Frequency modulation spectroscopy (FMS) is a new combination of optical absorption spectroscopy and radio technology which increases absorption sensitivity by orders of magnitude. Extremely weak atomic and molecular lines and stronger absorbers at low number densities, previously accessible only to fluorescence techniques, are readily detected.

Information obtainable through FMS include species number density, lineshape and temperature. Data interpretation is straightforward without regard to such problems as quenching, alternate excited state reaction channel loss and source/absorber solid angle and spectral response calibration, since the technique probes the lower state population via line of sight, volume integration. Application of the technique to the study of radical species such as CH, CN, and NO2 produced in flames and low pressure reactor vessels has made absorption studies of these systems feasible for the first time. The technique has also been used to study such diverse problems as the effect of fuel additives on the production of soot, and the concentration and distribution of the chemical intermediates, BH, BO, and BO2, in microwave driven reactive flows. Feasibility studies to determine if FMS can assist in the diagnostic and control problems of chemical vapor deposition in the semiconductor industry have recently been initiated.

Laser-induced fluorescence (LIF) provides a means for sensitive, selective and nonintrusive measurements of reactive intermediates, often free radicals, which control the chemistry of combustion. The same approach can be used to detect in the gas phase, atoms, radicals and ions which are reactive intermediates in materials processing. This article briefly examines the use of LIF in combustion. It includes a description of the LIF spectroscopic method as employed for quantitative measurements, and the use of LIF in a semiquantitative manner with extension to the simultaneous detection of more than one species.

Pump/probe methods are commonly employed to measure subnanosecond excited state processes in liquid and gas phase systems. Asynchronous Optical Sampling (ASOPS) is a newly-developed pump/probe method that will potentially allow the determination of number densities and relaxation rates in turbulent, high-pressure flames. In addition, ASOPS should yield a better signal-to-noise ratio than laser-induced fluorescence in practical combustion environments. The ASOPS method utilizes a coherent, signal-carrying beam and thus requires no more optical access than LDV measurements. The current ASOPS instrument consists of two dye lasers synchronously pumped by two frequency-doubled, mode-locked Nd:YAG lasers. The two lasers operate at slightly different repetition rates, causing a relative phase walk-out between the pump and probe beams. This strategy allows the mapping of subnanosecond excitation processes in a time scale on the order of milliseconds.

Multiphoton excitation provides spectroscopic access to several species that are very important in flame studies, but that cannot be detected in combustion environments using conventional optical techniques. This paper describes applications of multiphoton excitation techniques for detecting such species in combustion environments, with an emphasis placed on measurements of atomic hydrogen in flames.

The use of spontaneous rotational and vibrational Raman scattering for nonperturbing measurements of temperature and gas phase concentrations in combustion and in materials processing is reviewed. Theory of Raman intensities is briefly discussed and examples of Raman scattering from metal halide vapors in high temperature furnaces, laminar flames, and turbulent flames are given to demonstrate its capabilities and limitations. At present spontaneous Raman scattering is the only method which can provide simultaneous nonperturbing measurements of temperature and many species concentrations with high spatial (≤ 0.1 mm3) and temporal (≤ 1μs) resolution. Rotational Raman scattering is generally preferred for low temperature measurements (≤ 1200 K), and vibrational Raman scattering is better for measurements of higher temperatures and of multiple-species concentrations. The disadvantages of Raman scattering stem for the weakness of the nonresonant Raman interaction which limits its sensitivity to major species concentrations, renders it vulnerable to interferences from other sources of visible radiation, and requires high powered lasers and sensitive detectors. Data from Raman scattering measurements, particularly when coupled with data from other laser diagnostic techniques and when compared to computer model calculations, have led to a better understanding of the complex interactions between fluid mechanics and chemical reactions which can govern the operation of turbulent combustors or chemical flow reactors.

Coherent anti-Stokes Raman spectroscopy (CARS) is a powerful analysis approach for nonintrusive temperature and species measurements in hostile environments. Widely utilized in combustion, it may be able to significantly impact materials processing research. CARS is described and its applications to combustion and plasma process environments are reviewed and contrasted.

The current status of planar laser-induced fluorescence (PLIF) imaging is reviewed, and example imaging results obtained in subsonic and supersonic jet-mixing flows with a new intensified high-resolution CCD-array camera are presented.

The evaluation of the properties of energetic materials, such as burn rate and ignition energy, is of primary importance in understanding their reactions and the functioning of devices containing them. One method for recording such information is high-speed photography at rates of up to 20,000 images per second. When a copper vapor laser is synchronized with the camera, laser-illuminated images can be recorded that detail the performance of a material and/or component in a manner never before possible. The laser can also be used for ignition of the energetic material, thus eliminating the need for bridgewires or electric squibs that can interfere with photography. Details of such ignitions are readily observable, and the burn rate of a material can be determined directly from the film. There are indications that information useful for the modeling of pyrotechnic reactions will become available as well. Recent results from high-speed photography of several pyrotechnic materials and devices will be presented.

The radial extension of the plasma edge being determined by the ionisation length of the atomic hydrogen, the rtanges of the main plasma parameters to be covered are: 1010 cm-3 < ne <. 1013cm-3 5 eV < Te- < 100 eV, density of neutral particles below 1010 cm-3. Physical quantities to be measured are the electron density ne and temperature Te, the density and temperature (velocity distribution) of hydrogen and of the main impurities (oxygen, carbon, metals). Relying mainly on emission spectroscopy, laser induced fluorescence and atomic beam methods, most of these quantities can be determined with the desired space and time resolution. Experimental arrangements will be described and typical results will be presented.

The developments of the laser diagnostics at Kyushu University for studies of high temperature plasmas and plasma-surface interactions are reviewed, followed by the recent results and future plans.

We have used a variety of electrostatic, calorimetric and sample exposure probes to diagnose the edge plasma of neutral-beam heated discharges on the tokamak fusion test reactor (TFTR).

In the past, spatial profiles of electron density and temperature, and other edge parameters, were generated by repositioning probes between discharges during experiments consisting of a large number of nominally identical discharges. Recently, we have employed a probe which executes a relatively rapid stroke to generate spatial and temporal profiles during a single discharge. This has greatly enhanced our ability to perform systematic studies of the edge plasma as various operating variables are changed. In addition, this rapid radial motion permits withdrawal of the probe during portions of the discharge when the intense neutral beam heating would otherwise damage the measuring tips.

Applications of such a technique to low temperature, process plasmas will be discussed.

An effect of sinusoidal r.f. voltages between a plasma and a floating Langmuir probe (or other electrode) is to produce a d.c. self bias voltage. This effect has previously been studied with r.f. applied to probes in d.c. plasmas, and the results have often been referred to in connection with d.c. probes in r.f. plasmas, although the situation is slightly different. In the current work a ‘d.c.’ probe in an r.f. plasma is examined. However, the r.f. potential difference between probe and plasma is first compensated for by superimposing a synchronous signal of appropriate amplitude and phase on to the d.c. circuit. Thus the probe can perform essentially d.c. type measurements on the r.f. plasma. The deliberate reintroduction of a measure of r.f. between probe and plasma is accomplished by overdriving the compensation signal thereby generating a controlled amount of self bias.

The observed variation of probe bias with r.f. overdrive is in good agreement with analytic theory based upon modified Bessel functions as used for d.c. generated plasmas. This work is of general interest in probe diagnostics of r.f. generated plasmas, and understanding the self bias of isolated electrodes.

A Tonks-Langmuir type model for a one-dimensional low pressure (collisionless) plasma containing at least two species of Maxwellian negative charge carriers is examined. The solutions of this model yield the positive ion energy distribution at the sheath edge without needing to specify the ionization process. This distribution has a width consistent with the potential drop across the plasma, and is shown to satisfy the generalised Bohm criterion for sheath formation. However by assuming a form for the ionization rate in the plasma, the potential profile across the discharge has been calculated. It has been found that for a range of plasma parameters the solution for the potential at which quasineutrality fails becomes triple valued; the physical solution in this regime is identified.

Many plasmas used for materials processing contain negative ions. It is important to understand how these ions influence the positive ion energy distribution at the substrate where the processing occurs.

This work is also of relevance to the behaviour of Langmuir probes in electronegative plasmas.

Imaging diagnostics are used for spatially-and temporally-resolved quantitative measurements of plasma properties such as the ionization particle source, particle and energy loss, and impurity radiation in magnetically confined fusion plasmas. Diagnostics equipped with multi-element solid-state detectors (often with image intensifiers) are well suited to the environment of large fusion machines with high magnetic fields and x-ray and neutron fluxes. We have used both conventional (16 ms/frame) and highspeed video cameras to measure neutral deuterium Hα, (6563 Å) emissions from fusion plasmas. Continuous high-speed measurements are made with video cameras operating at 0.1–0.5 ms/frame; gated cameras provide snapshots of 10–100 μs during each 16-ms video frame. Digital data acquisition and absolute intensity calibrations of the cameras enable detailed quantitative source measurements; these are extremely important in determining the particle balance of the plasma. In a linear confinement device, radial transport can be determined from the total particle balance. In a toroidal confinement device, the details of particle recycling can be determined. Optical imaging in other regions of the spectrum are also important, particularly for the divertor region of large tokamaks. Absolutely calibrated infrared cameras have been used to image the temperature changes in the walls and thereby determine the heat flux. Absolutely calibrated imaging ultraviolet spectrometers measure impurity concentrations; both spatial and spectral imaging instruments are employed. Representative data from each of these diagnostic systems will be presented.

Optical interferometry techniques allow phase changes in a medium to be accurately determined. Since plasmas introduce a phase modification proportional to electron density, interferometry has been used routinely to determine the time development of electron density profiles in magnetic confinement fusion plasmas. The present paper describes the basic principles of interferometry of plasmas and establishes criteria regarding wavelength selection, vibrational phase noise, phase averaging etc. In addition, recent advances at UCLA in interferometry of fusion plasmas will be described. For example, high spatial sampling has allowed the detailed study of MHD phenomena such as sawteeth and Mimov oscillations, and 2-D imaging interferometry has permitted tomographic reconstruction of electron density contours. Difficulties associated with interferometry on future fusion devices, such as CIT, will also be discussed together with possible solutions.

A low-cost heterodyne microwave interferometer system capable of measuring and/or controlling the plasma density over a dynamic range covering two orders of magnitude is demonstrated. The microwave frequency is chosen to match the size and density of plasma to be monitored. Large amplitude, high frequency fluctuations can be quantitatively followed and the longer-time-scale density can be held constant over hours of operation, for example during an inline production process to maintain uniformity and stoichiometry of films. A linear relationship is shown between plasma density and discharge current in a specific plasma device. This simple relationship makes control of the plasma straightforward using the interferometer as a density monitor. Other plasma processes could equally well benefit from such density control capability. By combining the interferometer measurement with diagnostics such as probes or optical spectroscopy, the total density profile and the constituent proportions of the various species in the plasma could be determined.