C.M. Fowler

Production of Very High Magnetic Fields by Implosion

Magnetic fields are produced in the 10–15 megagauss range by use of high explosives which compress the flux obtained from initial fields of approximately a hundred thousand gauss. The fields described here occupy a cylindrical volume and are essentially axial. A typical field might have these general characteristics: Peak field 14 megagauss; 2 μsec duration from 10–14 megagauss; field volume around peak, 6 mm diameter, 50 mm estimated length.

Dynamic Polymorphism of Some Binary Iron Alloys

Investigations of the pressure‐induced polymorphism of the Fe‐V, Fe‐Mo, Fe‐Co, and Fe‐C alloy systems have been carried out by dynamic techniques. The addition of vanadium to iron is found to raise the transition pressure very strongly, to some 575 kbar for 25 wt% vanadium content. The Fe‐Co study indicates that long‐range order does not have an appreciable effect on the transformation. The initial mixed phase state of the Fe‐Mo system is clearly manifested in the transition behavior, generally fulfilling expectations. Variations of the transition parameters by various sample heat treatments are demonstrated in the Fe‐C system investigation. The dynamic transformation parameters for pure iron have been reappraised, and are found to be Pb=129±1 kbar, v/v0=0.9360±0.0005. A summary of transition pressures for binary iron alloys is provided.

Results of railgun experiments powered by magnetic flux compression generators

Researchers from the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory initiated a joint railgun research and development program to explore the potential of electromagnetic railguns to accelerate projectiles to hypervelocities. The effort was intended to 1) determine experimentally the limits of railgun operation, 2) verify calculations of railgun performance, and 3) establish a data base at megampere currents. The program has led to the selection of a particular magnetic flux compression generator (MFCG) design for a set of initial experiments and the design of small- and large-square-bore railguns to match the expected MFCG power profile. The bore sizes are 12.7 and 50 mm, respectively. In this paper, we briefly describe the design of the railguns and the diagnostic and data reduction techniques, followed by the results of eight experiments with the two railgun types.

Ultrahigh magnetic fields produced in a gas .. puff Z pinch

Controlled, ultrahigh axial magnetic fields have been produced and measured in a gas‐puff Z pinch. A 0.5‐MA, 2‐cm‐radius annular gas‐puff Z pinch with a 3‐min repetition rate was imploded radially onto an axial seed field, causing the field to compress. Axial magnetic field compressions up to 180 and peak magnetic fields up to 1.6 MG were measured. Faraday rotation of an Argon laser (5154 A) in a quartz fiber on‐axis was the principal magnetic field diagnostic. Other diagnostics included a nitrogen laser interferometer, x‐ray diodes, and magnetic field probes. The magnetic field compression results are consistent with simple snowplow and self‐similar analytic models, which are presented here. Even small axial fields help stabilize the pinches, some of which exhibit several stable radial bounces during a current pulse. The method of compressing axial fields in a gas‐puff Z pinch is extrapolable to the order of 100 MG. Scaling laws are presented. Potential applications of ultrahigh axial fields in Z pinches...

The mark IX generator

A large, explosive-driven helical generator (the Mark IX) is described. The stator ID is 35.6 cm and the armature OD is 17.3 cm. The overall length is 112 cm. This generator delivered 11, 23.5, and 30 MA to 120-, 56-, 35-nH loads, respectively. A Marxing technique is used that enables us to employ over 1 MJ of capacitor bank energy for the initial current without destroying the generator prematurely by magnetic forces. The shot data were analyzed by first computing the gener tor inductance vs. time curve and then deriving the resistance vs. time curve from the data. This approach yields a useful characterization of the generator.

TECHNIQUE FOR MEASURING MEGAGAUSS MAGNETIC FIELDS USING ZEEMAN EFFECT

Rapidly varying magnetic fields with peak values in the range from 1 to 5 MG are measured by use of a sweeping image spectrographic method. Atomic spectral lines from an exploding wire light source situated in the experimental region are recorded as the magnetic field varies in a few microseconds from a moderate initial value of a few tens of kilogauss to the peak values. Field measurements are generally accurate to within 2–3% as determined by the consistency of measurements made from several different spectral lines. The sodium D lines and the indium I 4102 A line have proven to be exceptionally useful for field determinations. The highest field determined to date by this method is 5.1 MG, corresponding to a measured separation of 164 A between the centers of the shorter and longer wavelength doublets which the NaD lines assume in very high fields. The doublets, of approximately 4 A separation, are not themselves resolved.

An Explosive‐Driven High‐Field System for Physics Applications

A simple explosive‐driven flux compression system is described for producing magnetic fields in the MG range. The flux‐trapping device is a seamless hollow stainless steel cylinder driven by a ring of explosive. The initial field is introduced by a coil pair supplied by a 90‐kJ capacitor bank. The assembly is readily evacuated. During implosion, the experimental volume is free of objectionable debris and asymmetries. Peak fields of 1.2 and 4 MG are achieved in working diameters of 8.9 and 3.2 mm, respectively. The usable length is about 15 mm at these fields. Several possible applications are mentioned.

Measurement of Faraday Rotation in Megagauss Fields over the Continuous Visible Spectrum

A method is described for making Faraday rotation measurements simultaneously over most of the visible spectrum at magnetic fields in the megagauss range. The fields are produced by explosive‐driven magnetic flux compression systems and typically reach 1.2 MG in a few tens of microseconds. The optical train consists of an explosive white light source, polarizer, sample, analyzer, and a sweeping image spectrograph. The technique is applicable to solid or liquid samples. Verdet coefficients are given for LiF, H2O, and fused quartz over the wave‐length range from about 4000 to 6300 A. The coefficients generally agree to within ±2% with previously published values except for fused quartz for which the present values are somewhat higher at the short wavelength end.

RANCHERO explosive pulsed power experiments

The authors are developing the RANCHERO high explosive pulsed power (HEPP) system to power cylindrically imploding solid-density liners for hydrodynamics experiments. Their near-term goal is to conduct experiments in the regime pertinent to the Atlas capacitor bank. That is, they will attempt to implode liners of /spl sim/50 g mass at velocities approaching 15 km/sec. The basic building block of the HEPP system is a coaxial generator with a 304.8 mm diameter stator, and an initial armature diameter of 152 mm. The armature is expanded by a high explosive (HE) charge detonated simultaneously along its axis. The authors have reported a variety of experiments conducted with generator modules 43 cm long and have presented an initial design for hydrodynamic liner experiments. In this paper, they give a synopsis of their first system test, and a status report on the development of a generator module that is 1.4 m long.

Design and testing of high-pressure railguns and projectiles

The results of high-pressure tests of four railgun designs and four projectile types are presented. All tests were conducted at the Los Alamos explosive magnetic-flux compression facility in Ancho Canyon. The data suggest that the high-strength projectiles have lower resistance to acceleration than the low-strength projectiles, which expand against the bore during acceleration. The railguns were powered by explosive magnetic-flux compression generators. 1,2 Calculations to predict railgun and power supply performance were performed by Kerrisk.3