Electron density measurement in a pulsed-power plasma by FIR laser beam deflection and/or interferometry

A pulsed-power experiment has been designed to produce arc-shaped magnetic flux tubes similar to ascending solar flares. The tubes are filled with hydrogen plasma (electron temperature ≤ 10 eV, electron density 2...3×10²⁰ m⁻³) and expand with a velocity of ~2.5 cm/μs, while keeping their cross section constant at a radius of about 1.5 cm. For measuring the spatial electron density distribution within the moving flux tube, a single cw laser beam can be used. The information taken from the laser beam, which traverses the vacuum vessel perpendicular to the plane of the plasma arch, can be either the phase shift or the beam deflection due to the density gradient. Assuming a parabolic distribution with a central electron density of 2 × 10²⁰ m⁻³, the maximum deflection angle occurring at an impact parameter of 0.7 amounts to γ_max/deg ≊ 10⁻⁵ × (λ/μm)². Hence, a FIR laser operating at λ = 433 μm would be deflected by γ_max = 1.9° only. Alternatively, a beam passing through the plasma centre would experience a plasma-induced phase shift of Δϕ_max/rad ≊ 10⁻² × (λ/μm), yielding 4.3 rad for a FIR laser (λ = 433 μm) and 0.1 rad for a CO₂ laser (λ = 10.6 μm). While the former is readily detectable in a standard interferometer, the latter requires a more advanced technique of measurement to achieve the necessary resolution. On the other hand, the short wavelength compared to FIR radiation allows for a very narrow beam and hence for a high spatial resolution. For these reasons a so-called coupled-cavity scheme for a CO₂ laser interferometer is presently under development.