Development of a Powerful Discharge in Deuterium

Transcription

1 P/2302 USSR Development of a Powerful Discharge in Deuterium By V. S. Komelkov, U. V. Skvortsov and S. S. Tserevitinov * The investigations carried out at the Institute of Atomic Energy of the Academy of Sciences of the USSR resulted in the discovery of both neutron and hard X-ray radiation occurring under certain conditions in a powerful pulse discharge in deuterium. 1-3 In the present work, the investigations in this field were continued with a view to studying these processes at greater currents and higher rates of increase of the current, by minimizing the circuit inductance and the size of the discharge chambers. Studies were made of the current distribution in the chamber, neutron radiation, electrode-metal vapor movement, and the effect of pre-ionization on the initial stages of the process. EXPERIMENTAL APPARATUS The arrangement consisted of capacitors connected in parallel with a total capacity of 130/Л. The total inductance of the circuit leads and the capacitors was about 15 cm. The spark gaps used for switching were of two types : air gaps, and gaps with a breakable hard dielectric. The construction of the latter switch is described elsewhere. Three types of chambers are used in the experiments: The chambers of the firsttype had an inside diameter of 60.0 cm, a height of 17.5 cm, and porcelain walls 1.5 cm thick, with a number of holes for probes. It had two flat aluminum electrodes. In the lower electrode there was a cylindrical crater 4.0 cm deep, with a diameter equal to the inside diameter of the porcelain cylinder. This electrode was provided with a window 3.4 cm high, whose lower edge was at the level of the flat electrode surface. The total length of the window was 50.0 cm and it was divided into separate sections by four metal baffles. The chambers of the second type had flat copper electrodes, were 4.7-cm high, and were of 18.5-cm inside diameter. The walls were 20.0-cm outer diameter glass cylinders, which were destroyed during each test. The porcelain chamber of the third type had flat copper electrodes and the same diameter as the second Original language: Russian. * Institute of Atomic Energy, Academy of Sciences of the USSR, Moscow. 374 type, but its height was 25.0 cm. In optical measurements the porcelain cylinder was replaced by a glass one of the same diameter and height. The current and voltage were recorded by double-beam oscilloscopes with the aid of voltage dividers and Rogovsky belts. The accuracy of the measurements was ± io%- Measurements of the field and current distribution in the discharge were carried out by the magnetic probe method evolved by Andrianov, Bazilevskaya and Prokhorov. 1 The 3x6 mm measuring coil had 10 to 20 turns. One end of the coil was connected to a copper tube 4 mm in diameter and 40.0-cm long, serving both as a shield and a conductor. The second lead of the coil was inside the tube. Both the tube and coil were placed in a capped porcelain tube to completely eliminate any contact with plasma. The connecting cable, amplifier and oscilloscope were insulated from ground. The coils were used to measure the magnetic field (accuracy i 20%), and its time derivative, which determines the time at which the magnetic field appears at a fixed point in the discharge (accuracy ^ 0.5 ^sec). The plasma motion and the variations in the diameter of the pinch were registered by a high-speed camera (SFR-2M)t operated either as a streak camera or as a framing camera. Kerr cells pulsed by a special circuit were also used. Explosive shutters 4 were used to cut oñ the background light. A 0.1 to 0.2-mm streak camera slit was placed at the center of the window (chamber 1) or of the translucent cylinder (chamber 2). To synchronize optical recordings with the current, a spark discharge with a duration of 0.2/^sec was recorded simultaneously by the streak camera and the oscilloscope. The propagation of metal vapor was investigated as follows : Light from the discharge chamber window fell on the entrance slit of a monochromator (UM-2)t passed through the entrance lens, prism, collimator lens and outlet slit of the monochromator to the photocathode of a photomultiplier tube (FEU-39) t and through the oscilloscope amplifier (20 Mc^band width). The vapor velocity measurements were t Ed. note: Transliterated designation.

2 DISCHARGES IN DEUTERIUM lo 3 - / / 0 f V 1 V Д (b) T.me^sec) carried out with the À 3961 Á aluminum line. The resolving power of the monochromator in this spectral region was 30 A/mm. Production of neutrons was observed with a stilbene scintillation counter using the FEU-19MÎ photomultiplier. The discharge current was simultaneously recorded on the oscilloscope. To protect the multiplier and the stilbene crystal from hard X-ray radiation, 20-mm thick lead screen was used. A B-type $ unit with a ^-counter placed in a paraffin block was used as a control. The time variation of the diameter of the charge channel current, voltage across the electrodes, current distribution, neutron radiation and metal vapor velocity along the discharge channel were recorded in chamber 1. The current, voltage across the electrodes, variations in the pinch diameter (by streak photographs and Kerr cell photographs) and neutron radiations were recorded in chamber 2. The discharge in chamber 3 was investigated with a framing camera and with 12.0 and 16.0-cm diameter Rogovsky belts. Time {fi sec) Figure 1. Current and voltage characteristics (a) 20 kv, 0.05 mm Hg; (b) 20 kv, 0.1 mm Hg;(c) 20 kv, 0.5 mm Hg; (d) 30 kv, 0.05 mm Hg; (e) 30 kv, 0.1 mm Hg EXPERIMENTAL RESULTS The experiments in chamber 1 were carried out at a working voltage of 20 and 30 kv at pressures between 0.05 and 0.5 mm Hg. The initial dl/dt and current Ed. note: Transliterated designation. 1600' 1600 ' ), a/cm 2. ' 1 ' i I U \ \ ÔT J - 4ÜÜ -Í i sec) = Figure 2. Radial distribution ; (r) of the current density; V = 30 kv, P = 0.1 mm Hg

3 376 SESSION A-6 P/2302 V. S. KOMELKOV et al. i ' H J, _ J n 0 гп " t { i sec) n П. 10 I H J Цзо]*" л lüj " I 1 Figure 3. V = 20 kv, P = 0.05 mm Hg amplitude for these voltages varied between the following limits: V = 20 kv, {di/dt) m3ix = X amp/sec, I max = kamp V == 30 kv, (dl/dt) màx = 5.5-6x amp/sec, /max = kamp. The oscillograms of the current and voltage are given in Fig. 1 and show the typical characteristics for discharges which contract due to magnetic forces produced by the current distribution. The results of measurements with the aid of integrating magnetic probes are shown in Figs. 2, 3, 4, 5 in the form of the radial distribution of the current density Q(T) in the chamber at different times. The oscillographic measurements were interpreted by assuming that cylindrical symmetry of the magnetic field and current are maintained during the discharge. About 50% of the discharge current remained in a layer 5-cm thick near the wall after the current started in the central region of the chamber. Typically, for pressures of 0.1 and 0.05 mm Hg, it is observed that negative currents are generated near the chamber walls. They appear before the main current reaches the center and before shock waves are reflected from the axis of the chamber. j(r), a/cm ÍT r (cm) 213 Ш* Tu 2a зо" t (^sec) = ; 8.55 Figure 4. V = 30 kv, P = 0.05 mm Hg

4 DISCHARGES IN DEUTERIUM j r),a/cm l> Л10 20 г зс Г 1 [ LF 10, 2011 ЗОГ -150 t ( i sec) = Figure 5. V = 20 kv, P = 0.5 mm Hg 2302 Figure 6. Oscillograms of the discharge current and neutron pulse; (a) 30 kv, 0.1 mm Hg; (b) 30 kv,0.05 mm Hg The oscillograms of the discharge current and the neutron pulse are shown in Fig. 6. For P = 0.5 mm Hg and V = 30 kv, the current appears in the center of the chamber at 8 / sec, at 9 //sec it reaches its maximum and remains at a high value until 12 / sec. In the external circuit, the current changes its sign at 8 //sec. Streak camera photographs of the discharge show that the optical picture of the motion is in reasonable agreement with the magnetic probe data (Fig. 7). If one takes into consideration the fact that the magnetic probe records the time at which the internal boundary of the moving layer passes a fixed point and that the streak photograph shows its external boundary, the agreement between the two measurements is within the error of the probe measurements. With V = 30 kv, the maximum rate of contraction is 10 7 cm/sec (P = 0.05 mm Hg) and 9 X 10 6 cm/sec (P = 0.1 mm Hg). In these experiments the intensity of neutron radiation was notably smaller than it had been when chambers 50 to 90 cm high were used in which the neutron yield sometimes reached 10 9 neutrons/pulse. 1 When the distance between the electrodes is reduced to 5.0 cm with V = 40 kv and P == 0.1 mm Hg, a small number of neutrons can still be observed within the sensitivity limit of the scintillation counter, which is 5 X 10 5 neutrons. When the distance between electrodes is 17.4 cm, a neutron yield of 10 6 in one discharge was recorded with only 30 kv initial voltage, in the pressure range 0.05 to 0.4 mm Hg. The neutrons are produced at 3.8 / sec and 4.3 / sec after the current flow begins for P = 0.05 mm Hg and for P = 0.1 mm Hg, respectively. In all cases, the neutron radiation starts before the maximum of the current in the center of the

5 378 SESSION A-6 P/2302 V. S. KOMELKOV et ai. (cm) P (mm Hg) T, fisec Table 1 Vav cm/sec. 6 x x x x x x 10 5 V, kev. Polarity For initial pressures of 10 and 1 mm Hg, many independent, relatively bright channels were observed in the constricting layer. Constriction of the gas occurs at an increasing speed with decreasing pressure. For a pressure of 10 mm Hg the maximum speed is 5.4 X 10 6 cm/sec. Full constriction is reached near the time of maximum current, 1.4 X 10 6 amp. By the time constriction is complete, the gas velocity reaches 8 X 10 6 cm/sec; with a current of 1.6 X 10 6 amp, pinch diameter of 3.0 cm and initial pressure of 1 mm Hg Time { i sec) Figure 7. Variation of the pinch radius with time from optical and probe measurements kv, 0.1 mm Hg; optical kv, 0.1 mm Hg; optical kv, 0.1 mm Hg; probe kv, 1.0 mm Hg; probe kv, 0.05 mm Hg; optical kv, 0.05 mm Hg; probe chamber is reached. It can also be seen from the streak photographs (Fig. 8) that the neutrons are produced before the contracted pinch flies apart. Spectroscopic measurements of the radiation from neutral aluminum atoms showed that the metal vapor in such discharges moves with velocities up to 10 6 cm/sec, and the velocity is independent of electrode polarity. The results of the measurements are given in Table 1, where T = time at which metal vapor appears at the electrodes, v ay = average velocity of metal vapor (0-1.5 cm from electrode). It was noted that the aluminum vapor appeared at a time which corresponded to a definite current density in the discharge, a/cm 2. It was also found that the average velocities of the neutral aluminum atoms increased with increasing voltage. Typical oscillograms of the current and vapor luminosity in a discharge are shown in Fig. 9. A discharge in small chambers 18.5 cm in diameter and 4.7 cm in height was investigated at a voltage of 40 kv, initial dl/dt of 1.4 X amp/sec and maximum currents of X 10 6 amp. The initial pressure in the chamber varied between 0.1 and 10 mm Hg. Figure 9. Oscillograms of the current and luminosity with V = 20 kv, P = 0.1 mm Hg (a) near the electrode, (b) 1.5 cm from the electrode Figure 8. Streak camera photograph of a discharge with V = 30 kv and P = 0.1 mm Hg Streak photographs, Kerr cell photographs, and oscillograms of the discharge current and voltages are shown in Fig. 10. The maximum compression ratio of the plasma was estimated to be 140 from the square of the ratio of the initial to final optical pinch diameter. The expansion is accompanied by stratification of the gas, part of which moves back to the center of the chamber, where the luminous pinch and some current still remain. The stratification takes place three times with a time interval of 0.2 to 1.0 / sec. The Kerr cell

6 DISCHARGES IN DEUTERIUM 379 Figure 10. (a) Streak camera photographs, (b) current and voltage osciliograms, (c, d) Kerr cell pictures. P = 1 mm Hg photographs show that the stratification is symmetrical. As the pressure fell to 0.1 mm Hg the rate of contraction rose to l.l-1.3xlo 7 cm. Three successive constrictions were observed, each originating from the wall. In the first constriction the minimum pinch diameter was 1.5 cm and the total current was 1.1 x 10 6 amp. In the second and third constrictions the currents were 1.2 x 10 6 and 1.3 X 10 6 amp, respectively. Neutrons were not observed with a lower limit of sensitivity of 10 6 neutrons/pulse. Experiments with a discharge chamber 18.5 cm in diameter and 25.0 cm in height were aimed largely at finding the efíect of pre-ionization on the subsequent development of the discharge. Pre-ionization was accomplished by means of a discharge from an auxiliary 22 [xî condenser bank rated at 20 kv. After the pinch formed by the auxiliary current detached itself from the wall and traveled some distance towards the center, the main 20 kv bank was discharged. The maximum dl/dt and current amplitude for the first and second discharges were 1.4 X amp/sec, 5.5 X 10 4 amp, and amp/sec, 4.5 X 10 5 amp, respectively. The experiments were carried out at an initial pressure of 1 mm Hg. Typical osciliograms which show the total current and the current in the Rogovsky belt, with the main and auxiliary capacitor banks operating separately and jointly, are shown in Fig. 11. The experimental results are summarized in Table 2, where т is the time lag between the auxiliary and main discharges, t 0 is the time at which current appears in the Rogovsky belt and I is the total discharge current at this time. With pre-ionization, the period during which the conducting sheath remains near the wall is cut at least by half. The velocity of the current sheath, as obtained from oscillographic measurements of the time at which current appears in the belts and from optical measurements of the motion of the luminous layer, diners by not more than i 15%. DISCUSSION In discharge chambers where the space between the electrodes is small, the influence of such processes as heat conduction from the constricted pinch to the electrodes and the occupation of the chamber by metal vapors should be taken into account. As has been demonstrated by S. I. Braginsky and V. D. Shafranov, heat conduction to electrodes during the current flow is insignificant for times t< {3ll5){e*N c*kt)k where I is the chamber length, T the particle temperature, с the velocity of light, e the electronic charge, Belt diameter 160 Table 2 Energy Source t(nsec) t o {f*sec) I(amp) Auxiliary discharge Main discharge Joint 6.4 ± 0.1 Auxiliary discharge Main discharge Joint 6.9 ± 0.4 ± ±-j ± X X X X

7 380 SESSION A-6 P/2302 V. S. KOMELKOV et al. Figure 11. Oscillograms of (1) total discharge current and (2) current in a Rogovsky belt obtained with (a) auxiliary discharge and 16 cm belt, (b) main discharge and 12 cm belt and (c) joint discharge and 12 cm belt k the Boltzmann constant, and N the number of particles in the discharge chamber. For chambers 4.7 cm in height, at a temperature of 300 ev and an initial pressure of 0.1 mm Hg, this time will be about 3 //sec. High-velocity metal vapors which move into the discharge are observed in regions where the current density is relatively high immediately before, or at the time of, maximum constriction. However, these velocities are such that in chambers of the first type, at a pressure of 0.03 mm Hg, the influence of the metal vapor is important 8 / sec after the time of maximum constriction. In chambers of the second type at a pressure of 0.1 mm Hg, metal vapors appear 3 //sec after the time of maximum constriction. This process is roughly similar to that previously described. г > 7 However, the experiments described here have provided the following additional information: 1. With the exception of the initial time when the discharge detaches itself from chamber walls, the luminosity and current boundaries coincide within the experimental accuracy. The initial discrepancy is explained by the fact that the differentiating magnetic probe responds to a build-up of current inside the stationary conductor. Optical measurements with chambers of the second type show that the width of the luminous layer between the shock front and external boundary, for pressures of 10, 1.0, and 0.1 mm Hg are 2.7, 2.2, and 1.0 cm, respectively. Should the plasma conductivity be determined from the width of the luminous layer, which can be 2 to 3 times the skin-depth, it will be X Ю 14 cgse, which corresponds to an electron temperature of ev. The main discharge characteristics are in reasonable agreement with the hydrodynamic theory developed by Leontovitch and Osovetz 5 and with calculations made by S. I. Braginsky, I. M. Gelfandt and R. P. Fedorenko. 2. The maximum compression ratio obtained from the optical measurements with chamber 2 was 140, which greatly exceeds the maximum compression ratio predicted by the hydrodynamic theory. The actual ratio of the initial and final (maximum) densities may be somewhat smaller because all the particles are not swept in by the contracting current sheath. 3. The current in chamber 1, with a pressure of 0.05 mm Hg and a voltage of 30 kv persists in the central zone (14 cm in diameter) for 4 //sec which markedly exceeds the constriction time obtained from calculations. Optical measurements carried out with chamber 2 give similar results. 4. About 50% of the total current is concentrated in the center of the chamber. Characteristically this portion of the current grows after the maximum constriction is achieved. In measurements obtained with chamber 3, about 20% of the current remains near the wall in a layer 0.5-cm thick. This is probably due to the evaporation of materials with high atomic number from the chamber wall. Pre-ionization of the gas by a discharge shortens the time during which large currents flow near the wall and this reduces the energy losses to the wall. The production of reverse currents at certain times will result in the formation of a closed circuit inside the chamber. This causes the current to grow in the center of the chamber at the end of the first and beginning of the second half cycles. As follows from theory, 5 the circuit parameters determine the rate of contraction and the gas temperature. In Fig. 12 the dependence of the maximum rate of contraction on voltage and the initial inductv mox (10 6 cm/sec) 8 Figure 12. -er- - dl/dt (10-11 * """ a/sec) Maximum speed of contraction as a function of {d\ldt) initiai for P = 1 mm Hg A -

8 DISCHARGES IN DEUTERIUM 381 anee is shown. This data was obtained with different set-ups in chambers with the same diameter at a pressure of 1 mm Hg. According to existing theory, the thermal energy of the gas at the time of maximum constriction is approximately twice as great as the maximum kinetic energy during the constriction process. From these assumptions the maximum temperature in chamber 2 at pressures of 10, 1.0 and 0.1 mm Hg is estimated to be 40, and 340 ev, respectively. In chambers of the first type, the gas temperature, estimated on the same basis, is 200 ev (at P == 0.05 mm Hg, V = 30 kv). The neutron radiation from thermonuclear processes that would be expected at such temperatures could not be recorded by the available apparatus. Therefore, the radiation which was detected probably has the same origin as described earlier. 1 ' 6 Its occurrence in pinches 5 cm long testifies to the fact that the accelerating processes which presumably take place occur in regions of this length or less. A decrease in the radiation intensity with decreasing pinch length probably testifies to the fact that the radiation sources are distributed along the entire pinch column, which is in accordance with data obtained previously. 6 REFERENCES 2. L. A. Artsimovich, A. M. Andrianov, O. A. Bazilevskaya, Y. G. Prokhorov and N. V. Filippov, Atomnaya Energ., No. 3, 76 (1956). L. A. Artsimovich, A. M. Andrianov, E. I. Dobrokhotov, S. Y. Lukyanov, I. M. Podgorny, V. I. Sinitsyn and N. V. Filippov, Atomnaya Energ., No. 3, 84 (1956). 3. V. S. Komelkov and G. N. Aretov, Doklady Akad. Nauk. S.S.R., 110, No. 4, 559 (1956). 4. V. S. Komelkov and B. P. Surnin, Pribory i tekhnika eksperimenta, No. 1, 78 (1956). 5. M. A. Leontovich and S. M. Osovets, Atomnaya Energ., No. 3, 81 (1956). 6. O. Anderson, W. Baker, S. Colegate, I. Ise, I. Pyle and P. Pyle, Proceedings of the Third International Conference on Ionization Phenomena. 7. S. Y. Lukyanov and V. I. Sinitsyn, Atomnaya Energ., No. 3, 88 (1956).