Electron bunch form monitoring at quasi-cw SRF accelerators: Status at the ELBE accelerator

In the past few years the quasi-cw SRF electron accelerator ELBE has been upgraded so that it now allows to compress electron bunches to the sub-picosecond regime at repetition rates of up to 13 MHz and bunch charges up to 100 pC. The actual optimization and control of the electron bunch form represents one of the largest challenges of the coming years at ELBE. In particular with respect to the midterm goal to utilize the ultra-short electron bunches for Laser-Thomson scattering experiments or high field THz experiments. Since 2012, the ELBE accelerator, together with its recently established super-radiant THz facility TELBE serves as a unique test facility for the development of novel diagnostic for quasi-cw electron beams. In this contribution different diagnostic approaches for the online determination of the electron bunch form are presented. The results of the different techniques, ranging from adapted classical laser-based and interferometer-based concepts to novel on-chip THz spectrometers are compared. The merits of the different concepts: e.g. pulse to pulse diagnostics at few 100 kHz repetition rates or a sensitivity for charges down to the few 100 femto coulomb regime are discussed based on the results of recent commissioning campaigns by a multi-institutional collaboration. A careful analysis of simultaneous measurements with the different techniques allows an educated guess for the presently shortest electron bunch duration achieved at ELBE of 1.2 ps (FWHM). It is also shown that interferometric concepts at this moderately short bunch durations systematically underestimate the bunch length due to diffraction effects at low THz frequencies.