Systematic studies of the microbunching instability at very low bunch charges

At KARA, the Karlsruhe Research Accelerator of the Karlsruhe Institute of Technology synchrotron, the so-called short-bunch operation mode allows the reduction of the bunch length down to a few picoseconds. The microbunching instability resulting from the high degree of longitudinal compression leads to fluctuations in the emitted terahertz radiation, referred to as bursting. For extremely compressed bunches at KARA, bursting occurs not only in one but in two different bunch-current ranges that are separated by a stable region. This work presents measurements of the thresholds and bursting frequencies in both regimes. Good agreement is found between the data and numerical solutions of the Vlasov-Fokker-Planck equation