A J D Farmer

Temperature determinations in a free-burning arc. I. Experimental techniques and results in argon

Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon at 1 atm pressure. The experiment provides evidence for departures from LTE in the arc, and demonstrates deficiencies in the theory describing continuum emission from high temperature plasmas. This paper describes the apparatus and data reduction procedures and compares the measurements with recent theoretical calculations.

Surface temperature measurements for tungsten-based cathodes of high-current free-burning arcs

Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

Temperature determinations in a free-burning arc : III. Measurements with molten anodes

Spectroscopic measurements have been made to compare the structure of a gas-tungsten arc burning on a molten anode with one burning on a water-cooled anode. Metal vapour emanating from the molten-anode pool can be detected in the arc column near the anode, with concentrations as high as 2500 parts per million. However, it is found that the presence of the metal vapour does not significantly influence the temperature distribution in the arc.

Local thermodynamic equilibrium in free‐burning arcs in argon

Measurements of normalized radial emission coefficients in a free‐burning arc in argon are presented as a function of position in the arc. The variation of these emission coefficients with pressure indicates departures from local thermodynamic equilibrium at 1‐atm pressure.

Large effect of cathode shape on plasma temperature in high-current free-burning arcs

The temperatures of the cathode surface and of the plasma for an atmospheric-pressure high-current free-burning argon arc have been measured for a range of cone angles for cathodes of thoriated tungsten. These measurements have shown that both the surface temperature of the cathode and the temperature of the plasma depend strongly on the cathode shape. For cathodes with conical shape, plasma temperatures were found to be a maximum for a cathode cone angle of 60 degrees .

Temperature measurements for high-current free-burning arcs in nitrogen

A Fowler-Milne technique using the relative intensity of the 746.8 nm N I line has been used to measure the plasma temperature of free-burning arcs in nitrogen at 1 atm pressure. Temperature profiles are presented for different arc currents. The maximum plasma temperature depends on the arc current and is 27000 K near the tip of a 3.2 mm diameter thoriated tungsten cathode with a 60 degrees cone angle for a 200 A, 5 mm long arc. Comparisons are made between high-current arcs burning in argon and in nitrogen.

Temperature measurement in thermal plasmas by Rayleigh scattering

The range of temperatures over which Rayleigh scattering is a useful diagnostic of temperature in a noble gas thermal plasma at atmospheric pressure is examined. The various scattering processes that can occur a light passes through such a plasma are outlined. It is shown that while Rayleigh scattering by ground-state atoms is the dominant process at low temperatures, Thomson scattering becomes important at temperatures at which significant ionization occurs. Resonance scattering is also a significant source of scattered radiation when the laser wavelength is approximately equal to the wavelength of a transition of a species present in the plasma. Radial profiles of the polarized and depolarized radiation scattered from argon and helium arcs have been measured and analysed to confirm the relative importance of the respective scattering mechanisms. The occurrence of Thomson scattering is found to set a fundamental upper-temperature limit to the applicability of Rayleigh-scattering techniques to temperature measurement. For a laser wavelength of 514.532 nm and a detection system bandwidth of less than 0.1 nm, Thomson scattering becomes significant at approximately 9000 K in argon and 11000 K in helium. Above these values, the derivation of plasma properties from scattering measurements requires that all scattering mechanisms be taken into account. The temperature of the argon arc is calculated to exemplify this.

Rayleigh scattering measurements in a free-burning argon arc

Rayleigh scattering of argon-ion laser radiation has been used to obtain number densities in the outer regions (T<9000 K) of a free-burning arc. Excellent agreement is found between the measured argon atom densities and those predicted by theoretical models (e.g. Kovitya and Cram, 1986). A considerable discrepancy exists between the number densities derived from these scattering measurements and those obtained from argon line emission measurements. This difference is shown to be consistent with the non-equilibrium theory of Cram and co-workers for the outer regions of these arcs.

Plasma Destruction of Ozone Depleting Substances

The literature on the plasma destruction of ozone depleting substances (ODS) such as CCl2F2 and CBrF3 is reviewed, and compared with more recent work on the decomposition of CCl2F2 and CBrClF2 in oxygen and steam. A comprehensive kinetic scheme for the decomposition of CBrClF2, which includes the decomposition of CCl2F2 and CBrF3, is presented. Simulations performed with this scheme, and experimental results, demonstrate the importance of allowing for the interconversion of ODS in the assessment of plasma destruction devices.Both experimental and modeling results show that the efficiency of operation of a practical plasma ODS destruction device can be quantified in terms of a throughput parameter, the feed to plasma power ratio (units mol (kWh)-1), or in terms of the thermochemical mixing temperature, Tm, of the plasma, ODS and oxidant. At low throughputs and high Tm, essentially complete destruction may be achieved, with below-ppm quantities of ODS remaining in the plasma exhaust gases. As throughput rises and Tm falls, a threshold is reached above which the ODS residual rises steeply towards the practical working limit set for ODS destruction by the Montreal Protocol (a destruction level of 99.99%). The assessment of this limit must include all ODS in the exhaust gases, weighted for ozone depleting potential. The use of steam, rather than oxygen, as the oxidizing gas gives superior destruction performance.

Laser‐scattering measurement of temperature profiles of a free‐burning arc

A method of determining the temperature of a thermal plasma by laser scattering, without frequency resolution of the scattered signal, is introduced. The method gives valid measurements of the gas temperature across the complete diameter of a free‐burning argon arc, even in regions in which local thermodynamic equilibrium does not exist. The results show that the maximum temperature in a 100 A arc, with a 60° conical thoriated‐tungsten cathode, is over 18 000 K, in agreement with our previous spectroscopic measurements.