Date of Award

Summer 2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Program/Concentration

Electrical Engineering

Committee Director

Mounir Laroussi

Committee Member

Sacharia Albin

Committee Member

Leposava Vuskovic

Committee Member

Frederic D. McKenzie

Abstract

The main objective of this dissertation is to understand the formation of the plasma jet from the plasma pencil, and the propagation of the plasma jet in the ambient atmosphere where the effect of the external electric field is almost zero. Before investigating the formation and propagation phenomenon of the plasma jet, common physical properties of plasma jets are determined by using the imaging technique and optical emission spectroscopy. The first goal of this dissertation is to establish the laminar helium gas flow channel through a plasma pencil.

The formation position, formation time, and the criterion of the plasma jet formation from the discharge chamber of a plasma pencil are investigated by imaging technique, optical emission spectroscopy, and electrical measurement technique. It shows that the plasma jet forms at the surface of the grounded dielectric as a positive plasma front. The formation time of the plasma jet decreases with applied voltage. The maximum power, total power, and average energy to the system and to the discharge are calculated from the total current, discharge current, input potential, and gap potential of the plasma pencil. The calculated average input power with applied voltage to the discharge shows that 56% of the input power is used in the discharge. The total charge in the discharge chamber is calculated by integrating the discharge current waveform. The critical charge in the discharge chamber required to generate a plasma jet is also determined.

The propagation phenomenon of the plasma bullet in the ambient atmosphere has been investigated from the velocity curves of the plasma bullet along the jet axis for different applied voltages, pulse widths, and feed gas flow rates. The plasma bullet's velocity is measured by using two different techniques: (i) imaging technique and (ii) electrical technique. In imaging technique, ultra-fast ICCD images of the plasma jet have been taken at different times and positions, and from the change of position of the plasma bullet's front with time plasma bullet velocity is calculated. In the electrical technique, from the spatial and temporal evolution of the jet current along the jet axis, the plasma bullet velocity is calculated. It shows that the plasma bullet velocity curve has three distinct phases: transition phase, propagation phase, and collapse of the plasma bullet. The transition phase and the propagation phase of the plasma bullet have been explained to understand the plasma properties in the plasma bullet. A correlation between the average plasma bullet velocity, length of the plasma jet, and the operating pulse width is established, which can be used in the modeling of the plasma bullet's propagation. The change of the shape and the inner structure of the plasma bullet are observed along the propagation pathway of the plasma bullet. The contraction of the plasma bullet is investigated from the head-on view image and the electric field due to the cross-section of the plasma bullet. It shows that the contraction of the plasma bullet occurs because of a cone-shape electric field line in front of the plasma bullet. The average reduced electric field and the power density have been estimated by using a simple model. The electron density of the plasma jet along the jet axis is estimated from the jet current density and the spatial and temporal differences between the consecutive jet current peaks along the jet axis. The estimated electron density in the plasma bullet is in the order of 1011 cm−3. The effect of applied voltage and gas flow rate on the electron density is also investigated.

The spatial evolution of the different species along the jet axis is investigated by optical emission spectroscopy. The evolution of the different species follows the same shape as the plasma bullet velocity curve. Along the transition phase, the plasma chemistry is dominated by excited state of ionized molecular nitrogen, helium metastable, and high energy electrons. Along the propagation phase, the plasma chemistry is dominated by the first negative system of nitrogen and some of the long lived metastable helium. The local reduced electric field and electron density along the jet axis are measured from the intensity of the nitrogen bands and it is compared with the results obtained from the measurement by using a dielectric probe. The maximum local reduced electric field is 350 Td for the applied voltage of 6 kV. The estimated electron density is in the same order (1011 cm−3) for both measurements.

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DOI

10.25777/8hka-6c89

ISBN

9781124291789

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