Coupling between plasma electron and ion oscillations

Voltut~ 12, m~aber 3

COUPLING

PHYSICS L E T T E R S

BETWEEN

PLASMA

ELECTRON

1 October 1964_

AND

ION

OSCILLATIONS

J. M. JONES and K. G. EMELEUS

Physics Department, Queen's University, Belfast l~coived 4 September 1964

Plasma electron oscillations generated spontaneously In beam-plasma systems are often assoclated with low frequency fluctuations, apparently ionic in origin, and it has been suggested that there i s a causal relationship between the two 1,2). This letter r e p o r t s briefly some observations which give some support to thin view. The present experiments were mostly done with a low-voltage discharge in mercury vapour (pressure ca. 1 #) between a plane 8 m m diameter oxide-coated cathode and a 3 c m diameter nickel anode 7 c m apart~ mounted in a 5 c m internal diameter cylindrical Pyrex tube. Tube voltages were from 17 to 2~ V. The electron oscillations were picked up by a Langmuir type probe, a few m m to the anode side of the main oscillation front (meniscus), loosely coupled to a U H F receiver. The receiver output was displa).od on the upper trace of a TekLronic type 555 oscilloscope. The low frequency oscillations were observed as voltage flu(tuations between anode and cathode, and displayed on the lower trace of the oscilloscope. Three fairly distinct low frequency regimes were found: (a) With small currents (less than 25 m A ) a small amplitude (ca. 100 m V ) monochromatic signal was recorded, whose frequency varied linearly with the squ~.re root of the anode current. This suggested ion oscillations at or near the upper limiting frequency; the frequency (ca. 1 Mc/s) was consistent with this. (b) At higher currents, the signal was of m u c h larger amplitude and of lower frequency (e.g. 160 kc/s) suggesting some ionic sound wave mechanism. It was ~sually very noisy. (c) For certain critical probe positions a pure sJnusoidat signal was observed with frequency in the same range as for (b), apparently a r i s i n g from a standing ion acoustic wave between probe and caLhode. Electron oscillations (frequency s o m e hundreds of Me/s) could ~e detected above about 18 n~t., and

were found to have properties determined by the particular L F regime obtaining. In case (a), the electron oscillations were frequency modulated at the ion frequency. F r o m linear theory one would expect no ac electron charge density at the ion plasma frequency. If however the frequency were a little below this, or the oscillations non-linsar, or if the oscillations were associated with some other process such as a two-stream instability in the cathode sheath, then the necessary variation in plasma electron concentration at the site of generation of the electron oscillations could occur. In the c o m m o n e r higher current case (b) the electron osciliatlons were recorded as short bursts of rather less than 1 ~sec duration, phased with respect to the L F wave form, an effect somewhat s i m i l a r to one described by Cutler 3), and possibly related to the interesting examples of intermittence described by Morris ,t). It was at f i r s t thought that this was due to the large amplitude L F fluctuations giving an electron frequency deviation which was large compared with the bandwidth of the receiver (3 Me/s). Ifowevur, it was found, using a wide-band detector, that the discontinuities in oscillation intensity could be genuine. When the electron oscillations occurred in two separate frequency bands, their respoctive pulses oce~wred in anti-coincidence, as with some discharges in magnetic fieids described by Ifsieh 5). In the special higher current case (c) evidence was found both for discontinuities and for large frequency modulation of the electron oscillations. The frequency modulation in this case was not harmonic, having instead a form suggesting that a relaxation process was involved, possibly connected With frequency pulling of the type described by Sumi 6) : I t is evident that the processes occurring are Of considerable complexity, and possibly the manifestation of s0me kind of turbulence ~), with the side of the oscillations moving about, as well as their frequency varying. 187

Voluble 12, number 3

pIIYSICS L E T T E R S

The research reported in this document has been sponsored by Aeronautical Research Laboratory, OAR through the European Office, Aeros[,aee Research, United S~:atcs Air Force. D T.K.Allen, R.A. Bailey avd K.G.Emeleus, Brit. J. Appl. Phys. 6 (1955) 320. 2) K.G.Emeleus and A.Gars~adden, Suppl. Nuovo Cimento 26 (1962) 40.

MOLECULAR

1 Cctol~r 19~4

3) W.H,Cufler. J, Appl. Phys. 35 (1964) 464. 4) G.T.Morr~, Bull, Am. Phya. Be,c. 8 (1963) 169; ProgTe~ r e v [ ~ , IHa~ma phynica lalmrat~ry, ~c~lng Scientific Research Latmratorlas (1963L 5) H. Y. It$ieh, Q~arterly p r e s s report rw, 72, Research Laboratory of Electr~lc~, M.I,T, (1964) I17. 6) M, Sami, J. Phys. ~ . Japan 14 (19~9) 1093. 7) K.G.Emeleus, Prec. Phys. SCC. (London) B64 (1951) 166.

M O T I O N S IN L I Q U I D A N D S O L I D O R T H O ~ H Y D R O G E N

P. A. EGELSTAFF, B.C. FL~,YWOOD, F . J . WEBB and A. H. BASTON A uclear Physics Division, A. E. R. E., Hat,yell Received 4 September 1984

Using the cold neutron apparatus on the DIDO reactor at Harwell described by Harris et al. 1), we have measured the distribution of cold neutrons scsttsred from sol~d hydrogen at 12OK and liquid hydrogen at 15o, i 8 ° and 21ceK. Orthohydrogen scattering is interesting for s e v e r a l reasons: f i r s t the t r i p l e t ortho state should be split in the liquid and solid by the interactions between neighhouring molecule,:, and give a r~easurable broadening of the qlmsi-elastic scattering peak, secondly the ortho-para transition (i.e., the rotational transition J= 1 to 0) at around 15 meV is de-exci~ed by n e u r o n scattering and should be easily distinguished from the quasielastic peak, and thirdly the broadening of these two peaks should provide some information on the diffusive and vibrational modes of the molecules. In the present experi;~epts the specimen of hydrogen was contained in a s e r i e s of parallel stainless steel tubes, 1 mm diameter with wall thickness 0.65 ram, in a cryostat. Gaseous hydrogen at 20oc was condensed down into this specimen holder in about half an hour and scattering runs lasting about ten hours were commenced. Thus the ratio ~f ortho to para hydrogen during the run was only slightly less than the room temperature value of S : 1 since the conversion rate at 20°K is around 1% per hour. The c r o s s section for the scattering of neutrons from ~ara hydrogen is about 4 barns for neutrons unable to excite the para-ortho transition 2). As this is ~ ~ 0 f the ortho hydrogen scattering c r e s s sectinn, the con188

tribution o~ para-hydrogen scatter ing to the p r e s ent experiments is 2%, and has been ignored. Typical experimental r e s u l t s a r e shown in fig. I, Fig. l a shows distributions in reciprocal velocity of 4 ~, neutrons scattered from liquid hydrogen at 16°K at various angles. The ortho-para conversion line at 5~.0 ~t~/m and the quasi-elastic peak at 1030 ~ s / m can be clearly resolved. Fig. l b shows the ortho-para conversicn line on an enlarged scale. It is clear from this figure that the time-of-flight corresponding to maximum intensity v a r i e s with angle, but it has been shown that this effect disappears when the data are corrected for the detailed balance factor and tim,:~-of-fiight is converted to a constant energy interval scale. All the r e s u l t s are consistent with an ortho-para conversion energy of 15.2 • 0.5 MeV and no change with temperature in the range 12 to 21OK has been detected. This compares with the value of the transition in the gas of 14.7 MeV deduced from the J = 3 to 1 ortho transition measured Lu infrared experiments 3). As can be seen, the width of the quasi-elastic and ortho-para conversion.lines i s a function of angle of scatter. The widths ~re made up of three components: f h ' ~ due to the ipstrumental r e s o l u tion which is almost independent of angler second due to the diffusive and acoustic motions of the molecules, third due t o the splitting of the t r i p l e t ortho state. The instrumental resolution is indicated in fig. 1. Diffusive motions of the at0ms'can be expected to produce a linewidth of the order of ~,Q2D where Q is the momentum t r a n s f e r r e d i n