Shane M. Tysk

Plasma interferometry at high pressures

A general formulation for the measurement of plasma density and effective collision frequency for lowly as well as highly, collisional plasmas using millimeter wave interferometry is presented. In the presence of high density and collisionality at high gas pressures where the collision frequency (ν) is of the order of both the plasma (ωp) and the wave frequency of the millimeter wave (ω) (ν∼ω,ωp), the measured line-average plasma density has a complex dependence on phase shift as well as the amplitude change of the millimeter wave signal. The measurement scheme and analysis presented in this article show that for collisional plasmas, simultaneous measurement of the phase change and the amplitude change data is required to uniquely determine the plasma density and collision frequency. The treatment allows the application of millimeter wave interferometry to a wide range of relative collision frequency, wave frequency and plasma frequency since it uniquely determines the line-average plasma density and effe...

Optical, wave measurements, and modeling of helicon plasmas for a wide range of magnetic fields

Helicon waves are excited in a plasma wave facility by a half-turn double-helix antenna operating at 13.56 MHz for static magnetic fields ranging from 200 to 1000 G. A non-perturbing optical probe located outside the Pyrex™ plasma chamber is used to observe 443 nm Ar II emission that is spatially and temporally correlated with the helicon wave. The Ar II emission is measured along with wave magnetic and Langmuir probe density measurements at various axial and radial positions. 105 GHz interferometry is used to verify the bulk temperature corrected Langmuir probe measurements. The measured peak Ar II emission phase velocity is compared to the measured wave magnetic field phase velocity and code predicted wave phase velocity for the transition and blue mode regimes. Very different properties of the optical emission peak phase and wave characteristics for the transition and helicon modes of operation are observed. Comparison of the experimental results with the ANTENAII code [Y. Mouzouris and J. E. Scharer, ...

Characterization of laser produced tetrakis (dimethylamino) ethylene plasma in a high-pressure background gas

We present an interferometric and spectroscopic characterization of ultraviolet (UV) laser photoionization of a low ionization potential organic vapor, tetrakis (dimethylamino) ethylene (TMAE), seeded in high-pressure air component gases. These experiments are performed to explore the feasibility of using an electrodeless UV laser preionization of TMAE to initiate a plasma seeded in atmospheric pressure gases that can later be sustained by radiofrequency (RF) power by inductive wave coupling, thereby reducing the initiation RF power budget. A large volume (500 cc), high-density (/spl sim/10/sup 13/cm/sup -3/), electrodeless plasma is created by single-photon, 193 nm excimer laser ionization. 105 GHz millimeter-wave interferometry along with optical spectroscopy is employed to investigate the plasma formation and decay characteristics. The TMAE plasma decay mechanisms including two-body and three-body recombination with and without high pressure gases are examined and the dominant loss processes discussed and evaluated. Both density and optical emission measurements show a delay of 140 /spl plusmn/ 10 ns in the peak plasma density and emission indicating that the dominant ionization process is delayed ionization via excitation of super-excited states. The experiment also shows that TMAE remains a viable seed gas for UV ionization in the presence of air for t/spl les/10 min.

Radio-Frequency Sustainment of Laser Initiated, High-Pressure Air Constituent Plasmas

In this paper we investigate the feasibility of creating a high‐density ∼ 1012−1014 cm−3, large volume seed plasma in air constituents by laser (300 mJ, 20(±2) ns) preionization of an organic gas seeded in high‐pressure gas mixtures and then sustained by efficient absorption of rf power (1–25 kW pulsed) through inductive coupling of the wave fields. A multi‐turn helical antenna is used to couple radio‐frequency power through a capacitive matching network. A 105 GHz interferometer is employed to obtain the plasma density in the presence of high collisionality utilizing phase shift and amplitude attenuation data. TMAE Plasma decay mechanisms with and without the background gas are examined.

Optical, wave measurements, and modeling of helicon plasmas over a wide range of magnetic fields

Summary form only given, as follows. helix antenna operating at 13.56 MHz for static magnetic fields ranging from 200-1000 G? Diagnostics include Langmuir, wave magnetic field, and optical probes, as well as electron energy analyzer and 105 GHz interferometly. The physics behind an apparent constant phase peak of the 443 nm AI II emission relative to the rf antenna drive current is investigated. This phenomenon, which occurs under higher magnetic fields, is contrasted against a traveling peak observed at lower magnetic field strengths. The transition point at which highdensity ?blue mode? helicon operation begins is investigated to determine the plasma physics responsible. The I-dimensional ANTENA2 and the 2-dimensional MAXEB computational codes are used to model the wave electric and magnetic fields, with the latter capable of axial and radial variation in both the static magnetic field and plasma density? Also, an alternative experimental setup is designed for a nonuniform static magnetic field configuration to examine the effect on radial confinement and ionization.

Experimental Measurements and Modeling of a Helicon Plasma Source with Large Axial Density Gradients

An investigation of wave magnetic field, density and temperature profiles, electron distribution function and wave‐correlated Argon optical emission in a helicon plasma source is carried out. Diagnostics include Langmuir and wave magnetic field probes, interferometer, monochromator, and retarding field energy analyzer. The UW helicon experimental facility operates with argon pressures in the range of 1–300 mTorr. A variable capacitor matching network is used to couple up to 1.3 kW of pulsed RF power to a half‐turn double‐helix antenna. A uniform axial magnetic field of 200–1000 G is applied. Densities in the range of 1011−3×1013 cm−3 are obtained. Wave‐correlated optical emission and modulation is externally measured at 443 nm corresponding to an upper state 35 eV above the neutral ground state and phase correlated with the 13.56 MHz antenna current. MAXEB, AntenaII, and nonlinear ionization codes are used to model the conditions present in the system and to provide a comprehensive picture of wave field b...

Optical emission, electron energy, density, wave magnetic field and spectrum measurements in a helicon plasma source

Summary form only given. Measurements and analysis of optical emission, electron energy analyzer, Langmuir and magnetic probe and wave spectra are presented for a wide range of helicon plasma source conditions. Helicon plasma source characteristics at lower argon neutral pressures of 2-6 mTorr at both low (200 G) and high (1.2 kG) magnetic field strengths and at high pressures (100 mTorr) are presented for a wide range of radiofrequency input powers. Plasma densities in the range of 10/sup 11/-10/sup 13//cm/sup 3/ are obtained in the UW helicon facility which utilizes a double half-turn helix. Observations of Ar II emission, its modulation and correlation with the RF phase are presented in both time and spatial domains for a variety of plasma conditions. The emission spectrum measured by optical probes either internal or external to the 10 cm diameter Pyrex cylinder plasma is compared with wave magnetic and plasma density as well as miniaturized electron energy analyzer measurements. In addition, both low frequency, fundamental, sideband and harmonic components of the RF wave are measured and analyzed to obtain a comprehensive picture of the helicon source operation for a variety of conditions. Network analyzer measurements, Antenna II wave modeling and wave-particle ionization models are used to analyze the properties of the plasma source.

Electron energy distribution function in a helicon discharge

Summary form only given. A miniature, fast time response, gridded energy analyzer (GEA) is designed and constructed to measure the electron energy distribution function (EEDF) in a helicon plasma and compare with density, wave magnetic field, and Ar II optical emission results. These plasmas demonstrate an ionization efficiency greater than would be expected for their typical 3 eV electron temperature. The enhanced ionization effects may be partially explained by elevated quantities of high temperature electrons in the 20-50 eV range. Energy analyzer measurements of the EEDF can provide a more complete picture of helicon plasma source properties in different regimes.

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.

Optical diagnostic for examination of fast and thermal electrons from a helicon plasma source

Summary form only given, as follows. Helicon waves are excited in an argon plasma discharge by a twisted double helix antenna operating at 13.56 MHz. Two optical probes are used. One can be inserted into a I cm diameter glass tube in the center of the plasma chamber (about 11 cm diameter). The other observes the plasma from the outside. The optical probes are measuring emission from the short-lifetime At II states. Software is used to correlate the optical emission to the phase of the RF source. The Ar II emission is measured along with B-dot and Langmuir probe measurements at different axial positions. 105 GHz and 10 GHz; interferometry are used to verify Langmuir probe measurements. The result is to compare Ar II emission to B/sub z/ phase velocity and plasma density to determine the traveling wave particle and background Maxwellian interactions. The background level and modulation index of the Ar II emission is measured for this reason.