Speaker
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. Utilising the wake field excited by an externally injected electron beam reduces the de-embedding complexity due to the requirements for THz in-coupling. Here, a beam-based experiment at ARES at DESY is proposed to study the conversion quality of the mode converter at 165 GHz, the design frequency envisioned for the TWAC project (**). The wake-driven excitation enables a wideband characterization around the design frequency.
A gently compressed ( < 1 ps rms) high charge (~50 pC) electron bunch at 105 MeV/c beam energy 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, leading to an estimated THz pulse energy of ~ 0.1 to 0.5 µJ. By detecting the THz radiation with different polarization states the relative mode content between the residual unconverted TM01 and the expected TE11 after conversion will be determined.
The result will be benchmarked against complementary lab-based experiments utilising an external table-top THz source.
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.
Footnotes
() M. J. Kellermeier et al. in Proc. IPAC'23, Venice, Italy, May 2023, pp. 2916-2919. doi:10.18429/JACoW-IPAC2023-WEPA113
(*) C. Bruni et al., in Proc. IPAC'23, Venice, Italy, May 2023, pp. 1468-1471. doi:10.18429/JACoW-IPAC2023-TUPA061
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