K.L. Kelly

Microwave reflections from a vacuum ultraviolet laser produced plasma sheet

A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas tetrakis- dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet with a width of 7.8 cm and an adjustable thickness of 0.7–1.4 cm. The axial scale length of the plasma density is a function of TMAE neutral pressure and is typically 50 cm. X-band (10 GHz) microwaves are incident on the plasma with the electric field polarized parallel to the laser beam axis. The power reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a response time of 10 ns is utilized to determine the amplitude and phase of the reflected wave. The peak plasma density is ne≈4×1013 cm−3, sufficiently above the critical density (ncrit=1.2×1012 cm−3) to produce reflections comparable to a conducting sheet placed in the same position as the plasma. A computer model is developed to interpret and optimize the plasma conditions which provide the highest backscatter and phase-...

Properties of a vacuum ultraviolet laser created plasma sheet for a microwave reflector

A 193 nm excimer laser and a custom fabricated cylindrical lens system is used to produce a plasma sheet of 8 cm×30 cm×0.4 cm in tetrakis(dimethylamino)ethylene (TMAE), a low ionization energy organic gas. Plasma density variation due to photon absorption is studied by scanning the filling pressure of TMAE between 12 and 150 mTorr. A high density (n≥2.0×1013 cm−3), low temperature (Te≊0.8 eV) plasma sheet of 4 mm thickness is obtained with less than 50% spatial density variation over the 30 cm axial length. Charge recombination is found to be the dominant process for t≤1.2 μs with the plasma diffusion playing a perturbational role. A one‐dimensional plasma model is utilized to model the experimental plasma data by treating the diffusion as a perturbation. This study shows that the recombination coefficient is 1.8±0.1×10−7 cm3 s−1 and the diffusion coefficient is 2.8±0.4×104 cm2 s−1. The plasma sheet has attractive properties for a microwave agile mirror.

Laser ionization and radio frequency sustainment of high-pressure seeded plasmas

The feasibility of using a photoionized, low-ionization potential organic seed gas to initiate a high pressure plasma discharge is examined and compared to radio frequency breakdown of high pressure argon alone. The seed gas, tetrakis(dimethylamino)ethylene, which has an ionization potential of 6.1 eV is ionized by an ultraviolet laser through 6.4 eV photon absorption, and forms a plasma column inside a vacuum chamber. The plasma absorbs additional power through inductive coupling of 13.56 MHz helical antenna radio frequency wave fields to the plasma through electron acceleration, ionization, and collisional damping. Laser initiation of 2–6 mTorr of the seed gas in 1–150 Torr of argon is accomplished and produces steady-state line-average plasma densities of ne≈4×1012 cm−3 in a volume of 300 cm3. The two-body recombination coefficient of the organic seed gas and its optimum partial pressure when mixed with argon are experimentally determined and analyzed. Particle loss and power requirements for maintaini...

Effects of rapidly decaying plasmas on Langmuir probe measurements

Sheath motion, displacement current, and probe edge effects on temporal Langmuir probe (LP) ion saturation current measurements are investigated for a pulsed laser produced plasma during the period 100 ns→1000 ns after the turn on of the laser pulse. The plasma has a large volume (hundreds of cm3) and a high initial plasma density (ne>1013 cm−3). The sheath motion and edge effects are found to be very important, but the displacement current is found to be very small. We present both a quantitative correction for the effects of rapidly decaying plasmas on LP ion saturation current measurements and a validity condition for this method. The results are compared with the densities predicted from electron saturation currents, and the former are ⩽30% lower than the latter. The corrected probe measurements are utilized to determine the plasma recombination coefficient. It is found to be in good agreement with the results obtained by Stalder and Eckstrom [J. Appl. Phys. 72, 3917 (1992)] who utilized a microwave m...

Diagnostics and analyses of decay process in laser produced tetrakis(dimethyl-amino)ethylene plasma

A large volume (hundreds of cm3) plasma is created by a 193 nm laser ionizing an organic vapor, tetrakis(dimethyl-amino)ethylene (TMAE). The plasma is characterized as high electron density (1013–1012 cm−3) and low electron temperature (∼0.1 eV). To investigate the plasma decay processes, a fast Langmuir probe technique is developed, including detailed considerations of probe structure, probe surface cleaning, shielding, frequency response of the detection system, physical processes in probe measurement, dummy probe corrections as well as noise analysis. The mechanisms for the plasma decay are studied and a delayed ionization process following the laser pulse is found to be important. This mechanism is also supported by optical emission measurements which show that nitrogen enhances the delayed emission from TMAE plasma. A model combining electron–ion recombination and delayed ionization is utilized together with experimental results to order the terms and calculate the relaxation times for delayed ioniza...

Microwave reflections from a VUV laser produced plasma sheet

Summary form only given. A 20 ns vacuum Ultra-Violet (VUV) laser pulse is used to create a plasma sheet in an organic gas. A bistatic antenna system is used for transmitting and receiving X-band microwaves which interact with the plasma. Reflected signals are measured for amplitude and phase analysis. Amplitude and phase shifts are compared to an aluminum conducting sheet placed in the same position as the plasma. The working gas is tetrakis (dimethylamino) ethylene (TMAE) with an ionization energy of 6.1 eV. The ionizing source is an excimer laser (W/sub max/=300 mJ) operating at 193 nanometers (6.4 eV). The laser beam is transformed into a sheet using VUV thin-film matched lenses. A plasma sheet with a peak density of 2.5/spl times/10/sup 13/ cm/sup -3/ and T/sub e/=0.3 eV is formed with dimensions 0.7-5 cm/spl times/7.8 cm/spl times/30 cm. Additional measurements of transmitted signals are utilized to determine plasma density and collision frequency. A finite-element computer model of the plasma profile to determine microwave transmission and reflection levels has been developed to optimize reflected signal levels as a function of density and thickness and to interpret experimental results. Comparison between the experimental results and the model show that this system is attractive for use as a microwave reflector. In addition, studies are being carried out to explore plasmas created with air components with this microwave.

Nitrogen influences on a laser produced TMAE plasma

Summary form only given. TMAE is a readily ionized organic gas, tetrakis(dimethylamino)ethylene, which can be single photon ionized by a 193 nm laser, so that a large volume (hundreds of cm/sup 3/), high initial plasma density (>10/sup 13/ cm/sup -3/) plasma can be created. Additional high pressure nitrogen admixture effects in this plasma are studied by measuring planar Langmuir probe electron saturation currents: which provide more reliable electron density measurements than those from ion saturation current, because the former is not dependent on ion species but the latter is. The former is much more difficult in the experiment due to the difficulty of accurately identifying the electron saturation current: but we have got very flat curves of the electron saturation current vs. the bias voltage so that the electron saturation currents can be accurately identified. The technique of these fast probe measurements which include a detailed considerations of probe structure, probe surface cleaning, shielding, probe perturbation, frequency response. Temporal and spatial resolutions, dummy probe corrections as well as noise analysis will be shown. The electron densities and temperatures vs. time at different TMAE pressures, nitrogen pressures and laser powers will be present.

Laser initiation and radiofrequency sustainment of seeded high pressure plasmas

Summary form only given. Laser initiation of a low ionization seed (2-6 mTorr) gas in high pressure(>10 Torr) gases is examined. An excimer laser operating at 193 nm is used to initiate a high density, low ionization energy (6.1 eV) organic seed gas plasma of 10/sup 13///cm/sup 3/ and a low electron temperatures of 100 Torr) to determine optimum gas mix and RF power requirements for producing high density plasmas in high pressure conditions. We utilize mm wave interferometry, optical spectroscopy and antenna loading impedance measurements and analyses to determine the ionization and recombination processes occurring in the high pressure discharges. The influence of the laser/RF timing and power levels, gas mixes as well as density and spectroscopic data are used to determine the ionic constituents and plasma decay processes for the high pressure plasma discharges.

Radio frequency sustainment of laser initiated seeded high-pressure discharge

Summary form only given, as follows. The radio-frequency power required to initiate a discharge at high gas pressure (>100 Torr) is quite high. But once the gas breakdown has occurred and discharge is initiated, the radiofrequency power can be much more efficiently absorbed by the plasma through inductive coupling of the wave fields. The ohmic heating of the antenna and the coil current required to sustain the discharge reduce significantly. The discharge is initiated by photoionization of a low ionization potential (6.1 eV) organic gas tetrakis(dimethylamino)ethylene or TMAE, using a high power excimer laser (wavelength 193 nm and a maximum laser energy of 300 mJ for 20(/spl plusmn/) ns half-pulse width) seeded in a high-pressure gas. The discharge is then sustained by coupling 1-25 kW of radio frequency power through a five turn helical antenna using a capacitive matching network. Recent results with the laser-initiated discharge of 2-6 mTorr of seed gas with a high-pressure background gas (/spl ges/760 Torr of nitrogen and argon mix) will be presented. A large volume plasma 300-500 cm/sup 3/ in the density range 10/sup 12/-10/sup 13/ cm/sup -3/ is produced. The influence of the accurate timing of the laser and radio frequency pulses along with the radio frequency power level and gas mix to produce the high-density plasma will be discussed.

Radiofrequency initiation and radiofrequency sustainment of laser initiated seeded high pressure plasma

We examine radiofrequency initiation of high pressure(1–70 Torr) inductive plasma discharges in argon, nitrogen, air and organic seed gas mixtures. Millimeter wave interferometry, optical emission and antenna wave impedance measurements for double half-turn helix and helical inductive antennas are used to interpret the rf/plasma coupling, measure the densities in the range of 1012 cm−3 and analyze the ionization and excited states of the gas mixtures. We have also carried out 193 nm excimer laser initiation of an organic gas seed plasma which is sustained at higher pressures(150 Torr) by radiofrequency coupling at 2.8 kW power levels.