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Physics Requirements for LCLS X-Ray Transport and Diagnostics

Physics Requirements for LCLS X-Ray Transport and Diagnostics. John Arthur. From the LCLS Global Requirements document:. The Project scope includes  facilities for production and transport of a bright, high-current electron beam

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Physics Requirements for LCLS X-Ray Transport and Diagnostics

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  1. Physics Requirements for LCLS X-Ray Transport and Diagnostics John Arthur

  2. From the LCLS Global Requirements document: • The Project scope includes •  facilities for production and transport of a bright, high-current electron beam •  an undulator system in which the electron beam will generate the x-ray beam •  facilities for transport, diagnostics and optical manipulation of the x-ray beam •  endstations and relatedfacilities for x-ray experiments •  conventional facilities for the accelerator systems and x-ray experiments •  a central lab office building to house support staff and researchers This talk will elaborate on the physics basis for the specification of the LCLS x-ray transport system and diagnostics

  3. The X-ray Transport Systems

  4. Where the X-ray transport systems are located Near Expt Hall Far Expt Hall ~420 m

  5. Basis for transport requirements: The LCLS x-ray beam must be transported to the experimental stations without degradation Vacuum transport line. Pressure requirement for minimal absorption of LCLS beam over ~ 500m is 10-5T. Additional requirement is that pressure at the ion pumps must be low enough to assure long pump life (>10 yrs). Transport line also serves a PPS function. Pipe must be large enough so that it cannot be hit by a mis-steered beam. Transport line must conform to SLAC vacuum standards, and LCLS standards for pumps, controllers, and gauges.

  6. The X-ray Optics

  7. Functions of the x-ray optics • Confinement  (masks, slits, local apertures) • Intensity attenuation  (gas attenuator, solid attenuator) • Focusing  (K-B mirror) • Spectral filter  (mirror low-pass filter, monochromator) • Beam direction (flipper mirrors) • Temporal filter  (pulse split/delay)

  8. High peak power (fluence) poses a challenge for x-ray optics • Response of material to ultra-high power x-ray pulse is untested • LLNL codes can describe all aspects of the response EXCEPT initial conversion of x-ray energy into hot electrons. Uncertainty due only to lack of understanding of non-linear response • We have good arguments that the non-linear response will be negligible • Therefore, we will use linear absorption cross sections with confidence • LLNL will do precise calculations (assuming linear cross sections) as part of optics design • Until those calculations are done, use conservative approximation based on known melting points of materials

  9. LLNL Approximation assumes FEL pulse energy instantly deposited in atoms within absorption volume (using linear absorption cross section). If resulting energy/atom much less than melt energy/atom, then the material will not be damaged. FEE NEH FEH Expected LCLS fluence compared with melt fluence for various materials

  10. Some proposed solutions to the peak power problem • Low-z materials (Be, B4C, C) • Grazing incidence • Gas attenuator • Distance from source Grazing-incidence slits Graded-density absorber

  11. Basic specifications for slits and attenuators Slit aperture range 2 x 4s beam size @ 800 eV Slit precision 1 µm Attenuator range up to 104 at any energy 800-8000 eV Attenuator precision 1% of attenuation, steps 3/10/100/103/104

  12. X-ray focusing • Produce high flux density K-B focusing mirrors Useful energy range 800 - 24000 eV Focus size < 1 µm Efficiency >10%

  13. X-ray mirrors for LCLS • Energy low-pass filter • Beam redirection Double-mirror low-pass filter Low-pass mirror critical energy variable 1200 eV -9000 eV Mirror mechanical stability beam jitter < 10% of beam size

  14. X-ray monochromators • Energy bandpass filter Energy range 800 eV -24000 eV Bandpass < 2 x10-4 Rapid scan range 10%

  15. X-ray pulse split and delay • Provides precise time delay between pulses Energy 8000, 24000 eV Delay range 0-200 ps Pulse split/delay using thin Si crystals

  16. The X-ray Diagnostics X-ray diagnostics are required for characterization of the FEL and spontaneous radiation, as means of assessing SASE performance

  17. Specifications for the x-ray diagnostics • Position of beam centroid 5% of beam size • Beam transverse dimensions 10% of beam size • Beam divergence 10% of divergence • Photon energy 0.02% of energy • Photon energy spread 20% of energy spread

  18. Summary • The XTOD group will provide facilities for transporting the LCLS x-ray beam, for measuring the beam characteristics, and for manipulating the characteristics in controlled ways • A vacuum beam path system will transport the beam without degradation • X-ray optical elements will aperture, attenuate, focus, and monochromate the x-rays • A suite of x-ray diagnostics will allow characterization of SASE performance

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