1. 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,

2. 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,

3. 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 Report is available at:
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,

4. 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,

5. 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 ( 𝑘 𝑛 𝑎 𝑛 ) 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,

6. 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 => 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,

7. 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,

8. 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. 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,

10. 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
~ kW
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

11. 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
0.26 / 0.9 (preliminary)
Repet. rate
MHz
0-0.93
0.93 (Cs2Te)
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

12. 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,

13. 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,

14. 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
; 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,

15. 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,

16. 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,

17. 4. Low frequency (quarter-wave) resonator (UoW, BNL..)
4. SRF-guns
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,

18. 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,

19. 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
Gun2 is in operation since June It continues operation with the Mg cathode, after it was contaminated by Cs2Te cathode in Jan
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
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,

20. 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,

21. 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 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
Bunch rep. rate
kHz
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,

22. 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 %
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,

23. 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,

24. 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,

25. 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 shots with 0.06 mJ/mm2,
2nd shots with 0.06 mJ/mm2, all at 248nm
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

26. Intrinsic (thermal) emittance
4.b R&D at DESY
Intrinsic (thermal) emittance
QE Spec
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,

27. 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 V
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,

28. 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,

29. 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 - QE
1.7E-4
1.3E-4
Decrease by 24%
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

30. 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,

31. 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,

32. Back up slides
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

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

34. 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
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,

35. 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 2K
Courtesy J. Hao of PKU
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

36. 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,

37. 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,

38. 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,

39. 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,

40. 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,

41. 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
Energy
MeV
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
Challenging parameters are marked in yellow
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,

42. 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,

43. 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,

44. 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,

45. Back up: BNL ɛn (100%) µm*rad Parameter Unit Spec Measured Ecath MV/m24 20 QE %
1<
1.25 fresh cathode
0.03 after multipacting
Charge nC
1-5
ɛn (100%)
µm*rad
< 5
H 0.5 / V 1nC
Energy
MeV 2
1.7
J. Sekutowicz, CW operating electron guns; A brief...., Shanghai,