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Longitudinal Beam Profile Measurements

One possibility to analyze in detail the longitudinal behaviour of the electron bunches in the CTF3 is to use a high resolution streak camera that gives access to a time resolution down to some picoseconds. The photons from the radiation to be analyzed are first converted to electrons, which are accelerated and deflected using a time-synchronized, ramped HV electric field. The signal from the electrons is then amplified with a micro channel plate (MCP), then converted to photons via a phosphor screen and finally detected using an imager like a CCD array, which converts the light into a voltage.

The aim of these measurement is to study in detail the longitudinal structure of the electron beam and to get information about the different components like to rf deflector or the wiggler. Due to the high radiation level in proximity to the beam and the sensitivity of the streak camera, adequate measures have to be taken in order to protect the camera. A measurement close to the accelerator clearly is not feasible. An overview of the present installations in CTF3 is shown in the following picture below.

The design steps can be summarized as follows:

  • Transmit the light over large distances using telescopic arrangements, i.e. two identical lenses placed apart two times their focal length.
  • Optimize the overall system between collecting, transmitting and demagnifying optics.
  • Minimize the number of optical elements in order to maximize light transmission and reduce aberrations. Each lens will absorb about ~10% of the incident light.
  • Optimize the optical resolution. Even though (transverse) aberrations are not the most critical point in streak camera measurements, one would still like to have a final image where these are minimized.

In order to be able to optimize the overall system, in-detail knowledge of the emitted radiation, characteristic parameters of the different lenses and the final image acquisition system is important.

One example of a measured profile is shown below. Besides the inter-bunch separation, this measurement allows to estimate the fraction of particles in the satelite bunches.


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Page last modified on October 04, 2006, at 08:59 AM EST