1. Electron sources for FCC-ee
Budker institute of nuclear physics
A.M. Barnyakov, D.A. Nikiforov, A.E. Levichev

2. Introduction
Project
Beam charge
Normalized Injector emittans
Energy spread
SuperKEKB
5 nC
~20 mm mrad 1%
C-τ Factory
3 nC
~10 mm mrad
0.1%
FCC
6.5 nC
~6 mm mrad
0.08%
1. Beam charge, normalized beam emittance and energy spread are depended on the electron gun design.
2. High beam charge is needed => large area of the photocathode => beam emittance increasing
3. Ultra low energy spread is necessary => short beam longitudinal size
4. Normalized emittance of the injector is not too low, but taking into account the lengths of the linacs and possible errors of the real devices, low beam emittance of the e-gun can be useful.
The new ideas of the e-gun design and the cathodes material are required
A. Levichev, Workshop on the Circular Electron-Positron Collider

3. Ordinary design of RF gun
Description of the structure:
1. Oscillation mode is π
2. Sequential RF power feeding
3. High coupling factors between the cavities
Electric field on axis
Distribution of the vertical electrical field component near the beam axis
Disadvantages of the structure:
1.High apertures of the coupling slots between the cavities
2. Nonlinear amplitude distribution along the cavities (likes cosine)
3. High transverse components of the electric field
4. Difficult cavities tuning and matching due to high coupling factor
5. Only external magnetic focusing system
Ez = 35MV/m
25 mm
Ex ≤ 24MV/m
A. Levichev, Workshop on the Circular Electron-Positron Collider

4. Main idea Ideal cavity without coupling
Cavity without coupling but with travelling drift tubes
Cavity without coupling but with travelling drift tubes
Ideal cavity without coupling
Cavity with high coupling factor
Cavity with high coupling factor
No coupling slots
No coupling slots
Large coupling slots
Large coupling slots
Small slots of the cavities
Uniform field distribution
No transverse electric fields
Nonuniform field distribution
High transverse electric fields
Quasi-uniform field distribution
Small transverse electric fields
A. Levichev, Workshop on the Circular Electron-Positron Collider

5. Gun for SuperKEKB
One of the leaders in the RF e-gun is KEK. The gun has new ideas of the design and cathode material
T. Miura. Progress of SuperKEKB. IPAC2015. Oral presentation
A. Levichev, Workshop on the Circular Electron-Positron Collider

6. Our suggestion
RF gun based on the parallel coupled accelerating structure (PCAS)
Features:
Parallel RF power feeding.
Cavities are not connected with each other by RF power: process in one cavity doesn’t influence on every cavities
Organization of the free electric field distribution along the structure can be obtained by changing the individual coupling slot
In order to develop accelerating structure only one accelerating cells have to be calculated due to absence of the cavities connection by electromagnetic field
Aperture of the structure is defined by only beam motion and can be considerably reduced
Design of the structure allows using internal permanent focusing magnets.
A. Levichev, Workshop on the Circular Electron-Positron Collider

7. Two guns comparison Ordinary accelerating structure
Parallel coupled accelerating structure
A. Levichev, Workshop on the Circular Electron-Positron Collider

8. Beam dynamics. 3D map field
Parallel coupled accelerating structure
Ordinary accelerating structure
Simulation based on the 3D field map
Simulation based on the field along axis
A. Levichev, Workshop on the Circular Electron-Positron Collider

9. Permanent magnets
A. Levichev, Workshop on the Circular Electron-Positron Collider

10. ASTRA simulation parameters
Beam dynamics for FCC
ASTRA simulation parameters
Initial emittance
0.6 mm mrad
Initial kinetic energy
0.6 eV
Total charge
6.5 nC
Cathode spot size
5 mm
Initial distribution
Rad. Uniform
Laser pulse duration
8 ps
Laser injection phase
variable
Magnetic field on the cathode
0 T
Peak accelerating field
100 MV/m
A. Levichev, Workshop on the Circular Electron-Positron Collider

11. Beam dynamics for FCC Parameter Value Beam length (sigma, mm) 1.5
Norm tr. Emittance (mm mrad) 3
Energy (MeV)
9.8
Energy spread (%)
0.6%
Injection phase (deg)
200
Charge, (nC)
6.5
Em_n=3.1 mm mrad
Em_n=2.2 mm mrad (95%)
A. Levichev, Workshop on the Circular Electron-Positron Collider

12. Prototype of parallel coupled accelerating structure with 2450 MHz
Beam current of 300 mA in the end of accelerating structure, duration is 2.5 ns (half of pulse), energy is about 4 MeV
Measured and calculated magnetic field from the permanent magnets
A. Levichev, Workshop on the Circular Electron-Positron Collider

13. Prototype of parallel coupled accelerating structure with 2450 MHz: after a few years of operating
After a few years of operating the structure were disassembled. Further, the cavities were tuned, assembled. The structure is working now.
A. Levichev, Workshop on the Circular Electron-Positron Collider

14. Prototype of parallel coupled accelerating structure with 2856 MHz
A. Levichev, Workshop on the Circular Electron-Positron Collider

15. Possible candidates for high efficiency and long life time photocathodes
Cathode type
Quantum efficiency
Vacuum
Advantages
Disadvantages
Cs2Te
20%-0.5%
10^-11 Torr
(>1000Hrs)
Good QE. Available for high acceleration gradient. High charge production
Very sensitive to vacuum conditions and breakdowns. Complicate installation.
Ir5Ce
5*10^-5 (before laser cleaning)
1.2*10^-4 (after laser cleaning)
@266 nm, [1]
10^-9 Torr
Acceptable QE and very good life time. Available for high acceleration gradient. High charge production. Not so sensitive to vacuum conditions, low work function – 2.64 eV, [2] (copper – 4.7 eV) – acceptable green laser.
Need to investigate Mg
0,1%
Very good QE for metal photocathode. Not so sensitive to vacuum conditions, it does not require something special equipment.
Apparently sensitive to very high acceleration gradients. (need to investigate)
[1]- D. Satoh and et. al. DEVELOPMENT OF BETTER QUANTUM EFFICIENCY AND LONG LIFETIME IRIDIUM CERIUM
PHOTOCATHODE FOR HIGH CHARGE ELECTRON RF GUN. Proceedings of IPAC2013, Shanghai, China
[2]-G.I.Kuznetsov, “IrCe Cathodes For EBIS”, Journal of Physics: Conference Series 2 (2004) 35–41.
A. Levichev, Workshop on the Circular Electron-Positron Collider

16. Production of Ir5Ce at BINP
Ir5Ce cathodes for Korean EBIS (left) and for BNL EBIS (right) was produced and tested at BINP (G. Kuznetsov)
In cooperation with institute of Laser physics SB RAS we are going to make a stress test and investigate the life time of such a cathodes
A. Levichev, Workshop on the Circular Electron-Positron Collider

17. Start of tests for Ir5Ce cathodes
The vacuum chamber for tests was prepared
Cathode test samples were produced (polished tablets 6 mm in diameter)
Next step -> Qe measurement with green laser
Data of D. Nikiforov
Results of experiments
Current Density
25 A/cm^2
Temperature
1650 C
Work function (measured)
2.64 eV
Termoemission
experiments with
Ir5Ce
A. Levichev, Workshop on the Circular Electron-Positron Collider

18. Conclusion
The design of parallel coupled accelerating structure allows using the permanent focusing magnets and create the strong magnetic field along the cavities and save the beam size.
The travelling tube aperture can be decreased as significantly as it allows the beam dynamics.
This design of the RF gun allows us to consider the cavities as independent. The length and field amplitude can be tuned separately for every cavity.
Preliminarily results of beam dynamic simulations gave reason to hope that this construction is acceptable solution for high charge generation.
The prototype of the accelerating structure based on new design with parallel coupling between the cavities have been produced in BINP with operating frequency of 2450 MHz and successfully tested.
It is currently planned to perform tests the new parallel coupled accelerating structure with 2856 MHz .
Ir5Ce cathode is developed
The work function of the cathode is measured. According to this measurements (2.4 eV), the green laser can be used for tests with the cathode
In plan
Stress tests with the cathode
Measuring of the quantum efficiency of the cathode
RF gun production
A. Levichev, Workshop on the Circular Electron-Positron Collider

19. Thank you for attention

20. Prototype of parallel coupled accelerating structure with 2450 MHz: breakdowns
Breakdown in the 5-th accelerating cavity. F1 – RF power from klystron, 1.6 MW, pulse duration 5 μs; F3 – reflected signal; F4 –
stored RF energy in 5-th accelerating cavity
Regime with breakdowns in several accelerating cavities. F1 – RF power from klystron, 1.6 MW, pulse duration 5 μs; F3 – reflected signal; F4 –
stored RF energy in 5-th accelerating cavity
The peak on the reflected signal (blue) corresponds to the breakdown in one of the accelerating cavities. One can see that only cavity with breakdown is “switched off”. On the left figure the first breakdown is in the fifth cavity and the its storage energy is terminated. On the right figure the first breakdown doesn’t influence on the fifth accelerating cavity.
A. Levichev, Workshop on the Circular Electron-Positron Collider