R. S. Caird

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.

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.

Explosive flux compression generators for rail gun power sources

A class of explosive magnetic flux compression generators is described that has been used successfully to power rail guns. A program to increase generator current magnitudes and pulse lengths is outlined. Various generator loss mechanisms are discussed and plans to control some of them are outlined. Included are various modifications of the conventional strip generators that are more resistant to undesirable expansion of generator components from magnetic forces. Finally, an integral rail gun is discussed that has coaxial geometry. Integral rail guns utilize the rails themselves as flux compression generator elements and, under ideal conditions, are theoretically capable of driving projectiles to arbitrarily high velocities. Integral coaxial rail guns should be superior in some regards to their square bore counterparts.

Optical Absorption Spectrum of MnF2 at High Fields

Absorption spectra covering the range from 3800 to 6400 A have been obtained for single‐crystal samples of MnF2 cooled to 6.5°K in fields above 1 MG. The field and light propagation vectors were parallel to the c axis. Molecular field theory predicts that the magnetic moments of the sublattices should tilt toward the field direction as the magnetic field increases, achieving complete parallelism at twice the exchange field (1.10 MG). Drastic changes are to be expected in the absorption structure as the sample approaches the aligned state. The present data on MnF2 are the first obtained in this field region and show the expected behavior. Most of the absorption lines and diffuse bands fade out between 0.7 and 1.0 MG. The last of the changes occur at 1.01±0.02 MG, indicating that alignment is essentially complete at this field. The sharp line at 5410 A moves toward the red, the shift amounting to 31 cm−1 at 830 kG.

Explosively produced megagauss fields and applications

We describe various explosive magnetic flux compression devices that produce pulsed megagauss fields, and a number of applications in which they have been used. Among the systems described are relatively simple ones that generate fields up to 250 T in large fixed volumes, and cylindrical implosion systems that produce fields in excess of 1000 T. Small fixed volume systems are described that may be used in the laboratory. They require only small amounts of explosive and can produce 100 T fields in coils 25 mm long and 10 mm diameter. We discuss measurements made on various materials in megagauss fields, often at cryogenic temperatures, including magnetoresistance, magnetic susceptibility, optical absorption, Faraday rotation, and Zeeman splittings. We also discuss experiments in which large magnetic pressures have been used to compress solid deuterium isentropically. In flux compression devices part of the energy of the explosives is converted to electromagnetic energy. This has led to their use as compact single-shot high power energy sources. At times, it is necessary to transformer couple loads to the device outputs. We describe successful operation of transformers in 165 T fields, and suggest that they can operate in much higher fields.

High field faraday rotation of some Zn(VI) compounds

Abstract Verdet coefficients at about 6.5 K were determined as a function of wavelength for single-crystal phases of ZnO, ZnSe and ZnS. The coefficients for all the materials studied here exhibit no magnetic field dependence below 100 T. ZnO and ZnS were investigated to fields substantially above 100 T. For ZnS, the Verdet coefficients were found to decrease at higher fields. A similar behavior did not occur for ZnO at fields to 180 T.

Evidence of hydrodynamic processes in multimegampere copper fuses

Experiments have been performed in which an explosively formed fuse transferred a fraction of the 10 MA output current from an explosively powered magnetic flux compression generator to a conventional exploding metallic fuse load. System malfunctions significantly reduced the current delivered to the load and changed the fuse performance. The resultant slowing of the fuse material’s trajectory through density‐temperature space allowed a correlation between optical and electrical diagnostics which indicated a significant delay between the onset of hydrodynamic motion and the onset of rapid resistance increase. The experiments have confirmed a computational model of the fuse load hydrodynamic behavior.