1 / 23

Superconducting RF Cavities for Particle Accelerators: An Introduction

Superconducting RF Cavities for Particle Accelerators: An Introduction. Ilan Ben-Zvi Brookhaven National Laboratory. In a word:. Superconducting RF (SRF) provides efficient, high-gradient accelerators at high duty-factor. SRF accelerator cavities are a success story.

Download Presentation

Superconducting RF Cavities for Particle Accelerators: An Introduction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Superconducting RF Cavities for Particle Accelerators: An Introduction Ilan Ben-Zvi Brookhaven National Laboratory

  2. In a word: • Superconducting RF (SRF) provides efficient, high-gradient accelerators at high duty-factor. • SRF accelerator cavities are a success story. • Large variety of SRF cavities, depending on: • Type of accelerator • Particle velocity • Current and Duty factor • Gradient • Acceleration or deflecting mode

  3. What is a resonant cavity and how do we accelerate beams? • A resonant cavity is the high-frequency analog of a LCR resonant circuit. • RF power at resonance builds up high electric fields used to accelerate charged particles. • Energy is stored in the electric & magnetic fields.

  4. Pill-box cavity G=257 Q=G/Rs Rs is the surface resistivity.

  5. Some important figures of merit • U=PQ/ • A cavity is characterized by its quality factor Q and the geometric factors R/Q and G • Dissipated power per cavity depends on voltage, surface resistivity and geometry factors. V2=P·Q·R/Q For a pillbox cavity R/Q=196 Per cavity: P = V2 · Rs · 1/G · Q/R Other quantities of interest for a pillbox cavity: Epeak /Eacceleration =1.6 (~2 in elliptical) Hpeak /Eacceleration = 30.5 Gauss / MV/m (~40 in elliptical cavities)

  6. RF Superconductivity Hc(T)=Hc(0)·[1-(T/Tc)2] • Superconducting electrons are paired in a coherent quantum state, for DC resistivity disappears bellow the critical field. • In RF, there is the BCS resistivity, arising from the unpaired electrons. For copper  = 5.8·107 -1 m-1 so at 1.5 GHz, Rs = 10 m For superconducting niobium Rs = RBCS + Rresidul and at 1.8K, 1.5 GHz, RBCS = 6 n Rresidual ~ 1 to 10 n

  7. Various SRF materials – only one practical and commonly used “Superheating” field for niobium at 0 K is 2.4 kGauss

  8. Design Considerations • Residual resistivity: RactualRBCS+Rresidual • Dependence on field – shape, material, preparation • “Q slope” Electropolishing, baking • Field emission- cleanliness, chemical processing • Thermal conductivity, thermal breakdown – High RRR • Multipacting – cavity shape, cleanliness, processing • Higher Order Modes – loss factor, couplers • Mechanical modes– stiffening, isolation, feedback

  9. Measure of performance:The Q vs. accelerating field plot Magnetic fields of 1.7 kGauss (multi-cell) to 1.9 kGauss (single cell) Can be achieved, and recently 2.09 kGauss achieved at Cornell.

  10. Limit on fields • Field emission – clean assembly • Magnetic field breakdown (ultimate limit) - good welds, reduce surface fields • Thermal conductivity – high RRR material • Local heating due to defects Fields of 20 to 25 MV/m at Q of over 1010 is routine

  11. Choice of material and preparation • High “RRR” material (300 and above) • Large grain material is an old – new approach • Buffered Chemical Polishing (BCP) (HF – HNO3 – H2PO4 , say 1:1:2) • Electropolishing (HF – H2SO4) • UHV baking (~800C) • Low temperature (~120C). • High pressure rinsing • Clean room assembly

  12. Multipacting • Multipacting is a resonant, low field conduction in vacuum due to secondary emission • Easily avoided in elliptical cavities with clean surfaces • May show up in couplers! Multipacting in Stanford SCA cavity, 1973 PAC

  13. Higher Order Modes (HOM) • Energy is transferred from beam to cavity modes • The power can be very high and must be dumped safely • Transverse modes can cause beam breakup Energy lost by charge q to cavity modes: Longitudinal and Transverse Solution: Strong damping of all HOM, Remove power from all HOM to loads Isolated from liquid helium environment.

  14. Electromechanical issues • Lorentz detuning • Pondermotive instabilities • Pressure and acoustic noise Solutions include • broadening resonance curve • feedback control • good mechanical design of cavity and cryostat

  15. Miscellaneous hardware • Fundamental mode couplers • Pick-up couplers • Higher-Order Mode couplers • Cryostats (including magnetic shields, thermal shields) • Helium refrigerators (1 watt at 2 K is ~900 watt from plug) • RF power amplifiers (very large for non energy recovered elements

  16. Some Examples • Low velocity • High acceleration gradient • Particle deflection • High current / Storage rings • High current / Energy Recovery Linac • RF electron gun

  17. Low  Resonators Quarter Wave Resonator Split Loop Resonator Spoke cavity Multi-spoke Elliptical Critical applications: Heavy ion accelerators, e.g. RIA High power protons, e.g. SNS, Project-X Radio Frequency Quadrupole

  18. High acceleration gradient Critical applications: Linear colliders e.g. ILC X-ray FELs e.g. DESY XFEL

  19. Deflecting Cavities Critical applications: Crab crossing (luminosity) e.g. KEK-B, LHC Short X-ray pulses from light sources

  20. Energy Recovery Linac:A transform to a boosted frame • Energy needed for acceleration is “borrowed” then returned to cavity. • Little power for field. Energy taken from cavity JLab ERL Demo Energy returned to cavity

  21. High current ERL cavities • Multi-ampere current possible in ERL Critical applications: High average power FELs (e.g. Jlab) High brightness light sources (e.g. Cornell) High luminosity e-P colliders (e.g. eRHIC)

  22. High current SRF photo-injector • Low emittance at high average current is required for FEL. • The high fields (over 20 MV/m) and large acceleration (2 MV) provide good emittance. • High current (0.5 ampere) is possible thanks to 1 MW power delivered to the beam. • Starting point for ERL’s beam.

  23. Summary • SRF cavities serve in a large variety of purposes with many shapes. • The future of particle accelerators is in SRF acceleration elements – light sources, colliders, linacs, ERLs and more. • While there is a lot of confidence in the technology, there is still a lot of science to be done.

More Related