CW operating electron injectors; A brief overview with focus on R&D at HZDR and DESY J. Sekutowicz Topical Workshop on High Repetition-Rate XFEL Physics and Technology Shanghai, 10.08.2017

R&D and Operation Experience at HZDR R&D at DESY Final remarks Outline Introduction DC-gun NCRF-gun SRF-guns R&D and Operation Experience at HZDR R&D at DESY Final remarks I will not discuss R&D at KEK, HZB and BNL. Short information on these projects is in the back up slides. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Introduction The goal of all R&D programs on photo-injectors are high brightness electron beams for various applications (FELs, UED, UEM). Recently one observes that more attention is paid to the goal. European Workshop on Photocathodes for Particle Accelerator Applications, Cockcroft, June 2016. Future of Electron Sources, BES/DOE Workshop, SLAC, September 2016. Report is available at: https://science.energy.gov/bes/community-resources/reports/ Photocathodes Physics for Photo-injectors (P3) Workshop, JLab, October 2016. UED=Ultra-fast electron diffraction UEM= Ultra-fast electron microscopy J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

An example: LCLS II-HE, 8 GeV, 100pC, bunch length <30µm. Introduction For FELs the “attention” is driven by potential saving on linac cost when we want to push operation to shorter wave length. An example: LCLS II-HE, 8 GeV, 100pC, bunch length <30µm. 25% longer linac Courtesy P. Emma, SLAC J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Conditions for low emittance beams (Courtesy D. Dowell) Introduction Conditions for low emittance beams (Courtesy D. Dowell) a. Theory says that thermal Roughness & photon energy Electric field & roughness 𝜀 𝑖𝑛𝑡 𝜎 𝑥 = 𝜀 𝑡ℎ𝑒𝑟𝑚𝑎𝑙+𝑟𝑜𝑢𝑔ℎ𝑛𝑒𝑠𝑠+𝑓𝑖𝑒𝑙𝑑 𝜎 𝑥 = (ℏ𝜔− ∅ 𝑒𝑓𝑓 ) 3𝑚 𝑐 2 1+ ( 𝑘 𝑛 𝑎 𝑛 ) 2 2 + 𝜋𝑒 𝐸 𝑜 4 𝑘 𝑛 𝑚 𝑐 2 ( 𝑘 𝑛 𝑎 𝑛 ) 2 ∅ 𝑒𝑓𝑓 = ∅ 𝑓 − ∅ 𝑆𝑐ℎ𝑜𝑡𝑡𝑘𝑦 = ∅ 𝑓 −𝑒 𝑒 𝐸 𝑜 4𝜋 𝜀 𝑜 2αn λn αn is roughness amplitude kn = 2π/λn σx is rms spot radius Eo is electric field on the cathode ℏω is energy of irradiating photons J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Conditions for low emittance beams, cont. Introduction Conditions for low emittance beams, cont. Energy of irradiating photon should be close to the effective work function: ℏω- Øeff => 0 to minimize the thermal emittance of the generated bunches. Unfortunately this causes drop of the quantum efficiency (QE). Roughness should be as small as possible Spot size σx should be also small to keep emittance low. Electric field on the cathode, Eo, should be high (40+MV/m) to compensate for the space charge force. However the max. Eo is limited by dark current. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

DC guns are in operation at Cornell, Jlab, KEK… Pros: The DC nature allows for very flexible operation, with beams from DC to any repetition rate or pulse structure is possible. Well established technology. DC-guns are not very sensitive to particulates produced by cathode exchanger. Easy to integrate any cathode (metal, alkali…) Cons: Gradients on cathodes are low, in the range of 5-10 MV/m. This limits achievable emittances for more populated bunches. A buncher is required to compress bunches before booster system. Low electron energy requires a booster to preserve emittance. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

DC-gun based injector at Cornell, HV 350 kV DC-guns DC-gun based injector at Cornell, HV 350 kV ~10m E~10 MeV 5x2-cell SRF-cavities DC-gun Data reported 2015 HV 350 kV J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

NCRF-cw: LBLN, Boeing (duty factor 25%), LANL NCRF-gun NCRF-cw: LBLN, Boeing (duty factor 25%), LANL Pros Well established copper technology. Easy to integrate any cathode (metal, alkali…). Suitable for longer bunches (lower charge density). Cons Gradients on cathodes are moderate, ~ 20MV/m. This limits achievable emittances for more populated bunches. A buncher is required to compress bunches before further acceleration. Significant RF-power consumption. Thermal stability of the cavity is challenging. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

An example: APEX-1 gun for LCLS II (courtesy F. Sannibale, LBNL) NCRF-gun An example: APEX-1 gun for LCLS II (courtesy F. Sannibale, LBNL) Frequency 186 MHz e- energy 0.75 MeV Ecath 19.5 MV/m RF Power ~100 kW APEX-2 future plans Frequency 162.5 MHz e- energy 2 MeV Ecath 34 MV/m RF Power ~250-300 kW J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Nominal, range and measured parameters for the LCLS II gun NCRF-gun APEX-1, cont. Nominal, range and measured parameters for the LCLS II gun   Unit Nominal Range Measured Charge pC 100 10-300 20 and 100 Energy MeV 0.75 0.84 ɛintrinsic /σx µm/mm < 1 ~0.7 ɛn (95%) µm·rad 0.4 0.2-0.6 0.26 / 0.9 (preliminary) Repet. rate MHz 0.1-0.62 0-0.93 0.93 (Cs2Te) J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

They seem to be the best choice for this spec, but 4. SRF-guns SRF photoinjectors have unique potential to generate high brightness electron beams at high duty factors q/bunch -> 1nC low ɛn + ≤ 1µm·rad high DF + -> 100% They seem to be the best choice for this spec, but for all SRF-gun designs, the integration of cathodes in a very clean sc cavity is the most challenging task. There are 4 designs of SRF-guns J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

DC Pierce gun attached to a sc cavity, R&D at PKU 4. SRF-guns DC Pierce gun attached to a sc cavity, R&D at PKU (+) High QE cathode does not penetrate interior of the sc cavity. (-) Low energy electrons in the Pierce gun drift before they enter high Eacc of the cavity; space charge force limits bunch charge. Pierce gun 3.5-cell cavity Cathode Electron drift distance Input coupler port HOM coupler Courtesy J. Hao of PKU J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Spec and demonstrated (in green) parameters of the PKU injector Unit 4. SRF-guns Spec and demonstrated (in green) parameters of the PKU injector   Unit Spec Test Bunch charge pC 20 6-50 Bunch length ps 1-3 Bunch rep. rate MHz 81.25 0.1625 ; 81.25 Trans. emittance µm·rad 1.7 2.0 Energy MeV 5 3.4 Beam current mA 1.6 0.55 QE at 266nm % >1 >2 Cath. life time h - >150 E on cathode MV/m 2.6 Eγ on cathode nJ 12 Spot size (rms) mm 1 Next goal is to increase HV from 50kV to 100kV to achieve the spec on beam energy. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

A. Arnold and J. Teichert of HZDR 4. SRF-guns Sc cavity + choke filter (+) High QE alkali cathodes allow for high beam currents. (-) Cathode penetrates cavity interior and to avoid an RF-leak a choke filter must be implemented. Unfortunately one often observes in this design multipacting or/and pollution of the clean sc cavity, leading to a degradation in the cavity performance. A. Arnold and J. Teichert of HZDR Courtesy J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Sc cathode integrated in sc cavity (all sc injector) 4. SRF-guns Sc cathode integrated in sc cavity (all sc injector) (+) A sc cathode simplifies the design, and can be exposed to high E. (-) Moderate QE limits beam current to a fraction of mA. No-exchanger design can be only implemented when lifetime of the cathode is several months or/and a facility has 2 guns. Nb (Tc=9.2K, Bc= 180mT) In gasket (Tc=3.4K, Bc=28mT) Pb coating (Tc=7.2K, Bc=80mT) Nb Plug J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

4. Low frequency (quarter-wave) resonator (UoW, BNL..) 4. SRF-guns 4. Low frequency (quarter-wave) resonator (UoW, BNL..) (+) Can operate in 4.2K. Suitable for longer bunches (lower charge density). (-) Demonstrated Eacc on cathode are still moderate (20+MV/m). Design showed multipacting close to the cathode (QE degradation). 200 MHz, Courtesy UoW, Wisconsin Now at SLAC for further studies. 112 MHz, Courtesy BNL J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

a R&D at HZDR The SRF-gun program for the ELBE facility is the most advanced from all SRF-gun programs. The gun can operate in 2 modes:  Spec Unit Mode 1 High charge Mode 2 FEL Cathode - Cs2Te Bunch charge pC 1000 77 Bunch length ps 10 2 Bunch rep. rate MHz 0.5 13 Trans. emittance µm·rad 2.5 1.0 Electron Energy MeV 9.5 Beam current µA 500 QE at 258nm % 1 Max. E on cathode MV/m 30 Challenging parameters are marked in yellow Because spec for the electron energy is high (9.5MeV), the ELBE gun cavity is 3.5-cell long. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Two cavities were built up to now. a R&D at HZDR Two cavities were built up to now. Gun1 (shown on the picture) was in operation from 2010-2013. Gun2 is in operation since June 2014. It continues operation with the Mg cathode, after it was contaminated by Cs2Te cathode in Jan. 2015.   Unit Operation Gun#  - Gun1 Gun2 Cathode - Cs2Te Mg Bunch charge pC up to 400 up to 100 Bunch rep. rate MHz up to 13 0.1 Trans. emittance µm·rad 3@80pC Energy MeV 3.5 4.0 Beam current µA 400 10 QE at 258nm % 1 Max. E on cathode MV/m 9.6 12 Courtesy A. Arnold and J. Teichert J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Summary of the Gun1 and Gun2 performance 4.a R&D at HZDR Summary of the Gun1 and Gun2 performance Progress Next In recent test Gun1 ->30MV/m (after EP at DESY) Gun2 will be replaced by Gun1 in 2017 Gun3 and 4 will be ordered in 2017 J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Recently proposed spec for the DESY SRF gun Unit Nominal Bunch charge 4.b R&D at DESY The SRF-gun program at DESY began in 2005. Motivation already at that time was an increased flexibility in the time structure of the FLASH/E-XFEL photon beams by enabling cw/lp operation. Recently proposed spec for the DESY SRF gun   Unit Nominal Bunch charge pC 100-250 Bunch rep. rate kHz 250-100 Trans. emittance µm·rad < 0.5 Gradient on cathode MV/m 40 Beam energy MeV 3 Beam current µA ->25 Beam power W ->75 Very low current J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Target QE of the Pb cathode and spec for the irradiating laser 4.b R&D at DESY Target QE of the Pb cathode and spec for the irradiating laser   Unit Spec QE % 0.018 @ 266nm Irradiating pulse energy on cathode µJ 2 for 100pC 5 for 250pC Laser P on cathode W 0.5 Laser P at IR (conversion, shaping, ƞ ~ 1%) 50 J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

1.5-cell , 1.3 GHz gun cavity 4.b R&D at DESY The 1.5-cell gun cavity prototype was built at TJNAF. The present plug version has very effective cooling of the cathode. Plug with LHe channels Nb/Pb cathode 1.5-cell , 1.3 GHz gun cavity J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

The test results of 1.5-cell gun cavity with Nb and Pb-coated cathode. 4.b R&D at DESY The test results of 1.5-cell gun cavity with Nb and Pb-coated cathode. Indium exposed to magnetic flux Deformed back wall, very thick Indium gasket. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

QE test at BNL of the Pb coating on plug 4.b R&D at DESY QE test at BNL of the Pb coating on plug Courtesy J. Smedley, J. Sinsheimer, M. Gaowei and V. Gofron Laser cleaning: 1st 1000 shots with 0.06 mJ/mm2, 2nd 10000 shots with 0.06 mJ/mm2, all at 248nm J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Intrinsic (thermal) emittance 4.b R&D at DESY Intrinsic (thermal) emittance QE Spec 1.8E-4 @266nm 0.27 @266nm σx=0.55mm, 40MV/m Preferred are lower energy irradiating photons, on the contrary what is needed to increase QE. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

4.b R&D at DESY: Support by NCBJ (Courtesy J. Lorkiewicz) Recent progress in smoothness of the Pb coating with new setup. arc Pb cathode anode coils pumping system cathode plug at negative bias cathode plug shield + 50 V - + 100V reference target at negative bias witness glass sample Image of a ca 500 nm thick Pb layer deposited on glass 1 mm arc plasma propagation The extrusions diam. r < 10 µm. Their heights is< 200 nm Their total area is << 0.1% of the total coating area. The film roughness is ca. 10 nm. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Test chamber with biased anode 4.b R&D at DESY: Support by HZDR (courtesy J. Teichert, ) Lifetime test of the Pb-cathode Bias Voltage 1 kV Laser λ 263 nm Laser spot r 0.5mm Laser power -> 0.2 W Rep. rate 100 kHz Pulse duration 3 ps Fluence -> 2.5µJ/mm2 UV-LED source P -> 78 µW Test chamber with biased anode The anode current was monitored during 550h of the irradiation with laser. Due to the short 3ps pulses and moderate 1kV voltage, the space charge force caused that only ca. 25% of the current reached the anode. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

4.b R&D at DESY: Support by NCBJ and HZDR Lifetime test of Pb-cathode at HZDR, cont. Anode current monitored during 550h Increase by 33 % 550 h of irradiation UV-LED source was used to measure QE too. Date 19.11.2016 19.07.2017 - QE 1.7E-4 1.3E-4 Decrease by 24% J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

PKU injector will operate at higher voltage of 100 kV. 5. Final remarks PKU injector will operate at higher voltage of 100 kV. At HZDR Gun-2 is in operation. Refurbished at DESY Gun-1 reached 30MV/m (at the cathode location) in the VT. HZDR plans to put it in operation in 2017. The SRF-gun at DESY shows the highest gradients on cathode and the goal QE for the superconducting metallic cathode. Lifetime test of the Pb-cathode is ongoing at HZDR. Cumulated time up to now is 550h, no significant change in QE was observed. NCBJ (Poland) works on smoothness of the Pb-coating. DESY builds 2 new guns, horizontal cryostat, new plugs. VT of guns is scheduled for September 2017. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Acknowledgment Thanks to A. Arnold, B. Dunham, J. Hao, C. Hernandez-Garcia, E. Kako, T. Konomi, D. Kostin, J. Lorkiewicz, L. Monaco, A. Neumann, R. Nietubyc, J. Teichert, F. Sannibale, J. Smedley……. for the input J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up slides J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up slides Boeing-433MHz J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

The type 1 SRF-gun at PKU is an electron source for the ERL 50MeV@1mA Back up: R&D at PKU The type 1 SRF-gun at PKU is an electron source for the ERL 50MeV@1mA SRF Cavity : 3.5-cell 100 kV Pierce DC gun with Cs2Te cathode Courtesy J. Hao of PKU J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Core part of the PKU injector is a large grain 3.5-cell cavity: Back up: R&D at PKU, cont. Core part of the PKU injector is a large grain 3.5-cell cavity: Test at TJNAF in a vertical cryostat: 800C/2h, BCP, HPR 23.5 MV/m with Qo >1E10 @ 2K Courtesy J. Hao of PKU J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

PKU team continues beam experiments since 2013 Back up: R&D at PKU, cont. PKU team continues beam experiments since 2013 Irradiating Laser Injector 2K Coldbox Courtesy J. Hao of PKU J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Following parameters have not been fully demonstrated up to now: Unit Back up: R&D at PKU, cont. Next steps Following parameters have not been fully demonstrated up to now: Unit Spec Measured Trans. emittance µrad 1.7 2.0 Beam energy MeV 5 3.4 To reach the specification: the Pierce gun has to operate at the nominal DC-voltage of 100kV the cavity has to operate at the nominal Eacc of 13MV/m. Both have not been reached yet, due to the break downs of the DC voltage higher than 50kV and issues with the FPC. J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Parameters of the SRF-injector at KEK Unit Mode 1 Mode 2 Bunch charge Back up: R&D at KEK SRF-gun program at KEK is new. The goal is an injector of type 2 for the ERL facility which will operate with beams up to 100mA at 3GeV. Parameters of the SRF-injector at KEK   Unit Mode 1 Mode 2 Bunch charge pC 77 7.7 Bunch length ps 3.2 Bunch rep. rate MHz 1300 Trans. emittance µrad 1.0 0.3 Beam current mA 100 10 Energy MeV 2 QE at 520nm (K2CsSb) % 3 E on cathode MV/m 25 Input power kW 200 Challenging parameters are marked in yellow J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up: R&D at KEK, cont. A simplified prototype (only one FPC port, no choke filter, no cathode) of the 1.5-cell gun cavity was built and tested recently. Surface field corresponding to the nominal cathode field of 25 MV/m Courtesy E. Kako, T. Konomi 1.3GHz gun cavity J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Transparent superconductor: LiTi2O4 , t ~100nm Back up: R&D at KEK, cont. In the next coming vertical tests, the cavity will be stepwise equipped with additional components. This will allow for study of the design complexity: Test#2: Cavity + cathode rod Test#3: Cavity + choke cell (w/o inner conductor) Test#4: Cavity + choke filter + cathode rod Test#5: Cavity + choke filter + cathode rod + transparent cathode irradiated from the back. Cathode Substrate: MgAl2O4 , t=0.5mm Transparent superconductor: LiTi2O4 , t ~100nm Photocathode: K2CsSb , thickness (t) ~100nm Photon e- Superconductor layer to block the RF J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

GunLab (new test stand) Back up: R&D at HZB The ongoing R&D SRF-gun (type 2) program is a part of the bERLinPro project, which will be a 50 MeV ERL, operating with Ibeam -> 100mA.   Unit GunLab (new test stand) Final version Bunch charge pC 0-100 77 Bunch length ps 2-10 4.6 Bunch rep. rate Hz 10-104 1.3·109 Transvers emittance µrad 0.4-10 0.5-1.0 Energy MeV 1.2-3.5 2.3 Beam current mA <0.04 100 Input power kW 20 230 E on cathode MV/m 14-34 24 Emitting material - CsK2Sb QE at 515nm % 1 (demonstrated 5%) Cathode life time day >7 Eγ on cathode nJ 400@258nm 200@515nm 20@515nm Challenging parameters are marked in yellow J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Backup: R&D at HZB, cont. The HZB injector employs 1.4-cell cavity, which will be equipped with 2 FPCs, a choke filter and load lock unit to exchange cathodes. Input coupler Choke filter 1.4-cell cavity Blade tuner Courtesy A. Neumann J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up: R&D at HZB, cont. The 1st cavity prototype was built and vertically tested at TJNAF and later tests continued at HZB in the horizontal cryostat HoBiCaT. The tests were performed at 1.8K, w/o cathode (cathode stalk). Surface field corresponding to the nominal cathode field of 24 MV/m J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up: R&D at HZB, cont. What is next: Testing of the 1st prototype in the HoBiCaT cryostat, equipped with 2 TTF-3 FPCs, cold tuner and solenoid, will continue this fall. Assembly of the 2nd prototype in a new horizontal cryostat will begin also in fall. The cavity will be fully equipped including a unit for the cathode exchange. The experiments at the new test stand, GunLab, will begin in summer next year. Courtesy A. Neumann J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017

Back up: BNL ɛn (100%) µm*rad Parameter Unit Spec Measured Ecath MV/m 24 20 QE % 1< 1.25 fresh cathode 0.03 after multipacting Charge nC 1-5 0.1-2.1 ɛn (100%) µm*rad < 5 H 0.5 / V 1.5 @ 1nC Energy MeV 2 1.7 J. Sekutowicz, CW operating electron guns; A brief...., Shanghai, 9-11.08.2017