Accelerator Physics Topic VIII Coherent Synchrotron Radiation - PowerPoint PPT Presentation

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Accelerator Physics Topic VIII Coherent Synchrotron Radiation

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  1. Accelerator PhysicsTopic VIIICoherent Synchrotron Radiation Joseph Bisognano Engineering Physics & Synchrotron Radiation Center University of Wisconsin J. J. Bisognano

  2. Coherent Synchrotron Radiation • Since the spectrum of synchrotron radiation in free space extends down to arbitrarily long wavelengths, a bunched beam would couple to angular frequencies of order 1/bunch time length • This was a major concern in the early days of talk about storage rings, since the coherent power scales as (Ne)2 rather than Ne 2 Shielding to the Rescue! Current frequency distribution shielding shielding S is normalized power spectrum (SYLee) J. J. Bisognano

  3. Basic Principle E(1), E(2) Energy~ E(1)**2+ E(2)**2 J. J. Bisognano

  4. Wakefields J. J. Bisognano

  5. A Picture J. J. Bisognano

  6. Shielding of CSR • In terms of a resistive coupling, the free space impedance is /Z/n/ ~ 300 n-2/3 ohms • Shielding sets in at n ~< (R/h) -3/2 ; h is distance from conducting wall, and /Z/n/ peaks out at ~150 h/R, 130 ohms resistive, 85 ohms reactive • This shielding prevents long bunches from radiating as a unit; only incoherent radiation • As machines have evolved to picosecond and subpicosecond bunches, with linac recirculation arcs and bunch compressors, coherent radiation is now important • This shielding is not the same as the cutoff frequency in a waveguide; it involves the bending radius in addition to chamber dimensions J. J. Bisognano

  7. A “Simple” Analysis of CSR R0=radius of pillbox; Rb =radius of particle orbit; 2b=height of pillbox J. J. Bisognano

  8. Potential Green’s Function We solve for modes and look for condition that particle velocity matches the phase velocity of the mode J. J. Bisognano

  9. Boundary Condition J. J. Bisognano

  10. Resonance Condition J. J. Bisognano

  11. Some Typical Values J. J. Bisognano

  12. What Does It Do? Good J. J. Bisognano

  13. Maybe Useful J. J. Bisognano

  14. What Does It Do? & Evil J. J. Bisognano

  15. Bunch Compression z z z M56 Voltage ramp J. J. Bisognano

  16. Bunch Compressors Pix Bentson, et al EPAC 2002 J. J. Bisognano

  17. Dispersion and Emittance Growth Bentson, et al EPAC 2002 J. J. Bisognano

  18. LCLS Bunch Compressor Bentson, et al EPAC 2002 J. J. Bisognano

  19. Dynamical Effects of CSR Presence of modulation (microbunches) in bunch density CSR may become significant Collective forces associated with CSR induce instability Instability feeds back, enhances microbunching

  20. Properties of CSR Impedance • Shielding cut off • Peak value J. J. Bisognano

  21. Storage Ring Microwave Instablity Similar instability in bunch compressors from finite R56-microbunching J. J. Bisognano

  22. Microwave Instability J. J. Bisognano

  23. Observations of CSR - NSLS VUV RingCarr et al. NIM-A 463 (2001) p. 387 Spectrum of CSR Signal (wavelength ~ 7 mm) Current Threshold for Detection of Coherent Signal J. J. Bisognano

  24. Observations of CSR - NSLS VUV RingCarr et al. NIM-A 463 (2001) p. 387 Detector Signal vs. Time • CSR is emitted in bursts. • Duration of bursts is • Separation of bursts is of the order of few ms but varies with current. J. J. Bisognano