Compact collimated vacuum ultraviolet diagnostics for localized impurity measurements in fusion boundary plasmas

Compact vacuum ultraviolet diagnostics for impurity emission measurements in boundary plasmas of nuclear fusion plasma devices are described. The instruments are designed for monitoring intensities of resonant impurity lines between 300 and 2000 Å. The intensities are used to infer basic yet important plasma parameters, such as density of impurity charge states, radiated power, or electron temperature and density estimates. All utilized components and materials satisfy ultrahigh vacuum and high bake-out temperature requirements, enabling the instruments to qualify for vacuum port or in-vessel placement, in close proximity to emitting plasmas. The instruments have high spatial (Δl ≤ 1 cm) and temporal (Δτ≤ 100 μs) resolution. The spectral resolution is Δλ ≤ 20 Å. Planar diffraction gratings at near-normal incidence are used for dispersion of incident radiation, collimated by high precision mechanical stacked grid collimators. Highly localized field of view, adequate throughput, and compactness distinguish these diagnostics from conventional slit instruments. A prototype monochromator for λ = 1550 Å has been built, evaluated, radiometrically calibrated, and used on the CDX-U spherical torus for monitoring C IV emission in ohmic and high harmonic fast wave heated plasmas with T_e(0) ≤ 100 eV. An attractive mechanical collimator based Wadsworth mount spectrometer concept is presented and its application to impurity content and transport measurements in tokamaks is discussed.