Proceedings of IBIC2012, Tsukuba, Japan

MOPB80HIGH DYNAMIC RANGE BEAM IMAGING WITH TWOSIMULTANEOUSLY SAMPLING CCDS

Abstract
Transverse beam profile measurement with sufficiently
high dynamic range (HDR) is a key diagnostic to measure
the beam halo, understand its sources and evolution. In
this contribution we describe our initial experience with
the HDR imaging of the electron beam at the JLab FEL.
Contrary to HDR measurements made with wire scanners
in counting mode, which provide only two or three 1D
projections of transverse beam distribution, imaging
allows to measure the distribution itself. That is especially
important for non-equilibrium beams in the LINACs. The
measurements were made by means of simultaneous
imaging with two CCD sensors with different exposure
time. Two images are combined then numerically in to
one HDR image. The system works as an online tool
providing HDR images at 4 Hz. An optically polished
YAG:Ce crystal with thickness of 100 µm was used forthe measurements. When tested with a laser beam, images
with a dynamic range (DR) of about 105 were obtained.
With the electron beam the DR was somewhat smaller
due to the limitations in the time structure of the tune-up
beam macro pulse.

MOTIVATIONHigh current CW SRF LINACs with average current ofseveral mA have been used to provide electron beam forhigh average brightness, high power IR FELs [1]. It is
proposed that LINACs with similar average current andbeam energy in the range 0.6 – 1.2 GeV can be used as
the drivers for next generation of high average brightness
light sources operated in X-ray wavelength range inseeded FEL configuration [2-5]. The existing pulsed
FELs, operating now in the soft and hard X-ray
wavelength ranges, utilize average currents many ordersof magnitude less than the above-mentioned mA. At thesame time, operation of the IR/UV-Upgrade at Jefferson
Lab with average current of up to 9 mA has provided anexperience base with high-current LINAC operation [1].
The primary operational difference between such highcurrent LINACs and storage rings, even with a few
hundred mA of average current, is that LINAC beams
have neither the time nor the mechanism to come toequilibrium, in contrast to storage ring beams, which areessentially Gaussian. This has significant operational
impact. When a LINAC is setup, by establishing thelongitudinal and transverse match, a tune-up beam withsmall average current is used. Such an accelerator setup isbased most frequently on measured mean and RMS
parameters such as beam size, bunch length, and energy
spread. When going from tune-up mode to higher duty____________________________________________*Work supported by US DOE office of Basic Energy Sciences underthe early career program; DOE award number FWP#JLAB-BES11-05# Pavel.Evtushenko@jlab.org

cycle and CW operation, it is frequently found that the“best” RMS-data-based setup must be changed to allow
for high current operation to eliminate beam losses. Even
when this modification is successful, it is time-consuming
process involving some trial and error. It is frequently
unclear what the sources of the problem are, and which
adjustments to the low-density parts of the phase space
distribution were effective in improving performance.
This is highly undesirable for any user facility where highavailability is required. Also of significance is that theresulting setup does not necessarily provide the best beam
brightness and is a compromise between acceptablebrightness and acceptably low beam losses.
Contributing to this problem is the fact that themeasurements used for machine setup are typically basedon methods with a DR of 103 or even less. It is not
surprising that the relevant (from the high current
operation and beam loss point of view) low-intensity andlarge-amplitude parts of the phase space are simply not
visible during machine tuning.
Therefore, we think that the proper solution to theaforementioned tune-up problem is to base the tuning on
the measurements with much larger, than routinely used
now, DR, such that the very low intensity and large
amplitude parts of phase space distribution are taken in toaccount from the very beginning. We are presently
developing such diagnostics at the JLab FEL. The
ultimate goal is to be able to measure both the transverseand longitudinal phase space with a DR of about 106.
Measurements of both phase spaces can be based on the
HDR transverse beam profile measurements, which is thefirst step in our program. One of the techniques we aredeveloping is the HDR beam imaging. Here we present
our technique and first results of transverse beam profile
measurements with extended DR and its application toemittance and Twiss parameters measurements.

EXPERIMENTAL SETUP
Operation of the JLab FEL relies very heavily on thetransverse beam profile measurements made in many
places around the machine. Even with a relatively
compact footprint the IR and UV recirculators beamlines
have 62 viewers and synchrotron light monitors in total.
This has allowed us to accumulate a lot of experimental
experience with transverse beam profile measurements.
From this experience we know the intensity of the beam
image on the CCD matrix from OTR or YAG:Ce viewer
with the typical beam size and with the amount of beam
charge in the tune-up macro pulse. It also agrees well withcalculations. Thus one can tell that, for the measurementswith the OTR and DR of 106 an additional gain in therange between 10 and 100 would be needed, and themeasurements with YAG:Ce may not need additional

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Copyright ○P. Evtushenko#, D. Douglas, JLab, Newport News, USA

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MOPB80

Proceedings of IBIC2012, Tsukuba, Japan

gain. One of our goals is to develop a diagnostic thatwould be affordable, hence could be used in many
locations at an accelerator, and would not be expensive torepair. For this reason we decided to use non-scientific
CCD cameras that are produced in large volumes. Amongsuch cameras we select ones with best signal to noise ratio(SNR), which is typically 57 dB. To make themeasurements with the DR of about 120 dB (~106), atleast two sensors are required. The original idea was tohave two cameras and an optical setup with the beam
splitter where the cameras would be aligned with anaccuracy better than one pixel and would have the same
magnification. The optical path of one of the sensors
would have an image intensifier to provide the additional
gain. The two cameras would be operated with different
gain or/and different exposure time. One of the sensors
with smaller gain would measure the brightest fraction of
the distribution, while the camera with higher gain or/and
exposure time would beam measuring less intense fractionof the distribution. Some fraction of the second CCD
sensor will be saturated. One concern here is that thesaturation might lead to the charge bleeding to theneighboring pixels and therefore affect linearity. We
check for this in the algorithm that combines images fromtwo sensors in to one HDR image. In practice, we did not
find any evidence that this an effect that, we had tocorrect for. As we were preparing for the experiment acamera with beam splitter and two CCD sensors had
become commercially available [6]. We decided to use
the camera for the first measurements. On one hand thissimplifies the optical setup, since the alignment of the
CCD sensors relative to each other is made, on the other
hand it has the disadvantage that one does not have theadditional gain from image intensifiers. However, since it
was planned to use a YAG:Ce viewer, it was not expectedthat the absence of the additional gain would be a
problem.
The camera has independent electronic gain control forboth sensors. However, its use does not improve thesensitivity of a CCD sensor, i.e., it does not make low
intensity light undetectable with zero gain detectable withhigher gain. A note should be made that, usually, higher
electronic gain in the camera worsens the SNR. This isvery undesirable especially for the HDR measurements.
The electronic gain of the camera is used to cross calibrate
the two sensors - to balance them. For this purpose theintegration time of the sensors is set equal and the gain of
one of the sensors is adjusted to minimize the differenceof intensities of the two sensors. The gain can be negative
as well as positive. We choose to adjust the gain of thesensors that needs to be adjusted negative to prevent SNR
degradation.
For the measurements, results of which are presented
here, we used 100 µm thin YAG:Ce crystal scintillator
optically polished on both sides. The crystal was inserted
in to the electron beam at normal incidence. Behind the
YAG:Ce crystal a stainless mirror was mounted at 45
degrees relative to the beam direction. Such, thescintillation photons were directed out of the beamline

Figure 1: HDR algorithm example.
through a viewport at 90 degrees. Orienting thescintillator normal to the beam direction (not 45 degree assometimes done) serves two purposes. First, it places all
the crystal in to the focal plane of the lens preventing any
problems due to the finite depth of field of the imaging
optics. Second, it mitigates the effect of the finite crystal
thickness on the transverse resolution. Since a scintillator
is transparent at the wavelength it emits, an infinitely
small beam passing though a 100 µm thin scintillator set
at 45 degrees to its direction, will appear to the observer
as infinitely thin in one direction and as a 100 2 µm longline in the orthogonal direction. That was an important
aspect for the viewer we used, as it was built formeasurements of beams with the transverse size down to,at least, 50 µm or smaller.
A 300 mm telephoto lens was used to image the crystal
on to the CCD matrix. The magnification is such that one
pixel of the matrix corresponds to about 11.7 µm on theviewer. The vertical field of view is about 8 mm and the
aspect ration of the field of view is 4:3.
Tune-up beam mode is used at the JLab FEL for
measurements with intercepting beam diagnostics. The
time structure of the beam is the following. The photo
cathode drive laser maximum repetition rate of 74.85
MHz is reduced by the factor of 2! with the help of fastelectro-optical cell. For the tune-up beam n is 4 or higher.
From the “infinite” pulse train of ~ 4.67 MHz 250 µs longmacro pulses are allowed through a Pockels cell. The
macro pulses follow at 2 Hz repetition rate. To improveaverage extinction ratio two mechanical shutters are used
to open a time window slightly longer than 250 µs. The
repetition rate of the micro pulses can be reduced further
from 4.67 MHz to ½, ¼ or ⅛ of this and the macro pulse
can be made correspondingly longer. When an image
intensifier is not used, the increase in the DR comesmainly from the integration time difference of two
sensors. The largest increase is possible with the largest
ratio of the integration times. This is limited by theminimum integration time of the camera (~20 µs) and by
longest allowable macro pulse length. At the time of themeasurements the macro pulse length was always limited
to 400 µs. This limited the DR increase to about 20. To

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Proceedings of IBIC2012, Tsukuba, Japan

MOPB80further improve the DR we average the data over 16
images. It takes 8 seconds to get a new image, which
seems to be acceptable for the improved DR.

COMBINING HDR ALGORITHM

BEAM SIZE AND LEVEL OF INTEREST
As mentioned earlier the LINAC beams in general do
not have Gaussian distribution. It is observed with the
JLab FEL beam regularly, and complicates the task oftransverse beam size measurements. Even more, itsuggests that the question of “what is the proper andrelevant measure of the size?” might need to be revised.
Here we use RMS width of the beam distribution
projections to the X and Y axis. To calculate the RMS
width one needs to decide on the fraction of the image tobe used for the projection, usually referred to as region ofinterest (ROI) and on the fraction of the 1D projectionused for the RMS width calculations, i.e., 1D-ROI. This iswhere the availability of HDR data makes a significant
difference, as shown below. We use another numericalalgorithm to remove the ambiguity from setting both the
2D and 1D-ROI. In the first step, the entire image is
projected to either axis and the two projections are sent
through a low pass filter to remove high frequency noise.
Then the maximum of a projection is found. It is assumed,that the beam distribution is far enough from the edges ofthe field of view, so that the edges can be used to

Figure 2: HDR beam profile.

Figure 3: RMS beam size vs. quad current and LOI.
determine level of the background. That is made on bothsides of the projection maximum. The range between the
maximum and the background level is defined as 1. Thenthe main adjustable parameter of the algorithm isintroduced; let’s call it the level of interest (LOI), which
can assume values between 0 and 1. Typically, as we try
to include as much of the beam as possible in the
projection, the level will be set to a few times 0.01, where0.01 means 1 % from the projection maximum. The LOI
and the filtered and normalized projection are used todetermine two transverse coordinates at which the
projection intensity equals the LOI. A numericalinterpolation is used for this step. Important here is thatthe DR of the data, or its overall SNR, limits how low the
LOI can be requested. For instance, in our standard
measurements with the DR of about 500, the smallest LOI
that typically can be used is 0.01 or higher. This depends
critically on the noise performance of the camera. With
the DR extended as described above, we found that we
can robustly set the LOI as low as 5×10-4. Consequencesof this for the beam measurements are presented in thenext section. An example of the image built by thealgorithm is shown in Fig. 2. Note the intensity scale inFig.2 is logarithmic.

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3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○An imperative element in such measurements is thealgorithm used to combine two images in to one withlarger DR. The algorithm that we use is illustrated in theFig. 1 and works in the following way. It operates on therows of an image and therefore is essentially 1D. As thefirst step, the intensity of each image is normalized to theintegration time, such that the images can be compared.
Then the algorithm looks for the regions of saturation in
the data of the sensor with longer integration time. If nosaturation is found, only the normalized data from thelonger integrating sensor are used. When saturation
regions are present the corresponding row of the
combined HDR image contains three kinds of regions.
First is the region where longer integrating sensor issaturated, for this part only the data from the shorter
integrating sensor are used. Second, the most important
kind, are regions where the data from both detectors
overlap. It is a requirement to setup the combination of
the integration times so that there are such regions and thedata overlap over substantial span of the amplitude. In ourcase we have overlapped the data by 0.5 – 1 orders ofmagnitude. The importance of this data is that it checks
the cross calibration of the two sensors and ensurescontinuity and linearity of the HDR image intensity. For
this region of the HDR image we use average of the datafrom two sensors. The third kind of data are from thelonger integrating sensor that are less intense than theones in the overlapping region. Here the data from theshorter integrating sensor have SNR of about 1 and are
not used.

MOPB80

Proceedings of IBIC2012, Tsukuba, Japanand Twiss parameters measurements. To demonstrate howmuch the LOI affects the RMS beam size measurements
and what impact this has on the emittance and Twiss
parameters values, we consider a quadrupole scan thatconsists of 20 HDR images. Note that the HDR and LOI
are directly related. The DR enables lower values of the
LOI. Selecting different LOI we extract from every image
not one value of the RMS beam size but an array, where
beam size is a function of the LOI. In the example here
we scan the LOI in the range from 0.001 through 0.03 insteps of 0.001. The results of such data evaluation are
given in Fig. 3. Using the standard approach, assuming
linear beam optics approximation, with the help of
nonlinear least square fit, we obtain transverse emittanceand Twiss parameters value. Results of such data
evaluation are shown in Fig. 4.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○DISCUSSION AND OUTLOOKAs can be seen from Figs. 3 and 4, including the low
intensity, large amplitude fraction of the beam in to themeasurements of beam size, emittance and Twiss
parameters makes a dramatic difference. This brings usback to the note made in the first section of this paper. As
the emittance and Twiss parameters of the beam including
halo are different from these on the beam without halo, it
requires a somewhat different match, compared to the best
peak brightness, to allow for high current operation. It is
critical that the difference in the match is not too big.
Then, when the match is adjusted for high current
operation, the peak brightness and best match to the FEL
undulator and the FEL performance are not alteredsignificantly. On the other hand, it is possible that theapplication of the HDR measurements to establishing thematch will help to fulfill these requirements.
We are planning to improve the DR further by adding
image intensifiers to the setup and by allowing more
flexibility in the tune-up beam time structure. We are
currently building a set of diagnostics stations to beinstalled around the machine to enable suchmeasurements in several places in the lattice.

REFERENCES

Figure 4: Emittance and Twiss parameters vs. LOI.

EMITTANCE AND TWISS PARAMETERS
A single HDR measurement of the transverse beamprofile can be revealing and interesting by itself, since at
the corresponding level the beam halo can be seen
directly. The next level of details and complexity would
be the application of such measurements to the emittance[1] S. V. Benson et al., “Operational aspects of highpower energy recovery LINACs”, Proceedings
LINAC06, Knoxville, TN, USA, August 21-25, 2006
[2] D. Kraemer et al., “The BESSY Soft X-ray Free
Electron Laser”, TDR, ISBN 3-9809534-0-8,
BESSY, Berlin[3] R. A. Bosch et al., “WiFEL: The Wisconsin Free
Electron Laser”, Proceedings FEL2009, Liverpool,
UK, August 23-28, 2009[4] S. V. Benson et al., “A proposed VUV oscillatorbased FEL upgrade at Jefferson Lab”, Journal ofModern Optics 58 1438 (2011).
[5] J. N. Corlett et al., “Next generation light source
R&D and design studies at LBNL”, Proceedings of
IPAC12, New Orleans, LA, USA, May 20-25, 2012[6] http://www.jai.com/en/products/ad-081ge

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