1. CEPC 650MHz High Efficiency Klystron R&D
Zusheng ZHOU(周祖圣)
RF Power Source Group
Accelerator Division, IHEP
May. 4, 2016

2. Contents
■Present situation
■Design proposal
■R&D Progress
■Summary

3. Present situation
The RF relevant FCC and CEPC machine parameters have well converged – they clearly indicate the need for Higher Energy Efficiency.
Tetrodes(四极管）
IOTs
Conventional klystrons
Solid State PA
Magnetrons
（磁控管）
DC – 400 MHz
(200 – 1500) MHz
300 MHz – 1 GHz
DC – 20 GHz
GHz range
1 MW
1.2 MW
1.5 MW
< 1MW
85% - 90% (class C)
70%
50%
60%
90%
Remark
Broadcast technology, widely discontinued
Oscillator, not amplifier!

4. Present situation 2014 saw a breakthrough in klystron theory:
The “congregated bunch” concept was re-introduced [V.A. Kochetova, 1981]
(later electrons faster when entering the output cavity).
The concept of “bunch core oscillations” was introduced [A. Yu. Baikov, et al.: “Simulation of conditions for the maximal efficiency of decimeter-wave klystrons”, Technical Physics, 2014]
(controlled periodic velocity modulation)
The “BAC” method was invented [I.A. Guzilov, O.Yu. Maslennikov, A.V. Konnov, “A way to increase the efficiency of klystrons”, IVEC 2013]
(Bunch, Align velocities, Collect outsiders)
These methods together promise a significant increase in klystron efficiency (approaching 90%)
An international collaboration has started – prototypes are being designed. (SLAC plans to convert an existing 5045 klystron– simulations are encouraging)

5. “Bunch core oscillations” explained
“Classical” bunching
RF=78.0%
Useful RF phase
Normalised velocity
RF period, rad
particle phase 2
Output cavity
RF=78.0%
Useful RF phase
New bunching method with core oscillations
particle phase 2
Output cavity
RF=89.6%
Normalised velocity
RF period, rad
Normalised velocity

6. HEIKA collaboration
HEIKA – “High Efficiency International Klystron Activity” is evaluating and implementing this “breakthrough”.
HEIKA Members: Labs (CERN, ESS, SLAC, CEA), Universities (MFUA, Lancaster), Industry (Thales, L3, CPI, VDBT)
It studies theoretically and experimentally high efficiency klystrons for both pulsed (e.g. CLIC, ESS) and CW applications (FCC).
I. Syratchev (CERN), A. Baikov (MFUA), I. Guzilov (VDBT), J. Neilson, A. Jensen (SLAC), G. Burt, D. Constable, C. Lingwood (U Lancaster), A. Mollard (CEA), R. Marchesin (Thales), Q. Vuillemin (Thales/CERN), C. Marrelli (ESS), R. Kowalczyk (L-3com), (Toshiba), T. Grant (CPI)

7. Klystron key design parameters Centre frequency (MHz)
Collider RF power source
Klystron key design parameters
Parameters mode
Now
Future
Centre frequency (MHz)
650+/-0.5
Output power (kW)
800
Beam voltage (kV) 80 70
Beam current (A) 16 15
Efficiency (%) 65

8. 实施途径 速调管效率提高主要通过以下途径： （1）低导流系数电子枪 （2）采用高次谐波腔 （3）采用多注速调管 （4）利用降压收集极
（5）束团内核振荡（COM）方案，比如BAC （Bunch Alignment Collector）方法
（6）绝热群聚方法
（7）Congregated bunch concept
目前低导流系数和高次谐波腔组合、低导流系数和多注的组合技术成熟，速调管效率65%以上。
要实现极高效率（80%以上）通常需要多种方法的组合，CEPC速调管将采用方案以下两种方案：
（1）单注低导流系数和高次谐波腔组合，理论模拟效率72%以上。
（2）单注低导流系数和BAC组合，期望效率高于80%。
CEPC速调管为CW速调管，功率较低，即使在低导流系数下电压也不算太高，因此可先采用较为简单的单注速调管，多注速调管可同步进行模拟设计。

9. Short term Schedule

10. Klystron Schedule and strategy
Because of klystron efficiency is more than 80%, in order to fulfill this program, there may have following problems:
we (China) have not an experienced to manufacture the high power, UHF klystrons. There is not the big furnace infrastructure in China also.
Design and simulation are not enough and matured, therefore we need to step up one by one.
Let’s start from beam tester, classical design and currently progressed design such as HEIKA.
In order to save the money and time, demountable structure is another way.

11. Klystron Schedule and strategy(2)
Merits
Saving the money
Saving the time
Possible to try 2 or 3 designs
(1) Beam tester
(2) #1 prototype
(3) #2 prototype

12. Klystron Concept Design
Power
perveance
efficiency
Cathode current density
frequency
800kW
0.68μP
65%
0.5A/cm^2 Hz
Rough Parameters
Parameters for Electron optics
Voltage
Current
Depressed Beam Potential
Cathode radius
Beam Radius
Tunnel Radius
b /a
阴极面积
电子枪面压缩比
Beam Current Density
80kV
15.385A
78.844kV
34mm
16mm
25mm
0.64
3674cm^2
4.57
1.91A/cm2
Relativistic Mass Factor σ
v/c βe γ γb γa
cut-off freq of tube
Brillouin Flux (Gauss)
1.154
0.499
27.275
23.629
0.378
0.591
5.4
7.07
3.52E+09
4.60E+09
Plasma Frequency
plasma propagation constant
plasma wavelength
Plasma Frequency Reduction Factor
Mbar
√Mbar
Reduced Plasma Frequency
Reduced Plasma propagation constant βq
reduced plasma wavelength
1.29E+09
8.58
0.732
0.222
0.874
0.935
2.85E+08
1.905
3.298
Parameters for interaction region
calculation

13. Electron Gun Design and Simulation
Egun simulation result:
Area compression ratio : 4.4
Perveance: μP
Cathode current density：0.39A/cm2~0.43A/cm2
Max. electric field : 2.520kV/mm
Cathode current density
-57kV
2.52E+03V/mm
-81.5kV
1.57E+03V/mm
0kV
Gun region beam trajectory simulation with Egun
Electrode E fied density optimization with Possion

14. Electron Optics Design and Simulation
200mm-1800mm
MAX
17.815
200mm-1000mm
1000mm-1800mm
MIN
16.947
AVE
17.305
17.315
17.295
MAX-MIN
1.0864
0.8378
ripple
6.28%
6.27%
4.84%
Laminar flow，Beam ripple <5.2%
whole tube beam trajectory simulation with EGUN
Magnetic field density of Brillouin Flux ：107.5 Gauss
Magnetic field density at cathode: Gauss
Magnetic field density at tube body： Gauss
K=0.75

15. Gun Assembly Design
Basic drawing of gun assembly using Auto Inventor. Geometry is come from the POISSON calculation.
Purpose is determine the ceramic sealing structure. And size to order to the company.
Based on this first drawing, this will be revised by IHEP Mechanical Group to more complete drawing
Current stage of drawing

16. Gun Assembly Design(2)

17. Gun Assembly Design(3)
In order to study gun assembly problems, several problems occurred at the HV ceramic due to the multipactor are planed to be analyzed.
Those are simulated by POISSON and CST, and in order to make clear the geometry, basic drawings.

18. Collector Design and Simulation
Collector shape and beam dissipation optimization
J collector optimization using universal beam spread curve
collector optimization using EGUN code
Analytic design
Numerical design
Collector Length
2147mm
1912mm
Collector radius
210mm
Total Beam power
1231kW
1230kW
Capability of power density in collector
150W/cm2
Max power density in collector
197W/cm2
207W/cm2
Crosscheck of beam trajectory with EGUN and MAGIC

19. Collector Design and Simulation(2)
Groove dimensions optimization
Groove number
Groove dimensions(a:b)
Total water flow rate
Water pressure loss for ideal smooth surface
180
1:2
1400kg/min
2.34E+4 Pa
Contour of temperature and water pressure loss

20. Collector Design and Simulation(3)
Collector Thermal analysis
Groove structure for 2 meter tapered collector in Ansys-CFX
contour of temperature on inner surface of copper domain
contour of temperature of water domain

21. Collector Design and Simulation(4)
Collector geometry of tapered part
2030mm A B C D E
Geometry of collector tapered part
Section A B C D E
groove number
180
150
120 60 40

22. Cavities design and optimization
Optimize cavity geometry to get the desired characteristic parameters and high R/Q.
TM010 electromagnetic field pattern
Table 1 Cavity geometry and characteristic parameters
Parameters
r_tube
L_right_cavity
r_nose
h_nose
Angle_nose
r_cavity
L_right_gap
f MHz
R/Q Q M
Knife edge
Output cavity
27.03
70.71 3 10
45deg
110
18.475
650
125
16984
0.8452
Knife edge input cavity
88.08
27.7
153.5
17399
0.7932
Knife edge 2nd harmonic cavity
36.463 60
9.25
1300 74
9146
0.616

23. Cavities design and optimization(2)
cavity1
cavity2
cavity3
cavity4
cavity5
cavity6
R/Q Qe Qo fo
Gap-Gap L
Gap Length d
146.8
175.5
4000
648
0.0889
0.4
143.4
1.00E+308
654
0.45
125.6
644
0.05
0.82
111.1
658
0.55
110.3
660
0.3
92.91
38.9
20000
650 M Gb Qb
Lmn Qu Qt
βq*L*360/(2*PI)
βe*d
0.622
2.922E-05
97.68
2.425
43.661
0.4
4000
225.24
49.119
0.829
1.780E-05
0.85
402.30
1.364
89.505
1.67
448.91
60.034
2.22
451.80
32.746
2.52
38.824
36.48
Total power gain57dB
-3dB bandwith:643MHz~658.4MHZ

24. Current Status -Beam tester
Concept
Simulation
Gun
HV Seal
Collector
Drawing
Gun HV Seal
Drawing
Collector drawing will be proceeded

25. Current Status -Beam tester
Latest drawing of Beam test stand

26. Summary Complete gun design Order the cathode and ceramic
Complete the collector design
Complete the designing / drawing and start manufacturing
In order to do the quick manufacturing, parallel schedule for manufacturing and cathode manufacturing and processing
Furnace problem
There is no big furnace in China, we are looking for appropriate company to build it.

27. Thanks for your attention!