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Gun Test Facility Layout Longitudinal Emittance measurement technique Longitudinal Emittance

Longitudinal and slice emittance measurements at the SLAC Gun Test Facility John Schmerge, SSRL/SLAC August 18, 2003. Gun Test Facility Layout Longitudinal Emittance measurement technique Longitudinal Emittance Linear Analysis Nonlinear Analysis Slice Emittance measurement technique

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Gun Test Facility Layout Longitudinal Emittance measurement technique Longitudinal Emittance

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  1. Longitudinal and slice emittance measurements at the SLAC Gun Test Facility John Schmerge, SSRL/SLACAugust 18, 2003 • Gun Test Facility Layout • Longitudinal Emittance measurement technique • Longitudinal Emittance • Linear Analysis • Nonlinear Analysis • Slice Emittance measurement technique • Slice Emittance • Emittance Measurements • Slice Offset Measurements John Schmerge, SLAC

  2. GTF Group • Paul Bolton Laser • John Castro GTF operator • Jym Clendenin Accelerator • Dave Dowell LCLS Injector Group Leader • Steve Gierman Accelerator • Cécile Limborg-Déprey Simulations • John Schmerge GTF Group Leader John Schmerge, SLAC

  3. Experimental Goals • Transverse Emittance • en < 1.2 mm-mrad (projected) • Q = 0.2-1 nC • Longitudinal Emittance • Dt < 10 ps • sg /g < 0.1% • Laser • sjitter < 1 ps • Temporal shape – flat-top • Elaser > 100 mJ (UV) John Schmerge, SLAC

  4. 3M LINAC RF GUN DIAGNOSTIC ROOM MOD MOD MOD KLY-3 KLY-2 2 3 1 GTF LASER ROOM Gun Test Facility at SSRL DIAGNOSTICS KLY-1 8 m Laser Transport System SSRL Injector Vault GTF Control Room John Schmerge, SLAC

  5. GTF Linac and Diagnostics Analyzing Magnet Solenoid Phosphor Screen Phosphor Faraday YAG Screen & OTR Screen Screens & Cup Quadrupole PCRF Faraday Phosphor Screen & Energy Filter Doublet Gun Cups Toroids 3m S-Band SLAC Linac John Schmerge, SLAC

  6. GTF 1.6 cell S-band gun Laser Port Photocathode Electron Beam Exit With single crystal (100) Cu cathode “Half” Cell Full Cell John Schmerge, SLAC

  7. GTF Beamline John Schmerge, SLAC

  8. Laser spot used for slice emittance measurement Horizontal Projection UV Laser Image Vertical Projection counts 2 mm pixels John Schmerge, SLAC

  9. Laser Pulse Length Streak camera resolution at 263 nm » 1 ps John Schmerge, SLAC

  10. Longitudinal Emittance Measurement Technique analagous to quadrupole scan of transverse emittance 3 m linac (vary flinac) Quad Doublet Gun Spectrometer Energy Screen Determine Longitudinal f-Space at Linac Entrance Measure Energy Spectra vs linac phase Linear analysis allows only linear time energy correlations Nonlinear analysis allows quadratic and cubic correlations John Schmerge, SLAC

  11. Energy Spectrum at 15 pC Time Energy John Schmerge, SLAC

  12. Energy Spectrum at 290 pC Time Energy John Schmerge, SLAC

  13. Linear Longitudinal Analysis Find t11, t12, t22 by fitting energy spread vs. linac phase John Schmerge, SLAC

  14. Centroid Energy vs Linac Phase 15 pC 290 pC Least Square Error Fit Egun, Elinac and Flinac John Schmerge, SLAC

  15. RMS Energy Spread vs Linac Phase 15 pC 290 pC Least Square Error Fit t11, t12 and t22 John Schmerge, SLAC

  16. Linear Fit 15 pC Linac Exit Slope = -242 keV/ps Flinac =+25° keV Linac Entrance Slope = -69 keV/ps ps John Schmerge, SLAC

  17. Linear Fit 290 pC Linac Exit Slope = -127 keV/ps Flinac = +4° keV Linac Entrance Slope = -97 keV/ps ps John Schmerge, SLAC

  18. Linear Analysis Fit results after linac John Schmerge, SLAC

  19. Spectrometer Resolution Longitudinal Emittance Measurements made at IQ2 = 0 A John Schmerge, SLAC

  20. Nonlinear Longitudinal Analysis Longitudinal beam ellipse Include distortions by adding quadratic and cubic terms Ray trace to fit 5 parameters John Schmerge, SLAC

  21. Distortions of the Longitudinal Phase Space Ellipse 20 Cubic Distortion t12 = 0 a = 0, b > 0 Quadratic Distortion t12 = 0 a > 0, b = 0 DE (keV) Undistorted Phase Space t12 = 0 a = 0, b = 0 Dt (ps) John Schmerge, SLAC

  22. Nonlinear Fit for 15 pC as a function of Flinac calc-blue,data-red66 -6 degrees calc-blue,data-red55 0 degrees 30.5 2000 30.7 1800 30.65 30.45 1000 1600 30.6 30.4 1400 800 30.55 1200 30.35 600 30.5 1000 30.3 800 30.45 400 600 30.25 30.4 400 200 30.35 30.2 200 0 30.3 30.3 30.4 30.5 30.6 -2 -1 0 1 2 0 30.15 30.2 30.25 30.3 30.35 -2 -1 0 1 2 calc-blue,data-red81 -9 degrees 30.26 calc-blue,data-red91 -11 degrees 1600 30.1 1400 30.24 1400 1200 30.22 1200 1000 30.2 30.05 1000 30.18 800 800 30.16 600 600 30.14 30 400 400 30.12 200 200 30.1 0 30.08 0 29.95 30.12 30.14 30.16 30.18 30.2 -2 -1 0 1 2 30 30.02 30.04 30.06 30.08 -2 -1 0 1 2 Flinac = 0° Flinac = -6° Fit Spectra Measured Spectra Flinac = -9° Flinac = -11° John Schmerge, SLAC

  23. Temporally Resolved Transverse Emittance Measurement • Introduce linear chirp • Measure beam size in dispersive section • Slice beam in energy (time) with software • Maintain chirp in the spectrometer • Small b in bend plane at quad • Large b in bend plane at quad • Quad scan in non-bend plane John Schmerge, SLAC

  24. Quad Scan Slice Emittance Measurement Booster Quad Doublet Gun Spectrometer Energy Screen Set Booster Phase for optimum linear chirp Determine Transverse f-Space at Quad Entrance Measure Transverse Beam Size vs quad strength for different energy/time slices (typically 10 slices) John Schmerge, SLAC

  25. Quad Scan Slice Emittance Measurement • Measure beam size as a function of quad current in a dominantly dispersive section with linearly chirped beam • Fit s11, s12, s22 • Also measure slice centroids with respect to the projected centroid • Fit x0, x0’ (centroid position and angle) • Actually 5 parameters to describe the slice ellipse not just 3 Twiss parameters John Schmerge, SLAC

  26. Centroid Fitting xn, x’n measured with respect to the projected centroid Least Square Error routine used to determine xn, x’n John Schmerge, SLAC

  27. Measured Beam Size and Centroids with Best Fits Q = 15 pC and Bsolenoid =1.669 kG John Schmerge, SLAC

  28. Measured Beam Size and Centroids with Best Fits Q = 300 pC and Bsolenoid =1.982 kG John Schmerge, SLAC

  29. Beam Images as a function of Quad Strength Horizontal Beam Size (pixels) Energy/Time Axis (pixels) John Schmerge, SLAC

  30. Slice Image Converging beam (underfocused) Head Tail Horizontal Beam Size (pixels) High Energy Low Energy Energy/Time Axis (pixels) John Schmerge, SLAC

  31. Head Converging beam (underfocused) Tail Horizontal Beam Size Projections (pixels) John Schmerge, SLAC

  32. Slice Image Beam near waist Head Tail Horizontal Beam Size (pixels) High Energy Low Energy Energy/Time Axis (pixels) John Schmerge, SLAC

  33. Head Beam near waist Tail Horizontal Beam Size Projections (pixels) John Schmerge, SLAC

  34. Beam at the Spectrometer Screen Q = 290 pC, Flinac = + 4 ° John Schmerge, SLAC

  35. Twiss parameters, offsets and current as function of time Q = 300 pC, Bsol = 1.937 kG John Schmerge, SLAC

  36. Twiss parameters, offsets and current as function of time Q = 300 pC, Bsol = 1.982 kG John Schmerge, SLAC

  37. Phase Space at Q = 300 pC Bsol = 1.937 kG Bsol = 1.982 kG John Schmerge, SLAC

  38. Twiss parameters, offsets and current as function of time Q = 15 pC, Bsol = 1.609 kG John Schmerge, SLAC

  39. Twiss parameters, offsets and current as function of time Q = 15 pC, Bsol = 1.669 kG John Schmerge, SLAC

  40. Phase Space at Q = 15 pC B solenoid = 1.609 kG B solenoid = 1.669 kG John Schmerge, SLAC

  41. Measured and Reconstructed Projected Emittance John Schmerge, SLAC

  42. Slice Emittance John Schmerge, SLAC

  43. Reconstructed Projected Emittance John Schmerge, SLAC

  44. Sources of Time Dependent Kicks • Transverse Wakefields • Linac • Normal Incidence Mirror • Gun • RF kicks • Gun Exit • Linac Entrance • Linac Exit • RF couplers • Laser • Alignment • Time-Space correlation John Schmerge, SLAC

  45. Xoffset vs Z for Q = 15 pC and Bsol = 1.669 kG John Schmerge, SLAC

  46. Xoffset vs Z for Q = 300 pC and Bsol = 1.982 kG John Schmerge, SLAC

  47. Time Dependent Angle in the linac for different Charges John Schmerge, SLAC

  48. Transverse Wakefield due to Linac Misalignment x offset in 3-m structure = 1 mm, q = 300 pC, Ipk 120 A x’ vs z xmax 35 mrad x vs z xmax 20 mm Courtesy of P. Emma John Schmerge, SLAC

  49. Back Propagate Linac Wakefield Simulation • Simulated Linac Wake produces • X0 = -20 mm at linac exit • X’0 = -35 mrad at linac exit • Back propagate output conditions and assume no wakefield to see where the beams intersect • X = 0 at z = 50 cm from linac exit John Schmerge, SLAC

  50. Slice Intersection Points 15 pC 300 pC John Schmerge, SLAC

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