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Powder” Lichtenberg Figures and the Ionization of Dielectric Surfaces Produced by Electrical Impulses* by A

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Powder” Lichtenberg Figures and the Ionization of Dielectric Surfaces Produced by Electrical Impulses* by A 6‘Heat developed” and “powder” Lichtenberg figures and the ionization of dielectric surfaces produced by electrical impulses* By A. MORRISTHOMAS, BSc., F.Inst.P., M.I.E.E., The British Electrical and Allied Industries Research Association, Greenford, Middlesex [Paper received 14 December, 19501 A brief outline of the characteristics of Lichtenberg figures is given. Some experiments on “heat developed” and “powder” figures are reported. With regard to the former it is shown that the effect is restricted to certain kinds of solid dielectrics and associated with the state of the surface; an explanation of the mode of their formation is suggested. The effect of repetition of impulses of alternating polarity is investigated by means of “powder” figures. It is shown that the effect of a discharge of given polarity is not neutralized or cancelled by a succeeding discharge of the opposite polarity. This leads to the conclusion that local electrical or mechanical failure in the surface may occur owing to the chance that positively and negatively charged particles will in the course of time be brought very close together. A difference between the Lichtenberg figures produced by negative impulses of short and long wave front and tail is demonstrated; an explanation based on the theory of spark discharge is given. The electrostatic charges produced on the dielectric surface by single and multiple impulses are measured. The results indicate that in order to produce the charge, electron emission from the dielectric surface, when this is the cathode, is necessary; this is evidence that formation of a cathode spot on the dielectric surface is possible, and may be capable of triggering the process which results in failure of the dielectric. 1. INTRODUCTION Having regard to the investigations of the mechanism The surface of a solid electrical insulating material is of the electrical breakdown of solid insulating materials affected in a characteristic way when it is adjacent to which are now in progress, any information appertaining or in contact with a conductor charged to a potential to the interaction of such materials with a gaseous sufFiciently high to produce an electrical discharge or discharge is of value; it was thought worth while there- ionization in the ambient gas. The effect is not imme- fore to undertake a short experimental investigation of diately visible but its existence is demonstrable by dusting these “heat developed” figures. This comprised tests on the surface with a dielectric powder. When the surplus a limited range of materials in order to obtain an indica- powder is gently blown away, the powder which remains tion of the type of materia1 which possessed the requisite adhering to the surface forms a pattern which is star-like, property, and further tests to enable this to be identified. diffuse or confused, according to the polarity and time During the progress of this work, however, other eEects variation of the applied potential. These patterns are were observed incidentally which prompted some further called “Lichtenberg figures” after their discoverer.(‘) If research with respect to Lichtenberg figure formation in the dielectric surface is unexposed photographic emulsion general, apart from the “heat developed” phenomena. and if this is developed and fixed by ordinary photo- For example, certain features of interest associated with graphic procedure after exposure to the electrical dis- the figures obtained in uniform or very slightly divergent charge, then images in the amidentical in size and shape fields are reported. These results appear to break new with the “powder” figures are obtained. With some ground since hitherto the study of Lichtenberg figures materials, e.g. glass, latent Lichtenberg figures may be has been more or less restricted to the patterns obtained produced owing to local physical changes of the dielectric in strongly divergent fields, presumably because under surface and be rendered visible merely by breathing on these conditions their size is directly related to the the surface, owing to selective condensation of moisture, amplitude of the applied impulse. In addition the effect or by the reaction of the surface with a silvering solution, of discharge repetitions is studied and measurements of the silver being selectively deposited;(2)in such cases a the electrostatic charge which is the primary cause both number of repetitions of the discharge is necessary before of the “powder” and “heat developed” Lichtenberg the effect is appreciable. figures are made. The present author observed during the course of an investigation on a silicone varnish that latent Lichtenberg 2. CHARACTERISTICS AND APPLICATIONS OF figures on the surface of certain kinds of solids could be LICHTENBERG FIGURES developed by the application of heat.(3) It has, however, The first comprehensive investigation of Lichtenberg since been ascertained that a similar observation was figures was undertaken by Pedersen.@) The photo- reported in 1897 by Swanc4) working with clear “Bor- graphic figures have been extensively used in “Klydono- deaux” resin containing some resin oil, but this effect graphs” for investigations of surge voltages on trans- does not appear to have received attention from any mission lines.(6) An interpretation of the mechanism of other investigators. their formation, based on modern theory of discharges * Based on Report Ref. L’T232 of the British Electrical and in gases, has been given by Merrill and Von Hippel;(’) Allied Industries Research Association. these authors indicate how ionization by electron impact, 98 BRITISH JOURNAL OF APPLIED PHYSICS Lichtenberg figures and the ionization of dielectric surfaces space charge and plasma formation, and charge neutral- 3. THE SURGE GENERATOR ization are all involved in the process. Lichtenberg figures are produced and studied most It is well established that Lichtenberg figures owe their conveniently by means of unipolar impulse potentials. origin to the movement of electrons in the ambient air The circuit of the impulse generator used in the present or gas when the applied electrostatic potential is of tests is shown in Fig. 1. The only feature which calls sufficient magnitude to produce ionization. The evi- for comment is the special sphere-gap switch. The dence for this is, first, the curvature of the rays of the central sphere is movable and when it is in contact with patterns which is observed when they are formed in the the left-hand sphere, capacitor C, is charged to 8.6 kV. presence of a magnetic field;(’, ‘I) secondly, that only It is attached to a spring so that on release of a stop it electrons possess the mobility needed to give the observed quickly moves towards the right-hand sphere but comes time of formation of the figures which is of the order of to rest without touching it; the gap between the spheres to sec; and, thirdly, that no figure is produced breaks down and C, is discharged through the resistors in the absence of an ionizing potential. Electrons are R, and R,, the latter being connected in parallel with deposited on or removed from the dielectric surface capacitor C, and the test specimen. Short wave front according to the poIarity of the applied surge, and the surges are obtained when R, and R, are 1 OOO and Lichtenberg figure is a result of the characteristic paths 3 OOO i2 respectively; for longer wave fronts these values which they traverse in the gas adjacent to the surface. are changed to 10 OOO and 30 OOO or 100 OOO and 300 OOO “Powder” figures are a consequence of the powder respectively. The waveforms were calculated and the particles being attracted to and retained by the surface results confirmed by a cathode-ray oscillograph; the charges; each particle adjacent to a localized charge values are given in Table 1. The surges are free from becomes a dipole by induction and so subject to an attractive force. The origin of the photographic figures has been a source of some controversy but, in the main, the experi- mental evidence indicates that they owe their existence to the emission of actinic radiation from the ionization generated by the electrons in their passage through the air or gas adjacent to the surface of the solid. The particular pattern produced by an applied impulse depends on the polarity and waveform; its overall size in a divergent field is related to the voltage amplitude. Rl,R2,R3, 20kR C1, 0.1pF A detailed discussion, including a fairly complete W,l,lOor1@@t;~.carbon/cv C2,@4@2pF bibliography, of the characteristics of Lichtenberg figures R5, 3,3@or300hQ, 11 C3, lOOpF (including test cap) has been given recently by Anstey(*) and need not be TI, 240/3500V VI.V?, U16 51. Special sphere-gap sv. itch repeated here. It may, however, be mentioned that Anstey has carried the subject a stage further by demon- Fig. 1. Surge generator strating that the wave front time of a surge potential may be estimated if a positive and a negative figure are oscillation. In Fig. 1 the generator is connected to give simultaneously obtained. a surge of negative polarity; a positive surge is obtained Two novel developments which are related to the by interchanging the connexions at a, b. As a safety Lichtenberg figure formation are of interest. First, precaution capacitor Cl is shunted by a leakage resistance Selen~i(~)has described a method of recording fast of 10MQ which serves to discharge this capacitor electrical phenomena which is based on the modulation quickly when the supply is switched off. of an electron or ion beam falling on a dielectric surface; Table 1. Waveforms of surges used in the tests the record is revealed by dusting; with the dielectric Wavefront Time-to-half Crest surface moving past the beam at a rate of 10m/sec an rime crest value voltase alternating current of 20 kc/s is resolved.
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