Nonexistence of looping trajectories in hydromagnetic waves of finite amplitude. Breaking of waves in a cold collision-free plasma in a magnetic field. On stabilities of periodic waves in a cold collision-free plasma in a magnetic field.

This dissertation consists of three parts. In Part I, it is shown that looping trajectories cannot exist in finite amplitude stationary hydromagnetic waves propagating across a magnetic field in a quasi-neutral cold collision-free plasma. In Part II, time-dependent solutions in series expansion are presented for the magnetic piston problem, which describes waves propagating into a quasi-neutral cold collision-free plasma, ensuing from magnetic disturbances on the boundary of the plasma. The expansion is equivalent to Picard's successive approximations. It is then shown that orbit crossings of plasma particles occur on the boundary for strong disturbances and inside the plasma for weak disturbances. In Part III, the existence of periodic waves propagating at an arbitrary angle to the magnetic field in a plasma is demonstrated by Stokes expansions in amplitude. Then stability analysis is made for such periodic waves with respect to side-band frequency disturbances. It is shown that waves of slow mode are unstable whereas waves of fast mode are stable if the frequency is below the cutoff frequency. The cutoff frequency depends on the propagation angle. For longitudinal propagation the cutoff frequency is equal to one-fourth of the electron's gyrofrequency. For transverse propagation the cutoff frequency is so high that waves of all frequencies are stable.

Resonance cones and mode conversion in a warm magnetized bounded plasma

The warm plasma resonance cone structure of the quasistatic field produced by a gap source in a bounded magnetized slab plasma is determined theoretically. This is initially determined for a homogeneous or mildly inhomogeneous plasma with source frequency lying between the lower hybrid frequency and the plasma frequency. It is then extended to the complicated case of an inhomogeneous plasma with two internal lower hybrid layers present, which is of interest to radio frequency heating of plasmas.

In the first case, the potential is obtained as a sum of multiply reflected warm plasma resonance cones, each of which has a similar structure, but a different size, amplitude, and position. An important interference between nearby multiply-reflected resonance cones is found. The cones are seen to spread out as they move away from the source, so that this interference increases and the individual resonance cones become obscured far away from the source.

In the second case, the potential is found to be expressible as a sum of multiply-reflected, multiply-tunnelled, and mode converted resonance cones, each of which has a unique but similar structure. The effects of both collisional and collisionless damping are included and their effects on the decay of the cone structure studied. Various properties of the cones such as how they move into and out of the hybrid layers, through the evanescent region, and transform at the hybrid layers are determined. It is found that cones can tunnel through the evanescent layer if the layer is thin, and the effect of the thin evanescent layer is to subdue the secondary maxima of cone relative to the main peak, while slightly broadening the main peak and shifting it closer to the cold plasma cone line.

Energy theorems for quasistatic fields are developed and applied to determine the power flow and absorption along the individual cones. This reveals the points of concentration of the flow and the various absorption mechanisms.

Propagation of the fast magnetosonic wave in a tokamak plasma

The propagation of the fast magnetosonic wave in a tokamak plasma has been investigated at low power, between 10 and 300 watts, as a prelude to future heating experiments.

The attention of the experiments has been focused on the understanding of the coupling between a loop antenna and a plasma-filled cavity. Special emphasis has been given to the measurement of the complex loading impedance of the plasma. The importance of this measurement is that once the complex loading impedance of the plasma is known, a matching network can be designed so that the r.f. generator impedance can be matched to one of the cavity modes, thus delivering maximum power to the plasma. For future heating experiments it will be essential to be able to match the generator impedance to a cavity mode in order to couple the r.f. energy efficiently to the plasma.

As a consequence of the complex impedance measurements, it was discovered that the designs of the transmitting antenna and the impedance matching network are both crucial. The losses in the antenna and the matching network must be kept below the plasma loading in order to be able to detect the complex plasma loading impedance. This is even more important in future heating experiments, because the fundamental basis for efficient heating before any other consideration is to deliver more energy into the plasma than is dissipated in the antenna system.

The characteristics of the magnetosonic cavity modes are confirmed by three different methods. First, the cavity modes are observed as voltage maxima at the output of a six-turn receiving probe. Second, they also appear as maxima in the input resistance of the transmitting antenna. Finally, when the real and imaginary parts of the measured complex input impedance of the antenna are plotted in the complex impedance plane, the resulting curves are approximately circles, indicating a resonance phenomenon.

The observed plasma loading resistances at the various cavity modes are as high as 3 to 4 times the basic antenna resistance (~ .4 Ω). The estimated cavity Q’s were between 400 and 700. This means that efficient energy coupling into the tokamak and low losses in the antenna system are possible.

Damping and saturation of a second order electron plasma wave echo

A description is given of experimental work on the damping of a second order electron plasma wave echo due to velocity space diffusion in a low temperature magnetoplasma. Sufficient precision was obtained to verify the theoretically predicted cubic rather than quadratic or quartic dependence of the damping on exciter separation. Compared to the damping predicted for Coulomb collisions in a thermal plasma in an infinite magnetic field, the magnitude of the damping was approximately as predicted, while the velocity dependence of the damping was weaker than predicted. The discrepancy is consistent with the actual non-Maxwellian electron distribution of the plasma.

In conjunction with the damping work, echo amplitude saturation was measured as a function of the velocity of the electrons contributing to the echo. Good agreement was obtained with the predicted J₁ Bessel function amplitude dependence, as well as a demonstration that saturation did not influence the damping results.

Fluctuations in a tokamak plasma

An experimental investigation of low frequency floating potential fluctuations (f ≤ 200 kHz) in a research tokamak plasma using two spatially separated electrostatic probes has been performed. The spectra, correlation length, and the phase velocity of the fluctuations in both the radial and azimuthal direction have been determined. The propagation velocity in the toroidal direction was also measured and was found to be in the direction of electron current flow. The waves traveled azimuthally in the ion diamagnetic drift direction, even after the usual E x B rotation was taken into account. The electron density fluctuations associated with these oscillations were large, δn/n ≃ 0.35 - 0.50.

The spectra were found to have regularly spaced peaks which seemed to be related to specific azimuthal modes (m =1,2,3,...,etc. ) A parametric study was made to determine what effect plasma parameters had on these peaks. During periods of high electron density in the first 2 msec of the plasma lifetime, strong sawtooth type oscillations were observed. These oscillations typically had frequencies of approximately 10 kHz and were also present when large amounts of neutral gas were added during the discharge by a process called "gas puffing."

The results are compared with experimental observations made on other plasma devices with electric and magnetic probes and with microwave and CO₂ laser scattering techniques. (The scattering measurements are complimentary to the probe measurements since, in the former case, the wavelength is fixed by the scattering angle, but the oscillations could not be spatially localized.) The oscillations in the Caltech torus were probably related to a drift-tearing type instability which is thought to play a major role in the anomalous particle and energy flux observed in tokamaks. Comparisons are made between current theory and the experimental results. However, the theory for the observed oscillations is still in a rudimentary stage of development, and it is hoped that the present investigation will stimulate future analytical work.

Radiation from a short electric dipole antenna in a hot uniaxial plasma

The effects of electron temperature on the radiation fields and the resistance of a short dipole antenna embedded in a uniaxial plasma have been studied. It is found that for ω < ω_p the antenna excites two waves, a slow wave and a fast wave. These waves propagate only within a cone whose axis is parallel to the biasing magnetostatic field B_o and whose semicone angle is slightly less than sin ^(-1) (ω/ω_p). In the case of ω > ω_p the antenna excites two separate modes of radiation. One of the modes is the electromagnetic mode, while the other mode is of hot plasma origin. A characteristic interference structure is noted in the angular distribution of the field. The far fields are evaluated by asymptotic methods, while the near fields are calculated numerically. The effects of antenna length ℓ, electron thermal speed, collisional and Landau damping on the near field patterns have been studied.

The input and the radiation resistances are calculated and are shown to remain finite for nonzero electron thermal velocities. The effect of Landau damping and the antenna length on the input and radiation resistances has been considered.

The radiation condition for solving Maxwell's equations is discussed and the phase and group velocities for propagation given. It is found that for ω < ω_p in the radial direction (cylindrical coordinates) the power flow is in the opposite direction to that of the phase propagation. For ω > ω_p the hot plasma mode has similar characteristics.

Upper hybrid resonance absorption and scattering from a plasma column

The electromagnetic scattering and absorption properties of small (kr~1/2) inhomogeneous magnetoplasma columns are calculated via the full set of Maxwell's equations with tensor dielectric constitutive relation. The cold plasma model with collisional damping is used to describe the column. The equations are solved numerically, subject to boundary conditions appropriate to an infinite parallel strip line and to an incident plane wave. The results are similar for several density profiles and exhibit semiquantitative agreement with measurements in waveguide. The absorption is spatially limited, especially for small collision frequency, to a narrow hybrid resonant layer and is essentially zero when there is no hybrid layer in the column. The reflection is also enhanced when the hybrid layer is present, but the value of the reflection coefficient is strongly modified by the presence of the glass tube. The nature of the solutions and an extensive discussion of the conditions under which the cold collisional model should yield valid results is presented.

Experimental, Numerical and Analytical Studies of the MHD-driven plasma jet, instabilities and waves

This thesis describes a series of experimental, numerical, and analytical studies involving the Caltech magnetohydrodynamically (MHD)-driven plasma jet experiment. The plasma jet is created via a capacitor discharge that powers a magnetized coaxial planar electrodes system. The jet is collimated and accelerated by the MHD forces.

We present three-dimensional ideal MHD finite-volume simulations of the plasma jet experiment using an astrophysical magnetic tower as the baseline model. A compact magnetic energy/helicity injection is exploited in the simulation analogous to both the experiment and to astrophysical situations. Detailed analysis provides a comprehensive description of the interplay of magnetic force, pressure, and flow effects. We delineate both the jet structure and the transition process that converts the injected magnetic energy to other forms.

When the experimental jet is sufficiently long, it undergoes a global kink instability and then a secondary local Rayleigh-Taylor instability caused by lateral acceleration of the kink instability. We present an MHD theory of the Rayleigh-Taylor instability on the cylindrical surface of a plasma flux rope in the presence of a lateral external gravity. The Rayleigh-Taylor instability is found to couple to the classic current-driven instability, resulting in a new type of hybrid instability. The coupled instability, produced by combination of helical magnetic field, curvature of the cylindrical geometry, and lateral gravity, is fundamentally different from the classic magnetic Rayleigh-Taylor instability occurring at a two-dimensional planar interface.

In the experiment, this instability cascade from macro-scale to micro-scale eventually leads to the failure of MHD. When the Rayleigh-Taylor instability becomes nonlinear, it compresses and pinches the plasma jet to a scale smaller than the ion skin depth and triggers a fast magnetic reconnection. We built a specially designed high-speed 3D magnetic probe and successfully detected the high frequency magnetic fluctuations of broadband whistler waves associated with the fast reconnection. The magnetic fluctuations exhibit power-law spectra. The magnetic components of single-frequency whistler waves are found to be circularly polarized regardless of the angle between the wave propagation direction and the background magnetic field.

Plasma loop and strapping field dynamics: reproducing solar eruptions in the laboratory

Coronal mass ejections (CMEs) are dramatic eruptions of large, plasma structures from the Sun. These eruptions are important because they can harm astronauts, damage electrical infrastructure, and cause auroras. A mysterious feature of these eruptions is that plasma-filled solar flux tubes first evolve slowly, but then suddenly erupt. One model, torus instability, predicts an explosive-like transition from slow expansion to fast acceleration, if the spatial decay of the ambient magnetic field exceeds a threshold.

We create arched, plasma filled, magnetic flux ropes similar to CMEs. Small, independently-powered auxiliary coils placed inside the vacuum chamber produce magnetic fields above the decay threshold that are strong enough to act on the plasma. When the strapping field is not too strong and not too weak, expansion force build up while the flux rope is in the strapping field region. When the flux rope moves to a critical height, the plasma accelerates quickly, corresponding to the observed slow-rise to fast-acceleration of most solar eruptions. This behavior is in agreement with the predictions of torus instability.

Historically, eruptions have been separated into gradual CMEs and impulsive CMEs, depending on the acceleration profile. Recent numerical studies question this separation. One study varies the strapping field profile to produce gradual eruptions and impulsive eruptions, while another study varies the temporal profile of the voltage applied to the flux tube footpoints to produce the two eruption types. Our experiment reproduced these different eruptions by changing the strapping field magnitude, and the temporal profile of the current trace. This suggests that the same physics underlies both types of CME and that the separation between impulsive and gradual classes of eruption is artificial.

Experimental and theoretical studies of the electrothermal instability in a nonequilibrium plasma

Experimental and theoretical studies have been made of the electrothermal waves occurring in a nonequilibrium MHD plasma. These waves are caused by an instability that occurs when a plasma having a dependence of conductivity on current density is subjected to crossed electric and magnetic fields. Theoretically, these waves were studied by developing and solving the equations of a steady, one-dimensional nonuniformity in electron density. From these nonlinear equations, predictions of the maximum amplitude and of the half width of steady waves could be obtained. Experimentally, the waves were studied in a nonequilibrium discharge produced in a potassium-seeded argon plasma at 2000°K and 1 atm. pressure. The behavior of such a discharge with four different configurations of electrodes was determined from photographs, photomultiplier measurements, and voltage probes. These four configurations were chosen to produce steady waves, to check the stability of steady waves, and to observe the manifestation of the waves in a MHD generator or accelerator configuration.

Steady, one-dimensional waves were found to exist in a number of situations, and where they existed, their characteristics agreed with the predictions of the steady theory. Some extensions of this theory were necessary, however, to describe the transient phenomena occurring in the inlet region of a discharge transverse to the gas flow. It was also found that in a discharge away from the stabilizing effect of the electrodes, steady waves became unstable for large Hall parameters. Methods of prediction of the effective electrical conductivity and Hall parameter of a plasma with nonuniformities caused by the electrothermal waves were also studied. Using these methods and the values of amplitude predicted by the steady theory, it was found that the measured decrease in transverse conductivity of a MHD device, 50 per cent at a Hall parameter of 5, could be accounted for in terms of the electrothermal instability.

Thermodynamic and kinetic behavior of an argon plasma jet. Decomposition of nitric oxide between 1300 - 1750 degrees K in an argon plasma

In the first part of the study, an RF coupled, atmospheric pressure, laminar plasma jet of argon was investigated for thermodynamic equilibrium and some rate processes.

Improved values of transition probabilities for 17 lines of argon I were developed from known values for 7 lines. The effect of inhomogeneity of the source was pointed out.

The temperatures, T, and the electron densities, n_e , were determined spectroscopically from the population densities of the higher excited states assuming the Saha-Boltzmann relationship to be valid for these states. The axial velocities, v_z, were measured by tracing the paths of particles of boron nitride using a three-dimentional mapping technique. The above quantities varied in the following ranges: 10¹² ˂ n_e ˂ 10¹⁵ particles/cm³, 3500 ˂ T ˂ 11000 °K, and 200 ˂ v_z ˂ 1200 cm/sec.

The absence of excitation equilibrium for the lower excitation population including the ground state under certain conditions of T and n_e was established and the departure from equilibrium was examined quantitatively. The ground state was shown to be highly underpopulated for the decaying plasma.

Rates of recombination between electrons and ions were obtained by solving the steady-state equation of continuity for electrons. The observed rates were consistent with a dissociative-molecular ion mechanism with a steady-state assumption for the molecular ions.

In the second part of the study, decomposition of NO was studied in the plasma at lower temperatures. The mole fractions of NO denoted by x_NO were determined gas-chromatographically and varied between 0.0012 ˂ x_NO ˂ 0.0055. The temperatures were measured pyrometrically and varied between 1300 ˂ T ˂ 1750°K. The observed rates of decomposition were orders of magnitude greater than those obtained by the previous workers under purely thermal reaction conditions. The overall activation energy was about 9 kcal/g mol which was considerably lower than the value under thermal conditions. The effect of excess nitrogen was to reduce the rate of decomposition of NO and to increase the order of the reaction with respect to NO from 1.33 to 1.85. The observed rates were consistent with a chain mechanism in which atomic nitrogen and oxygen act as chain carriers. The increased rates of decomposition and the reduced activation energy in the presence of the plasma could be explained on the basis of the observed large amount of atomic nitrogen which was probably formed as the result of reactions between excited atoms and ions of argon and the molecular nitrogen.

Plasma inverse scattering theory

The object of this report is to calculate the electron density profile of plane stratified inhomogeneous plasmas. The electron density profile is obtained through a numerical solution of the inverse scattering algorithm.

The inverse scattering algorithm connects the time dependent reflected field resulting from a δ-function field incident normally on the plasma to the inhomogeneous plasma density.

Examples show that the method produces uniquely the electron density on or behind maxima of the plasma frequency.

It is shown that the δ-function incident field used in the inverse scattering algorithm can be replaced by a thin square pulse.