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, IEEE Trans. Plasma Sci. 24, 152 ͑1996͔͒ is carried out for the wave field measurements for the two regimes of operation.

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A laser initiation and radio frequency ͑rf͒ sustainment technique has been developed and improved from our previous work to create and sustain large-volume, high-pressure air and nitrogen plasmas. This technique utilizes a laser-initiated, 15 mTorr partial pressure tetrakis ͑dimethylamino͒ ethylene seed plasma with a 75 Torr background gas pressure to achieve high-pressure air/nitrogen plasma breakdown and reduce the rf power requirement needed to sustain the plasma. Upon the laser plasma initiation, the chamber pressure is raised to 760 Torr in 0.5 s through a pulsed gas valve, and the end of the chamber is subsequently opened to the ambient air. The atmospheric-pressure plasma is then maintained with the 13.56 MHz rf power. Using this technique, large-volume ͑1000 cm 3 ͒, high electron density ͑on the order of 10 11-12 cm −3 ͒, 760 Torr air and nitrogen plasmas have been created while rf power reflection is minimized during the entire plasma pulse utilizing a dynamic matching method. This plasma can project far away from the antenna region ͑30 cm͒, and the rf power budget is 5 W / cm 3. Temporal evolution of the plasma electron density and total electron-neutral collision frequency during the pulsed plasma is diagnosed using millimeter wave interferometry. Optical emission spectroscopy ͑OES͒ aided by SPECAIR, a special OES simulation program for air-constituent plasmas, is used to analyze the radiating species and thermodynamic characteristics of the plasma. Rotational and vibrational temperatures of 4400-4600Ϯ 100 K are obtained from the emission spectra from the N 2 ͑2+͒ and N 2 + ͑1−͒ transitions by matching the experimental spectrum results with the SPECAIR simulation results. Based on the relation between the electron collision frequency and the neutral density, utilizing millimeter wave interferometry, the electron temperature of the 760 Torr nitrogen plasma is found to be 8700Ϯ 100 K ͑0.75Ϯ 0.1 eV͒. Therefore, the plasma deviates significantly from local thermal equilibrium.

Abstract-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 ( 10 13 cm 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 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 10 min.Index Terms-High-pressure plasmas, laser ionization and breakdown of plasmas, plasma emission.

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