1. Development of klystrons with ultimately high - 90% RF power production efficiency
A. Baikov (MUFA), I. Syratchev (CERN), C. Lingwood, D. Constable (Lancaster University)
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2. Introduction FCC has high power requirements due to high SR in FCC-ee.
100 MW CW
Traditional Klystrons -> 70%
Theoretical efficiencies up to 80%
MBIOT -> 80%
Simple minded calculation of wall plug power
@ 70% = ~140 MW
@ 90% = ~110 MW

3. State of art: L-band 10 MW MBK klystrons for ILC.
In terms of achieved efficiency at 10 MW peak RF power level, the existing MBK klystrons provides values very close to 70%.

4. Scaling of the klystron parameters thanks to (C Marelli)
Design values are in black
To go higher in efficiency, the intrinsic limits of the bunching processes and deceleration in the output cavity need to be understood

5. Process in the traditional Klystron
Bunching monotonic – electrons move to center of bunch
Many electrons miss bunch. Significant energy left in bunch!
Significant charge outside bunch. Velocities aligned…

6. Process in the traditional Klystron
Bunching monotonic – electrons move to center of bunch
Significant charge outside bunch. Velocities aligned…
Many electrons miss bunch. Significant energy left in bunch!

7. Limitations For high efficiency traditionally we chase low perveance
High voltages
Low currents (many beams)
For high power both become “unpleasant”.
Limited by the slowest electrons (must avoid trapping or reflecting electrons)
Ultimate theoretical efficiency
limited to 80%

8. Methods to get high efficiency Space charge Debunching
Bunching split into two distinct regimes:
non-monotonic: core of the bunch periodically contract and expand (in time) around center of the bunch
outsiders monotonically go to the center of the bunch
Core experiences higher space charge forces which naturally debunch
Outsiders have larger phase shift as space charge forces are small
Very long very efficient tubes result.
Phase
Traditional bunching
Core oscillations
Cavity ё
Space

9. 90% Efficient Klystron
Efficiency increases with number of core oscillations and reaches 88-90% for 4-5 oscillations

10. Process in the high efficiency klystron
Final compression and bunch rotation prepare congregating FS bunch.
After deceleration all the electrons have identical velocities.
Mission accomplished
Electron velocity/density
The fully saturated (FS) bunch

11. Process in the high efficiency klystron
Final compression and bunch rotation prepare congregating FS bunch.
After deceleration all the electrons have identical velocities.
Mission accomplished
Electron velocity/density
Output of traditional klystron
The fully saturated (FS) bunch

12. Comparison of the two bunching methods.
Core oscillations
RF current harmonics
For the ultimate high efficiency, there is a substantial increase of the bunching length.
Efficiency degradation up to perveance as high as 110-6 appeared to be rather small (about 3%).
Standard practice of reducing the klystron perveance is not the way to achieve very high, above 80%, efficiency.

13. Methods to get high efficiency BAC Method (I. Guzilov)
Again based on core oscillations
Interaction space is wasted “waiting” for space charge forces to debunch.
A cavity can achieve the same thing in a shorter space by aligning electron velocities
Structure half the length while maintaining efficiency. ё 13

14. Potential FCC Klystron Specification
Using core oscillation method
Frequency: 0.8 GHz
RF power: 1.5 MW
Beam voltage: 40 kV
Number of beams: 16
Total current: 42A
Per beam 2.6 A
Efficiency: 90%
Perveance: microPerv
Duty 100%
~0.3

15. Tube configuration Cathode loading: 2 A/cm^2 Beam radius: 3 mm
Filling factor 8 mm
Length: 2.3 m
Beam circle radius: 75 mm
Solenoid field (2x): 600 G
Solenoid radius: 150 mm
Collector: common
Nominal load: 170 kW
Peak load: 1.5 MW (no RF)

16. MBK Structure: Cavity Coaxial cavity
Best trade off between compactness, R/Q and manufacturability
R/Q 22 ~800 MHz
Single beam equivalent 352 Ohms Ez

17. Proposed Interaction Structure
3 Oscillations of core.
90.42% efficient

18. AJDisk comparison A dubious 92.2% but some validation
Some reflected electrons
Greens function method limited at high efficiency

19. PIC

20. Outlook 90% is theoretical but:
No new materials needed
No new manufacturing techniques needed
No additional complexity
Simply existing technology reconfigured
Would normally expect to lose 5% points (so 85%) from simulation to reality
Tube also well suited to ESS, potential for prototype?
Prototypes planned for proof of concept

21. HEIKA Collaboration ( as of January 2015)
CEA
PEAUGER Franck
PLOUIN Juliette
DALENA Barbara
Thales
MARCHESIN Rodolphe
VUILLEMIN Quentin
Lancaster
LINGWOOD Christopher
CONSTABLE Dave
HILL Victoria
ESS
MARRELLI Chiara
CCR Inc.
READ Michael
CERN
SYRATCHEV Igor
MUFA, Moscow
BAIKOV Andrey
JSC ‘VDBT’
GUZILOV Igor

22. HEIKA’s tubes selection
L-band:
CLIC: Frequency 1.0 GHz, pulse length 150 microsecond, 20 MW Multi-beam klystron with beams. Microperveance per beam , operating voltage below 60 kV. Expected efficiency above 85%.
FCC (ESS): Frequency 0.8 GHz, continuous wave, 1.5 MW Multi-beam klystron with beams. Microperveance per beam ~0.2, operating voltage kV. Expected efficiency above 90%.
S-band:
3 GHz technology demonstrator. 6 microsecond, 6 MW Multi-beam klystron with 40 beams. Microperveance per beam <0.3, operating voltage 52 kV. Expected efficiency >70% (with PPM focusing).
X-band:
12 GHz klystron with adiabatic bunching. 5 microsecond, 12 MW. Microperveance per beam ~1.5, operating voltage 170 kV. Expected efficiency >75%.
Low perveance MBK
High perveance single beam

23. Prototype (KIU-147A) I Guzilov (JSC)F
2.856 GHz
Power
6 MW
Voltage
52 kV
Efficiency
> 76 % F
2.856 GHz
Power
6 MW
Voltage
52 kV
Efficiency
45 %
Beams 40

24. Roadmap for high-efficiency high RF power klystron development
L-band, CW/long pulse
FCC, ESS
<20 beams; <50 kV
Optionally – gun with controlled electrode (2.5 kV)
3 years
3 years
L-band. CLIC.
40 beams; 60 kV
1.0 year
X-band
Kladistron
S-band Demonstrator
40 beams; <60 kV
L-band
CLIC
6-10 beams; ~160 kV
L-band
ILC
6 beams; 116 kV
1.0 year
Exists 2 6 20 40

25. Conclusion
Using new bunching theory 90% (at least in simulation) looks possible for FCC/CLIC/ESS klystrons
Low voltages achievable
No new technology, simply a design breakthrough
Prototypes and further validation required
International collaboration at work