A resolution to the order k6 of the three-level equation of Kane allows the authors to obtain a new expression for the conduction band energy and so new analytical expressions for the electron effective masses in the range of carrier concentration 1016-1019 cm-3. The calculated values of the conduction effective mass as a function of carrier density agree well, for the first time, with experimental values, which were obtained from Shubnikov-de Haas experiments by using a new expression for the oscillation amplitude. In this expression the authors take into account the effect of medium-range potential fluctuations induced by crystal inhomogeneities.

https://iopscience.iop.org/article/10.1088/0022-3719/12/12/014/pdf

The electron effective mass in heavily doped GaAs

An instrument has been developed that reliably generates pulsed magnetic fields in the range 150-250 T for experimental applications. A 100 kJ, 40 kV capacitor bank with 18 nH internal inductance and 2.9 m Omega internal resistance is discharged into single turn coils made of copper sheet of 3 mm thickness. 150 T are obtained in 10 mm diameter with a rise time (quarter period) of 2.5 mu s, 200 T are obtained in 6 mm ID within 2 mu s and 250 T in 4 mm ID within 1.8 mu s. Although the coils explode violently and generate a strong axial blast, there is no movement towards the magnetic field volume and even delicate samples can easily be protected to survive an experiment unharmed. This is demonstrated with two experiments: infra-red cyclotron resonance in a GaAs-AlGaAs superlattice and Faraday rotation in GaP.

A laboratory instrument for generating magnetic fields over 200 T with single turn coils

A comparative survey of known experimental and theoretical values of heavy-hole and electron effective mass in GaAs, InAs, and A1As is presented in this work. Recommended room-temperature values of these parameters are given for the above binary solutions and for their ternary compounds.

Effective masses of electrons and heavy holes in GaAs, InAs, A1As and their ternary compounds

Novel experimental data are reported that reveal helical instability formation on imploding $z$-pinch liners that are premagnetized with an axial field. Such instabilities differ dramatically from the mostly azimuthally symmetric instabilities that form on unmagnetized liners. The helical structure persists at nearly constant pitch as the liner implodes. This is surprising since, at the liner surface, the azimuthal drive field presumably dwarfs the axial field for all but the earliest stages of the experiment. These fundamentally 3D results provide a unique and challenging test for 3D-magnetohydrodynamics simulations.

Observations of modified three-dimensional instability structure for imploding z-pinch liners that are premagnetized with an axial field.

The demonstration of magnetic field compression to many tens of megagauss in cylindrical implosions of inertial confinement fusion targets is reported for the first time. The OMEGA laser [T.R. Boehly et al., Opt. Commun. 133, 495 (1997)] was used to implode cylindrical CH targets filled with deuterium gas and seeded with a strong external field (>50 kG) from a specially developed magnetic pulse generator. This seed field was trapped (frozen) in the shock-heated gas fill and compressed by the imploding shell at a high implosion velocity, minimizing the effect of resistive flux diffusion. The magnetic fields in the compressed core were probed via proton deflectrometry using the fusion products from an imploding D3He target. Line-averaged magnetic fields between 30 and 40 MG were observed.

/pdf/laser-driven-magnetic-flux-compression-in-high-energy-49hvus113j.pdf

Laser-driven magnetic-flux compression in high-energy-density plasmas.

Methods of generating magnetic fields in excess of one megagauss are described and discussed. Particular attention is given to explosively driven flux-compression experiments, which at present can generate the highest fields. The discussion of the experimental techniques is divided into three parts: high explosives, initial field and the compression process. Flux-compression theory deals with the dynamics of the metallic `liner' which compresses the flux, and with the magnetic field diffusion into the liner, which results in heating the liner material above the boiling point. Theoretical limits on the highest attainable fields are discussed. Explosively driven current generators which can produce energy pulses of several megajoules are also described. Finally, experiments to generate megagauss fields by means of capacitor-bank discharges are reviewed, and an outlook is given on applications of megagauss fields in various domains of experimental physics.

https://iopscience.iop.org/article/10.1088/0034-4885/31/1/307/pdf

Megagauss magnetic fields

This paper describes an inductive magnetization sensor, designed to measure thin superconducting films (of order 3000 A) in very strong magnetic fields. For magnetic fields in the range 250–500 kG, the sensitivity of the probe is 10−2 emu. Accuracy and high sensitivity were obtained by combining a sample extraction system, vibration isolation, and an inductive sensor with optimized flux coupling geometry. The magnetic flux coupling to the sensor was verified to exceed 70%, both by calculation and experimental calibration.

A probe for magnetization measurements of thin superconducting films in pulsed high magnetic fields

Cyclotron resonance in InSb and GaAs with magnetic fields up to 140 T

The high pressures and current densities associated with MG fields limit their useful lifetime to 1 ?sec or less. The pulse duration of the SLAC electron beam being of the same order of magnitude, a successful marriage of the two techniques was achieved in experiments that use transverse fields in the range 1 to 2 MG as targets for a 19 GeV electron beam. The fields were generated in volumes of dia. 3 to 5 mm by discharging a very fast capacitor bank into small single turn coils, or by flux compression with electromagnetically driven aluminum foils. The termination of the capacitor bank, designed to minimize the destructive effects of the exploding coils, permitted a repetition rate of one shot per hour. Magnetic bremsstrahlung emitted by the electrons was recorded on X-ray film and nuclear emulsions. In an additional experiment, nuclear emulsions mounted in the high field region survived the violent self-destruction of the coils. In these, the magnetic deflection by far exceeds the multiple scattering, which is normally the dominant effect in nuclear emulsions.

https://epaper.kek.jp/p71/pdf/pac1971_0809.pdf

Experiments with Megagauss Targets at SLAC

A set of explosively driven flux‐compression experiments with stainless steel liners and final fields in the range 2–6 MG is discussed. From experimentally determined field curves, the motion of the liner is derived by calculating the flux loss. Up to 80% of the initial flux is lost in these experiments. The variation of resistivity with temperature is taken into account, and the Joule heating energy is also determined. As can be verified by the energy balance, a consistent picture of the flux‐compression process is obtained, including the phenomenon of field turnaround. The speed of sound in the liner material is comparable to the implosion speed; therefore, energy can be transferred to the magnetic field only from a thin inner layer of the liner. The energy accumulated in the magnetic field and in Joule heat increases very steeply at the end of the compression; thus, the inner layer of the liner is suddenly decelerated. There is still a large amount of kinetic energy in the outer layers of the liner at this instant. An empirical turnaround criterion is derived on the basis of these considerations. This shows that the peak field is practically independent of the initial field over a large range of initial fields. The final field depends essentially on the kinetic energy density of the liner. The influence of resistivity on flux compression is also found to be small. Nevertheless, an upper limit for the resistivity of stainless steel in a megagauss field environment can be derived from the experiments. A value of 120 μΩ·cm is found to be in best agreement with the experimental results. The methods developed in this paper can also be used to predict the performance of flux‐compression devices. This is demonstrated with a few examples.

Fritz Herlach

Papers

Megagauss magnetic fields

A probe for magnetization measurements of thin superconducting films in pulsed high magnetic fields

Cyclotron resonance in InSb and GaAs with magnetic fields up to 140 T

Experiments with Megagauss Targets at SLAC

Flux loss and energy balance in magnetic flux-compression experiments.