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FIELD EMISSION OF LIQUID METALS IN ALTERNATING FIELDS
A. Kontonistov, I. Radchenko, G. Fursey, L. Shirochin
To cite this version:
A. Kontonistov, I. Radchenko, G. Fursey, L. Shirochin. FIELD EMISSION OF LIQUID MET- ALS IN ALTERNATING FIELDS. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-203-C8-207.
�10.1051/jphyscol:1989835�. �jpa-00229933�
COLLOQUE DE PHYSIQUE
Colloque C8, supplement au n o 11, Tome 50, novembre 1989
FIELD EMISSION OF LIQUID METALS IN ALTERNATING FIELDS
A.A. KONTONISTOV, I.N. RADCHENKO, G.N. FURSEY and L.A. SHIROCHIN Research Scientific Centre for Electrophysical problems of Surface,
(RSC EPS), 191065, Moyka 61, Leningrad, USSR
Abstract
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It is experimentally shown that with oscillating electric fields of several GHz frequency at liquid conducting surfaces, the field emission of electrons and ions is possible due to the forming effect of the a.c. field on the surface.The effect of field emission is usualig observed when the electric fields at the surface of emitters E
L
10'-
l0 V/m. In practice this g%quires_,the creation of microtips having very small radii of curvature (rclO-
10 m),which are realized by traditional methods: electrochemical etching, etching in an oxygen atmosphere, tip formation in strong electric fields /l/. These methods allow reproducible emission characteristics of solid state emitters in a wide range of fields and currents / 2 / . The use of liquid metals allows solution of the problem of the creation of super strong electric fields at the surface of an emitter, applying the phenomenon of hydrodynamic instability of liquid metal surfaces In the electric field
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i.e. theTonks-Frenkel mechanism / 3 / . In particular, in liquid metal ion sources '(LMIS~, the typical dimensions of submicron inhomogeneities produced have ralO m, which provides for the creation of the fields required for emission.
It has been recently reported /4/ that field electron emission from liquid metals in static electric fields was observed using the same LMIS-technique, but changing the polarity of the voltage at the emitter.
In a.c. fields, an alternative mechanism of the creation of conducting liquid surface microgeometry by means of parametric resonance is possible 5 As the analysis shows, the development of such an instability in super high frequency (S!%) fields results in a capillary wave swing, with typical dimensions of c10 m, that, even with relatively low SHF-field strengths, may cause field emission.
The experiments were conducted in an SHF prismatic resonat05 (HlO3-mode), oscillation wavelength, A = 6.4 cm, input power Pin ( 1.5 X 10 W /6/. D.C.
components of the cathode current (I ) and those of the beam collector (I placed in the drift space of Ehe beam outside the resonator, were meszured. Liquid capillary emitters (K-Na alloy and fusible alloys :
capillary indium) and needle type with micron layer of Bi-Sn-Pb alloy on the solid base, were investigated.
The materials having low melting temperature (up to 1 5 0 ~ ~ ) were chosen to exclude thermoemission effects. Fusible emitters were first melted by an extra external heater.
In Fig 1 watt-ampere characteristics (WAC) are shown for various field electron emission cathodes in Fowler-Nordeim co-ordinates (here it has keen taken into account that the squafe of the field in the resonator is proportional to the input: power E %Pin). The experimental points in the graph are in correspondence with the mean values of the impulse current (see Fig 2). The measurements were taken in ~ingle-~SHF power impulse operation, having pulse-duration, T
,
not higher than 10 S, with the aim of excluding the transition of the fieyd electron emission (FEE) into explosive emission (EE) / 7 / . Here it should be noted that, in contrast to the case of high melting field emission cathodes, (for example, classical tungsten) upon EE occurrence, the parameters of liquid metal emitters are fully restored Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989835after the explosion. WAC's of the liquid cathodes reproduced, in a SHF fiefd, are observed in the vacuum up to the residual gas pressures of P %
10 Torr.
In comparison to the results obtained in static conditions / 4 / , in our case the relatively small spread of experimental points in the WAC can be seen.
(Fig 1)
.
Fig 1 Current-voltage characteristics for field electron emission from liquid metal surgaces subjected to SHF fighds.
1. K-Na, T = 3 X 10 S. 2. Bi-Sn-Pb, r = 10 S.
P P
This is, obviously, related to the special features inherent in the formation of the emission surface in SHF fields when, at the liquid metal, the "multicusp" system, having submicron dimensions, is created / 5 / , with statistically well reproducible emission area. (from pulse to pulse.)
As is seen from field emission oscillograms (Fig 21, there e x i e s a delay time ( T ~ )
,
for the creation of a noticeable current of FEE ( ~ 1 0 - A ) , which is typical for each level of SHF power and, correspondingly, of electrical field. In accordance with the concepts developed in /5/, this is determined by the amplitude swing of parametric oscillation of the liquid, up to the level when the field amplification factor at the inhomogeneities approaches a value sufficient for the emission current to be registered by an oscilloscope.Fig 2. Electron current and input power Pin oscillograms for (Bi-Sn-Pb) -cathode
~R'SHF
field.1. Pin = 1000 W, 2. Pin = 1220 W, 3. Pin = 1390 W.
In Fig 3, the typical dependence of T on the input power, P. for (Bi-Sn-Pb) cathode is shown, which is simiAr to the results obtainedl?or EE in / 5 / .
The specific feature of the field emission of liquid metal is the noticeable fluctuation of the current level (Fig 2), which is related, in our opinion, to the peculiarities of the behaviour of the liquid conducting surface in the field and requires further study.
Control experiments performed in static fields, with the same cathodes, have shown that the emission process in this case is not excited, even for d.c.
fields more than an order of magnitude larger than the rms value of the oscillating SHF field. This fact allows us to state that, in the present experiments, the effective SHF field at the cathode was less than the critical Tonks-Frenkel field for the limited surface / 3 / .
Fig 3. The dependence of T on the input power Pin for (Bi-Sn-Pb)
-
cathode. PO = 870
a.
Tt is quite evident that the excitation of a liquid conducting surface by SHF field makes possible the selection of ion current, provided there is sufficient enhancement of the field at the emitter within the positive half-period of SHF oscillations. However, in the total electron-ion flow in the resonator, the portion of the ion current is very small, due to the small velocities gained by ions in the SHF field. Applying an additional constant positive bias, U
,
to the cathode, we suppressed the electron emission and increased the Eon current to the collector, II In Fig 4 the oscillograms of field electron emission current,
1Zol (ayob'f input power, P .,
(b) and of ion currents, 1Iol (C) shown are excited at the same SHF pbGer level but with different vaEues of U at the indium emitter. From the very beginning, we shall note that the de?ay time ( T ~ ) for the ion current is considerably greater than in the case of FEE (compare Figs 4a and 4c).Fig 4. Current of collector and input power oscillograms for indium emitter.
a) FEE current Icol, Uc e = 0; b) input power pulse Pin; C) ion currents Icol. 1. Uc i = 1000 V, Pin = 540 W; 2. Uc = 3500 V, Pin = 540 W; 3. 'Uc = 1000 V, Pin = 700 W.
Fig 5. Retarding potential energy curve for indium emitter.
1. Pin = 400 W, Uc = 2600 V, = 6.10-~ 1: 2. Pin = 500 W, UC
= 2850 V, 1; = 2 0 . 1 0 - ~ ~ ; 3 . Pin = 600 wC, Uc = 2600 V, :1 = 7 5 . 1 0 ~ ~ ~ .
Evidently, this is due to the fact that ion emission requires the larger field value, i.e. the longer time for microprotrusion growth.
In contrast to electron emission, which is of a quasistationary nature (see Figs 2 and 4a), ion currents have unstable and ever-growing character.
Depending on the level of Pin with the value of U given, at a certain instant of time, the velocity of current growth is ihreased and, as in the case of EE /7/, at a certain density, the ion current is increased explosively by 2-3 orders of magnitude.
In Fig 5 the results of studies on the energy distribution of the ion beam from the resonator to the collector are shown. These were obtained py means of a delaying potential, U applied to the analyzing grid.
9'
The relationship of the collector, 1; and cathode, I ~ , currents depend on the value of the retarding potentiaq as a fraction %f the value of the positive bias at the emitter U (oscillatory energy of ions gained in SHF field has been neglected), as s h w n on the graph.
From the graph, it follows that the emission current growth results in the emission of ions, most of which have small energies. Obviously, this is due to the fact that the flow of neutral particles ionised at considerable distances from the emitter is increased. In the case of low currents, it may be considered that noticeable evaporation of neutrals is absent and all the ions appear directly at the emitter. The presence of a high-energy 'tail" can be explained by ionization and acceleration of the ions within the gap between the analysing grid and the collector.
Thus, in the present work, for the first time, it is experimentally shown that, in SHF fields at liquid conducting surfaces, field emission of electrons and ions is possible due to the forming effect of the a.c. field on such surfaces.
REFERENCES
1. Muller E.V., Tsong, T.T. Field ion microscopy. Elsevier, Amsterdam 1969.
2. "Non-accumulated cathodes", Elinson M. I (ed.).
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M. "~ov.radio", 1974, 165.3. Tonks L. Phys.Rev., (1935)
48,
No. 6, 562.Frenkel J., Phys. 2s. "Sowietunion", (19351, No.8, 675.
4. Hata K., Ohya R., Hishigaki S., Tamura H., Noda T., Japan.Journ.of Appl.Phys., (1987), 26, No.6, L 896.
5. Baskin L.M., Kontonistov A.A., Fursey G.N., Shirochin L.A., DAN USSR, (1987), 296, No.6, 1352.
6. Ananyev L.L. at al.
-
Sov.Journ.Instrum.and Exper.Techn. (PTE), (1983) ,No.5,165.7. Fursey G.N., IEE Trans .on Electr. Insul., (1985)
