James Welch [email protected]August 27,2009 FEL 2009 Undulator K-Parameter Measurements at Undulator K-Parameter Measurements at LCLS LCLS J. Welch, SLAC National Accelerator Laboratory J. Welch, SLAC National Accelerator Laboratory Contributors: R. Bionta, A. Brachmann, F.-J. Decker, Y. Ding, P. Emma, A. Fisher, Z. Huang, R. Iverson, H. Loos, H.-D. Nuhn, H. Sinn, P. Stefan, D. Ratner, J. Turner, J. Wu, D. Xiang This work is supported by the U.S. Department of Energy, contract DE-AC02- 76SF00515, and was performed under the auspices of the U.S. Department of Energy, by University of California, Lawrence Livermore National Laboratory under Contract W- 7405-Eng-48, in support of the LCLS project at SLAC. THOA05
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Undulator K-Parameter Measurements at LCLS J. Welch, SLAC National Accelerator Laboratory
Undulator K-Parameter Measurements at LCLS J. Welch, SLAC National Accelerator Laboratory. Contributors: R. Bionta, A. Brachmann, F.-J. Decker, Y. Ding, P. Emma, A. Fisher, Z. Huang, R. Iverson, H. Loos, H.-D. Nuhn, H. Sinn, P. Stefan, D. Ratner, J. Turner, J. Wu, D. Xiang. - PowerPoint PPT Presentation
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Undulator K-Parameter Measurements at LCLSUndulator K-Parameter Measurements at LCLSJ. Welch, SLAC National Accelerator LaboratoryJ. Welch, SLAC National Accelerator Laboratory
Contributors: R. Bionta, A. Brachmann, F.-J. Decker, Y. Ding, P. Emma, A. Fisher, Z. Huang, R. Iverson, H. Loos, H.-D.
Nuhn, H. Sinn, P. Stefan, D. Ratner, J. Turner, J. Wu, D. Xiang
This work is supported by the U.S. Department of Energy, contract DE-AC02-76SF00515, and was performed under the auspices of the U.S. Department of Energy, by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48, in support of the LCLS project at SLAC.
Motivation for Motivation for in-situin-situ K Measurements K Measurements
The 130 m long undulator consists of 33, essentially identical, independently tunable segments.FEL gain is lost if K/K (RMS) 1.5x10-4 K Tolerance was well met, we lased right away, but…
Temperature, alignment, position, radiation, can change K.
We have a validation program, whereby segments are ocassionally removed to the laboratory and tested.
In-situ K measurements will allow timely tuning correction, and guide segment selection for removal and validation.
First, only the REF segment is put online and a spectrum is measured. The Reference “inflection point” is determined.Next, the Ref removed and theTest segment is put online.Then, we measure a series of spectra for different horizontal positions the Test segment and find the match position.
K Monochromator Transmission K Monochromator Transmission To find electron energy for transmission, aim a bit high and look at imager. Next search for the transmission angle.3 rad rotation easy to see on imager. (FWHM is ~70 rad. ) Alternately, scan angle and measure photodiode signal.
Wakefield energy loss and peak bunch current jitter Photodiode noise
Mitigation….Dogleg bends bpms provide 3x10-5 relative energy resolution and freedom from betatron motion.Bias electron energy scan to match K steps.
Systematic ErrorsSpontaneous radiation Wakefield energy loss Temperature differences Observation angle
Mitigation3000 A peak bunch current is normal for FEL operation. Can easily tune to 500 A Both bunch current jitter and wakefield energy loss per meter are reduced.
Roll out all but two nearby segmentsVerify pointing angles using slit scanning to maximize photon energy.Precisely measure electron beam energy jitter, pulse to pulseDetect xrays around the first harmonic using narrow bandwidth crystal spectrometerConstruct the xray spectrum by correlating the no. of detected photons with the measured energy jitter.
Change K of second segment a known amount by shifting horizontally.Obtain another spectrum and move again (≈ 9 X). Find steepest slope of each spectrum. Fit steepest slopes vs K data to find position where K’s are matched. Advance to next pair of segmentsRepeat until all segments are measured.
Error sources Error sources Beam energy jitter, 0.1% rms.
Detector is assumed to be narrow bandwidth ( << 1/N), high efficiency, Si crystal, Bragg diffractionMeasure each pulse to 3x10-5 and use to reconstruct the spectrumNatural beam energy jitter is sufficient to sample region of steepest slope.
Phase differences between segmentsShown to be neglible
Alignment/Pointing errorsMore than about 8 rad beam angle will scrape core SR on the vacuum chamber and distort the high energy edge of the measured spectrum.
Measure energy deviation of each pulse in dispersive region.
Dogleg bends bpms provide 3x10-5 relative energy resolution and freedom from betatron motion.
Run at low bunch 3000 A peak bunch current is normal for FEL operation. Can easily tune to 500 A (longer bunch).Both bunch current jitter and wakefield energy loss per meter are reduced.
Inflection point can be sensitive to range of data used for fit when data is noisy.Biasing the electron energy scan range avoid biasing the fit.One measurement takes about 5 minutes. (Slow stage travel.)