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LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23, 2006

LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23, 2006. LCLS Bunch length monitor system:. Two subsystems: Deflecting structure ‘LOLA’ Accurate, calibrated, complex Destructive – ‘pulse stealing mode ok’ Expensive Tested Radiation monitors

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LCLS Bunch Length Monitor Conceptual Design Review Instrument Design Considerations February 23, 2006

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  1. LCLS Bunch Length Monitor Conceptual Design Review Instrument Design ConsiderationsFebruary 23, 2006 LCLS Bunch Length Monitor Marc Ross - SLAC

  2. LCLS Bunch length monitor system: • Two subsystems: • Deflecting structure ‘LOLA’ • Accurate, calibrated, complex • Destructive – ‘pulse stealing mode ok’ • Expensive • Tested • Radiation monitors • Dipole radiation • Gap radiation • Simple sensors • Cheap • Tested Complementary devices LCLS Bunch Length Monitor Marc Ross - SLAC

  3. Bunch length monitor system Dipole radiation Gap radiation LOLA Video signal mm wave optics WG Pyro-electric detector High frequency diode Single signal device LCLS Bunch Length Monitor Marc Ross - SLAC

  4. Two coherent radiation monitors for BC1 • Simple ceramic gap surrounded by mm-wave diode detectors (paired) • Total radiated energy is 2 uJ • For 1nC, 200 micron bunch. • (energy scales as Q^2/(bunch length) • Tested at SLC and ESA (actually many years…) • Annular reflector directs dipole radiation onto mm-wave ‘optics’ – pyroelectric detector • similar total radiated energy • Tested at FFTB/SPPS LCLS Bunch Length Monitor Marc Ross - SLAC

  5. Bunch charge distribution Simple indicator: central frequency of radiated energy

  6. Coherent radiation detection strategy: • Each individual detector has ~ factor 2 range • Long bunches: use diode sequence • (100, 200, 400, 1000 GHz) • Down to 100 um rms • Short bunches: use reflector and pyro-electric detectors • Below 150 um rms • RD required to match – see 2007 testing plan LCLS Bunch Length Monitor Marc Ross - SLAC

  7. CERN Bunch length RF gap

  8. ESA 100 GHz Gap and Detector gap LCLS Bunch Length Monitor Marc Ross - SLAC

  9. ESA gap monitor and detector gap • Gap / horn / WG-10 closeup

  10. 400 GHz diode LCLS Bunch Length Monitor Marc Ross - SLAC

  11. ESA 100 GHz System – Jan 8, 2006 Sig_z_min ~300 um gap

  12. 1nC / 200 um example (Total radiated energy 2uJ) “Energy in” assumes catalog item waveguide horn Detector sensitivity is 2.3e-15J into 50 Ohm out. Good S/N for 100, 200, 400… 1000GHz ? gap Multi-frequency diode ‘xylophone’ LCLS Bunch Length Monitor Marc Ross - SLAC

  13. gap ‘Beam view’ of multi-diode / waveguide assembly LCLS Bunch Length Monitor Marc Ross - SLAC

  14. Reflector • (in use at FFTB for plasma wake exp. – Hogan) • Thin Ti foil ‘in the beam’ – polished. • Si window • Simple, direct, detector optics • Typically shorter than LCLS BC1 (not BC2) • LCLS annular reflector will be 30mm diameter with 14 mm aperture • Capture 50% ~ 1 uJ – at best • Low frequency performance reported to be poor (<300 GHz) LCLS Bunch Length Monitor Marc Ross - SLAC

  15. Alignment Laser 1 µm Titanium Foil at 45º e- 12.5 µm Mylar 1mm HDPE Vacuum Window (3/4” dia) Reference Pyro Detector 12.5 µm Mylar Beam Splitters RT≈0.17 Variable Position Mirror ∆z Interferometer Pyro Detector CTR MICHELSON INTERFEROMETER sx=60 µm, sy=170 µm N≈1.91010 e- • Interference signal normalized to the reference signal • Motion resolution ∆zmin=1 µm or ≈14 fs (round trip) • Mylar: R≈22%, T≈78%, RT≈0.17 reflector

  16. reflector CTR Energy Correlates with Bunch Length Relative Energy @ end of linac

  17. reflector Pyro Is Not The Whole Story Need to Look at Details of the Spectra Example: Jitter from North Damping Ring: X-ray Relative Energy [GeV] • Pyro amplitude is ambiguous • Energy spectra are not • They are complimentary diagnostics • Clear correlation between energy spectrum and E-164X outcome

  18. reflector pyro jitter distribution – SLC NRTL stability

  19. reflector Spectra vs pyro-electric signal

  20. reflector One pyro vs another • meets 5 to 10% resolution goal

  21. reflector Pyro for one band vs another • Pyro response as a function of linac ‘chirp’ (phase - offset)

  22. reflector pyro response has position correlations:

  23. MM Wave detector Sensitivity • Pyroelectric: Basically a charge source, approximately 1.5uC/Joule. • Capacitance is 120pf for 3mm detector. • Charge amplifier (AmpTek A250, external FET), has noise 300 electrons RMS. • Corresponds to 30pJ of mm wave energy • Typically pyro detectors are supplied with included amplifier, performance tends to be worse. • Detector is a thermal integrator. Dynamic range is limited by the dynamic range of the amplifier. • For the AmpTek A250, this is approximately 60,000:1. • Commercial pyroelectric detectors (Scientech PHF02) have noise level of 3nJ, approx 100uJ maximum signal. • Note, sensitivity is 100X worse than theoretical, dynamic range is 30,000:1 LCLS Bunch Length Monitor Marc Ross - SLAC

  24. MM wave diode detectors • Sensitivity of ~2V/W (into 50 Ohms) with ~100GHz bandwidth (at 300GHz). • For a 100ps pulse, Bandwidth ~5GHz. • Noise is 2.3e-15J. • Diodes typically linear to ~30mV output. • 1.5pJ. Dynamic range 700:1 • Expect realistic amplifier (10dB noise figure) to limit dynamic range to 250:1 • Waveguide for 300GHz is WR-2.8, 0.7X0.35mm. • expected attenuation 0.2dB/cm. • Waveguide for 900MHz is WR-1.0, 0.25X0.13mm. • Expected attenuation 1.1dB/cm • Need something like 20cm of waveguide for dispersion • The initial pulse is very short, with extremely high peak power. Waveguide dispersion spreads the pulse in time, while keeping the original frequency content. LCLS Bunch Length Monitor Marc Ross - SLAC

  25. Comparison of pyro and diode detectors • Pyro detectors have much larger dynamic range (>30,000:1, vs ~200:1). • Noise energy diodes is 10^4 lower than for pyro detectors • Pyro area (for sample detector) is ~10 mm^2. • Diode (waveguide) • At 300GHz 0.2mm^2 • At 900GHz 0.03mm^2 • Diode Sensitivity / Area is 250x at 300GHz, 30X at 900GHz • Not clear how much gain available from horn antenna. (~10dB?) • Diodes more sensitive than pyros at 300GHz. • At 900GHz, diodes probably slightly less sensitive. • Dynamic range of pyro detectors is better • Diode alignment of waveguide is much easier. LCLS Bunch Length Monitor Marc Ross - SLAC

  26. Controls and data acquisition • These systems are ‘single signal’ systems, i.e. only a simple gated digitizer is needed* • (Some concerns over gating precision and noise – to be tested) • BC1 Feedback will require beam intensity normalization and (possibly) steering correction / feedback • integration with LOLA improves the systematics greatly  we strongly recommend an aggressive approach to LOLA data acq./integration. • Pyro/diode systematics will be different and may require different procedures. • 2007 testing plan LCLS Bunch Length Monitor Marc Ross - SLAC

  27. Testing plan • ESA • Minimum bunch length ~200 um ? • Multi-channel resolution test • No independent high accuracy reference • April and July 2006 • LCLS injector • Minimum bunch length ~ 50 um • High accuracy reference (29-4 LOLA Transverse cavity LCLS Bunch Length Monitor Marc Ross - SLAC

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