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Ion surface waves
PHYSICS
Volume 21, number 6
ION
Physics
Department,
LETTERS
SURFACE
1 July 1966
WAVES
K.G.EMELEUS School of Physics Queen’s University,
and Applied Belfast
Mathematics,
Received 21 May 1966
Attention is called to a possible example of ion surface waves and to the likelihood that these contribute to the complex sub-megacycle noise spectrum of bounded low-pressure discharges in general.
In a recent theoretical paper on ion wave on bounded plasmas, Andersson and Weissglas [l] have shown that, in addition to the infinite number of ion wave modes which degenerate into longitudinal waves in an infinite plasma, there should be a surface mode, corresponding to the well-known electron surface mode. This letter calls attention to the existence of oscillations which may be these ion surface waves, perhaps modified by ion-neutral collisions and finite ion temperature, in one class of cold-cathode discharges. They were first noticed in discharges through iodine vapour at a pressure of 200 microns in a cylindrical tube [4] and have been found since in carbon dioxide [5]. Fuller details of their generation and properties will be found in [4]. They originated from small, unstable irregularities close to the edge of an unbevelled disc cathode which did not have complete lateral dielectric shielding. Frequencies were of order of several 100 kc/s, that is, in an ion oscillation range for the plasmas in question. The plasma column and generating oscillations were certainiy coupled via the cathode fall space and possibly also by the external circuit. The location of the generating oscillations near the edge of the tube would favour the production of surface waves, whatever the exact nature of the latter. Recent experiments with a moveable probe by E. W. Gray
have shown that the oscillations in the positive column in iodine have a pronounced maximum near the surface. Andersson and Weissglas [l] refer to a possible example of surface waves in a low-pressure mercury plasma [6]. Irrespective of the correctness of that particular identification, it seems likely that surface waves will be found to be a constituent, possibly common, of the complex sub-megacycle noise spectrum of low pressure bounded discharges in general. The author is indebted to Dr. J.R.M.Coulter and Mr. E. W. Gray for discussion and to Mr. E. W. Gray for making available the results of his experiments.
References 1.
2. 3. 4. 5. 6.
B, Andersson and P. Weissglas, Phys. of Fluids 9 (1966) 271. A. W.Trivelpiece and R, W.Gould, J. Appl. Phys. 30 (1959) 1784. J.M.Jones, J.Elect.Control 17 (1964) 193. E.W.Gray, J.R.M.Coulter and K.G.Emeleus, Electronics Letters 1 (1966) 30. J.R.M.Coulter and S.P.Carswell, unpublished observations. P.F.Little and G.H. Jones, Proc.Phys.Soc. 85 (1965) 979.
633
Volume 21, number 6
ION
Physics
Department,
LETTERS
SURFACE
1 July 1966
WAVES
K.G.EMELEUS School of Physics Queen’s University,
and Applied Belfast
Mathematics,
Received 21 May 1966
Attention is called to a possible example of ion surface waves and to the likelihood that these contribute to the complex sub-megacycle noise spectrum of bounded low-pressure discharges in general.
In a recent theoretical paper on ion wave on bounded plasmas, Andersson and Weissglas [l] have shown that, in addition to the infinite number of ion wave modes which degenerate into longitudinal waves in an infinite plasma, there should be a surface mode, corresponding to the well-known electron surface mode. This letter calls attention to the existence of oscillations which may be these ion surface waves, perhaps modified by ion-neutral collisions and finite ion temperature, in one class of cold-cathode discharges. They were first noticed in discharges through iodine vapour at a pressure of 200 microns in a cylindrical tube [4] and have been found since in carbon dioxide [5]. Fuller details of their generation and properties will be found in [4]. They originated from small, unstable irregularities close to the edge of an unbevelled disc cathode which did not have complete lateral dielectric shielding. Frequencies were of order of several 100 kc/s, that is, in an ion oscillation range for the plasmas in question. The plasma column and generating oscillations were certainiy coupled via the cathode fall space and possibly also by the external circuit. The location of the generating oscillations near the edge of the tube would favour the production of surface waves, whatever the exact nature of the latter. Recent experiments with a moveable probe by E. W. Gray
have shown that the oscillations in the positive column in iodine have a pronounced maximum near the surface. Andersson and Weissglas [l] refer to a possible example of surface waves in a low-pressure mercury plasma [6]. Irrespective of the correctness of that particular identification, it seems likely that surface waves will be found to be a constituent, possibly common, of the complex sub-megacycle noise spectrum of low pressure bounded discharges in general. The author is indebted to Dr. J.R.M.Coulter and Mr. E. W. Gray for discussion and to Mr. E. W. Gray for making available the results of his experiments.
References 1.
2. 3. 4. 5. 6.
B, Andersson and P. Weissglas, Phys. of Fluids 9 (1966) 271. A. W.Trivelpiece and R, W.Gould, J. Appl. Phys. 30 (1959) 1784. J.M.Jones, J.Elect.Control 17 (1964) 193. E.W.Gray, J.R.M.Coulter and K.G.Emeleus, Electronics Letters 1 (1966) 30. J.R.M.Coulter and S.P.Carswell, unpublished observations. P.F.Little and G.H. Jones, Proc.Phys.Soc. 85 (1965) 979.
633