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This licentiate thesis presents our investigation of Saturn's inner plasma disk. The thesis gives an overview of the Cassini-Huygens project, what a plasma is and how we use the Langmuir probe to investigate it, various difficulties related to the measurements, the structure of the magnetosphere of Saturn, with special focus on the inner magnetosphere and the region around the Saturnian moon Enceladus. For our investigation we use the Cassini Langmuir probe to derive ion density and ion velocity in the region from 2.5 to 12 Saturn radii. We show that the dominant part of the plasma torus, ion density above ~15 particle/cm3, is located in between 2.5 and 8 Saturn radii (1 RS = 60,268 km) from the planet, with a north-southward extension of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cpm" />2 RS. The plume of the moon Enceladus is clearly visible as an ion density maximum of 105 cm-3, only present at the south side of the ring plane, as expected since the Enceladus plumes are located in the south polar region. Also the azimuthal ion velocity vi,Θ is estimated, showing a clear general trend in the region between 3 and 7 RS, described by vi,Θ =1.5R2-8.7R+39. The average vi,Θ starts to deviate from corotation speed at around 3 RS and reaches down to ~68 % of corotation close to 5 RS. The Langmuir probe data show a clear day/night side asymmetry in both ion density and ion velocity, most prominent in the radial region 4-6 RS from the center of Saturn. The ion densities ni varies from an average of ~35 cm-3 for the lowest dayside values close to noon up to ~70 cm-3 for the highest nightside values around midnight. The azimuthal ion velocities vi,Θ varies from ~28-32 km/s at the lowest dayside values around noon to ~36-40 km/s at the highest nightside values around midnight. This gives an azimuthal ion velocity difference between noon and midnight of Δvi,Θ ~5-10 km/s. The day/night asymmetry is suggested to be due to dust-plasma interaction.

Studies of emission corresponding to the second (3w0) and three-halves (3/2w0) harmonics are reported from laser produced plasmas. These plasmas were produced by means of a neodymium laser with an irradiance up to 2.10¹⁶ W/cm-2, in pulses of nominally 100ps duration. The spectral resolution of these harmonics from a range of targets has enabled various theories tobe verified and the location of the interaction identified. Acomputer code has been used to account for refraction of the emerging radiation in the plasma density profile. The expansion velocity and temporal variation of this profile has also been taken into account. Temporal resolution of the 2Wo and 3/2-Wo spectra has shown both harmonics to occur in pulses of less than the instrumental resolution limit which was ~10ps. This result gives some indication of the timescale for growth and saturation of parametrically excited instabilities.

The instabilities which occur in the interaction of CO2 laser radiation with a dense plasma have been studied. A TEA CO2 laser provided pulses of up to 30 joules of energy with a duration of 50 nanoseconds. By focussing the radiation on to a plane target a focal spot of about 180 micrometers diameter was formed with a irradiance of 10 to 10 W cm. The scattered radiation was collected by a laser focussing lens and analysed with a grating spectrometer. Linear relationships have been found between the incident irradiance and the back scattered energies. This is in contrast to the results of other workers who have found quadratic relations at high irradiance. The back scattered energies were of the order compared with the incident energy. The spectrum of shows a displacement to longer wavelength which is attributed to stimulated Brillouin scattering, there being an ion acoustic wave which is driven by the laser radiation. A similar displacement occurs in the 2 W spectrum and is attributed to the scattering of plasmons from the ion acoustic wave. The measured displacement is in agreement with a theory due to Silin. Fine structure also exists in the 2W spectrum, a shoulder being found on the red side of the line and this may result from a non-Maxwellian electron temperature distribution. There is also a satellite which is displaced towards longer wavelengths by about 0.1 micrometers from the precise value of 2W Explanations of this feature are offered. The self-generated magnetic field has been measured and its effect on the interactions have been examined. The plasma temperature has been determined by X-ray measurements and compared with the values estimated from the scattered spectrum.

The results of a study of non-thermal Carbon and Fluorine plasmas in an electromagnetic acceleration device are presented, the plasmas being created by ablation from the surface of a solid dielectric. The dielectric is close to a steady discharge through which ablated material, on entry, is heated and accelerated, and it is concluded that the high level of radiative power loss, which consists mainly of optically thick resonance line radiation, is in part responsible for the uniformity and stability of the plasma plume. Plasma velocities around 1-2 x 10⁶ cm/s. are attained. In addition to conventional electrical and spectroscopic diagnostic methods, several new spectroscopic techniques are described, providing greater accuracy in the measurement of particle temperatures and densities in non-thermal, non-hydrogenic plasmas. In order to make these methods possible, as well as allowing estimates of the radiation losses, a model describing the papulation distribution among the bound ionic states was constructed and solved for a wide range of plasma conditions. From these results new values of collisional-radiative ionization and recombination coefficients are obtained, which are in broad agreement with experimental values. A model was developed which described the rate of ablation from the solid and which formed part of a larger numerical scheme for calculating the temperature, density and velocity of the plasma plume. This model gives results which are in good agreement with the observations and is used to predict the plasma behaviour under different operating conditions and with different dielectric materials. The extension of certain parts of this work to other situations is also discussed, in particular, laser heating of plasmas, problems of ablation in high-temperature plasma containment devices, and as a source for selective excitation spectroscopy. The possibility of using the device as a soft X-ray laser is also investigated.

An investigation into wave interactions between self-excited ionization waves - striations - in the positive column of a direct current argon discharge is described. Since such self-excited ionization waves are determined by the ionization processes in the discharge, they are inherently nonlinear and a variety of nonlinear effects may be observed. After reviewing the theory and experimental observations for small amplitude linear ionization waves in gas discharges, experimental data obtained from an argon discharge, 5 cm in diameter, 110 cms in length, run at gas pressures between 0.1-1.5 torr and discharge currents of 20-250mA, are presented. Linear interactions between large amplitude self-excited waves of the same frequency, and nonlinear mode coupling between waves of different frequencies and wave numbers are reported and the characteristics of each wave type are described. In order to interpret some of the phenomena observed for the self-excited waves, experiments where an external alternating voltage was applied across the discharge were performed. Following a general discussion of wave modulation the wave-wave mode coupling is interpreted as due to nonlinear interactions which may arise during the initial nonlinear growth of ionization waves. A physical model for the nonlinear behaviour is described. By including a nonlinear term in the basic theory of striations expressions are derived which account for the mode-coupling and support the physical interpretation. Theoretical techniques previously used to describe the nonlinear behaviour of water waves are outlined, and then applied to ionization waves to predict further nonlinear effects which could not be obtained from the modified linear theory. Finally, a general overall view of two-wave interactions between ionization waves in a discharge is presented using, as an example, the results from a neon discharge. The experimental observations are related to the linear and nonlinear growth of the waves.

The high pressure plasma produced by an r.f. induction plasma torch has been investigated in a pulsed magnetic field using spectroscopy, high speed photography, inductive probes and a diamagnetic loop. Radiation and temperature of the plasma have been measured under various conditions of the torch and various intensities of the pulsed field. In most cases Joule-heating has been found the main mechanism of energy transfer to the plasma. From the decay of temperature in the plasma afterglow a value of radiative recombination in dense argon plasma has been found. Various properties of the torch in the absence of a pulsed magnetic field have also been investigated. These investigations cover gas breakdown at the torch initiation, measurements of the r.f. magnetic field in the plasma and the plasma instabilities. The measurements of the r.f. magnetic field provide information leading to estimates of plasma conductivity, electrical parameters and efficiency of the torch. The study of plasma instabilities reveals their sources and helps to suppress the acoustic noise from the torch.

Cette étude vise à étudier le procédé innovant de catalyse post-plasma pour l’abattement de faibles teneurs de trichloroéthylène (TCE), présent dans des effluents gazeux, à la pression atmosphérique et à la température la plus basse possible de fonctionnement du catalyseur. L’objectif est de trouver une alternative aux méthodes traditionnelles de remédiation de COVs non adaptées dans ces conditions. Le réacteur Plasma Non Thermique (PNT) utilisé dans ce travail est à multipointes-vers-plaque à décharge luminescente à courant continu. Bien qu’attractif pour la décomposition du TCE du fait de sa facilité d’utilisation et de son faible coût de fonctionnement, il conduit cependant à une oxydation incomplète du COV. Le catalyseur doit palier aux insuffisances du PNT. L’ajout d’un catalyseur à base d’oxyde de manganèse en aval du réacteur PNT à 150°C voire à température ambiante améliore significativement l’efficacité du procédé. Les meilleurs formulations catalytiques minimisent les phénomènes inhibiteurs de l’eau et décomposent fortement l’ozone. / This study concerns the innovative post-plasma catalysis process for abatement of low levels of trichloroethylene (TCE) in gaseous exhausts at atmospheric pressure and at the lowest operating temperature of the catalyst. The aim is to find an alternative to the conventional VOCs remediation methods which are not adapted in these conditions. The Non Thermal Plasma (NTP) reactor used in this work is a direct current luminescent glow discharge multipins to plate reactor. Although attractive for TCE decomposition due to its ease of use and its low operating cost, it leads however to incomplete VOC oxidation. The catalyst must overcome the weaknesses of NTP. Adding a manganese oxide type catalyst downstream the NTP reactor at 150°C even at room temperature significantly increases the effectiveness of the process. It has been shown that the best catalytic formulations lower the inhibiting factors of water and are efficient catalysts for ozone decomposition.

Pulsed plasmas were investigated as a means of controlling the composition of the surfaces generated via plasma polymerisation. A variety of precursors were studied under a range of plasma conditions using both continuous wave and pulsed plasmas. Surface and bulk analytical techniques were used to characterise the deposited plasma polymers whilst deposition rate measurements aided in understanding the effects of altering the various plasma parameters. Continuous wave plasma polymerisation of saturated cyclic fluorocarbons yielded plasma polymers with high fluorine/carbon ratios. Plasma instability at low powers limits the extent to which continuous wave power can be used to achieve good selectivity in the polymerisation process. Pulsed plasma polymerisation of perfluoroallylbenzene was studied in detail to investigate the influence of pulsing parameters on the surface composition. Highly aromatic surfaces were obtained through retention of the perfluorophenyl group from the precursor. Deposition rate experiments confirmed polymerisation was taking place in the off-portion of the duty cycle for precursors with a functional group susceptible to radical initiated reactions. A cyclic siloxane precursor with vinyl substituents was used to generate surface consisting of siloxane rings in an organic matrix. The monomer structure was retained through the reaction of the vinyl groups in the off-portion of the duty cycle. For low duty cycle pulsed plasma polymers the Si:0 ratio of the plasma polymers was identical to that of the monomer, indicating successflil retention of monomer structure using pulsed plasmas. Preliminary investigations into the pulsed plasma polymerisation of styrene oxide yielded a range of polymer compositions with varying oxygen contents. The properties of the surfaces varied with oxygen content. The results indicate that pulsed plasmas can give significant enhancements over continuous wave plasmas in controlling surface composition and properties.

The Advanced Wakefield Experiment, or AWAKE, is an experiment based at CERN (European Organization for Nuclear Research) whose purpose is to demon- strate the acceleration of electrons using plasma wakefields driven by a charged par- ticle bunch. As a proof-of-principle experiment, AWAKE will be propagating a high- energy proton bunch through 10 meters of plasma to drive the wakefields for electron acceleration. To accelerate the electrons, we want to inject them into regions of both focusing and acceleration within these wakefields behind the proton bunch. In order for the electrons to stay within this optimal accelerating/focusing region, we need to maintain uniform plasma density within 0.2%, and we need to inject when the wakefield phase-velocity is constant. To preserve uniform plasma density, we use a liquid heat-exchanging pipe which can maintain stable temperatures, and therefore uniform rubidium vapor/plasma densities, to within 0.2%. We show that this is pos- sible using Galden HT270 as a heat-exchanging liquid. We also show that additional components required for this system will need external heating to prevent heat-loss, and therefore temperature non-uniformity. Furthermore, using the PIC simulation OSIRIS, we study how changing size parameters of the initial proton bunch by ±5% a ects the phase-velocity of the wakefield. It is seen that these parameter variations will not significantly affect the optimal region size and energy gain of injected elec- trons; so long as the electrons are injected at regions of ξ near σzb of the proton bunch and after 4 m of bunch propagation length in the plasma. / Graduate