Laser Thermometry of solids in plasma (Review)

Principles, features, and limitations of certain new methods for determinining the temperatures of solids are considered. Thermometry is based on remote measurements of temperature-dependent parameters of solids by using a probe light beam. Problems and conditions of surface temperature measurements in gas discharges and methods for checking the influence of plasma on the optical characteristics of solids are discussed. The methods are evaluated by several criteria, which are important for practical application under electromagnetic noises, discharge-emitted optical radiation, and frequent replacement of the samples under study. The range of the measured temperature exceeds 1000 K for some methods, and temperature steps are usually 0.1–1 K. The relative sensitivity of most laser techniques is no lower and sometimes 10–100 times higher than that of conventional thermometry methods. Results of methodological importance for the development of the temperature diagnostics of processes occurring at the plasma-surface interface are presented. The unsolved problems of the new thermometry technique are formulated.     

Spectral pyrometry of the objects with hot spots

Optical emission spectra of microwave discharges at the powder mixture surface are experimentally recorded, indicating spatial temperature nonuniformity the within the field of vision. Within the field of vision, the maximal and minimal temperature values calculated from the continuous spectrum (wavelength range 380–620 nm) differ by 20–25%. Modeling of the integral emission spectra of a heated surface with hot spots is performed. Spectral pyrometry of the objects with nonuniform temperature, when used within a narrow spectral interval, gives no possbility of determining the exact nature of the measured temperature because the temperature calculated from this spectrum may differ substantially from both averaged and maximal values. To determine the averaged and maximal temperature, it is necessary to significantly widen the recorded spectral interval.     

Spectral pyrometry (Review)

In the emission spectra of many objects, there are regions where the intensity distribution is similar to the blackbody spectrum. This allows one to determine the temperature by recording the emission spectrum and comparing it to a Planck spectrum. Experimental or calculated data on the emissivity of such objects are not necessary because the temperature is determined as a parameter of the distribution observed. This property characterizes the emission spectra of gaseous and solid-phase flames, erosion plasmas of surface discharges, metals, semiconductors, dielectrics, micro- and nanoparticles, and heterogeneous media (powder mixtures and ceramics) at temperatures both lower and higher than the melting temperature. Spectrometers with photodetector arrays sensitive in the wavelength range of 200–1100 nm are used to record spectra. Such spectrometers allow spectrum recording and determination of the radiator temperature in the temperature range of 800 K–140 kK within a time of ～1 ms. The specific features of the method, examples of its application, measurement characteristics, unsolved problems, and prospects are discussed in the review.     

Laser Thermometry of Solids: State of the Art and Problems

Novel active techniques are surveyed for measuring the temperatures of solids. They are based on using an optical beam, usually from a laser. Laser thermometry methods are compared with contact and radiation ones. Problems are discussed in relation to the general use of the new methods in engineering monitoring. The main problem is the lack of metrological support.     

Solar-blind UV flame detector based on natural diamond

We consider the properties and use of a natural-diamond-based photodetector sensitive in a wavelength range of λ < 300 nm for detecting gas-burner flame with a temperature of 1900 K. In the region of maximum sensitivity (λ = 210 nm), the current responsivity of the photodetector is 2 A/W. The possibility of flame detection is based on the fact that nonequilibrium flame radiation intensity in the region of maximum sensitivity of the photodetector is six orders of magnitude greater than the intensity of the equilibrium (thermal) emission.     

Pulsed interferometric calorimetry

The possibility of using semiconductor single crystals (Si, GaP, etc.) as calorimeters for pulsed heat flux measurements is discussed. The method of laser interferometric thermometry in the reflected light at a wavelength of 1.15 μm makes it possible to detect changes δθ≈2×10⁻⁵ K in the temperature of a 1-mm-thick Si single crystal. The sensitivity of a calorimeter with laser readout is sufficient for detection of the absorbed energy δE≈15 μJ/cm² and is independent of the Si plate thickness. The method for selecting the working point in the resonance curve for achieving maximum sensitivity in detection of pulse fluxes is discussed.     

Solid-state chemical reactions initiated by optical and microwave radiation

It is experimentally demonstrated that the simultaneous exposure of dielectric powders and mixtures to optical and microwave radiation pulses can lead to the development of chemical reactions. This phenomenon is explained by the light-induced absorption of microwave radiation in the dielectric powder, which leads to heating of the mixture and the initiation of a chemical reaction that can subsequently proceed at the expense of the intrinsic heat evolution.     

Measurement of the characteristics of a point source of x-rays using a picosecond laser plasma

The emissive properties of silicon and aluminum plasmas, produced by 40 psec laser pulses with a peak intensity of greater than 10¹⁴ W/cm², are investigated. The x-ray line spectra of H- and He-like ions, measured with high resolution, are analyzed to determine the plasma parameters. The form of the resonance lines and their intensity with respect to the corresponding dielectronic satellites and the intercombination line are compared with model calculations.Translated from Izmeritelnaya Tekhnika, No. 1, pp. 50–55, January, 2005.