Bingxin Yang High resolution effective K September 22-23, 2004 High-Resolution Effective K Measurements Using Spontaneous Undulator Radiation Bingxin Yang Advanced Photon Source Argonne National Lab

Bingxin Yang High resolution effective K September 22-23, 2004 Two Essential Elements for Far-Field Measurements (Adapted from x-ray diagnostics planning meeting, Feb. 2004, SLAC) Roll away undulators Spontaneous radiation is most useful when background is clean, with each undulator rolled in individually. Adequate Far-field X-ray Diagnostics extracts the beam / undulator information –Electron trajectory inside the undulator (  m /  rad accuracy) –Undulator K-value (  K/K ~ 1.5 × ) –Relative phase of undulators (  ~ 10°) –X-ray intensity measurements (  E/E ~ 0.1%, z-dependent) –Micro-bunching measurements (z-dependent)

Bingxin Yang High resolution effective K September 22-23, 2004 Scope Introduction: A simple feature of the spontaneous spectrum Effect of beam quality: emittance, energy spread… Simulated experiments (  K/K ~ ?!) Key components Final remarks (conditional conclusion) Contents Relative measurements of undulator effective K using far- field spontaneous radiation (8 keV, 40 m to 60 m from undulator exit). Bonus: Wide bandwidth monochromator for z-dependent x-ray intensity measurement (  E/E ~ 0.1%).

Bingxin Yang High resolution effective K September 22-23, 2004 Main Tools Analytical work (back of an envelope) Numerical simulations (MathCAD) Undulator Radiation Modeling (XOP) Angle integrated spectra: XOP/XUS Undulator radiation intensity profile: XOP/XURGENT Reference: M. Sanchez del Rio and R. J. Dejus "XOP: Recent Developments," SPIE proceedings Vol. 3448, pp , 1998.

Bingxin Yang High resolution effective K September 22-23, 2004 Spontaneous Radiation Spectrum

Bingxin Yang High resolution effective K September 22-23, 2004 A Closer Look at the Spectral Edge Monitor the edge of angle-integrated spectrum Shifts  E/E ~ – 2  K/K. 50 – 100 data points, 5 – 15 minutes to acquire a spectrum! Monitor the intensity at fundamental photon energy Change  F/F ~ 400  K/K  < 6% intensity change needed Takes 1 – 2 seconds to acquire data?

Bingxin Yang High resolution effective K September 22-23, 2004 Impact of Aperture Change (Size and Center) Lower energy photons come in larger angles. Spectra independent of aperture size / location as long as the beam is fully contained. Spectra independent of emittance for adequate aperture.

Bingxin Yang High resolution effective K September 22-23, 2004 Impact of Finite Energy Resolution Electron beam energy spread (0.06% RMS) X-ray energy spread = 25 eV FWHM Monochromator resolution (  E/E ~ 0.1% or 8 eV) Small effect on 70-eV wide edge!

Bingxin Yang High resolution effective K September 22-23, 2004 Impact of Electron Energy Jitter Location of the spectrum edge is very sensitive to e-beam energy change (0.1% jitter):  /  = 2·  /  X-ray intensity is proportional to electron bunch charge. Current monitor data (20% fluctuation) can be used to normalize the x-ray intensity data. Impact of Electron Bunch Charge Fluctuation Most damaging instrument effect!

Bingxin Yang High resolution effective K September 22-23, 2004 A Simulation: Input and Approach

Bingxin Yang High resolution effective K September 22-23, 2004 A look at the output intensity jitter Intensity distribution depends strongly on photon energy!

Bingxin Yang High resolution effective K September 22-23, 2004 Effect of multi-shots integration An acceptable spectrum needs integration of 256 – 1024 shots, resulting scan time = 7 – Hz.

Bingxin Yang High resolution effective K September 22-23, 2004 Summary of One-Undulator Simulations Intensity noise (jitter) at the spectrum edge is largely due to electron beam energy jitter. With sufficient integration time, the measured spectrum is accurate enough to resolve effective K change at a level of  K/K ~ 1.5 × Average will take longer if LINAC jitter has time structure. A faster and more accurate technique is desirable.

Bingxin Yang High resolution effective K September 22-23, 2004 Electricity 101  V/V ~ 0.001,  I/I ~ 0.001, R = 3.50xxx? Compare two passive devices: (R-R0)/R ~ I

Bingxin Yang High resolution effective K September 22-23, 2004 Differential Measurements of Two Undulators Insert only two segments in for the entire undulator. Kick the e-beam to separate the x-rays Use one mono to pick the same x-ray energy Use two detectors to detect the x-ray flux separately Use differential electronics to get the difference in flux

Bingxin Yang High resolution effective K September 22-23, 2004 Differential Measurements: Signal Select x-ray energy at the edge (Point A). Record difference in flux from two undulators. Make histogram to analyze signal quality Signals are statistically significant when peaks are distinctly resolved  K/K =  1.5  10 -4

Bingxin Yang High resolution effective K September 22-23, 2004 Summing multi-shots improves resolution Summing difference signals over 64 bunches (0.5 sec.) Distinct peaks make it possible to calculate the difference  K at the level of Example: Average improves resolution for  K/K =  10 -5

Bingxin Yang High resolution effective K September 22-23, 2004 Simulation II Recap Use one perfect reference undulator to test another perfect undulator (two Perfect Periodic Undulators) Set monochromator energy at the spectral edge Accumulate difference count from the two undulators for ~64 bunches (0.5 second).  K/K =  3  The signal is statistically significant in resolving undulators with Is it still meaningful? Can we detect minor radiation damage?

Bingxin Yang High resolution effective K September 22-23, 2004 Key Component: Reference Undulator Last segment in the undulator Period length and B-field same as other segments Zero cant angle Field characterized with high accuracy Upstream corrector capable of 400  rad kicks.

Bingxin Yang High resolution effective K September 22-23, 2004 Key Component: Monochromator Large acceptance aperture (30 mm  15 mm) Wide bandwidth (  E/E = 0.1%) Asymmetrically cut Ge(111) crystals (2 – 8 keV) Multilayer reflectors (0.8 – 2.5 keV) Low power only Large dynamic range detector(s) Low noise amplifier and 16-bit digitizers

Bingxin Yang High resolution effective K September 22-23, 2004 Asymmetrically Cut Ge(111)

Bingxin Yang High resolution effective K September 22-23, 2004 Final Remarks We proposed a differential measurement technique for effective K. It is based on comparison of angle- integrated flux intensity from a test undulator with that from a reference undulator. Within the perfect undulator approximation, its potential resolution,  K/K =  3  or better, is sufficient for LCLS applications. It is essential to have remotely controlled roll away undulators for this technique to be practical. For not so perfect undulators, we need to extend the definition of Keff, or define a new figure of merit. The limitation of this proposed technique will need to be re-examined in that context.