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. = 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.