Efficient RF sources for Linear
Accelerators
Dr Chris Lingwood
Motivation
From CLIC CDR (2012)
• LARGE numbers of RF
sources are required for
future linear colliders.
• According to CDR 2012 CLIC requires 1638 @
15 MW
• Supply large amount of
power at affordable
cost (high efficiency)
• Current state of the art
– 15 MW klystrons can
achieve 65% efficiency
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)
Why many beams?
• Low perveance leads to higher efficiency.
𝑝𝑒𝑟𝑣𝑒𝑎𝑛𝑐𝑒 𝐾 =
𝐼
𝑉 3/2
• Low current -> lower space charge forces -> better bunching ->
higher efficiency
• 20 beams – trade off between beam voltage and complexity
due to beams
Ib = 8.2A
Vb = 115V
Cavity choices
• Comparison of multiple
cavity types.
• Re-entrant & HOM
cavities -> Low R/Q
• Recessed re-entrant
and coax cavity ->
high R/Q
1. Re-entrant
2. Recessed Re-entrant
TM 0 1
3. & 4. Coaxial Cavity
TM 0 1
TM 10 1
5. Whispering Gallery
Interaction structure
• Optimised 6 cavity
• (single 2nd harmonic)
• Low R/Q structure 70%
• High R/Q structure 20
beam structure up to
80%
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
– 5000-10,000 evaluations
• Novel publishable optimisation concepts
(recombination operator).
• Impractical without high throughput computing (CI HTCondor Pool)
•
Use spare clock cycles of desktop pcs
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?
Cavity HOMs
CPI
(estimated)
Can model coaxial cavity as a
piece of ridged waveguide
Very good agreement even for
HOMs
Ours
Normalised frequency
Quadrupole
R
Cavity radius
• Large diameter (35cm) at 15MW
• More power -> more beams ->
larger still
– Dipole mode gets closer for
larger cavities
Current Collaboration with CERN
• Working with I. Syratchev, C. Marelli
• Attempting to formalise empirical relationship
between efficiency and perveance
85
80
75
xNo harm
x2nd Harm
70
65
60
0
0.2
0.4
0.6
0.8
1
1.2
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
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
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?
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?
Klystron Configuration
• Some interesting configuration
options under consideration.
• Some beyond state of the art
• Some just beyond ….
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.
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
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
Klystron
IOT
• Just like klystron MBK -> MB-IOT
• 10 beams -> 1MW, more plausible.
• ESS seriously considering.
• Road block - guns
• Worth doing the sums.
Figures from IOT based High Power Amplifiers, Morten
Jensen, TIARA Workshop on RF Power Generation for
Accelerators – Uppsala June 2013
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.
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
Financial
Staff (Lingwood)
RA3
Student (TBD)
Materials
Travel
1
6+6
12
5+7
1+2
1 RA and 1 Phd student for three years
1
6+6
12
4+5
1+2
1
6+6
12
0+0
1+2
3
18+18
36
9+12
3+6
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.