The form factor, representing the statistical characteristics of a bunch's longitudinal distribution, is one of the most essential properties of a pre-bunched electron beam and is used for many types of frontier accelerator applications. We demonstrated the measurement of a pre-bunched beam's longitudinal form factor component based on coherent radiation from a widely tunable-gap undulator. The radiation energy from bunches with different longitudinal properties was measured as a function of undulator gap. The root-mean-square length of a 60 pC ultrashort quasi-Gaussian bunch generated by linac and chicane compression ranged from 75 fs to 240 fs, as obtained by fitting the radiation energy curve. Furthermore, the form factor component of the bunch train based on nonlinear longitudinal space charge oscillation was measured, and a higher-order harmonic component was observed with the proposed method than with the widely used coherent transition radiation method. The proposed method may satisfy the requirements of sub-fs bunch length measurement with proper undulator design.

In recent experiments at Tsinghua University Accelerator Laboratory, the 31 MeV electron beam, which has been compressed to subpicosecond pulse durations, has been used to generate high peak power, narrow band Terahertz (THz) radiation by transit through different slow wave structures, specifically quartz capillaries metallized on the outside. Despite the high peak powers that have been produced, the THz pulse energy is negligible compared to the energy of the electron beam. Therefore, the THz generation process can be complementary to other beamline applications like plasma wakefield acceleration studies and Compton x-ray free electron lasers. This approach can be used at x-ray free electron laser beamlines, where THz radiation can be generated without disturbing the x-ray generation process. In the experiment reported here, a high peak current electron beam generated strong narrow band (∼1% bandwidth) THz signals in the form of a mixture of TM01 and TM02 modes. Each slow wave structure is completed with a mode converter at the end of the structure that allows for efficient (>90%) power extraction into free space. In the experiment, both modes in these two dielectric-loaded waveguides TM01 (0.3 THz/0.5 THz) and TM02 (0.9 THz/1.3 THz) were explicitly measured with an interferometer. The THz pulse energy was measured with a calibrated Golay cell at a few μJ.

High peak power, narrow band THz radiation can be generated by an ultrarelativistic electron beam passing through slow wave structure. In this paper we will review a series of recent experiments at Tsinghua University Accelerator Laboratory where a 31 MeV electron beam compressed to a sub-picosecond duration had been transported through dielectric loaded waveguide structures of various dimensions. Such high peak current electron beam generates wakefield, - a mix of TM0,N modes in the slow wave structure. This radiation is extracted from the slow wave structure and is characterized with interferometer. Narrowband signals in the range from 0.3 - 1.5 THz were measured with typical duration of few tens of picoseconds and few microjoules energy per pulse.

High-gradient, beam-driven wakefield acceleration in THz structures is a promising technology for future free electron lasers and colliders. In this scheme, the main beam is accelerated by the wakefield of the high current drive beam. The time separation between the main and drive beams has to be chosen carefully to ensure that the main beam is in an accelerating phase of the drive's wakefield. THz accelerating structures provide high gradient acceleration due to their small apertures, but their phase control is difficult due to the picosecond-scale period. Here, we report on a wakefield acceleration experiment in a 460 GHz dielectric wakefield accelerator (DWA). The optimum phase of the main beam during the experiment is determined with a two-beam wakefield interferometry (TBI) measurement. This is performed without the measurement of the main and drive beam bunch lengths or their separation. In TBI, the interference of the wakefields produced by the drive and main beams is measured with an integrating THz detector. The TBI signal, as a function of separation between the drive and main beams, exhibits a minimum due to destructive interference of these wakefields, which corresponds to maximum acceleration of the main beam as is confirmed by the energy spectrometer measurement. The maximum energy gain of 0.8 MeV and maximum energy loss of 1.2 MeV for the main beam have been measured, which agrees well with theoretical predictions.

We report the first operation of a widely-tunable 8-period undulator at terahertz (THz) frequencies in the Tsinghua University beamline. We observed the coherent undulator radiation from sub-picosecond electron bunches of 30MeV. The measured radiation curve shows clearly that the radiation energy is proportional to the charge square, and the THz frequency can be changed from 0.4 THz to 10 THz with narrow-band spectrums. Our results demonstrate a high power and tunable coherent THz source, which could be useful for many applications in the future.