Advances in the DTT poloidal interferometer/polarimeter design

Abstract

A multi-channels poloidal interferometer/polarimeter is under development for the Divertor Tokamak Test (DTT) facility, a new tokamak device whose construction is starting in Italy. The aim of the diagnostics is the simultaneous measurement of the line integrated electron density, Faraday rotation and Cotton Mouton effect. The measurement of two polarimetric signals together with the interferometric one would allow for a robust electron density estimate, for the internal magnetic field measurement as well as for the magnetic equilibrium reconstruction. In this work, we present the advances in the diagnostics design whose characteristics are constrained by scientific requirements as well as the DTT structural aspects. In this regards, we have analyzed the possibility of accommodating up to 16 lines of sight in the available space and the contribution of different chords positions in the magnetic equilibrium reconstruction. In particular, we have used the VMEC and V3FIT codes to evaluate the expected interferometric/polarimetric signals and their contribution in the reconstruction of the q profile. These results have been used to optimize the chords positions. Eventually, we present a realistic CAD-driven design of the diagnostic, with details of the most critical components (e.g. the corner cube retroreflectors in the high field side), which would allow for the selected chords configurations implementation.

High-frequency measurement of the interferometric phase, the Faraday rotation, and the Cotton-Mouton effect with a single detector in a far-infrared interferometer-polarimeter

“…For simplicity, we assume that all the beam-splitters, mirrors, combiners, and the retroreflector are ideal elements and that, by properly selecting their transmissivity/reflectivity, the E-field amplitude of the component at the unshifted frequency ω is p √ 2 times the E-field amplitude of the rotating azimuth component. The probing beam is first sent to a reference detector and then, accordingly to the chord layout chosen for the interferometer-polarimeter of DTT, is directed to traverse the plasma in a double-pass configuration, backreflected by a corner cube, and is finally detected by a probe detector [19]. Both detectors employ a polarization analyzer constituted by a linear polarizer whose axis is oriented to select a specific linear polarization.…”

“…From these data, we have calculated 2D maps of B R , B Z , and B T (radial, vertical, and toroidal B components) and n e in a poloidal plasma cross-section. Then we have considered a probing chord belonging to one of the different chord configurations studied for the interferometer-polarimeter [19] and we have calculated the B components and the electron density n e along the laser beam. Figure 2 shows the spatial profiles of the B R , B Z , B T components and of n e along a plasma equatorial diameter.…”

“…The choice of the more suitable laser wavelength, the optimization of the arrangement of the measuring chords and a preliminary assessment of the signal-to-noise ratio achievable in a typical DTT plasma reference scenario have been the object of previous studies [17,18]. Based on these analyses, for DTT a CH 3 OH far-infrared (FIR) laser (λ = 118.8 µm) and a multichord arrangement in which each probing laser beam is retroreflected to cross the plasma in a double-pass configuration, have been selected [19]. These studies have also indicated that in the DTT plasma both the Faraday and Cotton-Mouton effects are remarkable and that, due to considerable birefringence, the perturbation of the interferometric phase cannot be neglected.…”

“…The interferometric information is contained in the phase ϕ. This is determined by comparing the phases of the cosine terms in the signals of equations ( 20) and (19). For an interferometer-polarimeter, the measured phase shift between the reference and probing beam is ϕ = ∆ + φ in which…”