##### 5 October 2021, J. Chrin, M. Leutwyler, E. Prat, T. Schietinger

#### Electron energy loss in unduators

## About

Adapted from the Matlab application by E. Prat.

## The Application

# Overall objective

The primary objective of the application is to measure the electon energy loss when traversing the undulators.

## E-loss measurement

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.

The energy of the electron bunch is derived from two BPM (beam position monitor) readings in the beam dump (where the electrons are deflected vertically and we have dispersion). This is our measurement, essentially two scalars read out under two different conditions (lasing and no-lasing).

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).

The measurement consists of the following steps:

- 1) Measure the electron beam energy while the electrons are lasing.
- 2) Detune the undulators.
- 3) Measure the electron beam energy while the electrons are not lasing.
- 4) Restore the undulators.

  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).

  In this application, the number of measurements is an adjustable parameter.

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