H. P. Furth

Finite‐Resistivity Instabilities of a Sheet Pinch

The stability of a plane current layer is analyzed in the hydromagnetic approximation, allowing for finite isotropic resistivity. The effect of a small layer curvature is simulated by a gravitational field. In an incompressible fluid, there can be three basic types of ``resistive instability: a long‐wave ``tearing mode, corresponding to breakup of the layer along current‐flow lines; a short‐wave ``rippling mode, due to the flow of current across the resistivity gradients of the layer; and a low‐g gravitational interchange mode that grows in spite of finite magnetic shear. The time scale is set by the resistive diffusion time τR and the hydromagnetic transit time τH of the layer. For large S = τR/τH, the growth rate of the ``tearing and ``rippling modes is of order τR−3/5τH−2/5, and that of the gravitational mode is of order τR−1/3τH−2/3. As S → ∞, the gravitational effect dominates and may be used to stabilize the two nongravitational modes. If the zero‐order configuration is in equilibrium, the...

Tearing mode in the cylindrical tokamak

Detailed computational results are presented on the stability and radial distribution of linear tearing modes in cylindrical tokamaks of various radial profiles. In the case of a skin‐current profile, a “double tearing mode”, with two points of discontinuity in the radial magnetic field perturbation is found. An analytical method is also derived for comparison of the stability of different radial profiles. It is further shown that the tearing mode can be driven by finite electron viscosity, as well as by the usual finite resistivity mechanism.

PRODUCTION AND USE OF HIGH TRANSIENT MAGNETIC FIELDS. I

Experiments involving pulsed magnetic fields of a half‐megagauss and more are shown to be practical. Some basic concepts are developed regarding the construction of impact resistant solenoids and the selection of the appropriate power supply. Operation of various high‐field magnets in the pulse range 50 to 10 000 μsec is discussed, and some techniques for associated solid state experiments are given. Magnetoresistance measurements up to 600 000 gauss have been made for germanium. Operation of a 175 000 gauss Helmholtz‐type magnet for nuclear track plate work with the 95‐Mev Harvard synchrocyclotron is described.

Prevalent Instability of Nonthermal Plasmas

In thermally anisotropic plasmas or sheet pinches, the zero‐order counterstreaming or directed streaming of like particles gives rise to instability modes that correspond essentially to the formation of local first‐order pinches within the plasma. The marginal stability criteria for all such modes can be interpreted in terms of Bennetts pinch condition. The range of examples discussed includes Weibels thermally anisotropic hot‐electron plasma instability in null field, which is generalized for hot ions and finite magnetic field, and which is applied to interpenetrating neutral plasma streams, to a spherical electrostatic plasma‐containment scheme, and to the Astron thermonuclear experiment. The familiar ``mirror instability is shown to be another example of a ``pinch‐type mode. Sheet pinches with or without longitudinal magnetic field are generally unstable against tearing into subsidiary linear pinches.

Adiabatic Compression of Tokamak Discharges

Adiabatic compression in minor and major radius is shown to facilitate the attainment of dense, high‐temperature plasmas in the tokamak configuration.

Closed Magnetic Vacuum Configurations with Periodic Multipole Stabilization

It is shown that a previously presented equation for plasma containment in regions of maximum field strength can also be satisfied with closed magnetic vacuum configurations with periodic multipole stabilization. (C.E.S.)

Overview of TFTR transport studies

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Stabilization of Pinch Discharges

The hydromagnetic stability properties of ``hard‐core pinches are shown to be more favorable than those of conventional ``stabilized pinches. Linear ``hard‐core pinch experiments with a wide range of configurations show a basic consistency with hydromagnetic theory, but all configurations studied so far become unstable at sufficiently high current densities and low particle densities, even if possessed of theoretical hydromagnetic stability. Pinches with nulls in Bθ tend to be much less stable than those with nulls in Bz. ``Inverse stabilized pinches yield perfectly reproducible magnetic probe traces at power levels several times those at which ``stabilized pinches are unstable. A technique for the study of plasma density distributions is given, based on the propagation of radial compressional waves. The theory of the steady‐state pinch is enlarged to include convective effects and is documented by experiment. The role of electrodes is considered, and an anode‐cathode asymmetry in instability be...

Correlations of heat and momentum transport in the TFTR tokamak

Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low‐recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≊χi>χe. Recent measurements have confirmed similar behavior in broad‐density L‐mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test‐particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked‐density TFTR ‘‘supershots’’ with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In hi...

Inverse Pinch Effect

In the conventional pinch effect an ionized gas is enclosed by a conducting cylinder, and a sufficiently large current passing through the gas and returning along the inner wall of the cylinder produces a magnetic field which compresses the gas into an axial filament. A device is described which produces an inverse pinch effect. Here the above conducting cylinder is replaced by an axial rod surrounded by the ionized gas. When a current passes through the gas and returns along the rod the magnetic field pushes the plasma outward, leaving a cylindrical vacuum region behind. The velocity and thickness of the expanding plasma front have been studied optically and by means of magnetic probes. Except at the highest gas densities, the velocity is in good agreement with the ``snow plow theory of Rosenbluth, and the thickness of the front is reasonably consistent with the ``snow plow model. At the higher densities it appears that diffusion of magnetic field into the plasma is significant. The advantages of the inverse pinch effect in studying plasma behavior and the idea of a magnetically stabilized inverse pinch are discussed.