<html><head><title>Energy Loss in Undulators</title></head>
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<h5  style="color:gray;">5 October 2022, J. Chrin, M. Leutwyler, E. Prat, T. Schietinger</h5>
<h4>Electron energy loss in unduators</h4>
<h2 style="color:blue;">About</h2>
<p> Adapted from the Matlab application by E. Prat.</p> 

<h2>The Application</h2>

<h1>Overall objective</h1>

The primary objective is to integrate a measurement known as "E loss 
measurement" into our python based application framework. If there is time, 
a second measurement ("Undulator contributions") may be envisaged.

<h2>E-loss measurement</h2>

What is measured with "E loss"? When the electrons in the undulators radiate 
photons, they lose energy. The lost energy corresponds to the energy contained 
in the photon pulse. Therefore, we can estimate the photon pulse energy simply 
by comparing the energy of the electron bunch when it is lasing to the the 
energy when it is not lasing. This measurement typically serves as a 
cross-check when there is some doubt on the reading of the gas detector, which 
we normally use for determining the photon pulse energy.
<p>
The energy of the electron bunch is derived from two BPM (beam position 
monitor) readings in the beam dump (where the electrons are deflected 
verticallyand we have dispersion). This is our measurement, essentially two
scalars read out under two different conditions (lasing and no-lasing). 
<p>
The no-lasing condition is achieved by detuning the undulators: we simply 
change the K values of adjacent undulators by alternating amounts positive and 
negative to completely avoid the build up of the FEL process (still some 
incoherent radiation but you have that also when lasing and it won't contribute
to the FEL pulse).
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The measurement therefore consists of the following steps:
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1) Measure the electron beam energy while the electrons are lasing.
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2) Detune the undulators.
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3) Measure the electron beam energy while the electrons are not lasing.
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4) Restore the undulators.
<p>
To have an idea of the stability, and to avoid outliers, the measurement is 
typically repeated a few times (the current default is five) and the results 
are averaged. We can also compare the two results from the two BPMs to check
for systematic effects (broken BPM for instance).
<p>
A typical ELOG output of such a measurement is shown here: 
https://elog-gfa.psi.ch/SwissFEL+commissioning+data/35186
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Apart from the five measurements, also the BPM readings are shown, just to 
have an idea of the stability of the machine during the measurement.
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In this new application, we would like to have the number of measurements to be 
an adjustable (expert) parameter, and then I wouldn't show the measurements as 
a time line, but rather as a histogram with a mean and an errorbar on top.


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