High sensitivity imaging Thomson scattering for low temperature plasma.

A highly sensitive imaging Thomson scattering system was developed for low temperature (0.1-10 eV) plasma applications at the Pilot-PSI linear plasma generator. The essential parts of the diagnostic are a neodymium doped yttrium aluminum garnet laser operating at the second harmonic (532 nm), a laser beam line with a unique stray light suppression system and a detection branch consisting of a Littrow spectrometer equipped with an efficient detector based on a "Generation III" image intensifier combined with an intensified charged coupled device camera. The system is capable of measuring electron density and temperature profiles of a plasma column of 30 mm in diameter with a spatial resolution of 0.6 mm and an observational error of 3% in the electron density (n(e)) and 6% in the electron temperature (T(e)) at n(e) = 4 x 10(19) m(-3). This is achievable at an accumulated laser input energy of 11 J (from 30 laser pulses at 10 Hz repetition frequency). The stray light contribution is below 9 x 10(17) m(-3) in electron density equivalents by the application of a unique stray light suppression system. The amount of laser energy that is required for a n(e) and T(e) measurement is 7 x 10(20)n(e) J, which means that single shot measurements are possible for n(e)>2 x 10(21) m(-3).

Mirror system for collecting Thomson-scattered light in a tangential direction.

We describe an optical system for collecting Thomson-scattering light in the tangential direction of a tokamak. The key part of the optics is a set of mirrors arranged as a Venetian blind. This system makes it possible to look around the corner of the tokamak vessel. Design considerations and test performance are presented.

High-transmission 20-channel polychromator for observing non-Maxwellian electron velocity distributions in plasmas by Thomson scattering.

A high-transmission (~45%) twenty-channel polychromator equipped with near-infrared sensitive photomultipliers has been constructed to record Thomson scattering spectra at the TORTUR tokamak. The high transmission was achieved by the use of mirrors instead of fiber optics to guide the spectrally resolved light to a set of photomultipliers. Spectral analysis is performed with a holographically ruled concave grating. Acceptable dimensions of the wavelength selection mirrors were obtained by magnifying the spectral image by a factor of 5 with a Mangin mirror. Electron temperatures up to 1000 eV at a density of 5 x 10(19) m(-3) can be measured with an accuracy of approximately l%. Both high sensitivity and high resolution enable the detection of irregularities in the velocity distribution. For example, satellites corresponding to partial densities of (5 +/- 1) x 10(17) m(-3) were found at 23 nm from the laser wavelength.

Stigmatic and coma-free imaging with a thick prism: a comparison of third-order theory and ray-tracing results.

Although the use of prisms is often avoided because of their aberrations, they are suitable for image-tilt correction. In contradiction to current recommendations, zero astigmatism together with zero coma cannot be achieved at minimum deviation. This paper describes two configurations that meet these conditions. A correction of earlier published third-order formulas is presented and compared to ray-tracing results. The results of the third-order theory are used to develop a model for image-tilt correction, while introducing minimal coma and maintaining a flat image field.