Cj Timmermans

The continuum emission of an arc plasma

We present free-bound Biberman factors (12,000, 13,500 and 14,500 K) for the wavelength range of 250–320 nm and 380–800 nm. These values were determined by measuring the absolute continuum intensity of a thermal argon plasma in a cascade arc (2mm dia) for a pressure range of 2–6×105 Pa and a current range of 20–60 A. The continuum emission is corrected for free-free contributions. Two highly accurate experimental reference values of the free-bound factors were used to check the electron density. Agreement with experiments in the u.v. is good. Comparison with theoretical values also shows good agreement below 430 nm. Above this wavelength, theory predicts an edge structure which is not apparent in the available experimental values. The agreement between our results and recent experimental values (for which the electron density was obtained with two-wavelength interferometry) is good. The influence of non-equilibrium has been found to be negligible. We conclude that absolute continuum measurements are well suited to determine electron densities (visible spectroscopy) and electron temperatures (u.v. spectroscopy) by using the known free-bound Biberman factors. Prediction of the absolute intensity as a function of wavelength is possible within 10%.

Characteristic quantities of a cascade arc used as a light source for spectroscopic techniques

The authors have determined the characteristic quantities (the electron temperature and the electron density) of an argon plasma in a cascade arc (diameter of 2 and 4 mm) for a pressure range of 1*105-8*105 Pa and a current range of 20-70 A. The absolute continuum intensity was also determined in the wavelength ranges from 250-320 nm and 380-800 nm for pressures of 2, 4 and 6 bar and 20, 40 and 60 A in the case of a 2 mm arc. The plasma is close to local thermal equilibrium (LTE). Using the mentioned quantities, prediction of the absolute intensity as a function of the wavelength is possible from 140 nm to the infrared within 10%. The use of the arc as a light source in photon induced chemical vapour deposition, spectroscopic ellipsometry and infrared absorption spectroscopy is discussed.

An investigation of non-equilibrium effects in thermal argon plasmas

The parameters and transport properties of a wall stabilized argon arc (40-200 A) at atmospheric pressure with diameters of 5 and 8 mm are studied by spectroscopy and interferometry. The plasma is assumed to be partial local thermal equilibrium and this assumption is verified with the aid of a collisional-radiative model. The departures from Saha-equilibrium of the argon neutral ground state are found to be associated with particle diffusion and the escape of recombination radiation. The measurement of the total excitation rate, from the ground level, including direct ionization, of neutral argon is in reasonable agreement with the literature value.

A phase quadrature feedback interferometer using a two-mode He-Ne laser

A new feedback interferometer is developed with which not only the magnitude but also the sign of a phase change caused by a variation in the optical path can be measured. This is an important feature, for example, in plasma diagnostics. Phase quadrature is obtained by using two axial modes of a He-Ne laser. The two interference signals can be separately detected by virtue of the orthogonal polarisation of the two axial modes of the laser without Brewster windows. The result is a versatile, stable and accurate instrument. In the experimental test the sensitivity proved to be approximately 1/5 fringe at a laser wavelength of 632.8 nm, equivalent to 0.1 mu m. In plasma applications the instrument would have a sensitivity of neL approximately=2*1020 m-2, where ne is the electron density and L the length of the laser cavity.

Plasma parameters and weakly non-ideal behaviour of a high density, super-atmospheric 2kA cascade arc in argon

Abstract Experimental results are reported for simultaneous pressure and current pulses up to 14 bar and 2200 A superimposed on an atmospheric pressure 60 A dc cascade arc. A current density over 108 A/m2, previously 107 A/m2 (power density 1012W m3, previously 1010W/m3) has been obtained. The electron temperature of the thermal plasma was deduced from the end-on measured radiance of argon lines, and the electron density from the absolute continuum emission. The values found for the quantities mentioned during the quasi-stationary phase of the current pulse, lasting ~ 1 ms, were 27000 K and 3 · 1023 m-3, respectively, at a pressure of ~ 1.5 bar (ionization degree 100%), and 18000 K and > 1024 m-3, respectively, for a pressure of 14 bar (ionization degree 60%). These values satisfy the LTE relation. Deviations from the Spitzer conductivity have been observed in this weakly non-ideal plasma. In general, the high ionized thermal plasma studied with its composition of neutral, singly ionized and doubly ionized argon atoms can serve as a useful medium for spectroscopic studies of highly ionized systems and as a valuable source of radiation in the visible a well as in the near and far ultraviolet parts of the spectrum.

On the Pressure Balance and Plasma Transport in Cylindrical Magnetized Arcs

Abstract Magnetized current-carrying plasmas exhibit usually significant E x B rotation velocities which often approach the ion thermal velocity. It is shown both experimentally and theoretically that this rotation in combination with inertia, viscosity and friction leads to an important reduction of the radial transport. If the radial electric field component is directed inward an inwardly directed force on the ions is set up. On the other hand, turbulence leads to enhanced transport especially at higher values of the electron Hall parameter. Also this effect is observed in this experiment and is shown to be in accordance with measured turbulence levels.