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Compact accelerators soon to become reality

Beam diagnostics method for accurate, non-invasive length measurement of very small electron bunches

PTBnews 1.2022
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electron beam diagnostics

particle accelerators

Laser excitation in plasmas may enable compact particle accelerators to be realized in the near future. Such accelerators are an alternative to current accelerator facilities that often extend over kilometers. A diagnostic procedure has therefore been developed at the Metrology Light Source. It can be applied to the geometric beam parameters of the electron bunches needed for this purpose.

Simulation of the electron bunches along the horizontal beam axis in a laser-generated plasma wakefield. The electron bunches having been accelerated to reach a high energy are electrically enclosed in a plasma bubble. The electron density ranges from low (blue) to high (magenta). (Credit: J. Ludwig, cc 4.0 Wikimedia)

Particle accelerators have become ever larger over the decades. With circumferences of several kilometers, ring accelerators have now reached their practical limit. Linear accelerators require considerable construction lengths too. And yet, there has been an alternative for several years: compact particle accelerators based on laser wakefield excitation in plasmas. Their design could be a lot more compact than that of other linear accelerators, and compact particle accelerators might be able to supplement linear accelerators both in industry and medicine. However, to be able to utilize the synchrotron radiation generated by this technology, the electron bunches thus formed must have a very well-defined and well-known shape.

At PTB’s Metrology Light Source, uniquely flexible setting possibilities are available for the stored electron beam. These settings were used within the scope of a project managed by the Helmholtz- Zentrum Berlin (HZB) to generate particularly small electron bunches that are very similar to those generated by laser wakefield accelerators. By measuring the synchrotron radiation generated by the electrons, it was possible to determine the lateral expansion of individual electron bunches with a resolution of a few micrometers.

This is done by exploiting the fact that the electron bunches generated have a length that is comparable to the wavelength of infrared radiation, which, in this spectral range, leads to coherence effects when the radiation is emitted. The coherent synchrotron radiation generates an interferometric pattern at a double slit. This pattern is detected by a highly sensitive single-photon camera. The pattern is evaluated by means of a special algorithm that reconstructs the lateral expansion of the radiation source – i.e., of the electron bunches themselves.

The results have shown that this procedure has the resolution and sensitivity required to serve as a diagnostic tool for electron bunches generated in the wakefield. In contrast to the procedures realized to date for measuring bunch geometry, this method is non-invasive, i.e., it has no influence on the electron beam, so that continuous measurement in operation is possible. This property is of paramount importance to specifically develop the wakefield technique further.


Arne Hoehl
Department 7.1
Radiometry with Synchrotron Radiation
Phone: +49 30 3481-7181
Opens local program for sending emailarne.hoehl(at)ptb.de

Scientific publication

J.-G. Hwang, K. Albrecht, A. Hoehl, B. A. Esuain, T. Kamps: Monitoring the size of low-intensity beams at plasma-wakefield accelerators using high-resolution interferometry. Communications Physics 4, 214 (2021)