Electronic Space-Charge Neutralisation in a Thermionic Converter

ELECTRONIC SPACE-CHARGE NEUTRALISATION IN A THERMIONIC CONVERTER

BY V. T. CI~PLONKAR AND S. K. DESAI* (Ion Physics and Electronics Laboratory, Physlcs Department, lnstitute of Science, Bombay) Received Januaty 17, 1966 (Communir by Dr. R. K. Astmdi, v.A.sc.)

ABSTRACT

The V-I characteristies and the spaee distribution of electron density n,, temperature To, ion density n~, etc., have been studied in a thermionie converter using (i) d.c. excitation, (ii) r.f. excitation, for the auxiliary discharge in blue Argon. The latter has been found to give higher yields. Higher yields have also been obtained when the r.f. signal was aUowed to leak into the interaetion space of the deviee. Ir has been shown that Ramsauer scattering accounts for a major part of the resistivity of the deviee.

1. INTRODUCTION

Ir is known that the neutralisation of the electron space-charge represents one of the important problems in the design of thermionic converters for the direct conversion of heat into electricity. Of the various methods that have been used for this purpose, the one based on the use of the positive ions of a gas ora vapour appears to be the most promising. 1-5 The neutralisation in this case is brought about by means of the positive ions, produced (i) directly in the converter diode space itself by the surface ionisation of the atoms of a vapour like caesium (externally introduced) from the high temperature emitter surface, (ii) from ah auxiliary discharge in an inert gas (from which they are allowed to diffuse into the converter space). The latter method has several advantages as it (i) avoids the problems created by the chemical activity of Cs; (ii) enables the use of lower emitter tempera- tures with a consequent improvement in its lifetime and (iii) utilises the small collision cross-section of the neutral rare gas atoms for low energy electrons (R/amsauer effect).

* Now at Ulitsa Polytechnichcskaya-21, Lcningrad K 21, U.S.S.R. 338 Electronic Space-charge Neutralisation in a Thermionic Converter 339 Assuming that the current in the converter is carried by the electrons, the conductivity of the device will be determined by the foUowing processes or interactions: (i) electron or positive ion space-charge effects, (ii) Ramsauer scattering of electrons by the neutral gas atoms, (i¡ interaction of electrons with the r.f. voltage signal when present due to plasma oscillations or other sources and (iv) Coulomb scattering of electrons by the ions. The investi- gation under discussion was carried out in order to assess the relative impor- tance of some of these effects.

2. EXPERIMENTAL The experimental assembly consisted of three electrodes: (i) emitter E (ii) collector C and (iii) auxiliary anode A, all with a planar geometry en- closed in a cylindrical glass envelope. The emitter was made of an oxide- coated nickel wire with an effective emitting area of 0.012 cm) (estimated from the saturation emission current). The emitter was heated electrically by a L.T. battery. Its temperature was determined (i) from the measure- ment of its resistance, (ii) by the visual observation with the optical pyrocolour (disappearing filament type) and (iiŸ Maxwell-Boltzmann slope of the V~.c --lE characteristic of the deviee. The collector was an oxide-eoated dise of nickel provided with a number of holes having an axial symmetry through which the positive ions produced in the auxiliary diseharge enter into the EC region. The distance EC was kept fixed at 1.2 cm. As no external source was applied to the converter, the potential difference which obtained across it was entirely due to the difference in the work functions of the emitter and the collector, under the actual working conditions. In order to determine the distribution of (i) the electric potential (V), (ii) eleetron temperature (Te), and density (ne) and (iii) ion temperature (Ti) and density ni, in the inter-eleetrode space a single Langmuir probe was provided which eould be moved along the axis in vacuum, by a micrometer screw arrangement from outside. A eopper constantan thermocouple, included in the eleetrical circuit immediately behind the probe, enabled the gas temperature in the electrode space to be determined. A variable load of one Kilo-ohm was provided between the collector and the emitter, the self-bias voltage developed across which was measured with a V.T.V.M. The coUector was earthed and the emitter current Is was measured with a milli-micro-ammeter. The initial thermionic current of approximately 10/~A was increased to approximately 10 mA on supply of the positive ions by the auxiliary discharge. The auxiliary discharge was excited by a Hartley oscillator (frequency range = 0.8-3"0 MHZ). The r.f. excitation was preferred because of (i) lower breakdown potentials, (ii)possibility of smaller operating pressures, 340 V. T. CHmLONKAR AND S. K. DFSAI (iii) smaller voltage disturbances in the EC space, (iv) the fact that the ions which enter the EC region will ave low kinetic energies. A spectroscopi- cally pure mixture of argon and neon (1% Ne) was used. Evacuation was obtained by means of acenco rotary oil pump; the pressure of the gas being measured with a thermocouple gauge. Although approp¡ electrostatic shielding was provided for the probe and the main emitter-collector space in order to prevent a pick-up of the r.f. voltage signal from the oscillator by them, a weak leakage r.f. voltage could be observed across C and E (only when the emitter was heated) with a C.R.O. This voltage could however be bypassed by connecting a suitable condenser across EC. The frequency of the leakage r.f. signal was equal to that of the oscillator used for the excitation andat no stage plasma oscillations were observed. If one does not bypass the r.f. voltage, values of the current Ix (for a given voltage Vzc between the emitter and the collector) were found to be much higher (8-120%) than the corresponding values, when the r.f. was bypassed. Itis knowns that the application of a r.f. voltage signal to an ionised gas influences the electron collision frequency and there- foro its temperature. We have therefore taken observations on the space distributions of the potential, the electron density, etc., with and without the bypass of the r.f. The parameters he, ni, Te, and VF were determined at different points in the CE region from the Langmuir probe characteristics. As has been usually observed under these conditions, the saturation value of the ionic eomponent of the probe current could be accurately determined, but that for the electric component had to be found out from the determination of the point of inflexion$ in the semi-log plot of the characteristic because of the increase in the probe current due to the onset of the ionisation by collision. he and Te were determined from the probe characte¡ and ni was calculated from the saturation ion current based on the assumption that the ion temperature was equal to the gas temperature. The floating potentials VF acquired by the probe observed at different points in the EC region were •onsidered to be fairly good representative of the relative space potentials in this region.

3. RESULTS AND DISCUSSION The effect of the gas pressure p and the nature and the frequency of the r~f. excitation of the auxiliary discharge on the saturation r (hence on the maximum powr output Pro of the device) is well shown by the curves I-VII (Fig. 1). Ir ma)' be mentioned that the volumr density of the posifive Electronic Space-charge Neutralisation in a Thermionic Converter 341 ions in the converter space was found to be muela greater than he, the density of the electrons, for aU the conditions of the experiment used. Curves I, V, II and IV show the advantage of the r.f. over the d.e. excitation. A com- parison of the curves II, IV, V whieh were obtained with the r.f. voltage bypassed, with III, VI, VII (Fig. 1), observed without the bypass, shows that the leakage of the r.f. power to the interaction space leads to significanfly higher values for IEs and also for Te, but has no significant effect on either n e or ni. The latter result has been more direcfly demonstrated by the probe measurements.

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The spaco distribution of he, ni, Te and V~ for some of the typical observations are shown in Fig. 2. Ir will be observed that as n~ is greater than n e (by a factor of lO s -- 103), there is a net positivc space-charge in this region. he, Te and V~ show insignificant variation with the distance except in the immediate vicinity of the emitter (which may be duc to the formation of a space-charge sheath near it).

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~£237237 -"~, ", ,, ,,, ,, , ,, 0"40 0'45 O SO O'SS 0"60 {;)ISTANCE OF PROBE FROM EMITTER IN C,rfl Electronic Space-charge Neutralisation in a Thermionic Converter 343 Because of the large value of the neutral particle density as compared with that of the ions, ir is expected that the contribution of the Ramsauer scattering to the resistivity of the converter wiU be more important than that due to the Coulomb scattering. Fora feebly ionised gas the conductivity due to coUisions of the electrons with the neutral atoms is given bys

o = 0.532 he e ~ 1 no" ~'C~' where no = number of neutral molecules per era. s ne = electron volume density 1 Q : heno ~e = mean free path for coUisions. the value of, calculated for one typical observation (p ----- 195 q 820 KHZ with bypass of the r.f.) for the region over which ne and Te are approximately eonstant, on the basis of the available data 9 for the collision cross-section of the electrons in argon, gives a value of 1.5 • 10-~ mho/cm. This value com- pares well with 1- 25 • 10-4 mho[cm estimated from a knowledge of the saturation current and the geometrical cross-seetion of the EC region. Ir wiU be observed that the Ramsauer scattering accounts for 80% of the resistivity of the device. We thank Prof. (Dr.) R.K. Asundi of A.E.E.T. (Speetroscopy Division) for bis keen interest in the work. We are thankful to Sh¡ S. B. Joshi for help in finalising this work.

REFERENCES

1. Wilson, V. C .... J. Appl. Phys., 1959, 30, 475. 2. Hernquist, K. G., Kanefsky, R.C.A. Review, 1958, 19, 244. M. and Norman, F. H. 3. Grover, G. M. Roehling, J. Appl. Phys., 1958, 29, 1611. D. J., Salmi, E. W. and Pidd, R. W. 4. Medicus, G. and Wchner, lbid., 1951, 22, 1389. G.K. 5. Gabor, G. Nature, 1961, 189, 868. 6. Francis, G. _ Ionisatton Phenomena in Gases, Butterworth, l.,ol~don, 1960v p. 187. Au~trallan phys., 1962, 15, |6.~, 344 V. T. CH~LONT~R AND S. K. DESAI

8. Wtight, J.K. ., Shock Tubes, Methuen Monograph, 1961, p. 134. 9. Brown, S.C. .. Basic Data of Plasma Physics, Tr Pr~,~s, U.S,A. 1959, p. 7.

EXPLANATION OF FIGURES

FIo. I. Vine--I• Characteristic for the thermionic converter gas for auxiliar3' discharge---blue argon Emitter temperature from (I) Resistance measurements---T~, (2) Optical Pyrocolour--T~. (3) Maxwell-Bo]tzmann SIope--TRM. Approximate gas temperature--Tg. Cm've I. p = 195 microns, d.c. excitation. T~ = 1090~ Tzp ~= 1360~ Tsu ~= 225X 101 K., ]~o = 650~ K. Curve II. p = 240 microns, r.f. excitation (1" 8 MHZ) with r.f. bypass. Tm ~ 1050~ K., T~ •- 1235~ T~ = 30• T o ~ 630~ Curve III. p = 240 microns, r.f. excitation (1"8 MHz) without r.f. pypass. T~---1050~ T~ = 1235~ T~H = 65• T a = 630~ K. Curve IV. p = 240 microns, r.f. excitation (1"0 MHz)with r.f. bypass. T~ ~= ll00~ T~ =~ 1220~ Tz~ = 35• s K., T o = 620~ Curve V. p = 195 microns, r.f. excitation (820 KHz) with r.f. bypass. T~ -- 1080 ~ K., T~v 1360~ Tsu = 55• T o = 650~ Curve VI. p= 240 microns, r.f. excitation (I'0MHZ) without r.f. bypass. T~ = ll00~ T~ = 1220~ T~~ = 150• Ta = 620~ Curve VIL p ---- 195 microns r.f. excitation (820 KHZ), without r.f. bypass. T~ = 1080 ~ K. T~, = 1360~ K., Tw~ = 105• 10SK., T o = 650~

FIo. 2. Curves ! to VI'II gas-blue argon, p = 195 microns. Space distribution of electron temperature Te Curve I. d.c. excitation, other data same as for Curve I, Fig. 1.

Curve II. r.f. excitation, (820 KHZ) with bypass of r.f., other data same as for Curve V, Fig. I. Space distribution of ion density n~ Curve IIL r.f. excitation (820 KHZ) with bypass of r.f. Curve IV. d.c. excitation. Space distribution of floatlng potenial Vv Curve V. r.f. excitation (820KHz) with bypass of r.f. Curve VI. d.c. excitation. $pacr dlstribution of electron density he Curve VIL r.f. excitation (820KHz) with bypass of r.f. Curve VIII. d.c. excitation. Effect of r.f bypass on Vv and Te Gas-blue argon, p----- 240 microns, r.f. ---- 1 "0 MHZ. Curvr IX. Vv distribution with bypass of r.f. Curv X. Vp distribution without bypass of r.f. Curv XI. Te distribution without bypass of r.f. Ctu'vr XII, Tf distribution with bypass of r.f.