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A.Shapovalov ([email protected]) for the BIW2010, Santa Fee, New Mexico, USA
Emittance Measurement Wizard at PITZ.
Abstract
The Photo Injector Test Facility at DESY, Zeuthen site (PITZ) develops
electron sources of high brightness beams, required for linac based free
electron lasers (FELs) like FLASH or the European XFEL. One of the key
issues in electron beam optimization is the minimization of the transverse
emittance. The main method to measure emittance at PITZ is a single slit
scan technique, implying local beam divergence measurement by insertion
of the slit mask at a definite location within the beam and measurements of
the transmitted beamlet profile downstream of the slit station. “Emittance
Measurement Wizard” (EMWiz) is the program used by PITZ operators for
automated emittance measurements. EMWiz combines an acquisition
program for beam and beamlet image recording and a postprocessing tool
for the analysis of the measured transverse phase space of the electron
beam. It provides a way to execute the difficult emittance measurements in
an automatic mode and to get a calculated emittance result.
Photo Injector Test facility at DESY, Zeuthen site (PITZ)
Three EMSY stations are located in the current setup. The first EMSY station (EMSY1)
behind the exit of the booster cavity is used in the standard emittance measurement
procedure. It is at 5.74 m downstream the photocathode corresponding to the expected
minimum emittance location.
Emittance Measurement at PITZ
Transverse emittance measurement
using slit scan technique
For this technique, the local divergence is estimated
by transversely cutting the electron beam into thin slices.
Then, the size of the beamlets created by the slits is
measured at the YAG screen at some distance
downstream the EMSY station. The 10 m slit and a
distance between the slit mask and the beamlet
observation screen of 2.64 m are used in the standard
emittance measurement. In the emittance measurement
setup and procedure, intrinsic cuts have been minimized
by e.g. using highly sensitive screens, a 12 bit signal
resolution CCD-camera and a large area of interest to
cover the whole beam distribution. Therefore, emittance
value is called 100% rms emittance. The momenta of the
beam after the gun and the booster acceleration are
measured using spectrometers with screen stations in the
dispersive arm downstream the RF-gun and in the first
dispersive arm downstream the booster cavity.
The normalized rms emittance is calculated using the formula:
222
2xxxx
x
xn
EMSY1
(z = 5.74 mm)
beamlet
screen
2.64 m
Schematic representation of the single slit scan
technique.
Measurement results in summer 2009
0.3
0.5
0.7
0.9
1.1
1.3
377 379 381 383 385 387 389
main solenoid current, A
rms
be
am
siz
e,
mm
Xrms Yrms XYrms
a)
b) c)
Measured beam size (top) and normalized projected
emittance (bottom) vs. main solenoid current at EMSY1
station for a gun phase of +6 degree off-crest and booster
phase on-crest (opened symbols for fast scan and solid
symbols for detailed scan).
0.8
1.0
1.2
1.4
1.6
1.8
2.0
377 379 381 383 385 387 389
main solenoid current, A
emit
tan
ce, m
m m
rad
Ex Ey Exy
Ex Ey Exy
Transverse distribution (a), horizontal (b), and vertical (c)
phase space of the best emittance point at the solenoid
current ~383.5 A.
The Emittance Measurement SYstem (EMSY) consists of
horizontal and vertical actuators. Stepper motors are
provided to move separately each one of the four axes of
EMSY which give the precise spatial positioning and
orientation of the components. On each of the actuators,
either a YAG or OTR screen is mounted to observe the
beam distribution. A single and a multi slit masks are
mounted consecutive to take samples from the transverse
phase space of the electron beam. A CCD camera is used to
observe the screens.
ID: TUPSM057
is the Lorentz factor corresponding to the beam energy.
and are the second central moments of the electron beam distribution in the trace space obtained from
the slit scan.
is the angle of the single electron trajectory with respect to the whole beam trajectory.
is the rms beam size measured at the slit location.
is applied to correct for possible sensitivity limitation of low intensity beamlets.
2x
yx ppx /
x2/ xx
2x
The Emittance Measurement System
It is possible to observe video stream in
a separate window.
EMWiz has own plot system. It is much
faster than ROOT, but the development is
still ongoing.
The spectrum tool shows the number of pixels in the image
with a given intensity. Major part of image pixels should have
intensity between 50% and 70% of maximum intensity. Control of
saturation is one important criteria for obtaining good data quality
The most complex tool is “PreScan”. It is a
hands-off procedure for signal quality testing and
finding the scan limits of emittance
measurements.
The Scan&Grab control panel collects
features for measurement procedures
and actuator control.
The scan info panel gives possibility to set
EMSY device, change scan limits, observe
the actuator position, control the laser
shutter.
The video system at PITZ has 24 digital cameras with 8 and 12 effective bits per pixel and 7 video servers.
The video part of EMWiz manages the cameras by reading properties, changing settings and connecting to a
video server. The client makes it possible to get the video image, to apply filters, to grab and to save video
images. Calculation of important values is going simultaneously with the drawing process.
The process of data collection in EMWiz stores all machine
parameters continuously and informs an operator about critical
fluctuations of controlled parameters, which can influence the
measurement reliability. The list of the parameters can be
changed via an XML-config file. This GUI is based on a config
file and can be easily transformed with new demands.
At first, the equipment should be adjusted before
the measurements. Some necessary actions are
executed by others programs and there
measurement results such as the Lorentz factor are
put in corresponding fields of this option panel.
EMWiz has a black box feature. All
actions of the operator to the program and
machine, system status, error events and
alarms with time-stamp are stored to a log-
file. With help of an expert, one can open a
hidden window, to support the shift crew
remotely for fixing of a problem.
The shared memory watcher is an expert program. At present, the wizard consists of
separated programs, each of them for one logical task. Communication between
program parts is realized by a shared memory. This approach gives the possibility to use
EMWiz components on different user stations with starting on one host. This increases
graphics productivity and decreases the CPU loading. In the future it is planned to add
network communication, which will give possibility to use parts of the wizard on different
machines.
Rescan window. An operator can
scan some beamlets again.
EMWiz is a multithread, multiprocessor,
multiwindow application with user-friendly
interface, flexible design which based on XML-
config-file. An operator can save and load GUI
setting, e.g. window positions and sizes, values
in fields.
The main panel of EMWiz informs an operator
about system status, program flows, errors,
recommendations. EMWiz is done for 64 bits
Scientific Linux CERN version 5.0.
The Fast Scan panel receives settings from the
basic part of EMWiz. An operator can make “Fast
Scan” procedure with the measurement time of about
20 seconds for one solenoid setting compared to
about 30 minutes with old “Step Scan” procedure.
This panel allows to set scan parameters
manually or automatically, which receives the
settings from the basic part of EMWiz through
shared memory.
Last step in the measurement is to calculate
the emittance. The tool “calculator” exists for this
purpose. The operator sets a folder with saved
data, the tool checks this folder and shows only
good sets (MOI image, EMSY image, slit scan
image file, background file, data file with
parameters of system). A lot of processing of the
data is ongoing during calculation. Different
filters, formulas and etc. is applied to the video
data. A user can select to plot all intermediate
results after each complex calculation. At the
end of the calculation the emittance plot is
shown by default .
Panel “Results” shows information about the
selected path, folders, files for data recording.
For plotting, ROOT is integrated to EMWiz. It gives possibility
to use all features of mathematical apparatus and graphic
potential of CERN’s ROOT.
Main Panel & Video System Alarm & Measurement System
Fast Scan & Emittance CalculatorCommunicator & Plot System
A.Shapovalov ([email protected]) for the BIW2010, Santa Fee, New Mexico, USA
Emittance Measurement Wizard at PITZ.
ID: TUPSM057
