No description available.

No description available.

No description available.

This thesis is a theoretical model and experimental study of the physics of a low pressure (5-50 mtorr), low electron temperature (1-10 eV), high density (J017 _J01Bm-J ) inductively coupled plasma. This type of plasma is similar to those much used in plasma etching, deposition, and other plasma aided materials processing of devices [1-5]. A two-dimensional, electromagnetic, finite-element model has been set up to simulate the operation of the inductively coupled plasma using the external coil configuration used in the experimental work. Given fixed external RF coil current and voltage and the measured plasma density profile, Maxwell's equations and magnetohydrodynamical (MHD) fluid equations are used to calculate the self-consistent electromagnetic field. A number of predictions are presented and compared with experiments. A symmetric, cylindrical inductively coupled discharge system has been set up. A single tum loop magnetic probe has been used to measure electromagnetic (EM) field in the discharge chamber. A Langmuir double probe has been designed to measure the plasma density and electron temperature. An emissive probe was used to measure the time averaged plasma potential, while a capacitive probe was used to measure the RF component of the plasma potential. A retarding field energy analyser has been used to measure the total ion flux flowing to the vessel on the midplane. Experimental results show that (1) the inductively coupled plasma is well confined inside the induction coil in the pressure range of 5-50 mtorr and RF power range of 10-400 W; (2) the measured electrostatic RP field «1.0 V/cm) in the whole discharge chamber is negligible, compared with the large induction RF field, which is in the order of 10 V/cm; (3) the RF power is coupled into the discharge through the nonlinear electron motion and corresponding collision processes; (4) it has been shown that the induction-field-ionization, electrostatic-field-modulation and various collision processes together influence the velocity distribution function of ions at the boundary surfaces.

No description available.

This thesis is a theoretical model and experimental study of the physics of a low pressure (5-50 mtorr), low electron temperature (1-10 eV), high density (10¹⁷-10¹⁸m³ ) inductively coupled plasma. This type of plasma is similar to those much used in plasma etching, deposition, and other plasma aided materials processing of devices [I-5]. A two-dimensional, electromagnetic, finite-element model has been set up to simulate the operation of the inductively coupled plasma using the external coil configuration used in the experimental work. Given fixed external RF coil current and voltage and the measured plasma density profile, Maxwell's equations and magnetohydrodynamical (MHD) fluid equations are used to calculate the self-consistent electromagnetic field. A number of predictions are presented and compared with experiments. A symmetric, cylindrical inductively coupled discharge system has been set up. A single turn loop magnetic probe has been used to measure electromagnetic (EM) field in the discharge chamber. A Langmuir double probe has been designed to measure the plasma density and electron temperature. An emissive probe was used to measure the time averaged plasma potential, while a capacitive probe was used to measure the RF component of the plasma potential. A retarding field energy analyser has been used to measure the total ion flux flowing to the vessel on the midplane. Experimental results show that (1) the inductively coupled plasma is well confined inside the induction coil in the pressure range of 5-50 mtorr and RF power range of 10-400 W; (2) the measured electrostatic RF field (<1.0 V/cm ) in the whole discharge chamber is negligible, compared with the large induction RF field, which is in the order of 10 V/cm; (3) the RF power is coupled into the discharge through the nonlinear electron motion and corresponding collision processes; (4) it has been shown that the induction-field-ionization, electrostatic-field-modulation and various collision processes together influence the velocity distribution function of ions at the boundary surfaces.

This thesis is an account of a detailed investigation of the properties of a recently-discovered gas discharge - a discharge that operates at a few volts or tens of volts, and which carries a current of the order of amps through a mixture consisting of a hot, atmospheric-pressure rare gas (the "diluent") to which a small amount of alkali metal vapour has been added as an easily-ionised "seed". It is shown that the establishment of the discharge under study is brought about by the breakdown of the gas, which occurs at a breakdown voltage that depends mainly on the electrode spacing, seed pressure, and choice of diluent gas. The discharge itself is shown to consist of two main regions, namely, a constricted, cylindrical positive column that extends from the anode to within a short distance of the cathode, and a thin, glowing sheath that covers the entire cathode surface; the two regions are separated by a dark space. The positive column is shown to expand as current increases, while the value of its electric field is shown to depend mainly on the discharge current, seed pressure, and choice of diluent gas, and hardly at all on the gas temperature or choice of seed metal. The cathode fall is shown to depend mainly on the discharge current, seed pressure, and choice of seed metal. The breakdown of the gas, the positive column of the discharge, and the cathode regions of the discharge are discussed in successive chapters.

This thesis presents experimental studies of various non-thermal atmospheric pressure gas discharges generated using short pulsed excitation as an alternative to widely used sinusoidal excitation. Several pulse generators are detailed that provide high voltage pulses ranging from hundreds of microseconds to less than ten nanoseconds in duration. A key enabler to the generation of a stable discharge is a suitably high repetition rate; this prerequisite precludes many conventional pulsed power technologies. Fortunately, recent advances in semiconductor technology have made it possible to construct solid state switches capable of producing high voltage pulses with repetition rates of many kilohertz. Pulsed excitation introduces many opportunities to tailor the applied voltage and consequently enhance the discharge which are not possible with sinusoidal excitation sources. Through detailed electrical and optical analysis it is shown that pulsed excitation is not only more energy efficient than a comparable sinusoidal source but produces a higher flux of excited species that are essential in many applications. When pulse widths are reduced to a sub-microsecond timescale a novel barrier-free mode of operation is observed. It is shown that diffuse large area plasmas are easily produced at kilohertz repetition rates without the usually indispensable dielectric barriers. Experimental results show that a short pulse width prevents the onset of the undesirable glow-to-arc transition thus introducing an added degree of stability. A further benefit of pulsed excitation is the ability to produce gas discharges with a high instantaneous peak power yet low average power consumption, resulting in a high density plasma that exhibits roomtemperature characteristics. Finally, as an acid test to highlight the many benefits of pulsed excitation several real-world applications are considered. It is shown that in all cases pulsed gas discharges provide real benefits compared to their sinusoidal counterparts.

A Global Enhanced Vibrational Kinetic (GEVKM) model is presented and applied to the simulation of a new High Current Negative Hydrogen Ion Source (HCNHIS) developed by Busek Co. Inc. and Worcester Polytechnic Institute. The HCNHIS consists of a high-pressure radio-frequency discharge (RFD) chamber in which the main production of high-lying vibrational states of the hydrogen molecules occurs, a bypass system, and a low-pressure negative hydrogen ion production (NIP) region where negative ions are generated by the dissociative attachment of low energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is developed from moment equations for multi-temperature chemically reacting plasmas and for a cylindrical geometry of an inductively coupled discharge chamber. The species included into the model are ground state hydrogen atoms H and molecules H2, 14 vibrationally excited hydrogen molecules H2(v), v=1-14, electronically excited hydrogen atoms H(2), H(3), ground state positive ions H+, H2+, H3+, ground state negative ions H-, and electrons e. The space-averaged steady-state continuity equations coupled with the electron energy equation, the total energy equation and heat transfer to the chamber walls, are solved simultaneously in order to obtain the space-averaged number densities of the plasma components, the electron and heavy particle temperatures as well as the wall temperature. The GEVKM is supplemented by a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. The GEVKM is verified and validated in the low pressure, in the intermediate to high-pressure (1-100 Torr) and high absorbed power density (8.26-22 W/cm3) regimes by comparisons with the numerical simulations and experimental measurements. The GEVKM is applied to the simulation of the RFD chamber of the HCNHIS. The GEVKM predictions of negative hydrogen ions number densities and electron temperatures in the RFD chamber of the HCNHIS are used to estimate the negative hydrogen ion current using the Bohm flux approximation. The estimated negative current compare well with the Faraday Cup measurements and provide additional validation of the model. The GEVKM is used in a parametric investigation of the RFD chamber of HCNHIS-2 with hydrogen inlet flow rates 5-1000 sccm and absorbed powers 200-1000 W.

This thesis presents experimental studies of low-temperature atmospheric pressure plasma sources with generic ability to effectively treat large-scale three-dimensional (3D) objects. The reported large-scale plasma sources are developed through parallelisation of single plasma jets. This strategy outshines the other reported strategies for treatment of uneven surfaces by being able to produce spatially extended plasma directly onto the surface of heavily three-dimensional objects. Comparable studies of the design of elemental plasma jets bring out a hybrid electrode configuration, the capillary-ring jet, as the best elemental jet to be used in the parallelisation. It is found that the introduction of a ballast resistor to the individual jet circuit or built-in capacitance is important to assure the jet-to-jet synchronism, stability and uniformity. Electrical and optical analyses of one-dimensional (1D) array of atmospheric pressure plasma jets demonstrate robust temporal and spatial jet-to-jet uniformity both for flat and sloped surfaces. Hexagonally-arranged two-dimensional (2D) arrays of atmospheric pressure plasma jets show good level of insusceptibility to variations of the downstream samples in their physical dimensions as well as structural and material properties. The reaction chemistry impact area of a 2D 37-jet array is estimated to be 18.6 cm2. These confirm the plasma jet arrays as a viable option as large-scale atmospheric plasma sources, well suited for many processing applications including plasma medicine. The spatially separated dual-frequency excitation further benefits the plasma jet in that separate control of different important plasma parameters is possible. Enhanced plasma properties achieved by the dual-frequency offer greater potential to the jet arrays.