Fig. 2.12.

Fig. 2.13.

36 ELECTRICAL METHODS OF NONIDEAL PLASMA GENERATION

ohm cm

g cm

.

. Pa

. .

.

.

.

.

.

p = 1.3.108

Isochores of the electrical conductivity of mercury (Hensel and Franck 1968).

eV

gcm

The transport, ∆E, and optical, ∆Eopt, gaps in the energy spectrum of mercury,

T = 1820 K. Experiments: 1 – ∆E, Kulik et al. (1984); 2 – ∆E, Kikoin and Senchenkov (1967); 3 – ∆Eopt, Uchtmann and Hensel (1975).

2.1.2Optical absorption measurements.

The optical properties of strongly nonideal mercury plasma were measured by Hensel (1970, 1971, 1977) and Ikezi et al. (1978). Optical cells in the 1.3–4.9 eV energy range were developed. The cell design, enabling one to perform the measurements of light transmission in mercury at temperature of up to 1700 ◦C and pressures of up to 0.2 GPa, is illustrated in Fig. 2.14.

PLASMA HEATING IN FURNACES

37

Fig. 2.14. Optical cell for determining the absorption in a dense mercury plasma (Hensel 1971): 1, high–pressure windows; 2, niobium cylinder; 3, measuring volume; 4, Teflon seals; 5, container with mercury; 6, autoclave; 7, furnace; 8, optical sapphires.

The cell consisted of a niobium cylinder (2) closed on both ends by windows of polished sapphires (8) whose outer ends stayed cold. They are sealed by Teflon spacers (4). Provided between the ends of the sapphires was a clearance whose width could be varied from 5 · 104 to 0.1 cm. Mercury was delivered to that clearance from a special container (5). The clearance served a measuring element for determining the absorption index of mercury plasma. The cell was heated by a furnace (7) inside an autoclave (6). Argon provided the pressure-maintaining medium. Light from the autoclave passed out through windows arranged in ends

(1).

The absorption index k could be determined from the measured radiation transmission T = I/I0

T = (1 − R)2 exp(−kd),

(2.3)

where d is the sample thickness, I0 and I are the incident and emitted radiation intensities, and R is the reflection coe cient. The values of k were determined by Hensel (1971) without using the values of R, but as a result of measurement of T with two values of d.

The sources of errors such as the uncertainty in the optical path, the nonuniformity of the temperature distribution and the inaccuracy of intensity recording led to errors in the absorption coe cient not exceeding 20%. The main source of uncertainty is provided, as in the case of measuring the electrophysical quantities, by errors in recording the temperature and density. The stimulated radiation correction to the absorption index, made by the factor [1 − exp(−β ω)] in the conditions under investigation, is small.

Figure 2.15 shows the frequency dependence of the absorption index of dense mercury vapors obtained in two di erent laboratories. The absorption index k(ω) varies exponentially upon variation of the incident light wavelength. The first thing to strike one’s eye is the abruptness of the variation of k(ω). A variation of k(ω) by an order of magnitude corresponds to a variation of ω by only several

Fig. 2.15.

38 ELECTRICAL METHODS OF NONIDEAL PLASMA GENERATION

cm

. . . . . . . .

			.		.
.	.	.	.	.	.	.
		.
					.	.
						.

.

.

eV

Absorption index of mercury as a function of the incident light frequency, k(ω), at T = 1600 ◦C and at di erent densities ρ, g cm−3: solid curves, Ikezi et al. (1978); broken curves, Uchtmann and Hensel (1975).

tenths of an electron volt. The data of the two experiments are in good agreement with each other.

The mercury reflection coe cient R was measured by Ikezi et al. (1978) and Hefner et al. (1980). Figure 2.16 shows the results of measurements of the reflection coe cient of incident radiation, which is almost normal to the surface of the plasma layer (Ikezi et al. 1978). These results cover the region in which mercury loses its metallic properties at temperatures somewhat exceeding the critical point and changes to the state of strongly nonideal plasma. As in the case of the results of electrical conductivity measurements, the density of 9 g cm−3 separates the metal and dielectric regions. The measured absorption and reflection coe cients were used by Hefner et al. (1980), Cheshnovsky et al. (1981), and Hefner and Hensel (1982) to obtain the frequency dependencies of dielectric permeability and electrical conductivity (cf. Chapter 8).

The emissivity of mercury was measured by Uchtmann et al. (1981) in an apparatus analogous to that shown in Fig. 2.14. The radiation intensity of a plane layer of plasma was determined as the di erence between its values recorded in an empty cell and in a mercury–filled cell. The emissivity was determined as the ratio between the intensity being measured (normal to the surface of plasma layer) and the intensity of the perfect radiator. The isochores of emissivity are presented in Fig.2.17.

A characteristic feature of these curves resides in the occurrence (at densities

	PLASMA HEATING IN FURNACES				39

eV

Fig. 2.16. Reflection coe cient of mercury as a function of frequency (Ikezi et al. 1978): solid curves, experiment; broken lines, calculated curves (Chapter 8); 1 – ρ = 13.6 g cm−3

/ p = 120 MPa / T = 20 ◦C; 2 – 12 / 186.5 / 650; 3 – 11 / 177.5 / 1090; 4 – 10.6 / 169 / 1220; 5 – 10 / 174 / 1325; 6 – 9.6 / 177 / 1390; 7 – 9 / 179 / 1440; 8 – 8.3 / 181 / 1475; 9

– 6.5 / 185 / 1510; 10 – 5 / 187.5 / 1575; 11 – 4 / 175 / 1550.

exceeding 3 g cm−3) of a continuum in the infrared region, the intensity of which grows sharply with further increase in density. This is in good agreement with the results of measuring the absorption index at the same densities and will be discussed in Chapter 8.

.
.	.
.
.	.		.	.		.	.
.		.
.	.	.	.		.	.	eV

Fig. 2.17. The emissivity of mercury plasma at 1600 ◦C and various densities ρ (Uchtmann

et al. 1981).