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Measurements of internal magnetic fields are of primary importance in laboratory plasma physics; the most common diagnostic method is Faraday rotation measurement by means of polarimetry. Faraday rotation measurements are integrated along a line of sight: the overall rotation is proportional to the plasma density, magnetic field and probing wavelength, as well as to the integration segment length and hence to the plasma dimensions. As a results, measurements of small magnetic fields in small, low density plasmas becomes non-trivial; on the other hand, this is typically the situation of small, self organized laboratory experiments. Being the probing wavelength practically limited to the THz region by diffraction phenomena, the use of a multipass scheme is actually the only way to improve the diagnostic sensitivity. This work discuss an application study of a multipass cavity to the PROTO-SPHERA experiment, but the results can be easily generalized to others of similar characteristics. In particular, different layouts are analyzed (straight, folded, annular cavity) discussing the overall stability region, the impact of changes in plasma refraction and the overall performances in terms of photon lifetime. The performance analysis features a discussion on continuous vs pulsed regime as well as on some basic detection schemes (direct, closed loop feedback).
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