video-based x-ray beam position monitoring at chess€¦ · nsls-ii xbpm mini workshop, 2009...

NSLS-II XBPM mini workshop, 2009

Video-based X-Ray Beam Position Monitoring at CHESS Peter Revesz, Cornell University, CHESS

Ithaca, NY 14850

Basic Types of XBPMs:

Intercepting barely intercepting non-interceptingMost fluorescent photo-electron types Gas luminescence screens wires

lateral photo-diodes


T=�Iox+�,B=�’Io(1-x)+�’

For symmetrical and linear detector:

x~D/S=(T-B)/(T+B)

Photo-Electron Beam Position Monitor for CHESS wiggler beam lines

guard-5kV

collector+ 5kV

guard-5kV

collector+ 5kV

TOP

BOTTOM

Disadvantage:Measures the fringes only:Hard bend contamination.

Benefits:Fast, robust, reliable

Possible problems:Linearity?


He gas luminescence

Diamond screen

The Classics form 2002 VBPM exhibition at G-line


The Principle of VBPMCentroid position Captured image Camera setup

Xcij � ()

iGii � () � ��

��

ij � ()

Gij � () � � Xc

ij � ()

iGii � () � ��

��

ij � ()

Gij � () � �Yc

ij � ()

jGii � () � ��

��

ij � ()

Gij � () � � Yc

ij � ()

jGii � () � ��

��

ij � ()

Gij � () � �

Xc and Yc in pixels, butIt is easy to cross-calibrate tomicrons by imaging a mm-grid.

No Z-jack is needed, the whole systemcan be mounted rigidly.

It is not just a “number” but visual information as well. Important also as a diagnostic tool.


Advantages and Disadvantages of VBPMs

PRO•Non-intercepting•Visual information•Position is basically what the user has•Provides beam profile•No Z-jack needed, easy calibration•Beam size information•Beam intensity information

CON•More complicated H/W•Requires special software •Requires computer•For analog cameras: noise creates artifact beam motion

•Non-vacuum•Possible radiation degradation•“Zingers”


05101520-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Relative centroid position (m

icrons)

Gaussian position (microns)

12-bit 8-bit

How sensitive is the centroid measurement?

8-bit digitalization: 255 Grayscale steps

1/255

Pixels: 8�m

Sub-pixel motion

Computer simulation

Gaussian shift


Bench test of centroid position sensitivity

Optical bench

LED on Z-jack


Linearity and Offset

140150160170180190200-8

-7

-6

-5

-4

-3

-2

-1

0

1

Effect of asymmetry on centroid

Centroid position (m

icrons)

Intensity (a.u.)

Ycentroid Xcentroid Ysquared centroid Xsquared centroid

The use of squared centroid helps to reduce the artifact due to offset an asymmetry

��

�) , (

) , (2

2

y x Iy x I x

xsqc Squared centroid :-Helps to reduce the effect of offset for asymmetric profiles.


VBPM program and architectureBeam linesAnalog cameras, Sensoray 4 input USB 2.0 frame capture devices

Frame Capture:•Get pixel data in ROI•Adjustments:

median, rotation, flip

•Calculate:centroids,Intensity,FWHMs,edgesstandard deviations

Display:Image, centroids, trace, profiles

Camera/Image controlFor multiple cameras:

•Enable/Disable cameras•Set ROIs•Adjustments:

Brightness, gain, offset, averaging

•Calculate:centroids, Intensity,FWHMs, edges

Display:Image, centroids, trace, profiles

Calibrate:Pixel-to-micron Intensity-to-mA

Communicate:•Accept UDP connection

from server•Accept and respond to:

SENDALLLISTALL and SENDBYNAME

•Send data

Lab-wideLAN

USB


VBPM Centroid program user interface

Allows to control 12 cameras,

Allows operator to visuallyinspect all camera imagesto optimize settings,

Transmits positions,width, intensity to signal collector program,

Saves data,Saves/retrieves systemconfiguration


CHESS-East Position monitors

VBPMP.E

D-line diamondVBPMs in tunnelAnd cave

F1-F3-lineHe VBPMsin cave

E-line HeVBPM in tunnelAlso source size measurement


VBPMP.E

CHESS-West Position monitors

He


F-W_VBPMA-Ver_VBPMA-Hor_VBPM

00.0050.01

0.0150.02

0.0250.03

0.0350.04

0.0450.05

020000400006000080000100000120000

tiem (seconds)

Beam

position (mm

)

FW_VBPMFHB_VBPM

0

0.05

0.1

0.15

0.2

0.25

Beam

Current (A

)

Current(A)


CHESS-G-line Position monitors

VBPMP.E


Diamond VBPM image at G2 beam passed multilayers and focusing mirrors

Diamond Video BPM image at G1 shutter location (passed multilayers and focusing). Spill-over is seen.

CHESS-G-line Position monitors


G-line Beam position monitors

-0.5

0

0.5

1

1.5

2

20000300004000050000

time (seconds)

Postition (mm

)

G_PEG_VBPMG2_VBPM_Vert

G-line PE vs. VBPM

-0.2

-0.1

0

0.1

0.2

0.3

20000300004000050000

time (seconds)

Position (m

m)

G_PEG_VBPM

G-line position signals


Some VBPM pitfalls

•Offset effect on position•Noise and ground-loops•“Zingers”•Intensity saturation•Contamination, humidity

•Eliminate background light, adjust offset•Short video cables, filters and video amplifiers, ultimately use digital camera

•Image filtering i.e. median, shilding•Optimize optics, shutter time.

To minimize these effects:


Humidity effects He luminescence !


Conclusion and Summary

Video BPMs give multitude of important operational information about X-ray beam conditions.

The future:Application of intelligent cameras, where the frame processing is inside the camera with built-in FGPA and DSP. This will reduce noise, the network data traffic volume and make possible faster frame capture.

P. Revesz and J.A. White An X-ray beam position monitor based on the photoluminescence of helium gas Nuclear Instruments & Methods in Physics Research, Section A 540, n 2-3, 470-9 (2005)

Jeffrey A. White, Peter Revesz, Ken FinkelsteinE-line: A new crystal collimator beam line for source size measurements at CHESSNuclear Instruments and Methods in Physics Research A 582(2007) 66–69

K.D. Finkelstein, Ivan Bazarov, Jeffrey White, Peter Revesz Crystal Collimator Measurement of CESR particle-beam Source Size CP705, Synchrotron Radiation Instrumentation: Eighth International Conference edited by T. Warwick et al. American Institute of Physics proceedings, 597-600 (2004)

references

video-based x-ray beam position monitoring at chess€¦ · nsls-ii xbpm mini workshop, 2009...

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