Present status of 200keV polarized electron gun at Nagoya University

Transcription

1 Present status of 200keV polarized electron gun at Nagoya University Nagoya University Masahiro Yamamoto, N. Yamamoto, T. Nakanishi, S. Okumi, F. Furuta, M. Miyamoto, M. Kuwahara, K. Naniwa, K. Yasui KEK M. Kuriki, H. Matsumoto, M. Yoshioka

2 Features of the 200kV Gun Ceramic Preparation system is always ground level in spite of 200kV running. Load lock system Clean-Z (re-melted SUS316L) Electro-buff polishing (R<0.12μm) Dark current < 1nA photocathode surface Double ceramics insulation Atomic hydrogen cleaning

3 -200kV power supply Gun Chamber Insulation tank Beam extraction Preparation Chamber Laser pulse

4 Laser (1) CW Nd:YVO 4 SHG ps Ti:Sapphire CW Ti:Sapphire CW Nd:YVO 4 laser (SHG) (Millenia VsJ: Spectra Physics) Power 5W(max), 532nm Pico-second Ti:Sapphire laser (Tsunami PS: Spectra Physics) Pulse width 20 ±1 ps repetition MHz power 800 CW Ti:Sapphire laser (Model 3900: Spectra Physics) tuning range 750 ~ 950 nm average power 500 mw

5 Laser (2) Q-switch YAG Laser (GCR-18S: Spectra Physics) Pulse width ~20 ns repetition 10 Hz power Ti:Sapphire & SHG (LT-2211: LOTIS TII) FF ~ 40 SH ~ 10 THG Ti:Sapphire & SHG SHG Q-switch YAG THG (LOTIS TII) ~ 1 Cs 2 Te Photocathode experiment

6 Problem & Improvement

7 Lifetime problem Quantum Efficiency SLSP#16 (200kV Run) Bulk GaAs (Dark Run) Bulk GaAs (200kV Run) CH :2.0% 4 H O:3.6% 2 CO :8.0% 2 N /CO:10.3% 2 O 2 :0.4% Ar:0.6% H 2 :75.1% H2 N2/CO CO2 H2O CH4 O2 Ar Time(hour) Lifetime < 40 hours Lifetime was limited by harmful residual gases? Base Pressure : Pa H 2 O : Pa CO 2 : Pa

8 QE(t) = QE 0 exp Photocathode Lifetime t τ 1 τ = τ + gas τ + DC τ is determined by... 1 τ = 1 τ gas τ ion 1. Adsorption molecules 2. Field emission 3. Ionization The problem should be conquered first. τ gas << τ DC, τ ion

9 Improvement of a vacuum system Pumping Speed Base Pressure Before 1200 l/s 3.2e-9 Pa After 4640 l/s 5.7e-10 Pa NEG module WP950(saes getters) pumping speed (H 2 ) 430 l/s 8 Total 3440 l/s

10 Improvement of a lifetime Nomalized QE 1 Y = M0*e M1*X M M R After Before Y = M0*e M1*X M0 M1 R O :0.8% Ar:0.6% 2 CH :7.6% 4 H O:4.5% 2 CO :8.9% 2 H :39.4% 2 N /CO:24.1% 2 He:14.1% H2 He N2/CO CO2 H2O CH4 O2 Ar Time (h) Base Pressure : Pa Lifetime was improved ~ 150 hours H 2 O : Pa Total harmful gases were reduced to 1/5. CO 2 : Pa

11 Degradation of NEA Surface dn dt = N(t) α Ι gas N 0 N 0 :the number of initial dipole site α:the coefficient of destruction per one molecule. Ι gas :the density of harmful molecules flux τ gas = N 0 N 0 = ~ 1.4 x 10 5 [s] α I α 9.4 x 10 8 [/s/cm 2 gas ] ( ~ 40 hours ) N 0 = ~ 5.4 x 10 5 [s] α 2.2 x 10 8 [/s/cm 2 ] ( ~ 150 hours ) If N 0 = 3.3 x [/cm 2 ], α = 2.5 ~ 2.8 is estimated. I gas ~ 3.5 x 10 7 [/s/cm 2 ] is required for achieving 1000 h lifetime

12 Emittance Measurement System

13 Emittance Measurement System Not bakable ~10-10 Pa ~10-9 Pa the gun vacuum condition was satisfied. ~10-7 Pa Pepper Pot Mask (10µm holes)

14 Emittance Measurement System Optics Pepper Pot Chamber Sub Vacuum Chamber e-beam

15 Preliminary Result E = 80keV I beam =100nA (CW mode) He-Ne Laser ε rms ~ 1.1 π mm mrad Pulse test is preparing now.

16 Future Plan

17 Electrodes for High Field Gradient Gun Present 200keV Gun Clean-Z (re-melted SUS316L) With electro-buff polishing Limited E max ~ 10MV/m Dark Current [pa] Field emission dependence of electrodes materials Clean-Z Ti Mo Mo-Ti Emax ~ 30MV/m would be possible by using Mo cathode & Ti anode Achieving 500keV gun would be possible Field gradient [MV/m] F.Furuta et al. The result will be published in N.I.M.-A

18 Summary (1) Lifetime problem due to residual molecules was improved. before after Pumping Speed 1200 l/s ~ 4600 l/s Base Pressure Pa Pa Lifetime < 40 h ~ 150 h achieving longer lifetime would be possible... (2) Upgrading new gun chamber and electrodes. Change to Mo cathode and Ti anode from Clean-Z electrodes. Avoiding field emission to the gun chamber wall. (3) Emittance measurement system by pepper pot method has been constructed.

19 Mistakes... The electric connection between feed-through and NEG supporter was melted NEG modules were not activated sufficiently this time. The NEG supporter detected field emission from cathode more than -100kV bias voltage Upgrading to new large radius gun chamber is in progress.