1. Efficient RF sources for Linear Accelerators
Dr Chris Lingwood

2. Motivation
From CLIC CDR (2012)
LARGE numbers of RF sources are required for future linear colliders.
According to CDR CLIC requires 15 MW
Supply large amount of power at affordable cost (high efficiency)
Current state of the art
15 MW klystrons can achieve 65% efficiency

3. CLIC MBK Study
Collaboration with CERN and Thales (Erk Jensen, Igor Syratchev, Phillipe Thouvenin, Rodohple Marchesin).
Efficiency as main target
Evaluated configuration options, multiple beam klystron
Targeted a conservative (plausible) design
Targeted TESLA/ILC specification
Theoretical efficiency: 80% (beyond state of the art)

4. Why many beams? Low perveance leads to higher efficiency.
Low current -> lower space charge forces -> better bunching -> higher efficiency
20 beams – trade off between beam voltage and complexity due to beams
𝑝𝑒𝑟𝑣𝑒𝑎𝑛𝑐𝑒 𝐾= 𝐼 𝑉 3/2
Ib = 8.2A
Vb = 115V

5. Cavity choices Comparison of multiple cavity types.
1. Re-entrant
2. Recessed Re-entrant
TM 0 1
Comparison of multiple cavity types.
Re-entrant & HOM cavities -> Low R/Q
Recessed re-entrant and coax cavity ->
high R/Q
TM 10 1
TM 0 1
3. & 4. Coaxial Cavity
5. Whispering Gallery

6. Interaction structure
Optimised 6 cavity
(single 2nd harmonic)
Low R/Q structure 70%
High R/Q structure 20 beam structure up to 80%

7. Optimisation
Developed and published a new way to design klystron amplifiers:
~14 Decisions (frequencies, drifts, Qe’s)
3-4 objectives (efficiency, length,
bandwidth, slowest electron
,000 evaluations
Novel publishable optimisation concepts
(recombination operator).
Impractical without high throughput computing (CI HTCondor Pool)
Use spare clock cycles of desktop pcs

8. So it’s all done then? Conservative approach lead to complex tube
Many, many, many beams
Push the voltage (always the plan)
Don’t rule out newer techniques
Be braver on layout
Fundamentally: do you want 50MW in an MBK?

9. Cavity HOMs Can model coaxial cavity as a piece of ridged waveguide
CPI
(estimated)
Ours
Quadrupole
Cavity radius
Normalised frequency
Can model coaxial cavity as a piece of ridged waveguide
Very good agreement even for HOMs R
Large diameter (35cm) at 15MW
More power -> more beams -> larger still
Dipole mode gets closer for larger cavities

10. Current Collaboration with CERN
Working with I. Syratchev, C. Marelli
Attempting to formalise empirical relationship between efficiency and perveance

11. Proposed task Many questions still surround the RF sources
Requirements still push state of the art
Power, efficiency, cost, lifetime
Evaluate options (SBK, MBK, MBIOT…)
Solution is probably klystrons
Configuration (MBK/SBK)
Work towards helping CERN become the intelligent customer

12. Maximising Efficiency
Tight bunching isn’t enough
The key to higher efficiency is:
slow all your electrons down as much as possible in the output gap without stopping or reflecting them
This isn’t trivial
Potential improvements
Low frequency penultimate cavities
Travelling wave output structures

13. Higher harmonic cavities
To get the tightest bunch from a single cavity
sawtooth waveform (includes high harmonics)
2nd harmonic cavities well understood
Will 3rd or higher harmonic cavities help more?
Effect on bandwidth?
Effect on velocity spread?

14. Reduce velocity spread
IVEC 2013
Detune penultimate cavity to achieve π phase change
When phase change is good, coupling to the beam is bad
Two gap cavity can help with control
Tested in 63 W C-band tube, increase efficiency by 8%, 25% reduced voltage.
Does this approach scale to MW?

15. Klystron Configuration
Some interesting configuration options under consideration.
Some beyond state of the art
Some just beyond ….

16. New software
A number of klystron specific codes exist. Only one generally available
AJDisk
For instance cavity voltages can be unreliable disagree as much as 50% between GdfidL and AJDisk and 1500% (!) with klys2D (Thales).
Closed source so difficult to identify the issue
Also difficult to integrate with other codes
Propose to develop new “open” disk model code for klystron research from existing code at Lancaster.

17. PIC Simulations Simplified models can only get us so far
Detailed verification of designs to demonstrate improvements
1 week simulation time for klystron 1GHz up to 10 μs
8 cores
Scope for improvement with HPC (available at Hartree Centre, Sci-Tech Daresbury)
Careful benchmarking of code (V-SIM) against MAGIC

18. MB-IOTs for Linear Accelerators
Reduce power lost to collector by switch beam
Short “excursions” above rated power allowable.
Very useful when high efficiency and variable output power are needed.
Even better if “headroom” is needed
IOTs exist up to about 100kW
Not a great deal in the context of proposed large LINACs
Just like klystron MBK -> MB-IOT
10 beams -> 1MW, more plausible.
ESS seriously considering.
Road block - guns
Worth doing the sums.
Klystron
IOT
Figures from IOT based High Power Amplifiers, Morten Jensen, TIARA Workshop on RF Power Generation for Accelerators – Uppsala June 2013

19. Magnetrons Injection locked magnetrons to achieve phase control using
Small, cheap, efficient, low voltage
run at saturation
Perhaps lock with IOT
Probably not cost
effective at CLIC scale

20. Milestones
2014 Investigate suitability of single and multi-beam klystrons, MBIOTs, magnetrons.
2014 Produce a bunched beam vacuum tube model. Transcode and improve existing code to interface with PIC codes for output cavity. Benchmark against existing codes.
2015 Evaluate new and existing techniques to improve efficiency.
2015 Benchmark V-SIM and CST against MAGIC for bunched vacuum tubes.
2016 Design most appropriate tube and validate using PIC.

21. Deliverables 2014: Tube model complete and open sourced
2015: Design of tube interaction structure for drive beam - report
2016: PIC simulations and verification of proposed interaction structure - report

22. Financial Staff (Lingwood) 1 3 RA3 6+6 18+18 Student (TBD) 12 36
Materials
5+7
4+5
0+0
9+12
Travel
1+2
3+6
1 RA and 1 Phd student for three years

23. Summary Re-evaluate options for RF sources
Push efficiency and plausibility
Scope for improvement in simulation times
Develop more flexible and open klystron simulation tools.
Produce candidate structure using lessons learned.