuriki , “Review of Photo-Cathode RF Gun Study”, 6 th Beam Physics Seminar at Shanghai, 1 Nov. 2005

【Development of a multipurpose chamber】 【Purpose】 • Measurement of photoemission properties. • Pulsed laser deposition (PLD). - Ir. Ce thin layer cathode • Load-lock chamber for vacuum deposition. - Test a Cs 2 Te photocathode, etc… Motors for rotating a PLD sample Laser Port× 5 HV in 【Photo of chamber】 Turbo pump Gate valve Load-lock chamber 【Overview】 Transfer Rod

【 Vacuum Deposition System 】 【Test : K 2 Cs. Sb Photocathode】 【Load-lock chamber】 Load-lock chamber Gate valve Transfer Rod 【System Overview in load-lock chamber】 【Results】 35 Shield Te Cathode Substrate Cs dispenser Current [u. A] 30 λ=410 nm 25 QE = 3. 80 %@410 nm 20 15 10 QE = 1. 64 %@532 nm 5 0 Alumina Pot 0 5 10 15 Laser Power [m. W] 20

【Summary】 [*] Thaddeus B. Massalski, et al. , "Binary Alloy Phase Diagrams Second Edition"

Thank you for attention.

【High temperature cathode operation】 Heating Method : Laser Pre-Pulse Heating 【Concept】 UV Laser pulse@266 nm 【Rough calculation】 【 1 D thermal diffusion equation】 IR laser pulse for pulse heating 【Strong Point】 ・ Efficient heating method. ・ Don’t need an electrical circuit in the gun chamber. ・ Little influence of dark current Surface Temperature (℃) 1200 T(t) 1000 800 Laser parameter λ 1064 nm σ 2. 5 ns r 4 mm W 288 m. J/pulse 600 400 200 0 0 10 Irradiation of pre-pulse 20 t (ns) 30 40

【Development of a multipurpose chamber】 Laser Port× 5 • Measurement of photoemission properties. • Pulsed laser deposition (PLD). - Ir. Ce thin layer cathode • Load-lock chamber for vacuum deposition. - Test a Cs 2 Te photocathode, etc… 【Overview】 Motors for rotating a PLD sample Turbo pump Gate valve HV in Load-lock chamber Transfer Rod