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Description
Laser-based high power THz generation advances rapidly, enabling THz-driven electron acceleration beyond the breakdown limits of conventional RF-driven structures. For this purpose the external THz pulse has to be coupled to the accelerating structure and its polarization has to be matched to the required TM01 mode. Vice versa, beam-driven high power THz generation also relies on the TM01 mode and requires efficient out-coupling and transport. A crucial step in both applications is the conversion between the fundamental mode, compatible with linearly polarized freespace beams, and the TM01 in a compact manner, which can be achieved efficiently in a double bend geometry (*).
However, the double bend mode converter is fully integrated with the chained neighbouring devices, being a horn antenna and a slow wave structure for THz-electron interaction. The embedding imposes challenges on the independent characterization of the converter. Here, a beam-based experiment at ARES is proposed for characterizing the conversion quality at 165 GHz, as planned the TWAC project.
A gently compressed (100s fs rms) high charge (~80 pC) electron bunch is passed through a dielectric loaded waveguide, thereby exciting the TM01 mode as longitudinal wake. The out-coupled THz pulse is band-pass filtered for the design frequency. By detecting the THz radiation with different polarization states the relative mode content between the remaining TM01 and the expected TE11 after conversion is determined.
Footnotes
(*) M. J. Kellermeier et al. in Proc. IPAC'23, Venice, Italy, May 2023, pp. 2916-2919. doi:10.18429/JACoW-IPAC2023-WEPA113
Funding Agency
The TWAC project is funded by the European Union‘s Horizon Europe research and innovation programme (EIC Pathfinder scheme) under grant agreement n. 101046504.
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