CHESS has pioneered the development of X-ray Video Beam Position Monitors (VBPMs). Unlike traditional photoelectron
beam position monitors that rely on photoelectrons generated by the fringe edges of the X-ray beam, with VBPMs
we collect information from the whole cross-section of the X-ray beam. VBPMs can also give real-time shape/size
We have developed three types of VBPMs:
(1) VBPMs based on helium luminescence from the intense white X-ray beam. In this case the CCD camera is viewing the luminescence from the side.
(2) VBPMs based on luminescence of a thin (~50 micron) CVD diamond sheet as the white beam passes through it. The CCD camera is placed outside the beam line vacuum and views the diamond fluorescence through a viewport.
(3) Scatter-based VBPMs. In this case the white X-ray beam passes through a thin graphite filter or Be window. The scattered X-rays create an image of the beam’s footprint on an X-ray sensitive fluorescent screen using a slit placed outside the beam line vacuum. For all VBPMs we use relatively inexpensive 1.3 Mega-pixel CCD cameras connected via USB to a Windows host for image acquisition and analysis. The VBPM host computers are networked and provide live images of the beam and streams of data about the beam position, profile and intensity to CHESS’s signal logging system and to the CHESS operator.
The operational use of VBPMs showed great advantage over the traditional BPMs by providing direct visual input for the CHESS operator. The VBPM precision in most cases is on the order of ~0.1 micron. On the down side, the data acquisition frequency (50-1000ms) is inferior to the photoelectron based BPMs. In the future with the use of more expensive fast cameras we will be able create VBPMs working in the few hundreds Hz scale.
We have used a direct optical measurement of the distortion of the first silicon crystal of the CHESS A2
monochromator. The total X-ray power absorbed by the crystal was in the range of 2 to 190 Watts. The X-ray powers
measured by a bolometer were in good agreement with the XOP calculations. In-situ optical measurements were used to
measure the deformation of the crystal under the heat load between a 3-15° angle of incidence. Simultaneously, ANSYS
modeling of the effect of the heat load on the monochromator crystal with the cooling assembly was done. The measured
slope error and the surface deformation profiles were in good agreement with the ANSYS simulations. A rocking curve
method was used to measure the effect of a heat load on the diffraction properties of the monochromator for a range of
beam-defining slit widths. We have found a good correlation between the FWHM of the rocking curves and the slope
errors from the optical measurements.
The CHESS A-line 49 pole wiggler produces a total power of 6.6 kW when operating at 5.3 GeV and 200 mA current. Half of this beam is directed into the A2 station operating with both crystal and multilayer optics. The heat load response of the multilayer optics was studied by changing the total power deposited on the first multilayer by varying the slit size. The W/B4C, 300 bi-layers, d=15
Å multilayers with the energy resolution of ΔE/E=0.5% were used in our experiment. The results from identical multilayers deposited on Si and SiC substrates, which differ by a factor of two in thermal conductivity, are presented and compared. The thermal distortions of the first multilayer were measured by using recently developed optical in-situ visualization technique and compared with ANSYS simulations. X-ray measurements of the monochromator throughput and effective source size confirm the results of the optical measurements and ANSYS simulations and demonstrate the superior behavior of
SiC-substrate based ML optics under high heat load.
Conference Committee Involvement (7)
Advances in X-Ray/EUV Optics and Components XIII
20 August 2018 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components XII
8 August 2017 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components XI
31 August 2016 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components X
11 August 2015 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components IX
19 August 2014 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components VIII
26 August 2013 | San Diego, California, United States
Advances in X-Ray/EUV Optics and Components VII
13 August 2012 | San Diego, California, United States