W. Hooke

Plasma-wave coupling and propagation using phased waveguide arrays

A study of the coupling and propagation of electron plasma waves excited by waveguide arrays is presented. The waves are generated in a low-temperature, linear plasma column in a homogeneous magnetic field. As predicted from a theoretical model, under appropriate conditions efficient coupling to plasma waves can be obtained. These studies are of relevance to plasma heating in that the modes are identical with those that must be generated in any lower hybrid heating experiment. We discuss the implications of these results with respect to future heating experiments on large devices.

A survey of experiments on ion cyclotron resonance in plasmas

This paper is a review of all published experiments concerning the production, propagation and utilization of the slow, or torsional plasma wave (the ion-cyclotron wave) at frequencies in the neighborhood of the ion cyclotron frequency. A brief summary of the theory of propagation and absorption of ion cyclotron waves is followed by a historical survey of the experimental work in this field. The various methods of wave generation are described, with major emphasis on the use of induction coils and radial electric fields. The experimental verification of the dispersion relation is discussed. The measurement of power in the wave, the damping of the wave by ion cyclotron absorption, and the heating of plasma by this means are described in detail. The results to date and some of the problems to be solved in the future are summarized. A complete bibliography of published work in this subject is appended.

Coaxial lower hybrid plasma source

We report the development of a coaxial radio frequency plasma source capable of producing pulsed plasmas of moderately high density (10(13)-10(14) cm(-3)). This device may be useful either as a general purpose laboratory plasma source or as an auxilliary source for a variety of fusion applications, such as providing target plasmas for mirror machines or producing cold plasma blankets to help isolate the hot plasma core of a toroidal plasma from the walls.

EXPERIMENTS ON ION CYCLOTRON WAVES

Experiments have been performed on the generation of ion cyclotron waves and their propagation into a magnetic beach. The experiments were carried out on the B‐66 machine, which is currently a magnetic mirror device. Studies of the production of neutrons have provided evidence for the absorption of the energy of these waves via ion cyclotron damping.Microwave phase‐shift measurements have now been made, and the addition of electron density completes the list of parameters required for direct comparison of experimental and theoretical dispersion relations. The experimental data yield a smooth monotonic relation between density and frequency which is qualitatively similar to that predicted by theory. There are, however, unexplained quantitative differences.Wave propagation into the magnetic beach region was observed with a single turn rf magnetic probe. The variation of the amplitude of these waves in the magnetic beach is in qualitative agreement with the theory of ion cyclotron wave propagation and cyclot...

Temperature and Power Measurements in a Plasma Heated by Absorption of Ion Cyclotron Waves

A plasma in a magnetic mirror field is heated by means of an induction coil which generates plasma waves at a frequency near the gyrofrequency of the ions. Several hundred kilowatts of power are transmitted to the plasma. Ion energies in the neighborhood of 1 keV are observed in a small volume within the plasma. These energies are believed to be associated with ions trapped within a small magnetic mirror region following their acceleration by ion cyclotron damping of the waves. In a deuterium plasma production of neutrons is observed in association with these trapped ions. Ion energies of 200–300 eV are found throughout the total 10‐liter plasma volume. The electron temperature is approximately 10 eV. Energy is lost from the plasma in times ranging from 30–50 μsec. This loss rate appears to result from particle losses as well as radiation from impurities. To explain the loss of energy it is not necessary to invoke an instability mechanism.

Measurements on the Fast Hydromagnetic Wave above the Ion Cyclotron Frequency

The propagation of the fast hydromagnetic wave in a cylindrical plasma is studied. A driving signal having a frequency omega /2 pi = 16 Mc is provided by means of an induction coil wrapped around the center of the plasma device. The ion cyclotron frequency ( OMEGA i/2 pi in the plasma is varied by varying the axial magnetic field intensity, and the experimental conditions are adjusted so that always omega > OMEGA i. The phase shift between the applied signal and the plasma excitation signal is measured as a function of the axial distance from the exciting coil. The experimental results are compared with the predictions of the dispersion relations for omega / OMEGA i = 3.3 and 12.

Observation of Ion Cyclotron Waves in a Hot Plasma

According to the theory of the propagation and absorption of ion cyclotron waves in a hot magnetized plasma, the wavelength approaches a minimum, nonzero value as the wave propagates into a magnetic beach where the wave frequency approaches the local ion cyclotron frequency. Furthermore, the damping of the waves is a sharply resonant function of the magnetic field strength, the width of the resonance being proportional to the square root of the ion temperature for motion parallel to the static magnetic field. These effects are observed in a plasma confined in a linear magnetic mirror geometry for perpendicular ion temperatures in the range 10–110 eV. The wave fields are measured with two magnetic probes, the plasma density is measured with two microwave interferometers, and the perpendicular ion temperature is measured with a diamagnetic probe. The theory is developed for the transport of spatially damped waves in a hot, finite plasma immersed in a slightly inhomogeneous magnetic field. Quantitative agreement between theory and experiment is obtained. These measurements represent the first experimental observations of thermally broadened collisionless cyclotron absorption in a laboratory plasma. Observations of collisional damping of ion cyclotron waves are also presented.

Perpendicular bremsstrahlung emission of suprathermal electrons

An interpretation of the x‐ray bremsstrahlung emission by suprathermal electrons perpendicular to a magnetic field is given in terms of the parallel and perpendicular temperature of a three‐temperature distribution function. The slope (i.e., the temperature) of the distribution can be determined relatively well. Factor‐of‐two uncertainties remain for the number of electrons.

ACTIVE-PASSIVE WAVEGUIDE ARRAY FOR WAVE EXCITATION IN PLASMAS

A modified version of the standard waveguide grill for exciting lower hybrid plasma waves is proposed. This version, consisting of alternate active and passive elements, permits a number of important engineering simplifications and also reduces the amplitude of the (undesirable) surface component of the wavenumber spectrum. Results from a simple two-element array excited at low power are presented.

Detection of lower hybrid waves within a plasma by microwave scattering

The spatial distribution and intensity of electrostatic waves injected into a hot plasma may be inferred from the scattering of millimeter microwaves. We report measurements on the 30 degrees scattering of 8.6-mm microwaves by a 500-W, 2.45-GHz slow wave excited in a linear plasma by a phased array of two waveguides. From the magnitude of the scattered signal and auxiliary measurements with probes, we infer that a large fraction of the injected power penetrates to the center of the overdense test plasma.