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BASIC PLASMA EXPERIMENTS 142 tions of a few degrees in velocity space), emissive probes, and electric dipole probes that are sensitive to the total electric field, including the magnetic component (i.e., E = ââ Î¦ â â A/ât). Computer-controlled probe drives capable of full, three-dimensional motion have been developed, and essentially the first- ever fully three-dimensional studies of several important plasma phenomena have been conducted. Laser-Based Optical Diagnostics The potential of laser-based optical diagnostics for plasma science has continued to develop. Two general categories of recent achievements are measurements of the spectrum of collective plasma density fluctuations and measurement of the single-particle distribution function, both by Thomson scattering and by laser-induced fluorescence. Scattering from collective plasma fluctuations has been used to study both thermal and nonthermal effects, such as waves and instabilities and entropy fluctuations. A variety of imaging diagnostics, such as phase-contrast and shearing-plate interferometry, has been developed for use in experimental plasma research. Single-particle scattering techniques have been developed to study both the ion and the electron velocity distributions. Thomson scattering techniques have been developed to measure electron density and temperature with unprecedented spatial and temporal resolution. The use of laser-induced fluorescence techniques to study plasma ions has been one of the major recent developments in plasma diagnostics. Not only does this technique permit temporally and spatially resolved measurement of the ion distribution function with high sensitivity, but it also permits a variety of extensions. (See Figure 8.3.) For example, the use of metastable states provides the capability of measuring nonlocal plasma properties, and metastable or spin- polarized ions produced by optical pumping may be used as test particles to trace ion orbits and to study particle transport. Laser-induced fluorescence has now been used to measure ion distribution functions in a variety of physical processes and to study the plasma dielectric response, and optically tagged test particles have been used to measure Fokker- Planck coefficients for collisional diffusion. Such test particles have also been used to measure the Lyapunov exponents that characterize chaotic particle motion and to measure the transport arising from a variety of physical processes. Recent developments in laser and optical technologies include the development of new, high-performance laser materials, such as Ti-sapphire, and a wide variety of solid-state lasers. Other important developments include nonlinear optical materials, which are essential to the production of tunable radiation at short wavelengths via frequency multiplication. Improvements in short-pulse laser technology have increased the time resolution of measurements to better than 10-14 s, which is shorter than virtually all of the natural time scales in most laboratory plasmas.