High Efficiency Xband Klystron Design Study Brandon Weatherford
High Efficiency X-band Klystron Design Study Brandon Weatherford, Rich Kowalczyk, Valery Dolgashev, and Jeff Neilson – SLAC National Accelerator Laboratory Aaron Jensen – Leidos, Inc. Igor Syratchev, Jinchi Cai and Walter Wuench - CERN Jan. 24, 2018
Agenda • Program overview • Design targets • Core Oscillation Method • Bunching circuit design • Multi-gap output cavity optimization • Conclusions 2
High Efficiency X-band Design Study - Overview • SLAC/CERN collaboration funded by CERN to explore achievable efficiencies for high peak power X-band klystron • Use recently developed techniques to improve existing SLAC XL series 50 MW, 38% efficient klystron • Improve efficiency with novel bunching techniques such as Core Oscillation Method (COM) • Improve output cavity lifetime using based on lessons learned from design and test of high gradient structures • Performance targets: • • Frequency: 12 GHz Pulse Width: 2 µs Efficiency: >70 percent Peak RF Power: 40 MW minimum, 50 MW goal 3
Design Study Plan • Preliminary COM bunching circuit designs in 1 D AJDisk - Effect of perveance - Fill Factor - Klystron length • Intermediate design with 2 D large signal codes (Kly. C, TESLA) - Radial effects - Benchmarking of Kly. C vs. MAGIC/VSIM vs. TESLA • Output cavity designs in MAGIC-2 D done in parallel with bunching circuit design - Standing wave and travelling wave designs - Evaluate for efficiency and peak surface fields • Final design on PIC codes (MAGIC, VSIM) - Stability analysis - Include calculations for depressed collector 4
Design Challenges • Minimizing radial stratification effects • Ultimate efficiency is limited, in part, by radial variation in beam interaction with klystron cavities • Generating optimal velocity distribution in fully saturated beam at the output • How do we slow down all electrons to the same energy after the output gap? • Given some energy spread, optimize the velocity profile of the incoming bunch • Existing XL klystron is already limited by high gradients in output cavity; even larger gradients will arise in a high-efficiency design • Increase output circuit length • Evaluate standing and traveling wave configurations • Consider distributed RF power extraction 5
Core Oscillation Method (COM) Klystron Design Preliminary 1 -D COM klystron designs are developed in AJDisk. Assume an “ideal” output cavity (M = 0. 9) and optimize bunching circuit. Applegate Diagram 3 x bunching cavities 7 x “oscillation” cavities Fundamental & Second Harmonic Current 0. 9 μK, 66% Fill Factor 11 Cavities 81. 9% RF Efficiency in AJDisk Electron Velocities 6
Kly. C-2 D Simulation: 0. 9 u. K, 11 Cavity Design 0. 9 μK, 66% Fill Factor 11 Cavities 3. 4% degradation in efficiency due to 2 D effects. Simulated RF efficiency is 78. 5%. 10 -Layer model: Radial Stratification = 1. 67 Spent Beam Velocity = 0. 42 c (outer) to 0. 67 c (inner) 7
Results - Variable Perveance, Fill Factor Design 0. 9 μK, 75% FF 0. 9 μK, 66% FF 1. 5 μK, 66% FF Efficiency (%) 79. 4* 78. 5 72. 6 r Stratification 1. 32 1. 67 1. 73 Maximum I 1/I 0 1. 83 1. 84 1. 82 Spent e- velocity 0. 44 – 0. 59 0. 42 – 0. 67 0. 41 – 0. 65 *For 75% FF, further improvement may be possible with finetuning of the output cavity 8
Results – 9 Cavities vs. 11 Cavities • 11 -Cavity klystron appears too long. Outer electrons start de-bunching before output; however, inner electrons continue to bunch • 9 -cavity klystron was modeled as well: only a 2. 5% drop in efficiency, to 76. 0% 11 Cavities: 805 mm Length 78. 5% 9 Cavities: 670 mm Length 76. 0% 9
Efficiency vs. Output Circuit • • • Efficiency strongly depends on the gap coupling factor, M, of the output cavity Impact of output cavity (M = 0. 7, 0. 75) was modeled in AJDisk (left) > 70% efficiency target was achieved in Kly. C-2 D with M = 0. 75 XL-4 M value is approximately 0. 8 – this is an achievable design! Optimization using Kly. C-2 D may yield a few % improvement AJDisk (1 D) output tuning Kly. C-2 D: 70. 7% efficiency No reflected electrons 10
Bunching Circuit - Summary • Kly. C-2 D simulations show that with a realistic output cavity, we can expect to reach a 70% efficient klystron design. • Multiple high efficiency COM topologies have been explored • Moving forward, output cavity design is most critical • Efficiency may further be increased by: - Re-optimization of bunching circuit with M=0. 75 output - Raising beam voltage (reducing perveance), within reason - Using superconducting magnets Design # Cavities Beam Voltage Beam Current Output Power Efficiency XL-4 7 420 k. V 335 A ~ 50 MW ~ 38% 0. 9 µK COM 9 – 11 363 k. V 197 A > 50 MW > 70% 11
Multi-gap Output Circuit – Existing XL-4 Design • MAGIC-2 D used to model the standard XL-4 multi-gap output cavity • Imported beam from XL-4 klystron simulation • TW circuit, with variable phase advance 119º 0. 7π 115 k. V 88º 0. 5π 104 k. V 117º 0. 6π π/2 mode = 0. 5π 115 k. V 120 k. V • Maximum gradient in MAGIC simulation is 72 MV/m • Location depends on phase • Beam parameters: 420 k. V, 335 A • Efficiency = 38% • Gradient = 72 MV/m 12
Parameterized Model • Output circuit optimization set up w/ parameterized model: • Axial taper (β) • Radial taper (α) • Cavity tuning (h) d 0 • Two goals: • Maximize RF efficiency, using standard XL-4 beam import • Minimize surface fields d 1=βd 0 d 2=βd 1 h r 0 r 1=αr 0 r 2=αr 1 r 3=αr 2 13
Comparison of 2 -Gap and 4 -Gap Circuits • Results show that output efficiency can increased • Can we re-shape cells to minimize surface gradient? • Distributed power extraction may be particularly effective 4 Cell, extraction from each cell 4 Cell 2 Cell Output circuit total length held roughly fixed at original 4 cell design 14
4 Cell, 58% Efficient, 220 MV/m Excited by Beam Extreme example gives 58% efficiency at 220 MV/m using poorly-bunched beam from the standard XL-4 Peak gradient in second cell is due to RF circuit impedance (i. e. a mismatch), not beam impedance We could lower the gradient by choosing a more appropriate cell pattern to eliminate mismatch Excited in first cavity Power Flow 15
Output Circuit – Next Steps • End goal is to design the output circuit using a “COM-like” bunched beam instead of the XL-4 beam • MAGIC-2 D model of 0. 9 μK klystron has been constructed, results are forthcoming • Beam from MAGIC model will be exported, and used for output cavity optimization (also in MAGIC) • “Cold” RF field distribution from the optimized output circuit can be imported into Kly. C or TESLA, and iterated upon for even more improvement in efficiency. 16
Conclusions • Kly. C-2 D simulations predict an efficiency of at least 70 percent with a realistic output circuit. • Efficiency of XL-4 traveling wave output circuit can be increased, based on 2 -D PIC simulations • Next steps: optimization of output circuit (TW, SW, and distributed coupling) using imported COM beam from MAGIC-2 D Many thanks to CERN for supporting this development effort. We are here. Onward and upward! 17
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