Investigation of pulsed high current gas discharges by high speed photography

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  • J. Nucl. Energy 11. 1959, Vol. 9, pp. 135 to 139. Perpamon Press Ltd., London. Printed in Northern Ireland

    INVESTIGATION OF PULSED HIGH CURRENT GAS DISCHARGES BY HIGH SPEED PHOTOGRAPHY*

    N. A. BORZUNOV, D. V. ORLINSKY and S. M. OSOVETS

    Abstract-High speed (lo6 frameslsec) photographic data on pulsed high energy gas discharges in deuterium and some of the inert gases have been combined with oscillograms of current and voltage to give a qualitative description of the basic processes occurring in the initial stages of a high current discharge. The experimental results confirm the basic conclusions of the inertial theory of plasma constriction worked out by LEONTOVICH and OSOVETS.

    THIS paper describes the results of studies on high current discharges in deuterium, neon, argon, krypton and xenon performed with a high speed tine-camera. The discharges studied carried currents of over lo5 A at gas pressures from 0.01 to 1 mm Hg. The basic phenomena in such discharges have been previously described.(l)

    Figure 1 gives a typical oscillogram of the discharge current and of the electrode

    40kV

    FIG. l.-Oscillogram of current and voltage for a discharge in deuterium (p,, = O-2 mm Hg).

    voltage as obtained at an initial pressure of 0.2 mm Hg in deuterium. In the initial stages, the curve of 1(t) shows several kinks, to each of which there corresponds a sharp drop in the voltage across the electrodes. In the following, we shall term these kinks in the current curve singularities and we shall call the time elapsed between the beginning of the discharge and the occurrence of the kink the time of the singu- larity.

    ARTSIMOVICH et u1.o) have shown that the time of a singularity and the current at that time change in a regular manner with varying initial gas pressure and with the voltage applied to the electrodes. LEONTOVICH and OSOVETS(~) have explained the regular behaviour in terms of gas motion from the periphery to the centre of the dis- charge under the action of electro-magnetic forces. They show that the main force counteracting the compression is inertial. A visual observation of discharges on a

    * Translated from Atomnaya Energiya 4, 149 (1958). 135

  • 136 N. A. BORZUNOV, D. V. ORLINSKY and S. M. Osovm

    series of consecutive photographs leads to a qualitative confirmation of this descrip- tion. The vivid picture of the development of the discharge provided by the photo- graphs is also of independent interest.

    The discharge was produced in a cylindrical glass tube with an internal diameter of 18.5 cm and with electrodes 97 cm apart. The current source was a condenser battery of 35 PF capacity charged up to 40 keV. The discharge current was measured with a measuring coil for which OJL> R, so that the magnitude of the current itself was measured, not its derivative, as would have been the case for COL < R. The signal from this coil was fed to the deflector plates of an OK 17 oscillograph. Under these conditions, the maximum current in the first half period was about 220 kA.

    The discharge was photographed with the very high speed camera SFR, working as a slow motion camera. This arrangement permits the photography of up to 2 x IO6 frames per second. In order to determine the particular phase of the discharge to which a given photograph corresponds, the following scheme was used. Immediately behind the exposure position of the am, an FEU 19 photo-multiplier was inserted whose photo-cathode was so covered that light could impinge on it only through one of the lenses of the optical system. The output signal from the photo-multiplier was passed through a cathode follower and amplifier and fed to the second pair of oscillo- graph plates (Fig. 2a). At the time, t,, when the photo-multiplier was illuminated, a certain frame was exposed but the frame adjacent to it (in position, not in time) remained unexposed (Fig. 2b). This method fixed the position on the photograph of the frame corresponding to the appearance of the pulse on the oscillogram. From the knowledge of the speed of revolution of the mirror and the time scale of the oscillogram, the stage of the discharge corresponding to any frame could be found. The accuracy of this method of identification depends on the speed of revolution of the mirror; at II = 60,000 rev/min, which was the speed at which all the photographs here given were taken, the error in identification of frames was no more than &O-25 ,usec.

    Figures 3-6 show photographs of discharges in deuterium, neon, argon and kryp- ton taken at various initial gas pressures. Figs. 7 and 8 give photographs of discharges in deuterium and in xenon obtained with the same SFR apparatus working so as to show a continuous picture of the discharge development. This was achieved by placing a slit between the first and second objectives in the plane of the intermediate image. The width of the slit was so chosen that only a 1-2 mm thick portion of the discharge chamber was projected on the rotating mirror. As a result of the rotation, the image was displaced along the film in the direction of the axis of the discharge chamber. Thus the film recorded a picture of the time-changes occurring in a small region of the discharge. (At n = 60,000 rev/min the image moved across the film at a rate of 3 X 105 cm/set.)

    The qualitative picture of the development of the discharge can be described as follows. In the very early stage, the gas is observed to glow only near the walls because at this time the skin effect causes the current to be confined to the periphery of the chamber. Then the plasma column begins to contract. The speed of contraction increases with decreasing mass of the gas. Curves showing the time dependence of the radius of the glowing region of the plasma column in a deuterium discharge are given in Fig. 9. During contraction, the inductance of the column increases and therefore the discharge current curve differs noticeably from sinusoidal shape (see Figs. 1 and 2). Simultaneously, the electrode voltage increases. The cessation of glow, and hence of

  • FIG. 2(a). 2 4 6 0

    FIG. 2(b).

    FIG. 2(a).-Oscillogram of current and timing pulse for a discharge in deuterium (pO = 0.03 mm Hg).

    FIG. 2(b).-Photograph of the same discharge as in Fig. 2(a).

    FIG. 3.-Photographs of discharges in deuterium with different initial pressures, pO.

  • FIG. 4.-Photographs of discharges in neon with different initial pressures, pO.

    ifferent initial pressures, po.

  • FIG. S.-Photographs of discharges in xenon with the uninterrupted picture arrangement.

  • Investigation of p&ed high current gas discharges by high spead photography 137

    current, at the periphery is only possible as a result of a pressure drop at the walls. This means that not only the ionized particles but also the neutral gas collects at the centre of the discharge. The measurements described by LUKYANOV and SINITSIN@) and by BORZUNOV et al. 6) show that in the initial stages of the discharge the degree of ionization is small. The neutral gas is propelled towards the centre of the discharge as a result of charge exchange ; when ions collide with neutral particles, the ions will be neutralized, but by momentum conservation their motion will persist while the origi- nally neutral particles become ions and join in the general plasma motion towards the axis of the discharge column. The effective cross-section for exchange collisions considerably exceeds the cross-sections for other possible forms of collision (apart from elastic scattering which by itself cannot play an important part in the process we are discussing). Thus the main mass of neutral particles is moved towards the axis.

    b a,= 9 2 cm

    ___---_-

    a-

    0 1 2 3 L 5 6 7

    ,/I set

    FIG. 9.-Curves of a(r) for deuterium discharges.

    Towards the end of the constriction process, however, there may be an accumulation of those neutral particles at the chamber walls which in the process of charge exchange were left with momentum directed outwards.

    As the photographs show, the plasma column contracts into a thin thread of radius, a Y 1 cm. The smallest value that this radius is visually observed to have is practically independent of the nature of the gas and of the initial pressure. The instant of greatest constriction corresponds in time to the first singularity in the current graph; after this the gas near the walls begins to glow again. Evidently breakdown occurs near the walls, and a plasma shell is formed. This is seen parti- cularly clearly on the photographs of the discharge in neon, argon and krypton (see also curves 1 of Figs. 10 and 11). Subsequently, this newly formed plasma shell contracts towards the axis. In those cases where the mass of gas in the discharge chamber is so great that the first contraction occurs at the end of the first half-period when the voltage on the electrodes is low, the second contraction is absent (see Curves 2 of Figs. 10 and 11).

    At the end of the first contraction of the plasma column, a short wave-length instability comes into evidence. After maximum contraction, when the thread begins to thicken, the instability shows an increase. The photographs reveal small random

  • I38 N. A. BORZUNOV, D. V. ORLINSKY and S. M. OSOVETS

    projections from the axis of the thread which grow from one frame to the next. The growth of the central thread continues until it begins to be constricted

    together with the outer shell of plasma. At low pressures, for example in a discharge through xenon at a pressure of 0405 mm Hg (see Fig. 8), the instant of the second singularity, i.e. of the second completed contraction, marks the beginning of

    0 I) 1 2 4 6 8 IO

    1; 14 16 18 20 22

    IJJ. set FIG. lO.-Curves of a(r) for argon discharges.

    further glow at the walls. If, on the other hand, the discharge is in a heavy gas at high pressures, the discharge current decreases so much, even before the second contraction can set in, that the electro-magnetic forces become too weak to cause further con- striction.

    In a deuterium discharge, when the second singularity occurs during the first quarter period of the current, the intensity of radiation in the visible spectral range is

    t,psec

    FIG. 1 l.-Curves of a(r) for xenon discharges.

    insufficient to permit observation of the third onset of glow at the walls. In that case, one sees some small pulsations of the plasma column near the axis (see Fig. 7) after which the glow nearly ceases, appearing again after about a microsecond, but at the periphery. Since the visible radiation is produced as a result of the excitation of neutral atoms in interactions with other particles, the decrease in radiation could be caused either by a diminution of the strength of these interactions or by a decrease in the number of neutral atoms in the region through which the current flows. Measure- ment of the current density distribution in the discharge chamber show@ that while

  • Investigation of pulsed high current gas discharges by high speed photography 139

    the amount of light radiated decreases, the current density near the axis continues to grow. Therefore the intensity of the interaction also should grow. The decrease in the intensity of the emitted light thus can be accounted for only by assuming that in the current carrying region there are practically no neutral atoms, so that the gas is fully ionized there. At the same time, the ratio of total number of ions and neutral atoms may remain small because the neutral atoms are removed from the region of maximum density towards the walls as a result of charge exchange. The glow at the periphery appears as a result of the interaction of the plasma with the walls after the plasma column has disintegrated. In this process, atoms of silicon and oxygent3) can appear in the region of the discharge, giving rise to a great deal of visible light.

    Neutrons and hard X-ray@ are produced just before the discharge column disintegrates. On the photograph of the deuterium discharge at 0.06 mm Hg initial pressure (Fig. 3), the frame coinciding with the neutron burst is marked n.

    In discharges in a heavy gas such as argon, krypton or xenon, the discharge column is not observed to disintegrate. Instead, the thread-like column thickens gradually until the discharge fills the whole volume of the chamber.

    The fact that at high initial pressures in a heavy gas the constriction of the dis- charge column occurs at the end of the first half-period of the current, so that the plasma continues to contract when the current is diminishing, indicates directly that inertial forces are important in the constriction process.

    It should be noted that the picture of the discharge given here is essentially con- firmed by later measurements of current distribution over the discharge tube.(l) Although these measurements were performed under somewhat different conditions, it can be maintained that there is qualitative agreement between the results obtained by high speed photography and those on electro-magnetic field distribution, although the latter naturally give a fuller picture of the process.

    REFERENCES

    1. ARTSIMOV~CH L. A., ANDRIANOV A. M., BAZILEVSKAYA 0. A., PROKHOROV Yu. G. and FILIP~~V N. V., .7. Nucl. Energy 4,203 (1957).

    2. LUKYANOV S. Yu. and PODCWRNIY I. M., J. Nucl. Energy 4,224 (1957). 3. LUKYANOV S. Yu. and SINITSIN V. I., J. Nucl. Energy 4,216 (1957). 4. LEONT~VICH M. A. and OSOVETS S. M., J. Nucl. Energy 4,209 (1957). 5. B~RZUNOV N. A., K~GAN V. I. and ORLINSKY D. V. J. AW. Energy 9, 143 (1959).

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