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

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 2

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 3

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 4

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 5

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 6

Two guns comparison Ordinary accelerating structure Parallel coupled accelerating structure A. Levichev, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 7

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 8

Permanent magnets A. Levichev, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 9

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 10

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 11

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, A.E.Levichev@inp.nsk.su 2018 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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 15

Prototype of parallel coupled accelerating structure with 2856 MHz A. Levichev, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 16

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 17

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 18

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 19

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 20

Thank you for attention

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, A.E.Levichev@inp.nsk.su 2018 Workshop on the Circular Electron-Positron Collider 14