Tomsk State University. Physical Department.
Laboratory of training physics experiment
[ Main page | Original | List-200 | Technology | History ]
Mach-Zehnder Interferometer     Back    

Temperature Fields in an Arc Plasma

A much more versatile instrument, in which the beams may be widely separated, is obtained when the beam-dividers and mirrors are separate elements. This is the basis of the Mach-Zehnder interferometer which is used to measure variations of refractive index, and hence of air density.

One of the basic problems of plasma diagnostics is the determination of temperature distributions. Good results are given by interference methods, applicable up to temperatures ~4000C, in combination with spectroscopic methods, effective in the temperature range beginning at 3000-4000C.

Using such a combination method, we investigated in detail the discharge of a dc arc (Veff = 580 V, Ieff = 0.116 A). Earlier studies offered a detailed examination of only one section of the discharge zone or considered only the high-temperature region, using the spectroscopic method

The stability of the temperature distribution was checked directly from the interference pattern with stroboscopic illumination. The shifts did not exceed 0.1-0.2 fringe even near the axis, and these lay within the limits of experimental error, which were 12-15%.

A MachZehnder interferometer was used for the interference measurements. The experimental setup consisted of a magnesium discharge-illuminator, a condenser, interferometer, field lens (F = 1 m), interference light filter at ?= 448.1 nm, and a Zenit camera with Tair-3 telephoto-lens (F = 300 cm)

Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5

Special interferogram formed in the line spectrum of magnesium without a light filter (Fig. 1). Interferograms were obtained in infinitely wide bands (Fig. 3), moire patterns (Fig. 4), and interference bands perpendicular and parallel to the arc axis (Figs. 2, 5). It was most convenient to process the interferograms of the last type, since the interferograms with infinitely wide bands and the moire patterns did not permit determination of the exact state of the medium in the intervals between successive interference bands.

Fig. 6
Fig. 7

The measured total displacement of the bands was converted by the Able method (axisymmetric discharge) using Bockasten's method and values of the index of refraction were obtained for various horizontal sections (16 were taken) of the arc discharge. These are presented in Fig. 6.

Also shown in Fig. 6 is the temperature as a function of the index of refraction, constructed from the equation T0 [(n0-l)/(n-l)]. Simultaneous use of the curves n -1 = f(r) and n 1 = F (T) makes construction of isotherms for the discharge plasma simple, permitting immediate determination of points with a given temperature in each section.

In the axial region (0 < r/r0 < 0.1, where r0 is the radius of the region disturbed by the discharge action), temperature was determined from the relative intensity of spectral lines by the Ornstein method.

The families of isotherms obtained for an arc discharge are presented in Fig. 7. From the shape of the arc discharge isotherms it is obvious that convection plays a significant role in thermal balance of a weak current arc.

[ Main page | Original | List-200 | Technology | History ]