The photocathode research for DCSRF Photoinjector at Peking
- Slides: 23
The photocathode research for DCSRF Photoinjector at Peking University Huamu XIE Institute of Heavy Ion Physics, School of Physics, Peking University 10/2/2020 P 3 meeting 2018, LANL 1
Outline Ø Ø Stable Operation of Cs 2 Te photocathode for PKU DC-SRF Photoinjector Bialkali photocathode for lower emittance beam with the new DC-SRF injector Photocathode research for high brightness electron source Summary 10/2/2020 P 3 meeting 2018, LANL 2
Cathode R&D plan Dowell, D. H. , et al. , Cathode R&D for future light sources. Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment, 2010. 622(3): p. 685 -697. 10/2/2020 P 3 meeting 2018, LANL 3
PKU DC-SRF Photoinjector 10/2/2020 P 3 meeting 2018, LANL 4
Beam experiment results Operation parameters of the DC-SRF injector 10/2/2020 P 3 meeting 2018, LANL 5
Long-term Behavior of Cs 2 Te Cathode Ø QE stabilize at ~4% for 12 days Ø Maximum current of 3~4 m. A electron beam was derived. Ø Dark Current: <1 n. A 10/2/2020 P 3 meeting 2018, LANL 6
Why bialkali photocathode? A New Photoinjector is designed for low emittance electron beam Ø Upgrade/optimization of the DC-SRF photoinjecter to lower emittance( < 1 mm. mrad @100 p. C ) Ø Improved drive laser system Ø New designed photo-cathode preparation system Ø New design of DC part for higher DC voltage Ø New cryomudule for lower heat loss 10/2/2020 P 3 meeting 2018, LANL 7
Optimization for lower emittance Ø DC + 3. 5 (1. 5) cell SRF + Solenoid Ø Drive laser: Transversely truncated Gaussian Longitudinally uniform Ø Bunch charge: 100 p. C Ø Cs 2 Te cathode / K 2 Cs. Sb cathode Genetic algorithm (GA) Astra 10/2/2020 P 3 meeting 2018, LANL 8
Parameters and Results Variables Min Max Units 5 15* ps Laser rms size 0. 5* 2 mm 3. 5 cell Ez, max 12 25* MV/m 3. 5 cell phase -30 30 degree Solenoid Bz, max 200 1500 Gs Solenoid position 1* 2 m Laser pulse length 10/2/2020 P 3 meeting 2018, LANL 9
Why bialkali photocathode? 2 K helium Photocathode, stabilized at ~20 K The PKU DC-SRF injector is a Cyro. DC-SRF injector 10/2/2020 P 3 meeting 2018, LANL 10
Cryo-photocathode and its behavior Gun type Photocathode Tcathode Performance BNL 704 MHz SRF gun Bialkali LN 2 QE, to 20% HZDR 1. 3 GHz SRF gun Cs 2 Te, Cu, Mg LN 2 No difference HZB 1. 3 GHz SRF gun Bialkali LN 2 Not test yet? Cornell Cryo DC gun Bialkali, etc ~20 K Low QE, low emittance PKU DC-SRF Cs 2 Te, Bialkali ~20 K Cs 2 Te, no difference. Bialkali, not test yet! 10/2/2020 P 3 meeting 2018, LANL 11
Photocathode behavior at Cryogenic temperature QE How will the bialkali photocathode performs at 20 K and its code description? The permittivity, phonon energy, etc. . . H. Xie, T. Rao, I. Ben-Zvi, E. Wang, Phys. Rev. Acce. and Beams 19, 103401(2016). 10/2/2020 P 3 meeting 2018, LANL 12
Why bialkali photocathode? Material Threshold QE Response time Metal: Cu 4. 7 e. V < 10 -4 <<1 ps NEA photocathode: Ga. As 2. 2 e. V 10 -1 ~70 ps Alkali Antimonide 2. 0 e. V 10 -1 <1 ps Near threshold photoemission 532 nm laser photon energy: 2. 32 e. V, The cryogenic effect: 0. 2 e. V Shottky effect: 0. 1 e. V. Alkali Antimonide photocathode is the best choice for cryogenic photocathode! Easy Excess energy tuning! 10/2/2020 P 3 meeting 2018, LANL 13
Photocathode behavior at Cryogenic temperature Emittance improvement as the tradeoff of QE • Photocathode QE will decrease at cryogenic temperature due to the change in the ionization energy( Eg+EA) of the semiconductive material. • The bunch charge of the electron beam needed for XFFEL can be reached even if the bialkali photocathode was LN 2 cooled. 10/2/2020 P 3 meeting 2018, LANL 14
New Photocathode deposition system Ø Vacuum in deposition chamber and transport chamber can reach up to low 10 -9 Pa with a sputtering ion pump (400 L/s) and a SAES NEG pump (3500 L/s and 2000 L/s) Ø SAES alkali sources and effusion sources can both be used in the system. Ø The temperature of the substrate puck can be controlled from 80 K to 800 K. Ø Alkali based photocathode, Cs 2 Te, K 2 Cs. Sb, K 2 Na. Sb, Ga. As etc can be grown on this system. Ø Can grow transmission mode photocathode New photocathode deposition system is being commissioned. 10/2/2020 P 3 meeting 2018, LANL 15
Photocathode transport suitcase To the gun To the Gun Suitcase 1. 10 -8 Pa vacuum 2. Magazine with up to 4 pucks 3. QE monitoring during the transporting process 10/2/2020 P 3 meeting 2018, LANL 16
Intrinsic emittance measurement system Ø Measure intrinsic emittance Ø Attached to the deposition chamber. Ø transferred under vacuum Ø With voltage up to 20 k. V Ø Solenoid and cryo-path added later J. Feng et al, Rev, Sci. Instrum. , 86(1) (2015) 015103 10/2/2020 P 3 meeting 2018, LANL 17
Photocathode research for high brightness electron source With the development of the photoinjectors, the intrinsic emittance of the photocathode has become the main limit of the final emittance of the electron beam. Main Direction: • Cryogenic performance of cathode • Transmission mode photocathode(QE can be measured in T. Mode during deposition) • bandgap engineering for semi-conductive photocathode • New photocathode material 10/2/2020 P 3 meeting 2018, LANL 18
Photocathode research for high brightness electron source Ultralow emittance electron beam (<0. 35 mm. mrad/mm) can be derived from the cryo-cooled photocathode (bialkali) with Shottky effect Hua-Mu Xie, Er-Dong Wang, Ke-Xin Liu, NUCL SCI TECH (2018) 29: 71 * Collaboration with Erdong Wang at BNL 10/2/2020 P 3 meeting 2018, LANL 19
Future work 1. Bialkali photocathode fabrication recipe commissioning. 2. Cryogenic performance of the bialkali photocathode 3. Intrinsic emittance vs wavelength measurement in the system 4. Test the bialkali photocathode in the DC-SRF gun 5. Bialkali photocathode performance at 20 K and its code description 10/2/2020 P 3 meeting 2018, LANL 20
Conclusion Ø High quality Cs 2 Te photocathodes with 3% QE and long lifetime are fabricated and transfered to the DC-SRF photoinjector. Ø Simulation shows that with bialkali photocathode (K 2 Cs. Sb), Laser shaping, Higher DC voltage(100 k. V), etc. Electron beam with emittance ~0. 5 µm @100 p. C can be achieved with the DC-SRF photoinjector(1. 3 GHz). Ø A bialkali photocathode deposition system is being commissioned at PKU. Ø Cs 2 Te and bialkali(multialkali) photocathode will be further studied at PKU for high brightness electron source application. 10/2/2020 P 3 meeting 2018, LANL 21
l Thanks for the colleagues at the PKU SRF group. l Thanks for the useful discussion on the new photocathode deposition system from Dr. Erdong Wang, Dr. Triveni, Dr. Ilan Ben-Zvi, Dr. Shukui Zhang etc. 10/2/2020 P 3 meeting 2018, LANL 22
Thank you! 10/2/2020 P 3 meeting 2018, LANL 23
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