Abstract
Coherent synchrotron radiation from an electron storage ring is observed in the THz spectral range when the bunch length is shortened down to the sub-mm-range. With increasing stored current, the bunch becomes longitudinally unstable and modulates the THz emission in the time domain. These micro-instabilities are investigated at the electron storage ring BESSY II by means of cross-correlation of the THz fields from successive bunches. The investigations allow deriving the longitudinal length scale of the micro bunch fluctuations and show that it grows faster than the current-dependent bunch length. Our findings will help to set the limits for the possible time resolution for pump-probe experiments achieved with coherent THz synchrotron radiation from a storage ring.
Highlights
Coherent synchrotron radiation (CSR) emission from a relativistic electron bunch in a storage ring is a natural process of collective in-phase emission of all electrons and occurs for wavelengths λ longer than the electron bunch length [1]
Coherent synchrotron radiation from an electron storage ring is observed in the THz spectral range when the bunch length is shortened down to the sub-mm-range
This paper reports on cross-correlation measurements from subsequent bunches emitting THz pulses at the storage ring BESSY II
Summary
Coherent synchrotron radiation (CSR) emission from a relativistic electron bunch in a storage ring is a natural process of collective in-phase emission of all electrons and occurs for wavelengths λ longer than the electron bunch length [1]. Short bunch lengths emitting coherently in the THz spectral range are obtained by reducing the momentum compaction factor from the value normally applied by tuning the storage ring optics [4] into the so-called “low α” mode. The electron distribution becomes unstable above a certain stored charge threshold and initial small density perturbations can cause micro-bunching due to the intense wake field emitting stochastically THz bursts [6,7] All these longitudinal density fluctuations in a scale shorter than the nominal bunch length are associated with a coherent increase of emitted THz power at higher photon energies than expected for the nominal Gaussian bunch shape. Matter 2020, 5, 24 phase space quantitative information on the dynamics of bunch instabilities, which are derived and compared to experimental findings
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