S-band RF gun based on
new type acceleratingstructure
A.M. Barnyakov, D.A. Nikiforov, A.E. Levichev

Accelerator seminar
KEK-BINP
26.10.2017
nikdanila@bk.ru

Outline• Motivation and requirements• Typical RF gun parameters• Parallel coupling accelerating structure (PCAS)• Beam dynamics for SUPER c-tau• Beam dynamics for FCC• Prototype of parallel coupled accelerating structure• Ir5Ce Cathode• Conlusion

Motivation
Photocathode rf guns are considering today as the mostadvanced type of electron injectors for modern electronpositron colliders.
Super KEKB (Japan)

FCC (CERN)

Super-с-τ-factory (BINP)High charge and low emittance are needed!

Motivation and requirements
Electric field on axis

Beam envelope along the gun

In the last cell beam can occupy a half of aperture

Distribution of the vertical electricalfield component near the beam axis

Motivation and requirements

Motivation and requirements

Main parameters of Super C-tau Injector Linac
Beam energy (GeV)

1.5-2.5

Bunch charge e-/e+ (nC)

1.5/3.5

Norm. emittance (pi mm mrad)
Energy spread<10 pi mm mrad
<1%

Typical RF gun parametersParameters

Value

Energy (MeV)

9-11Charge (nC)

6.5 (FCC*), 1.5 (Super c-tau)

Horizontal emittance (mm mrad)<6 (FCC), <10(Super c-tau)

Vertical emittance (mm mrad)<10(FCC),<10 (Super c-tau)Longitudinal sigma (mm)~1.3Transverse sigma (mm)1-2

RMS Energy spread<1%• To extract such a big charge, beam size will be too large , due to space charge
limit ( several mm)• Focusing magnetic field along the cavities or/ strong RF focusing are needed
to preserve or decrease transverse beam dimensions.
* - PRELIMINARY

Parallel coupling accelerating structure(PCAS)
Tuning rods

Exciting cavity

Ре-е-Permanent focusing magnets

Coupling slots

Accelerating cells

Features:
1) Parallel RF power feeding.
2) Cavities are not connected with each other by RF power: process in one cavity doesn’t influence on every cavities
3) Organization of the free electric field distribution along the structure can be obtained by changing the individual
coupling slot
4) In order to develop accelerating structure only one accelerating cells have to be calculated due to absence of thecavities connection by electromagnetic field5) Aperture of the structure is defined by only beam motion and can be considerably reduced6) Design of the structure allows using internal permanent focusing magnets.
For more details of PCAS excitation you can see:
Chernousov Yu D., Ivannikov V. I., Shebolaev I. V., Levichev A. E., Pavlov V. M.. Bandpass characteristics of coupled resonators. JOURNAL OFCOMMUNICATIONS TECHNOLOGY AND ELECTRONICS, Volume 55, Issue 8, pp. 863-869. DOI: 10.1134/S1064226910080036.
Published: AUG 2010
A. M. Barnyakov, A. E. Levichev, D. A. Nikiforov.. Intercavity Coupling Constant of the Cavities. Journal of Communications Technology andElectronics, 2016, Vol. 61, No. 7, pp. 783–788

RF gun based on (PCAS)

0,31000,25000,0-0,1-50-0,2-100-0,30,00,20,40,6z, [m]0,81,0Bz, [T]Ez, [MV/m]

0,1

Beam dynamics for SUPER c-tau
ASTRA simulation parameters
Initial emittance

0.6 mm mrad

Initial kinetic energy

0.6 eV

Total charge

1.5 nC

Cathode spot size

3 mm

Initial distribution

Rad. Uniform

Laser pulse duration10 ps

Laser injection phasevariable

Magnetic field on the cathode 0 T
Peak accelerating field

70 MV/m

Focusing solenoid field

0.35 TBeam dynamics for SUPER c-tau factory
0,180
60

0,0-0,1200-0,2-20-40-0,3-60-80-0,40,00,10,20,30,40,50,60,7z, m

Em_n=2.3 mm mrad

0,8Bz, TEz, MV/m

40Parameter

Value

Beam length (sigma, mm)

1.5Norm tr. Emittance (mmmrad)

2.3

Energy (MeV)

6.5

Energy spread (%)~1%Injection phase (deg)200

Beam dynamics for FCCASTRA 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

14 psLaser injection phasevariable

Magnetic field on the cathode 0 T
Peak accelerating field

100 MV/m

Focusing solenoid field

0.5 TBeam dynamics for FCC
1000,3Parameter

Value0,2

Beam length (sigma, mm)

1.5

Norm tr. Emittance (mmmrad)

3Energy (MeV)

9.8

Energy spread (%)~1%

Injection phase (deg)2005000,0-0,1-50-0,2-100-0,30,00,20,40,60,8z, [m]Em_n=3.1 mm mrad
Em_n=2.2 mm mrad (95%)

1,0Bz, [T]Ez, [MV/m]

0,1

Beam dynamics for FCC

Em_n=3 mm mrad

Em_n=2.2 mm mrad (95%)

3 m. long SLAC type acc. structure

Parameter

Value

Beam length (sigma, mm)

1.5

Norm tr. Emittance (mmmrad)

3

Energy (MeV)

9.8

Energy spread (%)

0.7%

Injection phase (deg)293Prototype of parallel coupled accelerating structure with 2450 MHz

Beam current of 300 mA in theend of accelerating structure,
duration is 2.5 ns (half of pulse),
energy is about 4 MeV

Measured and calculated magnetic
field from the permanent nagnets

Prototype of parallel coupled accelerating structure with 2450 MHz:
breakdowns

Breakdown in the 5-th accelerating cavity. yellow– RF power from klystron, 1.6 MW, pulse
duration 5 μs; blue – reflected signal; green –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 5μs; 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 theaccelerating cavities. One can see that only cavity with breakdown is “switched off”. Onthe left figure the first breakdown is in the fifth cavity and the its storage energy isterminated. On the right figure the first breakdown doesn’t influence on the fifth
accelerating cavity.

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.

Prototype of parallel coupled accelerating structure with 2856 MHz

Ir5Ce and LaB6 as a PhotocathodeIr5Ce 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 aregoing to make a stress test and investigate the life time of sucha cathodes

Conclusion
1. The design of parallel coupled accelerating structure allows using thepermanent focusing magnets and create the strong magnetic field alongthe cavities and save the beam size.
2. The travelling tube aperture can be decreased as significantly as it allows
the beam dynamics.
3. This design of the RF gun allows us to consider the cavities asindependent. The length and field amplitude can be tuned separately forevery cavity.
4. Preliminarily results of beam dynamic simulations gave reason to hopethat this construction is acceptable solution for high charge generation.
5. The prototype of the accelerating structure based on new design withparallel coupling between the cavities have been produced in BINP with
operating frequency of 2450 MHz and successfully tested.
6. It is currently planned to perform tests the new parallel coupled
accelerating structure with 2856 MHz .

But:
1. More investigations for high power operation mode are neededPossible cooperation• The procedure of manufacturing, tuning and measurement ofPCAS is well established at BINP.
• There is good experience of operation of such structure(especially with indium variant).
• At BINP we have a good experience of production andexploiting of high quality IrCe cathode with high current density
and long lifetime.
But…
• For further testing of such a structure with high power andintense beam extraction, the klystron and laser system areneeded.
And…
• KEK has a great experience of modern electron-positron
injector development. BINP is very interesting in the possibility
to adopt experience of KEK in this field.

References
1) H. A. Bethe, "Theory of Diffraction by Small Holes", Phys. Rev., Vol. 66, PP. 163-182, 1944.
2) ChernousovYuD., IvannikovV. I., ShebolaevI.V., LevichevA. E., PavlovV. M.. Bandpass characteristics ofcoupled resonators. JOURNAL OF COMMUNICATIONS TECHNOLOGY AND ELECTRONICS, Volume 55, Issue8, pp. 863-869. DOI: 10.1134/S1064226910080036. Published: AUG 2010
3) Characteristics of the model of accelerator based on parallel coupled accelerating structure with beamloading. A.E. Levichev, V.M. Pavlov, V.I. Ivannikov, I.V. Shebolaev, Yu.D. Chernousov. Proceedings of RuPac
2012, JACoW publication, pp. 164-166
4) Localization of the RF breakdown in the parallel coupled accelerating structure. A.M. Barnyakov, A.E.
Levichev, V.M. Pavlov, V.I. Ivannikov, I.V. Shebolaev, Yu.D. Chernousov. Proceedings of RuPac 2012, JACoWpublication, pp. 281-2835) Linear accelerator based on parallel coupled accelerating structure. A.E. Levichev, V.M. Pavlov, V.I.
Ivannikov, I.V. Shebolaev, Yu.D. Chernousov. Proceedings of LINAC 2012, JACoW publication, pp. 282-2846) A. Levichev, D. Nikiforov, A. Barnyakov, Yu. Chernousov, I. Shebolaev. Transient Process in a Parallel
Coupled Accelerating System with Regard to Beam Current Loading. TechnicalPhysics, 2015, Vol. 60, No. 1,
pp. 137–140.
7) A. M. Barnyakov, A. E. Levichev, D. A. Nikiforov.. Intercavity Coupling Constant of the Cavities. Journal ofCommunications Technology and Electronics, 2016, Vol. 61, No. 7, pp. 783–788

Thank you for attention

Initial distribution

Overvoltage factor
Ku≈1.2Ku≈1.3
Overvoltage factor of PCAS ishigher than for the ordinary
structure. Shape of the cavities canbe optimized.

Beam dynamics in the gun without any magnetic fields
ASTRA
Em_n=6 mm mrad
Em = 0.3 mm mrad

CST
Em_n=6 mm mrad
Em = 0.3 mm mrad

Two guns comparison
Parallel coupled accelerating structure

Ordinary accelerating structure

Emittance growth in combinedsolenoid-cavity section

SLAC Estimation of the electric field distribution for Gaussian beam

Q=1.5 nC
Sigma=1.5 mm

Q=6.5 nC
Sigma=1.5 mm