Xband linac technology for a high repetition rate
X-band linac technology for a high repetition rate light source Christou Diamond Light Source
NLS Desired Baseline Specification • High brightness (up to 1012 photons/pulse) • Coverage from THz to ~ 1 ke. V in the fundamental • ~ 1 k. Hz repetition rate with even pulse spacing • Capable of smooth tuning across most of the spectral range • Pulse durations down to ~ 20 fs • Two-colour capability for pump-probe experiments • High degree of transverse coherence • High degree of longitudinal coherence, at least up to 400 e. V • Synchronised to short pulse lasers
The normal conducting option reconsidered Advantages of a superconducting linac • Low input power • Weak wake fields • Relaxed alignment tolerances Advantages of a normal conducting linac • • High accelerating gradient Shorter filling time Standard fabrication methods Easier operation and maintenance High repetition rate requires a superconducting accelerating structure … or does it? • High shunt impedance of x-band structures allow an acceptable gradient to be generated with low power dissipation in structure walls • “Room temperature high-repetition rate RF sources for light sources”, S. G. Tantawi, 48 th ICFA Advanced Beam Dynamics Workshop on Future Light Sources, SLAC National Accelerator Laboratory, March 2010.
RF power supplies: modulators High repetition rate multi-MW modulators are already on the market
RF power supplies: klystrons High pulsed power klystrons are available at X-band High average power klystrons are available at X-band 50 MW * 2µs * 1 k. Hz = 100 k. W CPI VKX-7863 B SLAC XL-4 Toshiba E 3761 Operating frequency Peak power 11. 424 GHz > 50 MW 57 MW Pulse length 1. 5 µs Repetition rate 60 Hz 50 Hz Efficiency > 40% 49% Operating frequency Average power 8. 6 GHz Duty cycle to CW Efficiency 44% 250 k. W Take the best parts of each design and develop a multi-MW k. W repetition rate klystron!
Maximise shunt impedance to minimise wall heating Close iris to minimum allowed by wake fields
Maximise shunt impedance to minimise wall heating Want minimum iris diameter Some gain to be had from iris detailing Wake field parameter follows shunt impedance if Q is unchanged
What can the klystrons deliver? Collector will see: • All of beam during modulator pulse rise time = ½ * 1µs * 50 MW = 25 J • All of beam during modulator pulse fall time = ½ * 1µs * 50 MW = 25 J • Half of beam during flat-top = 50% * tpulse * 50 MW 500 ns pulse requires ~60 k. W dissipated in the collector • Is this a realistic goal? SLAC XL-4 test at 75 MW Envisage operation at 50 MW with up to 50% beam to RF efficiency Alternatives: 1. Shorter pulse (expect ~100 ns filling time) 2. Lower per klystron (increased cost) 3. Power extraction during pulse rise and fall (power loading in structure increases)
What can the accelerating structures take? Need a realistic estimate of shunt impedance over the accelerating length: • CLIC 90 – 110 MΩ/m (unloaded, <a>/λ = 0. 11) • NLC 70 – 90 MΩ/m (unloaded, <a>/λ = 0. 18) • Further enhancements possible if high-field design constraints are removed • Take a round 100 MΩ/m as a baseline Particularly concerned with temperature at iris tip • Minimum local heating but inaccessible • Simple thermal model would be a first step
Safe power level from thermal model Equilibrium iris temperature [K] 1 k. Hz repetition rate Plan to run with tens of MV/m gradient • Typical power loading of the order of 100 MW/m 2 maximum • Uniform external cooling What is an acceptable temperature rise? • Appears safe to 40 MV/m • Higher gradient with shorter pulse?
Structure power handling 16. 2 W/cm 2 -860 MHz / mm (r) 4. 13 W/cm 2 -50. 1 MHz / mm (l) 4. 06 W/cm 2 -429 MHz / mm (r) 17. 0 W/cm 2 -293 MHz / mm (l) 4. 39 W/cm 2 -96. 3 MHz / mm (l) 1. 38 W/cm 2 352 MHz / mm (r) 1. 08 W/cm 2 22. 2 MHz / mm (r) • Figures for nominal 1 MV/m gradient • Structure will deform with heating • Need to reach design dimensions at working temperatures • Requires precise temperature control during operation
Equilibrium temperature profile Operating parameters: • Repetition rate = 1 k. Hz (modulator) • Pulse length = 0. 5 µs (klystron) • Shunt impedance = 100 MΩ/m (NLC/CLIC with “low gradient premium”) • Accelerating gradient = 35 MV/m (structure) Pulsed heating is not an issue Simplified thermal model of cell Iris inner radius = 3. 25 mm Cavity radius = 9. 97 mm Outer radius = 100 mm
Accelerating structure assembly 1. NLC-type • Larger iris radius, higher group velocity, higher RF phase advance, longer structure 2. CLIC-type • Smaller iris radius, lower group velocity, lower RF phase advance, shorter structure 12200 12100 Group velocity 2, 50% 12000 Group velocity / c Resonant frequency [MHz] 3, 00% Dispersion curve 2, 00% 11900 1, 50% 11800 1, 00% 11700 11600 0, 50% 11500 0 0, 5 1 1, 5 2 2, 5 Phase advance [radians/cell] 3 3, 5 0, 00% 0 Iris a/λ = 0. 13 is intermediate • Group velocity of 2π/3 mode is 2. 4% of c • Field attenuation per unit length in 2π/3 mode is 76% More efficient use of RF in 5π/6 mode • Group velocity of 5π/6 mode is 1. 4% of c • Field attenuation per unit length in 2π/3 mode is 118% 0, 5 1 1, 5 2 2, 5 Phase advance [radians/cell] 3 3, 5
Arrangement of RF unit How many structures? How long is a structure? Constraints: • Unit of RF power is 50 MW klystron • Power gradient along structure is 12. 25 MW/m to give 35 MV/m accelerating gradient • RF phase advance determines attenuation along structure • Linac total energy is 2. 2 Ge. V 2π/3 mode Use 4 structures per klystron • 2π/3 mode length ≤ 60 cm • 5π/6 mode length ≤ 85 cm 5π/6 mode
Conclusion • Very rough optimisation yields a working k. Hz linac light source using normal conducting Xband technology • Build on NLC/JLC and CLIC work but operate at a lower gradient • Technology is not quite there but does not seem impossible
- Slides: 15