Operation of a Single Pass, Bunch-by-bunch x-ray
Beam Size Monitor for the CESR Test Accelerator
Research Program

October 3, 2012

GoalsGoals For This Presentation:
1.Provide an overview of the efforts required to design, commission andoperate an x-ray beam size monitor in support of a scientific program
2.Highlight the instrument capabilities, limitations and characteristics

3.Provide examples of present and future measurements

October 3rd, 2012Nate Rider, CLASSE2

CESR & CHESSCornell Electron Storage Ring (CESR)•Dual beam (positron/electron) storage ring with a 768 m circumference•
Configurable beam energies from 1.8 GeV to 6 GeV•Beam currents up to 400 mA for counter-rotating beams•
Bunch spacings down to 4 nS in increments of 2 nSCornell High Energy Synchrotron Source (CHESS):•High intensity x-ray source•
7 instrumented x-ray beam lines with 11 user experimental stations•
The positron instrument is installed in D-Line and the electron instrument isinstalled in C-Line
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Overview

Storage Ring Elements

Detector Box• Detector• Filter/Slit Stage• ElectronicsX-RaysOptics Box• Pinhole• LE Optics Chip
• HE Optics Chip

The basic setup consists of the following:•X-ray source dipole magnet which is part of CESR•Continuous vacuum vessel which extends from the source to the detector•
Optics box contains moveable stages with several optics elements which can be inserted into the x-ray beam•
Detector box contains movable stages which hold filters and the beam size detector and associated amplifiers•Control and analysis software running remotely

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Vacuum OverviewX-rays• Continuous vacuum vessel from x-ray source to detector• Multi stage differential pumping with gate valve protection controlled via programmable logic controllers• Allows for non UHV compatible electronics to be installed in the detector box WITHOUT contaminating thestorage ring vacuum• Provides a “windowless” x-ray transmission path from source to detector

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Vacuum Pressures Along Beam Line

Detector

CESR

Vacuum Pressure Along Beam LineOctober 3rd, 2012Nate Rider, CLASSE

6Detector

Installed Detector•••••

Detector Diode Segments

The standard detector is a vertical array of 32 InGaAs diodes with a 50µm
pitch and horizontal width of 400µm.
The diode array is mounted to a printed circuit board and the diodeconnections made with wire bonds
The InGaAs layer is 3.5 µm thick, which absorbs 73% of photons at 2.5keV;there is a 160nm Si3N4 passivation layer.
The time response of the detector is sub-nanosecond
The detector is mounted on a rotatable stage, studies thus far have focused onvertical beam size measurements in the vertical orientation

Mounted Detector
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Optics Stages• For all beam energies an adjustable vertically limiting slit (pinhole) is available (typ height = 45 µm)• For less than 2.5 GeV, a low energy Fresnel zone plate and a coded aperture are available• At or above 4 GeV a high power coded aperture is available on a high energy chip
• These elements reside in the storage ring vacuum and can be selected and aligned remotely to meetthe requirements of various measurements

October 3rd, 2012Nate Rider, CLASSE8Optics Elements• There are three features, a box, a coded aperture and aFresnel zone plate

Low Energy Optics Chip Layout

Coded Aperture• Creates a diffraction pattern at the detector• 2.5 µm Si substrate with a 0.7 µm thick layer of electroplated gold
• 310µm x 1200µm, 8 transmitting elements (10µm to 40 µm)• Usable with non-monochromatic x-ray beams and small beam sizes• Useful for low photon count, low energy measurements• High power device features are half the scale• 625 µm substrate with a 10 µm thick layer of electroplated gold

• True focusing device• The Fresnel zone plate has been problematic due to low
photon counts when using the required monochromator• The Fresnel zone plate pattern has 120 transmitting rings in adiameter of 1200µm.
Fresnel Zone Plate
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Filters/Slits

Filter/Slit Stage• Stage with multiple elements which can be inserted into the x-ray beam between the optical elements
and the detector• Horizontally limiting tungsten slits (35 µm – 171 µm) used for high energy and or high current operation• Filters used to change the spectral content of the x-ray beam and thus control the detector response• 4 µm diamond filter• 2 µm molybdenum filter• 6 µm aluminum filter (soon)• Used to derive the x-ray spectrum used in analysis

October 3rd, 2012Nate Rider, CLASSE

10Data Acquisition• The xbsm data acquisition system is capable of collecting beam size measurements on a single pass,bunch by bunch, turn by turn basis• There is an independent data acquisition channel for each of the 32 detector diodes

Variable Gain Amplifier• Digitally controlled• -4 dB to 20 dB
Diode Segment

Transimpedance Amplifier

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Analog To Digital Converter• 12 bit• 300 MSPSFPGA• Local control• Register interface• Memory interface

Clock Delay• Digitally controlled• 10 pS increments

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SRAM Memory• 1 M sample storage

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Synchronization

CESR
MasterOscillator

24 MHzInstrumentation
Clock

Positron
Local
TimingCircuitry

ElectronLocal
TimingCircuitry

ADC Sampling
ClocksTurn Marker

ADC Sampling
ClocksTurn Marker

Local Timing Circuitry:• Receives 24 MHz instrumentation clock• Embedded turn marker• Hardware triggers for instrument synchronization (xBSM, BPM)
• Generates 32 programmable sampling clocks• 250 MHz, 125 MHz, etc (based on bunch spacing)• Configurable clock delay for alignment and sampling• Adjustable turn marker for synchronization

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12Peak Sampling

Sample Point
Bunch 1

Sample Point
Bunch 2

• We use the timing controls and bunch pattern gating to align the sampling points with the beam
• This alignment allows for bunch by bunch sampling of the amplitude peak of the diode response signal• All 32 channels are aligned and sampled synchronously resulting in the capture of a detector image
NOTE: This image is a composite made by shifting the sample point in 10 pS increments with some tolerance

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SoftwareMatlab Control GUI

xBSM Root Fitter Single Bunch Size Summary

Data acquisition and instrument control software written in Matlab•
Graphical user interface panels for timing, acquisition and instrument motor control•Provides basic analysis and display of a sample of the turn by turn measurements•Allows for the possibility of real time particle beam tuning•
Analysis of large data sets is slow

Offline analysis is done via locally developed XbsmRootFitter• C++ based, utilizing the CERN ROOT package to book, store, fit and display histograms• ROOT uses MINUIT for fitting and FFTW frequency analysis• Very flexible package which allows for a variety of plots• Routines for systematic compensations• Pedestal subtraction, channel calibrations, gain-range scaling, bunch to bunch crosstalk compensation
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Pinhole Analysis

Model and Function•••••

Detector Data With Fitting Function

The pinhole is a simple optical element with a single slit and an opaque masking material which produces adiffraction pattern at the detector
In order to generate a model for the image, we apply the derived x-ray spectrum to a numerical calculationof Fraunhofer diffraction
The outer diffraction features are smeared due to the energy spread.
This allows us to approximate the pinhole image as a sum of two gaussians ( broad and narrow) plus abackground
The resulting fitting function is used to fit the detector image data and calculate a beam size and image
offsetOctober 3rd, 2012Nate Rider, CLASSE

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Coded Aperture Analysis••••••

The coded aperture is a multiple slit optical device which
generates a diffraction pattern at the detector
The diffraction pattern is calculable using the derived x-ray
spectrum and the detailed geometry and materials of thecoded apertureThis pattern can be calculated for a range of beam profiles
and positions and used to generate a searchable matrix oftemplates (see J Flanagan, proc. DIPAC11)
Alternatively, we have parameterized the diffraction pattern
as a sum of 12 gaussians with no un-modeled background
This is convenient for analytical chi squared fitting
The resulting fitting function is used to fit the detector image
data and calculate a beam size and image offset

9 µm beam size

21 µm beam size

Model and Function

58 µm beam size

Detector Data With Fitting Function
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Pinhole Optimization••

The pinhole image is sensitive to average x-ray energy and the height of the opening
Modeled image response to variations in pinhole height

Height = 200 µm•••

Height = 45 µmHeight = 15 µm

CESRTA requires operation at several different beamenergies
The pinhole height is controllable remotely
The height is optimized for each operating beamenergySlit Height (Motor Position Units)

Image Size Vs Pinhole Height Scan
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PH and CA Comparison
Blue Line = 1:1

Controlled Beam Size Scan

••
•

•

Coded Aperture Size Vs PH Size

Cross check pinhole results versus coded aperture results for identical conditions
Artificially increase and decrease the beam size by introducing a closed dispersion bump in a wiggler
Plot coded aperture results versus pinhole results– Points represent turn averaged data
– Brackets represent the RMS spread of the single bunch single turn measurements
We also used a specially tuned beam to capture two very small beam size measurements and added
them to the plot to extend the range of comparison
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Basic Measurements

Vertical Tune

Beam Position At Detector
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Basic Measurements

Beam Size
October 3rd, 2012Nate Rider, CLASSE20Intra Beam Scattering

Preliminary

Preliminary

RunFFT Of Position Data

Beam Size Versus Current••••Intra Beam Scattering is a phenomenon of current dependent beam size seen in very small beams
A typical experiment consists of filling the storage ring with single bunch charged to 1011 particles and allowing
the bunch to decay down to 109 particles while recording the turn by turn change in vertical beam size
The xBSM is used to collect beam size versus current of individual bunches over many turns
Turn by turn measurements allow for frequency analysis of beam dynamics

Disclaimer: The physics phenomena which are evident on this slide are outside the scope of this presentation
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Beam Dynamics•••

This program studies electron cloud induced multi-bunch phenomenon
A typical experiment involves loading the storage ring with a multi bunch train (20, 30 or 45 bunches) with aspacing of 4 nS to 28 nS (4 nS increments) between bunches and bunch currents of approximately 1 mA
The beam size for each bunch is measured on a turn by turn basis for up to 4096 turnsPreliminary
This Is Real

Positron Bunch Size (Pinhole Optic) Along 30 Bunch Train

Disclaimer: The physics phenomena which are evident on this slide are outside the scope of this presentation
October 3rd, 2012Nate Rider, CLASSE

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Future Measurements

1x12 Telecom Diode Array

Bunch Slicing:• Acquire a size profile for a single bunch on a single turn• A measurement requires multiple longitudinal slices of a single bunch• Testing high speed diodes (tr/tf ~35 pS) with parallel sampling
Multiple Longitudinal Slices Per Turn

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SummarySummary:•The xbsm is routinely used in the CESRTA program for precision measurement of beams with a vertical size
as small as 9 µm•

There is an ongoing effort to understand systematic effects and to optimize the analysis of images•

Now that we have achieved basic functionality of the instrument, we need to undertake a more detailed
characterization of the operational characteristics (resolution, accuracy, repeatability, etc)

•The device has proven to be essential to the CESRTA low emittance tuning program and to understandintrabeam scattering and the emittance dilution effect of the electron cloud

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xBSM TeamNameOrganization

Responsibility

Mike Billing

LEPP

Experimenter

Chris Conolly

CHESS

Operations

Eric Edwards

CHESS

Vacuum Controls

Michael Ehrlichman

LEPP

Experimenter

John Flanagan

KEK

Analysis

Brian Heltsley

LEPP

AnalysisAaron Lyndaker

CHESS

HardwareMark Palmer

LEPP/FNAL

Project Manager

Dan Peterson

LEPP

Analysis

Nate Rider

LEPP

Project Coordinator

Dave Rubin

LEPP

Principal Investigator

Robert Seeley

CHESS

VacuumJames Shanks

LEPP

Experimenter

Kiran SonnadLEPP

Experimenter

Chris Whiting

CHESS

VacuumPrimary Team Members
LEPP = Laboratory For Elementary Particle Physics
CHESS = Cornell High Energy Synchrotron Source
KEK = High Energy Accelerator Research Organization
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EndThank You

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Appendix

Additional Slides

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Abstract

Abstract:
The CESR Test Accelerator (CesrTA) program targets the study of beam physics issues relevant tolinear collider damping rings and other low emittance storage rings. This endeavor requires new
instrumentation to study the beam dynamics along trains of ultra low emittance bunches. A key elementof the program has been the design, commissioning and operation of an x-ray beam size monitor
capable, on a turn by turn basis, of collecting single pass measurements of each individual bunch in a
train over many thousands of turns. This new instrument utilizes custom, high bandwidth amplifiers anddigitization hardware and firmware to collect signals from a linear InGaAs diode array. The instrument
has been optimized to allow measurements with 3x109 to 1x1011 particles per bunch. This paper reportson the operational capabilities of this instrument, improvements for its performance, and the methods
utilized in data analysis. Examples of key measurements which illustrate the instrument's performanceare presented. This device demonstrates measurement capabilities applicable to future high energy
physics accelerators and light sources.

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Coded Aperture Analysis

The calculation of the diffraction results in the blue points. (Calculated values have beensmoothed with an RMS of 17.68 microns at the image or 7.0 microns at the source.) The
calculated image is fit to the sum of 12 Gaussians, shown in various colors in the figure. The
resulting sum of Gaussians (also smoothed) shown in the red curve closely matches the calculateddiffraction pattern.
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Coded Aperture Analysis

The model provides a function describing the image for zero beam size, shown superimposedon an image. To fit the image, this function is convoluted with a single Gaussian representing
the beam size, magnified on the detector.
Examples are shown for beam sizes of 9.85 and 13.77 microns.
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Derived x-ray spectrum

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Spectrum Development

Table values represent pinhole image relative integrated area for various filter and particle beam energy conditions(constant pinhole slit width)(particle beam current normalized)
Jackson formula, with applied transmission: (Ex-ray)2.31
data in black, {model} in red
1.800 GeV
2.085 GeV
2.300 GeV

no filter
0.304 { 0.284 }1{ 1}--{ 2.356 }4 μm diamond filter
0.079 { 0.076 }0.520 { 0.521 }*1.917 { 1.595 }2 μm molybdenum filter----- { 0.0081}
0.050 { 0.066 }0.299 { 0.273 }* this is a tuned value
We find we can fit this data, reasonably well for the conditions we have examined, with the Jackson formula modulated by anempirical description of the unknown other filters and detector response.
We will collect more complete data in the future (missing points, aluminum filter).
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Spectrum Development

pinhole image relative integrated area(constant pinhole slit width)(particle beam current normalized)
average x-ray energy in blue{model} in red Jackson formula, with applied transmission: (Ex-ray)2.31
data in black

no filter

4 μm diamond filter

6 μm aluminum filter

2 μm molybdenum filter

1.800 GeV

1.96 keV
0.304 { 0.284 }2.42 keV
0.079 { 0.076 }2.23 keV--- { 0.036 }2.70 keV--{ 0.0081}

2.085 GeV

3.09 keV1{ 13.50 keV
0.520 { 0.521 }*3.86 keV--- { 0.210 }

4.47 keV
0.050 { 0.066 }2.300 GeV

3.95 keV--{ 2.356 }4.23 keV
1.917 { 1.595 }

4.69 keV--- { 0.736 }

5.27 keV
0.299 { 0.273 }particle energy}* this is a tuned value
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Data Acquisition

Data Acquisition Functional Diagram

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Data Acquisition

Local Timing Circuitry

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