The near field of coaxial air jets, with swirl in the outer one, was investigated experimentally. Axial and azimuthal velocities were mapped using hot-wire anemometry, and static pressure measurements were obtained using a pitot tube. The flow was visualized using a double-pass schlieren system. The flow is sensitive to both the amount of swirl, characterized by the swirl number S, and the mass flow ratio between the outer and inner jets, mr. A necessary condition for recirculation to occur was that S > 0.58 and mr > 8.5. The magnitude of a pressure deficit in the centerline strongly depends on mr, while the existence of swirl appears to have a triggering effect on setting up this pressure gradient. Spectral analysis shows distinct characteristics dependent on the occurrence of recirculation. Because these features were observed upstream of the recirculation region, the vortex breakdown in this experiment may be related to flow instabilities.

The present study measured the critical diameter and critical thickness of a variety of explosives. The explosives tested included two "unstable" homogeneous explosives (nitromethane and a nitromethane/nitroethane blend); a model heterogeneous explosive consisting of a packed bed of glass beads (Φ ~ 80 μm) saturated with the homogeneous nitromethane/nitroethane blend; and a commercial heterogeneous explosive, Apex Elite(TM). The comparison of the critical diameter and thickness of an explosive is used to identify the dominant propagation and failure mechanisms of the various explosives. The ratio of critical diameter to critical thickness for nitromethane, the nitromethane/nitroethane blend, the beaded heterogeneous explosive, and Apex Elite(TM) were found to be 3.2 +/- 0.6, 3.6 +/- 0.4, 2.3 +/- 0.1, and 3.5 +/- 1.2 respectively. According to accepted detonation failure theories, the energy losses associated with detonation front curvature are responsible for detonation failure. The curvature model, which is elaborated upon in the present work, leads to a predicted critical diameter to critical thickness ratio of exactly 2. The present study has shown that the only explosive which follows the behaviour predicted by curvature failure models is the beaded heterogeneous explosive, which exhibits fine scale heterogeneities. This seems to indicate that unstable liquid explosives and heterogeneous explosives with large scale heterogeneities do not fail simply due to the wave front curvature, but rather by a local mechanism of failure and reinitiation which dominates the detonation propagation.

<p> Using the unique properties of the interaction between intense, short-pulse lasers and nanometer scale van-der-Waals bonded aggregates (or 'clusters'), modulated waveguides in hydrogen, argon and nitrogen plasmas were produced and extreme ultraviolet (EUV) light was generated in deeply ionized nitrogen plasmas. A jet of clusters behaves as an array of mass-limited, solid-density targets with the average density of a gas. </p><p> Two highly versatile experimental techniques are demonstrated for making preformed plasma waveguides with periodic structure within a laser-ionized cluster jet. The propagation of ultra-intense femtosecond laser pulses with intensities up to 2 x10¹⁷ W/cm² has been experimentally demonstrated in waveguides generated using both methods, limited by available laser energy. The first uses a 'ring grating' to impose radial intensity modulations on the channel-generating laser pulse, which leads to axial intensity modulations at the laser focus within the cluster jet target. This creates a waveguide with axial modulations in diameter with a period between 35 &mu;m and 2 mm, determined by the choice of ring grating. The second method creates modulated waveguides by focusing a uniform laser pulse within a jet of clusters with ow that has been modulated by periodically spaced wire obstructions. These wires make sharp, stable voids as short as 50 &mu;m with a period as small as 200 &mu;m within waveguides of hydrogen, nitrogen, and argon plasma. The gaps persist as the plasma expands for the full lifetime of the waveguide. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides. Simulations show that these 'slow wave' guiding structures could allow direct laser acceleration of electrons, achieving gradients of 80 MV/cm and 10 MV/cm for laser pulse powers of 1.9 TW and 30 GW, respectively. </p><p> Results are also presented from experiments in which a nitrogen cluster jet from a cryogenically cooled gas valve was irradiated with relativistically intense (up to 2 x 10¹⁸ W/cm²) femtosecond laser pulses. The original purpose of these experiments was to create a transient recombination-pumped nitrogen soft x-ray laser on the 2_p3/2 &rarr; 1_s1/2 (&lambda; = 24.779 &Aring;) and 2_p1/2 &rarr; 1_s1/2 (&lambda; = 24.785 &Aring;) transitions in H-like nitrogen (N⁶⁺). Although no amplification was observed, trends in EUV emission from H-like, He-like and Li-like nitrogen ions in the 15 &ndash;150 &Aring;spectral range were measured as a function of laser intensity and cluster size. These results were compared with calculations run in a 1-D fluid laser-cluster interaction code to study the time-dependent ionization, recombination, and evolution of nitrogen cluster plasmas. </p>

This work describes the evolution of an ultracold neutral plasma as it expands freely in vacuum. It presents a comprehensive study of the electron temperature evolution under different initial conditions. Ultracold neutral plasmas are created by photoionizing laser-cooled neutral atoms in ultrahigh vacuum. The ions are typically at a temperature of &sim; 1K while the electron temperature can be set from 1--1000 K. After photoionization, some of the highly energetic electrons escape from the cloud, leaving a net positive charge in the cloud. This creates a Coulomb well which traps the rest of the electrons, and a plasma is formed. Since the electrons have a lot of kinetic energy, they tend to leave the cloud, however, the Coulomb force from the ion pulls the electrons back into the cloud. This exerts a recoil force on the ions, and the whole plasma starts expanding radially outwards. Since the expansion is caused by the thermal pressure of the electrons, a study of the plasma expansion unravels the complicated electron temperature evolution, under different initial conditions. Many collisional processes become significant as a plasma expands. These physical processes tend to heat or cool the ions and electrons, leading to very different kinds of evolution depending on the initial conditions of the plasma. This work demonstrates three different regions of parameter space where the degree of significance of these physical processes is different during the ultracold neutral plasma evolution. The experimental results are verified by theoretical simulations, performed by Thomas Pohl, which untangle the complicated electron temperature evolution.

A formalism has been developed here in which the collisional ionization rate is determined directly, as a probability of transition due to changes in the plasma's local electric field. The calculations described here are performed within a model which treats hydrogenic ions only, but the generalization to more complex ions is straightforward. The initial state is a hydrogenic state with a reduced ionization potential, and the final state is that of a free particle. This model is effective in treating many scenerios that occur in laser fusion and sub-picosecond laser-matter experiments where high-density conditions exist. The results show that plasma screening of the interaction between target and free electrons serves to reduce the ionization rate while the drop in ionization potential serves to increase the ionization rate. The lowering of the continuum dominates in all calculations performed here and indicates that the enhancement in ionization rate can be as much as an order of magnitude in physically interesting regimes. (Abstract shortened by UMI.)

The Harris (1962) model, which is consistent with local thermodynamic equilibrium (LTE) does not represent the earth's magnetotail, which has a magnetic field component normal to the equatorial plane. However, inclusion of a normal magnetic $B\sb{z}$ component can be made without violating the slow-flow MHD approximation, under the condition ${B\sb{z}\over B\sb{x}}\ll 1$. A procedure is developed for constructing a family of two-dimensional asymptotic equilibrium solutions that are ordered with respect to the magnetic disturbance index $K\sb{p}$ and based on fitting to a three-dimensional magnetospheric model magnetotail. The low quality of fits for magnetically active configurations is indicative of their consistency with the assumed pressure function. This, in turn, implies that high magnetic activity levels of the real magnetosphere are ruled by different thermodynamic conditions than those associated with LTE.

The spectral, temporal, and spatial characteristics of plasma-induced self-phase and cross-phase modulation in rare gases have been investigated using a femtosecond KrF excimer laser focused to peak intensities of 10$\sp{14}$-10$\sp{15}$ W cm$\sp{-2}.$ The quiver energy of a free electron under these conditions is less than the ionization potential of all rare gases, ensuring that ionization occurs only by optical field-induced processes. Spectral blueshifts of up to 2 nm have been observed, and the blueshifted spectra show an oscillatory structure. The blueshifted spectra are shown to be the result of plasma-induced self-phase modulation and can be modeled by assuming tunneling ionization and one dimensional pulse propagation. The newly discovered oscillatory structure in the spectra is related to that observed in earlier experiments on self-phase modulation in optical fibers. To investigate the temporal behavior of the field ionization process, pump-probe experiments have been performed with a 100 fs probe pulse at 497 nm and a 400 fs pump pulse at 248 nm. Under conditions of weak ionization (Z $\ll$ 1), pump-probe experiments and theoretical calculations show that the ionization rate of the field ionized gas is maximum at the peak of the laser pulse and that the degree of ionization changes over a time equal to about half of the pump pulse width. By observing changes in the transmission of the probe pulse caused by plasma absorption, the electron temperature of a field ionized rare gas is determined to be on the order of 1 eV. The time varying electron density in the pump-probe experiments also causes plasma-induced cross-phase modulation, or spectral blueshifting of the probe pulse spectrum of up to 15 nm. The pump-probe experiments show that plasma defocusing causes the spectral blueshifting to be spatially dependent. Experimental results and a two dimensional pulse propagation model indicate that the most defocused beam components also show the maximum spectral blueshift. Plasma-induced cross-phase modulation has also been used to characterize the amplitude and phase of a 1 ps chirped pulse at 497 nm and the pulse width of a 400 fs pulse at 147 nm generated by four wave frequency mixing in xenon.

The 10 kilowatt prototype of the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) engine, abbreviated as VX-10, is designed to eject plasma at exhaust velocities of tens of kilometers per second. In this device, energy is imparted to the plasma ions by two mechanisms: ion cyclotron resonant heating (ICRH), and acceleration in an ambipolar electric field. Measurements from two different electrostatic probes are combined to determine how much each mechanism contributes to the total ion energy. The first probe is a gridded retarding potential analyzer (RPA) that incorporates a multi-channel collimator to obtain precise measurement of the ion and electron parallel energy distributions. The second is an emissive Langmuir probe that measures the DC and RF components of the plasma potential. The plasma potential obtained from the emitting probe allows calculation of the parallel velocity distribution once the parallel energy distribution is obtained from the energy analyzer data. Biasing the RPA housing is shown to minimize the plasma perturbation, as monitored by an auxiliary probe. When this minimization is done, the RPA measurements become compatible with the emissive probe's measurement of plasma potential. The collimated RPA and emissive probe have been used to examine the effects of a double dual half-turn (DDHT) antenna encircling the plasma. When power at the ion cyclotron frequency is applied, changes are seen in the saturation current and mean ion energy of the collimated RPA characteristic. The evolution of these changes as the RPA is moved downstream from the antenna is interpreted as firm evidence of ion cyclotron heating, albeit at absorbed energies of less than 1 electronvolt per ion. The emissive probe shows that, within experimental error, all of the increased ion energy is accounted for by an increase in the plasma potential that occurs when the ICRF power is applied. The combined RPA and emissive probe data also show that there is a jet of flowing plasma in the VX-10 when operated with the helicon source alone but that the signal from this jet is overwhelmed by a rapidly growing stationary plasma within the first second of the discharge.

High-speed video microscopy has been used to study drop breakup in dilute Newtonian emulsions under steady shear. Fundamental experimental studies on drop breakup have been limited to breakup in quiescent matrix or under pseudo-equilibrium conditions. This thesis represents the first direct visualization of drop breakup under steady shear at high capillary numbers (Ca). The mechanisms of drop breakup depend on Ca and the viscosity ratio (lambda). At Ca &sim; Cac , drops are broken up via necking. At Ca &lt; 2Cac, drop breakup is caused by end pinching. At Ca > 2Cac, the capillary instability is the dominant breakup mechanism. For Ca > 2Cac, breakup dynamics are strongly controlled by lambda. For 0.1 &lt; lambda &lt; 1, drops with different initial sizes deform into threads with the same radius at breakup. The wavelength of the capillary instability is uniform along the length of a thread and from thread to thread. Fairly monodisperse dilute emulsions are obtained due to this size selection mechanism, with the average drop size being inversely proportional to the shear rate. For 1 &lt; lambda &lt; 3.5, the breakup mechanism is similar to that for 0.1 &lt; lambda &lt; 1.0, except that the satellite drops are substantially larger, resulting in polydisperse emulsions. For lambda &lt; 0.1, the daughter drops are formed from long wavelength capillary instability and may break again. This induces collisions between drops, which in turn results in irregular drop re-breaking and coalescence, producing polydisperse emulsions. This re-breaking mechanism has not been observed in previous studies in the literature. Drops reach a pseudo-steady state before the capillary instability starts to grow. At this pseudo-steady state, the shear stress and the capillary pressure almost balance each other, determining a definite thread radius, which is independent of the initial drop size. We define a dimensionless thread number as the ratio of the two forces. The thread number is only a function of lambda, and shows a minimum in lambda. The measured thread number is in agreement with the slender body theory of Hinch and Acrivos (1980). Drops deform pseudo-affinely for 0.1 &lt; lambda &lt; 1.0, but deformation deviates from being pseudo-affine otherwise.

The present thesis considers the phenomenon of flow-rate enhancement of polymer solutions in a pipe due to pulsating pressure gradients. It presents an historical review of the problem. The unexplained experimental dependence of enhancement on pulsation frequency reported by Barnes et al is examined, as are later theoretical attempts to reproduce their results. We find that the results can be reproduced only by omitting the important inertial term. The Modified Moment Method is applied to the problem. The results confirm the predictions of other models. The enhancement is of second order in the pulsation amplitude, exhibits a maximum when the pressure gradient is varied, and declines with increasing pulsation frequency. An expansion in powers of the pulsation amplitude gives a satisfactory approximation. Less power is consumed at the same rate of flow if the pressure gradient is constant and not pulsated.