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Efficient RF sources for Linear Accelerators Dr Chris Lingwood.

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1 Efficient RF sources for Linear Accelerators Dr Chris Lingwood

2 Motivation 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 From CLIC CDR (2012)

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 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 Why many beams? I b = 8.2A V b = 115V

5 Cavity choices 5. Whispering Gallery TM 10 1TM & 4. Coaxial Cavity 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

6 Interaction structure Optimised 6 cavity (single 2 nd 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 CPI (estimated) Ours Quadrupole Cavity radius Normalised frequency R Can model coaxial cavity as a piece of ridged waveguide Very good agreement even for HOMs 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) 2 nd harmonic cavities well understood Will 3 rd 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 Produce a bunched beam vacuum tube model. Transcode and improve existing code to interface with PIC codes for output cavity. Benchmark against existing codes Evaluate new and existing techniques to improve efficiency Benchmark V-SIM and CST against MAGIC for bunched vacuum tubes 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)1113 RA Student (TBD)12 36 Materials Travel 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.


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