Experimental observation of surface plasma wave phase constant limit

Volume 90A, number 4

5 July 1982

PHYSICS LETTERS

EXPERIMENTAL OBSERVATION OF SURFACE PLASMA WAVE PHASE CONSTANT LIMIT P.K. CIBIN Institute of AppliedPhysics, P.O. Box 58, 11071 Beograd, Yugoslavia

and B.Lj. ~IVKOVI~ Boris Kidrië Institute, P.O. Box 522, 11001 Beograd, Yugoslavia Received 2 October 1981 Revised manuscript received 24 April 1982 It is shown experimentally that the phase constant of surface plasma waves has a maximum value and that the dispersion relation of these waves has a backward part.

Much theoretical and experimental work has been done on the study of surface plasma waves (waves with their amplitude maximum at the plasma— dielectric interface) and it has been obtained that the phase constant attains infinity at resonant frequency [1—7].In our theoretical investigations we have found that the presence of dissipative processes in the plasma and/or surrounding dielectric limits the phase constant of surface plasma waves [8—11].The aim of our experiment is to check our theoretical predictions. In our experiment, the plasma column is confined in a pyrex tube with an inner radius a = 25 mm and an outer radius b = 27 mm. We excite the axially symmetric mode of surface plasma waves using an axially symmetric coupler. For surface plasma wavelength measurements we use a moveable axially symmetric receiving coupler.. According to our theory [12,13], the attenuation constant in the backward wave region is larger than the phase constant and therefore for wavelength measurements we use a homodyne with amplitude modulation (fig, 1). Fig. 2 shows the measured phase characteristic of the axially symmetric mode of surface plasma waves for discharge current ‘a =~ mA and mercury-vapor pressure p = 0.3 Pa. This figure shows that the phase constant of 1these waves attains its 69 maximum values at the frequencyf= MHz, and above 13 = 170 rn— 0 031-9163/82/0000—0000/$02.75 © 1982 North-Holland

this frequency surface plasma waves are backward waves (waves with negative group velocity). Backward surface plasma waves have been observed in a number of experiments [4—6,14], but these waves have been identified as dipolar surface plasma waves, or as the axially symmetric mode coupled with the dipolar mode [141.

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Fig. 1. Homodyne with amplitude modulation for wavelength measurements of surface plasma waves.

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Volume 90A, number 4

PHYSICS LETTERS

5 July 1982

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Fig. 2. Experimental phase characteristic of surface plasma waves propagating along a plasma column contained in a pyrex tube with an inner radius a = 25 mm and an outer radius b = 27 mm, for discharge current ‘a = 4 mA and mercury-vapour pressure p = 0.3 Pa.

In our experiment, the surface plasma wave attenuation increases as the operating frequency is raised and we can conclude that in this case we have the axially symmetric surface plasma waves. Our theory [12,13], relates the largest value of the surface plasma wave phase constant to the ratio of the collision and plasma frequency. The calculation based on the data from fig. 2, gives the plasma frequency w = 10~s~ and collision frequencyi’ 42X 106 5i~ In the same case, Langmuir probe measurements give the electron temperature Te 3= 4[15]. eV and The electron correconcentrationne 2 Xplasma 108 cm— sponding collision=and frequency are v = X106s1andw~=8X108s1[16]. We wish to acknowledge many useful discussion with Professor B.A. Ani~in.

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(1959) 1784.

[31 Ya.B. Fainberg 29 (1959) 549. and

M.F. Gorbatenko, Zh. Tech. Fiz.

[4] Y. Akao andY. Ida, J. AppI. Phys. 34(1963) 2119. [5] Y. Akao and Y. Ida, J. App!. Phys. 35 (1964) 1384. [6] R.N. Carlile, J. App!. Phys. 35 (1964) 1384. [7) B.A. Ani~in,Proc. VII Int. Conf. Phen. in Ion Gases, Beograd, III (1965) 143. [8] P.K. Cibin, Proc. VIII Yug. Sump. Phys. Ion. Gases, Dubrovnik,p. 515. [9] P.K. Cibin, Proc. IX Yug. Symp. Phys. Ion. Gases, Dubrovnik, p. Phys. 313. 40 suppL 7(1979)599. [10] P.K. Cibin, J. [11] P.K. Cibin, Phys. Lett. 70A (1979)103. [12] P.K. Cibin, Proc. XIII Int. Conf. Phen. in Ion. Gases, Berlin,793. [13] P.K. Cibin, Plasma Phys. 22 (1980) 609. [141 B.B.O’Brien, Plasma Phys. 9(1969)369. [15] F.F. Chen. Plasma diagnostic technices, eds. R.H. Huddlestone and F.L. Leonard (Academic Press, New York, 1965) p. 113. [16] I.R. Gekker, Interaction of strong electromagnetic fields with plasma (Atomizdat, Moscow, 1978).