Synchrotron Radiation at BESSY II
Regular Synchrotron Radiation
Synchrotron radiation is electromagnetic radiation being emitted from deflected charged particles moving at relativistic velocities. As a consequence of relativistic motion, the radiation is being emitted in a narrow cone in forward direction from a point like source, the electron beam. At BESSY II the electron’s energy is 1.7 GeV and synchrotron radiation for users at ~50 monochromators is generated from magnetic deflections in simple electromagnetic bending magnets, from undulators consisting of long magnetic rows of permanent magnets as well as from superconducting magnet structures like wavelength shifters and multipole wigglers.
Coherent Synchrotron Radiation
Synchrotron radiation from undulators is at least partially transversely coherent. However, temporal coherence like in laser pulses is only available if the electron bunch is shorter than the wavelength of the radiation. After long year efforts of accelerator experts, this condition is fulfilled at BESSY II for long wavelength (~ 0.3-2mm) in a so-called low-alpha operation mode of the storage ring, where electrons are compressed within shorter bunches of only 1-2 ps duration. The far infrared wave trains emitted in magnets from individual electrons may then coherently overlap. Hence, temporarily coherent, Fourier-limited pulses in the sub-THz range up to mW average power are available at the IR and THz – beamlines.
Short pulse generation
The statistical processes caused by the emission of synchrotron radiation itself limit the available equilibrium electron bunch length and hence, also the minimum photon pulse length available from a storage ring. Typically, the latter is ~70 ps (FWHM) at BESSY II corresponding to ~30 ps (rms) electron bunch length in regular user operation. There are two workarounds to generate shorter X-ray pulses from a storage ring: (i) in the low alpha mode all beamlines receive ps X-ray pulses as each bunch emits temporarily incoherent but short pulses of ps duration and (ii) using the so-called slicing principle, a fs part of the electron bunch is used to generate fs pulses of undulator radiation.