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First experiment towards „Steady State Micro Bunching“ successfully performed at the Metrology Light Source


Photo diode signal of the undulator radiation originating from different stored electron bunches as recorded by a fast oscilloscope. Those bunches which interacted with the laser pulse during the previous revolution create a much larger signal because of coherent emission enhancement due to the imposed micro structure.

In cooperation with the Helmholtz-Center, Berlin, and the Tsinghua University, Beijing, a so-called proof of principle experiment towards Steady State Micro Bunching (SSMB) was successfully performed at the Metrology Light Source (MLS), the electron storage ring of PTB in Berlin-Adlershof. It could be proven that approximately one µm long structures created within a stored electron bunch of a few mm in length by the interaction of a co-propagating pulsed laser in the undulator section of the MLS, can radiate coherently in the same undulator one revolution later. Coherent radiation is a very promising type of synchrotron radiation as the intensity increases with the square of the number of the radiating electrons, thus amplifying the intensity by many orders of magnitude in comparison to the usual incoherent undulator radiation which increases linearly with the number of radiating electrons.

Maintaining such small structures during their storage in an electron storage ring is a very demanding task, as the photon emission process in the magnetic guiding fields usually will destroy them very fast just after a few meters. Indeed this was the first time worldwide that the conservation of micro structures in electron bunches over a complete revolution in a storage ring could be confirmed and it is a first important step towards a new type of a very intense synchrotron light source based on the SSMB scheme, the efforts for which are coordinated by Tsinghua University.

For this purpose the MLS had to be operated in a special mode, the “low alpha mode”, where the revolution time is nearly independent on the particle energy. The MLS is the first and only storage ring worldwide especially designed to operate in this mode and the experiments for SSMB will be continued at the MLS.

The electron bunches in the MLS are separated 2 ns in time and the pulse width of the 1064 nm wavelength laser was approx. 10 ns. One revolution of an electron bunch around the MLS, which has a circumference of 48 m, takes 160 ns. The undulator was tuned to be in resonance with the laser wavelength for its first harmonics. Those bunches that interacted with the laser beam and thus developed the micro bunch structure with the size of the laser wavelength, irradiate partly coherently in the undulator after one revolution around the MLS and thus show a larger signal level than those without prior interaction with the laser pulse (see figure).


R. Klein, 7.22, E-Mail: Opens window for sending emailRoman.Klein(at)ptb.de