Neutrons from Plasma Compressed by an Axial Magnetic Field (Scylla)

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1 P/356 USA Neutrons from Plasma Compressed by an Axial Magnetic Field (Scylla) By W. C. Elmore, E. M. Little and W. E. Quinn* Earlier work 1 at this laboratory established that shock waves of considerable intensity may be produced in deuterium gas by the abrupt application of a strong magnetic field. Such shock waves ionize the gas and can heat the resulting plasma to a temperature of several electron volts or more, provided the shock waves have sufficient intensity. The plasma so produced can then be used as the starting condition for further heating, for instance, by adiabatic compression. The first part of the work reported here deals briefly with an arrangement for magnetically driving high-intensity shock waves in deuterium gas at reduced pressure. The second part deals in more detail with early experimental results in which the shockheated plasma is adiabatically compressed by an axial magnetic field to a point where neutron emission is observed. Single-Coil System A simple theoretical analysis of a magnetic driver for shock waves shows that the product of the linear coil dimension and the initial rate of increase of magnetic field intensity should have as high a value as possible. If V is the potential of a capacitor bank, Lo is the external source inductance, and L\ is the inductance of a single turn of a closely coupled coil of n turns of radius r, then for similar geometries one finds that r d 4= T «\ T (i) at Lo+n Li when the bank is suddenly connected to the coil. Equation (1) is independent of the capacitance of the capacitor bank, except through the contribution of the bank to the source inductance Lo. Clearly, V should be made high, and L\ and Lo kept as low as possible. A lower limit to L\ is normally set by other considerations, such as the thickness of the tube walls, the thickness of necessary electrical insulation, and the desired volume of plasma to be shock excited. For given values of Lo and Li, n should be chosen so that for oo > Li/Lo >, n 1; for \ > Li/Lo >, n = 2; etc. * Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico. The paper includes an addendum by K. Boyer and the above authors. Results of experiments with several different coil configurations have been consistent with these design criteria. In a system where L± > Lo, we find that a single band of copper surrounding a cylindrical glass tube makes a satisfactory driver. Shock fronts having measured speeds up to 10 cm/ftsec have been produced in a tube of 5 cm inside diameter, containing deuterium gas at pressures in the range 10 to 1000 microns of mercury. In a particular experiment, the driving coil had a radius of 3.0 cm and consisted of a band of copper cm thick and 2.5 cm wide. The coil was connected by a short parallel-plate transmission line to a 0.85-/ f capacitor of 100 kv rating, surmounted in coaxial geometry by a triggered spark gap. Such a system had a total inductance of 0.23 / h, of which the coil contributed about /xh. A weak axial magnetic field (500 to 2000 gauss) was applied to the tube over an interval 5 to 20 cm from the driving coil. This field served to keep the shock wave free of the walls of the tube. 2 Shock Wave Studies Various properties of shock waves produced in the manner described have been studied using magnetic probes, electric probes and a pair of photomultiplier tubes. In addition, a moving-image camera with a writing speed of 0.6 cm/fisec has been used to record axial shock-front motion, and to establish the radial extent of the shock front. Normally, the gas is partially pre-ionized by radio-frequency power applied to external capacitive electrodes situated near each end of the shock tube. Without this provision, the gas often breaks down only after one or more oscillations of the applied field, particularly at low gas pressures. Moving-image records and magnetic probe measurements show that the shock speed some distance from the driver coil is very nearly proportional to the voltage to which the capacitor is charged. Observations were made in the range 30 to 70 kv. The speed at 70 kv was about 10 cm//xsec at a position 13 cm from the coil. Except near the coil, the shock front is nearly plane, with a small central cusp whose origin is associated with the magnetic field used to keep the shock wave from the tube wall. Near the coil, conditions are not simple and the shock front possesses a converging radial component. A number of unexplained shock 337

338 SESSION A-10 P/356 W. С ELMORE et al. SPARK -L 6APS ^ 4.4/tf 4.4/if -TRIGGER PULSE VWVWr n i -ЛЛЛЛЛг ' I 1 - The coils give a mirror ratio of about 1.4. The inductance of each coil is 0.

2 338 SESSION A-10 P/356 W. С ELMORE et al. SPARK -L 6APS ^ 4.4/tf 4.4/if -TRIGGER PULSE VWVWr n i -ЛЛЛЛЛг ' I 1 - The coils give a mirror ratio of about 1.4. The inductance of each coil is 0.08/xh, as compared with a source inductance of 0.13 / h for each bank. Thus only 38% of the capacitor voltage appears initially across the coil terminals. With the capacitors charged to 65 kv (the highest potential that could be used in the present set-up because of sparking difficulties) a maximum central compression field of about 38 kilogauss is obtained 1.5 /xsec after the spark gaps fire. Radiofrequency power is used to pre-excite the gas in the tube. [_OEUTERIUM FROM Pd LEAK Figure 1. Functional diagram of Scyíla TO VACUUM PUMP phenomena have been observed and deserve further study. However, these exploratory experiments, taken in conjunction with some unpublished calculations of the plasma temperature behind a plane shock front in deuterium gas, 3 indicate that it should be possible to attain plasma temperatures in the ev range, immediately after the passage of a magnetically driven shock front. In the earlier experiments, 1 a study was made of the compression produced by a magnetic field on the plasma in a tube through which successive shock fronts had been driven in opposing directions. It was estimated that temperatures of perhaps 40 ev were attained. In these experiments, the axial compression field reached a peak value of 40 kilogauss in 10 / sec over a 7.5-cm length of a tube having an inside diameter of 8.75 cm. Calculation showed, however, that even if a mirror geometry 4 were used, the escape of particles by collision and deflection into the mirror escape cone would prevent the attainment of a detectable thermonuclear reaction unless the rise time of the magnetic field were held down to a few microseconds. Scylla Twin-Coll System This consideration has led to a new but very simple arrangement for shock excitation followed by adiabatic compression. The device has been named Scylla. In it, two identical single-turn coils mounted coaxially are used both to shock excite plasma to a temperature in the ev range and, by use of the subsequent rise in magnetic field, to compress adiabatically the plasma formed between the coüs. In such an arrangement, the spacing of the coils sets the mirror ratio, that is, the ratio of the maximum axial magnetic field to the axial field at the mid-plane. A diagram showing the general functional arrangement of the experiment is given in Fig. 1. Two capacitor banks of 4.4 d each and of 100 kv rating are separately connected through triggered spark gaps 5 to the two single-turn coils. The coils consist of copper bands 0.63 cm thick, 2.5 cm wide and with a separation about equal to the mean coil diameter of 7.0 cm. Operation The arrangement described appears to function in the following manner. When the spark gapsfiresimultaneously, an initial circumferential electric field of about 1.2 kv/cm exists near the inner wall of the tube at each coil. The deuterium gas there breaks down and a plasma current arises. Magnetic repulsion between this current and the current in the coils produces a rapid motion of the plasma away from the vicinity of the coils thereby setting up shock fronts. The shock fronts pass through the gas in the region between the coils and also proceed in opposing directions down the tube away from the central region. In the central region, whose volume is about 150 cm 3, the gas is ionized and heated to perhaps 20 ev within 0.1 to 0.2 / sec. The resulting plasma is now a good electrical conductor and is further heated by the compression produced by the increasing magnetic field. Just prior to peak compression, neutron emission is observed. Preliminary measurements made with a leadshielded liquid scintillation detector show that, under the conditions of operation described, more than 10 4 neutrons are produced per burst. The neutron yield is very sensitive to capacitor voltage and disappears below about 50 kv. If the firing of the twp spark gaps differs in time by 0.2 / sec with the capacitors at 60 kv, the neutron yield is considerably reduced, and it disappears if the time difference is as much as 0.5 / sec. The yield is not very sensitive to initial deuterium pressure in the range 20 to 80 microns of mercury. Below 20 microns it begins to fall off; it disappears at pressures somewhat below 10 microns. The yield also begins to fall off fairly rapidly at pressures above 80 microns, and disappears somewhat above 100 microns. In addition to neutrons, X-rays have been\observed at gas pressures below 50 microns. As in the case of the neutrons, the X-rays first appear approximately at the time of peak compression. A simple absorption measurement in lead shows that many of the X-rays have an energy of at least 200 kev. The X-rays appear to come from regions of the tube about 13 cm each side of the midplane of the coils, as shown by the darkening of photographic film placed along the tube. In these regions, the walls of the Pyrex tube are darkened and have a bluish tinge suggesting bombardment by electrons that have been accelerated into the central region, and then scattered so that they can escape along magnetic field lines. It is surmised that

3 NEUTRONS FROM SCYLLA 339 I 10 TRIGGER CABLES-4 METERS LONG INPUT TRIGGER -8 CABLES, 4 METERS LONG CHARGING <+ 70 KV ) CHARGING (+70KV) 10 UNITS ( 100 KV RATING) "10 COMPRESSION COIL OIL IMMERSED JUNCTION Figure 2. Schematic circuit diagram of Scylla the absence of X-rays at higher initial gas pressures is due to electron-ion collisions that prevent the acceleration of electrons to high energies. There seems to be no correlation between X-ray and neutron yield. A few observations have been made without radiofrequency pre-excitation. It has been found that, at lower pressures, the neutron yield decreases or vanishes and that the X-ray yield increases considerably both in intensity and in energy. Evidently, gas breakdown is delayed so that a significant fraction of the finalcompression field exists at the time the ions are formed. Such a field would reduce the ultimate plasma compression. Furthermore, the particle trajectories in such a case have a somewhat higher escape probability. If the energetic deuterons responsible for neutrons arise by a run-away process, a close correlation between the neutron and X-ray yields should be expected. The lack of such correlation with respect to pressure dependence and with respect to rf pre-excitation makes a thermonuclear origin of the neutrons appear plausible. It is clear, however, that further diagnostic work must be done to establish conditions in the plasma at the time of peak compression. In addition, better information is needed regarding the duration of the neutron burst, the dependence of yield on the peak compression field, the effect of impurities on yield and the geometrical origin of the neutrons. Work along these lines is in progress with a new version of Scylla in which high-voltage insulation has been improved and the source inductance of the capacitor bank lowered by the use of ten spark-gap switches operating simultaneously. ACKNOWLEDGEMENTS The authors are greatly indebted to Keith Boyer for encouragement and advice in extending earlier experiments on the adiabatic compression of shock-heated plasmas by employing higher capacitor potentials and shortening the time for compression. They are also indebted to James L. Tuck for emphasizing the importance of the rate requirements in the experiment. ADDENDUMf During the past several months a number of advances have been made in the Scylla experiment. The t By K. Boyer, W. С Elmore, E. M. Little and W. E. Quinn.

$340 SESSION A-10 P/356 W. С ELMORE et al. У/Л Шк 5 cm \ - Ю cm - Figure 3. Axial cross section of coil for Scylla capacitor bank and its switching have been improved.$

4 340 SESSION A-10 P/356 W. С ELMORE et al. У/Л Шк 5 cm \ - Ю cm - Figure 3. Axial cross section of coil for Scylla capacitor bank and its switching have been improved. These changes together with a new coil geometry have greatly increased the neutron yield. With the new Scylla, observations have been made on (1) the dependence of neutron yield on pressure, (2) the time of occurrence of neutron and X-ray bursts, (3) the approximate shape and position of the neutron source, (4) the neutron energy spectrum for both signs of the applied voltage, to determine whether ordered acceleration processes are responsible for neutron production, and (5) the magnetic fieldconfiguration at the time of maximum compression. Equipment Capacitor Bank and Switching The original capacitor bank, which had ten 0.88 f, 100 kv capacitors connected in two equal sections with a spark-gap switch for each section, has been reassembled with one four-element spark-gap switch for each capacitor. From each switch mounted on its capacitor, eight 4-meter lengths of RG-14/U cable (RG-17/U in the Geneva Exhibit model) run to a common oil-immersed junction, connecting with a short parallel-plate transmission line that leads to the compression coil. This arrangement reduces the source inductance of the bank from /xh to / h and permits operation, without sparking, at voltages approaching the limit set by the capacitor ratings. The connecting cables can accommodate a time jitter of 0.04 /xsec in the firing of the triggered gaps. After considerable experimentation, the circuit of Fig. 2 was found to trigger the gaps reliably within this time interval. A five-stage cascade capacitor bank (Marx circuit) is used to charge the high-voltage bank to a potential in the range kv in 50 / sec. The highvoltage bank is then discharged through the compression coil to produce a peak current of about 10 amperes per volt on the condensers, rising to this value in 1.25 / sec, after which time it continues to oscillate with a damped sinusoidal waveform. Compression Coil It has been found that a compression coil having the shape shown in Fig. 3 results in an increase in neutron yield over that produced by the two single-turn coils used originally. In such a coil, the magneticfieldlines PRESSURE IN MICRONS Figure 4. Neutron yield as a function of initiai gas pressure fail to penetrate the thick metal walls, so that the average mirror ratio of the axialfieldis approximately set by the ratio of the internal area at the mid-plane to that at the end sections. The predicted ratio is 1.4 for the coil of Fig. 3, whose dimensions have been roughly optimized for greatest neutron yield by trying a series of coils of modified shapes. The measured mirror ratio is 1.33 with a circuit inductance of jh. Results Dependence of Neutron Yield on Pressure and Contaminants The neutron yield produced with the modified capacitor bank and shaped compression coil is sufficient to permit the use of a neutron detector consisting of silver foils surrounding four Geiger tubes mounted in a cadmium-shielded block of paraffin. The detector was calibrated using a Cockcroft Walton D-D neutron source, and gives one count for 4500 neutrons from a point source at the distance used with Scylla. The greatest neutron yield appears to occur at a deuterium pressure of about 100 microns. The observed yield, as a function of pressure, for the bank charged to 70 kv is given in Fig. 4. Ten to twenty observations were made at each pressure, and the five highest yields are indicated. At higher bank voltages (75 to 85 kv), occasional yields as high as 2xlO 7 are noted. It is found that at 100 microns, an admixture of 1 % of dry air reduces the yield by 56% and 5% of air reduces it by 97%. A more detailed study of the effect of impurities is planned. Time of Occurrence of Neutrons and X-rays Oscilloscope records made with a multi-channel oscilloscope have shown that the neutrons emerge on the second and to a much smaller extent on the third

5 NEUTRONS FROM SCYLLA 341 compression of the magnetic field. The pulses of neutrons have a symmetrical bell-shaped distribution which is centered on peak magnetic field, and which occupies less than one-quarter period. X-ray pulses, which appear only at low pressures, have an asymmetrical intensity distribution starting near magnetic field reversal and terminating before peak field. Often a smaller but similar pulse of X-rays occurs after peak field and extends to the next field reversal. Evidently the X-rays are strongest when the mean free path is long and during times when there is a strong electric field due to a high rate of change of magnetic flux. The absence of neutrons on the first current maximum suggests that the first half-cycle is required to ionize the gas fully, and to establish a starting temperature such that magnetic compression is possible during the second half current cycle. This would permit the adiabatic compression to raise the temperature to neutron producing levels. This conclusion is substantiated by the magnetic fieldmeasurements. Shape and Position of Neutron Source A massive neutron collimator, made from paraffin loaded with lithium to stop scattered neutrons and followed by lead shielding to stop X-rays, has been used to survey the source of neutrons within the compression coil. A plastic scintillator and fourteen-stage photomultiplier coupled to a cathode-ray oscilloscope records neutrons that pass through the 1 cm diameter cylindrical aperture of the collimator. By this means, the neutrons are found to come from a central region in the compression coil roughly 1.5 cm in diameter and 3 cm long, as shown in Fig. 5. These observations clearly establish that the neutrons originate in the gas away from the tube walls and that the compression is three-dimensional. 20 ie i г 1 1 n г AXIAL DIRECTION RADIAL DIRECTION Neutron Energy Spectrum A shadow-bar experiment, using nuclear emulsions as detectors, has been performed to look for acceleration processes produced by the induced electric field. A thick paraffin plate was mounted with its upper surface parallel to, and 0.33 cm above a horizontal plane passing through the axis of symmetry of Scylla. Nuclear emulsions were mounted on this plate at distances of 15 and 20 cm from the axis and Scylla was operated 1000 times for each of the two possible magnetic field orientations. Analysis of the nuclear emulsion data shown in Fig. 6 demonstrates an acceleration of deuterons, in the direction of the electric field, due to the rate of change of magnetic flux. If the assumption is made that all the particles are rotating with a uniform velocity, the actual energy shift will be twice the observed shift of 150 ±60 ev. Analysis of the probe data indicate circulating currents which could produce rotational energies of about 40 ev for the ions, which is approaching the lower limit of the observed energy shift. However, this is too low an energy, by a factor of 10, to have any appreciable effect on the neutron production rate. The neutron energy spectrum does not seem to permit more energetic collisions than those corresponding to 6 kev deuterons incident on deuterons at rest. Magnetic Probe Measurements An attempt has been made to determine the magnetic field distribution at the time of peak compression. Preliminary results are given here with some reservations. The difficulty arises from the extreme corrosion experienced by any device introduced into the hot plasma and the disturbing effects of the evaporated material on the plasma. Most of the measurements were taken by a small probe with thin metal walls (stainless steel) extending along a line parallel to the axis and passing to the mid-plane of the plasma region. Probes with quartz or ceramic walls were always destroyed by a single discharge. The surface of the steel probe tube was melted to a depth of about cm in 1 or 2 / sec and waves were NEUTRONS EMITTED IN DIRECTION OF ACCELERATION 165 TRACKS MEAN ENERGY» NEUTRONS EMITTED IN DIRECTION OPPOSITE TO ACCELERATION 102 TRACKS MEAN ENERGY « CENTIMETERS Figure 5. Relative neutron intensity from Scylla in the radial and axial direction J I I 1 1 I i i ' ' E n (MEV) Figure 6. Neutron energy spectrum for Scylla for the two signs of applied voltage

342 SESSION A-10 P/356 W. С ELMORE et al. PARTICLE PRESSURE MAGNETIC FIELD INTENSITY MAGNETIC INTENSITY ON MID PLAINE 2 3 4 5 RADIUS IN CENTIMETERS 100 80 40 g 5 Figure 7.

6 342 SESSION A-10 P/356 W. С ELMORE et al. PARTICLE PRESSURE MAGNETIC FIELD INTENSITY MAGNETIC INTENSITY ON MID PLAINE RADIUS IN CENTIMETERS g 5 Figure 7. Magnetic probe measurements on the medium plane of Scylla at peak compression and the particle pressure calculated from the magnetic field distribution formed on the molten surface by the high velocity gas escaping axially after compression, indicating that the plasma was indeed heated to quite high temperatures. The magnetic field distribution on the mid-plane and the resulting particle pressure deduced from pressure balance are shown in Fig. 7. While these data are preliminary, the major features are reproducible and are believed to be substantially correct. 20 SUMMARY The neutron yield, the size and shape of the neutron emitting source, the neutron energy spectrum and the magnetic fieldconfiguration are all consistent with the assumption that the plasma has a temperature of about 1.3 kev at maximum compression and a density of about 6xlO 16 deuterons per cm 3. It is also of interest to note that the peak compression lasts long enough for the average ion to make one thermalizing collision. If the neutrons prove to be of non-thermonuclear origin, they must then arise from random acceleration processes acting on the deuterons to give them energies lying between 2.5 and 6 kev in order to be consistent with the data. ACKNOWLEDGEMENTS The authors wish to express their appreciation to L. Cranberg for his help in the design and construction of the neutron collimator and to D. A. Baker, G. A. Sawyer and J. E. Hammel for their assistance in obtaining and analyzing the magnetic probe data. We are especially indebted to the Microscopy Group working under Louis Rosen for reading and analyzing the nuclear emulsion data. 1. E. M. Little and D. B. Thomson, Magnetic Compression of Shock Pre-Heated Plasma, Los Alamos Scientific Laboratory Report No F. R. Scott, W. P. Basmann, E. M. Little and D. B. Thomson, Magnetic Channeling of a Strong Shock, in The Plasma in a Magnetic Field, Stanford University Press, Palo Alto, Calif. (1958). REFERENCES 3. D. Venable and L. R. Stein, private communication. R. F. Post, Mirror Machine Studies, Washington Meeting of the American Physical Society (May 1958). R. S. Dike, published. A. E. Schofield and J. L. Tuck, to be

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