Siqi Luo

Laser-rf creation and diagnostics of seeded atmospheric pressure air and nitrogen plasmas

A laser initiation and radio frequency (rf) 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 (dimethylamino) 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 cm3), high electron density (on the order of 1011–12 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 ...

Experimental Study of Laser-Initiated Radiofrequency-Sustained High-Pressure Plasmas

Experiments are performed using 193-nm ultraviolet laser preionization of a seed gas in atmospheric pressure range argon and nitrogen to initiate a discharge that is sustained by 13.56-MHz radiofrequency (RF) power using efficient inductive wave coupling. High-density (4.5times1012/cm3 line average density) large-volume (~500 cm3) 760-torr argon plasma is initiated and maintained for more than 400 ms with 2.2 kW of net RF power coupled to the plasma. Using the same technique, a 50-torr nitrogen plasma with line average electron density of 3.5times1011/cm3 is obtained. The nitrogen plasma volume of 1500 cm3 is initiated by the laser and maintained by a net RF power of 3.5 kW for 350 ms. Measurements of the time-varying plasma impedance and optimization of the RF matching for the transition from laser-initiated to RF-sustained plasma are carried out. Both laser-initiated plasmas provide much larger plasma volumes at lower RF power densities than can be obtained by RF alone. Millimeter wave interferometry is used to determine the electron density and the total electron-neutral collision frequency. A new diagnostic technique based on interferometry is developed to evaluate the electron temperature in high-pressure plasmas with inclusion of the neutral heating. Broadband plasma emission spectroscopy is used to illustrate the changes in the ionized species character immediately after the laser pulse and later during the RF pulse

Measurements of Air Breakdown and Scaling to Microwaves Using 193 nm Focused Laser Radiation

Summary form only given. The measurements and analysis of air breakdown processes by focusing 193 nm, 200 mJ, 10 MW high power UV laser radiation on to a 10-20 mum spot size that produces laser power densities up to 1012-1013 W/cm2, well above the threshold power flux for air ionization will be presented. The breakdown threshold is measured and compared with classical and quantum theoretical models. A universal scaling analysis of these results allows one to predict some aspects of high power microwave breakdown based on measured laser breakdown observations. The air breakdown threshold is being measured for a wide pressure range from 90 torr to 5 atmospheres. Above atmospheric pressures and their increased collisional frequencies relative to the high laser frequency (1015 Hz) allow enhanced ionization and plasma formation. Multi-photon ionization processes can also play a substantial role at 193 nm due to the high photon energies (6.4 eV). The laser breakdown threshold data for air at 193 nm is correlated with corresponding microwave breakdown values using the concept of universal scaling, for which extensive microwave data is available [1-2] and current microwave breakdown measurements are being obtained at TexasTech University. An extensive range of optical and spectroscopic diagnostics with ns fast gating and 13 mum ICCD resolution is utilized to characterize the plasma. Laser shadowgraphy technique is used to characterize the spatial and temporal evolution of the laser focused plasma ionization shockwave. Plasma temperature is measure using optical emission spectroscopy method of the N2 second positive system N2 (2+) (0,0) at 337.1 nm band. Initial measurements of vibrational temperatures obtained from nitrogen lines at 500 mus are found to be 0.22 eV with electronic temperatures of 0.8 eV and rotational temperatures of 0.1 eV. Plasma density and neutral pressure variations are measured using two color laser interferometry. Phase shifts produced by the laser focused plasma for red (632 nm) and green (532 nm) wavelengths are being measured, which is used to measure the plasma and neutral pressure densities. The peak plasma densities are anticipated to be in the 1016-18/cc range. A comparison of the laser breakdown case to the microwave breakdown case utilizing the universal scaling law is being carried out and was presented.

Measurements of Air Breakdown Process using 193 NM Focused Laser Radiation

Summary form only given. We report the measurements and analysis of air breakdown process by focusing 193 nm, 200 mJ, 10 MW high power UV laser radiation on to a 10-20 mum spot size that can produce a maximum laser intensity of 1012-1013 W/cm2, and well above the threshold flux for air ionization. The breakdown threshold is measured and compared with the theoretical models such as classical and quantum analyses. The air breakdown threshold is measured for wide pressures range from 90 Torr to 5 atmosphere pressures. Above atmospheric pressures are required to increase the collisional frequency comparative to the high laser frequency (1015 Hz). Multiphoton ionization processes are also expected to play a substantial role at 193 nm due to the high photon energy (6.4 eV). The breakdown threshold data for air at 193 nm is correlated with the microwave breakdown using the concept of universal scaling, for which extensive microwave breakdown data is available at frequencies of 0.99, 2.8 and 9.4 GHz and current microwave breakdown results obtained at Texas Tech University. An extensive range of optical and spectroscopy diagnostics with 5 ns time scale gating and 13 mum ICCD resolution has been constructed to characterize the plasma. The spatial and temporal evolution of the laser focused plasma is measured using the shadowgraphy technique. The plasma temperatures can be obtained by measuring the velocity of the shock wave front and also by using optical emission spectroscopy. Optical emission spectroscopy is performed to diagnose the plasma temperature using the emission lines of the N2 second positive system N2 (2+) (0,0) at 337.1 nm. Plasma density is measured using two color laser interferometry. Phase shift produced by the laser focused plasma for two different probe laser wavelengths such as red (632 nm) and green (532 nm) wavelengths will be measured, which is used to measure the plasma density.

Optimization and Diagnostics of Fast Pulsed High Pressure Air Constituents Plasmas

Summary form only given. A fast-pulsed, UV laser-initiated, RF-sustained high pressure plasma source has been developed and optimized for more efficient large volume (ges1000 cc), high pressure plasma generation. We have shown that during the tast-pulsed plasma formation, the plasma load impedance as seen by the helical antenna varies significantly from 0.5+j125 Omega to 4.0+j130 Omega on short time scales. As a result, it presents a challenge to the RF matching system. In order to ensure efficient RF power coupling to the plasma, a dynamic RF impedance measurement and matching technique has been designed and implemented: a dual directional coupler is used to precisely measure the complex incident RF reflection coefficient at the input end of the matchbox. A pair of high voltage probes is used to monitor the time-dependent plasma impedance so that the matching circuit can be tuned accordingly to match the varying plasma load impedance. As a result of this effort, a large volume (1500 cc), high electron density (1.6times1011 /cc) laser-initiated, RF-sustained high pressure (50 torr) air constituents plasma is created utilizing an initiating UV laser pulse energy of 100 mJ and subsequently maintained by a net RF power level of 3 kW for several seconds. The RF power budget is 2 Watts/cc. Optical emission spectroscopy is used to analyze the plasma, aided by the Specair program developed by Laux et al. The rotational temperature of N2 C-B (second positive) is obtained by matching the experimental spectrum results with the Specair code-simulated results. Time evolution of the plasmas electron density and total electron-neutral collision frequency is diagnosed using a millimeter wave interferometry technique. A new mathematical method is developed to compute the time-dependent electron density and total electron-neutral collision frequency, and subsequently to evaluate electron temperature levels based on the interferometric experimental results.

Optical diagnostics of laser initiated, RF sustained high pressure seeded plasmas

Summary form only given. High-pressure, inductively-coupled plasmas (ICP) have been used for variety of scientific and industrial applications over a large gas pressure range from 50-760 torr. High density ~1011-1014 cm-3, large (500-2500 cc) volume, atmospheric plasmas are of great interest in applications such as material processing, biological decontamination, radar and stealth antennas. We present a technique for producing these plasmas and measuring their excitation temperature, density and plasma evolution from a seed gas by means of optical emission and millimeter wave interferometry. The plasma is produced by utilizing a seed (15 mtorr) organic gas, tetrakis (dimethyl-amino) ethylene (TMAE), in high-pressure argon or nitrogen gas is pre-ionized by a 193 nm excimer laser (300 mJ), 20 ns pulse width. The seed plasma is then sustained by the efficient absorption of the pulsed 13.56 MHz radio frequency (RF) power (1-25 kW) through inductive coupling of the wave fields, which reduces the RF power requirement for plasma initiation to a great extent. Optical emission spectroscopy is used to characterize the temporal evolution of the plasma and measuring the excitation temperature. The optically emitted spectral lines illustrate the temporal plasma evolution from a TMAE seed plasma initiated by the laser and early RF power to the steady-state RF plasma of neutral background gases such as argon and nitrogen. A three channel, wide band (200-850 nm) ST2000 Ocean optics spectrometer is used to record the plasma spectral emission perpendicular to the plasma column axis. Each channel is connected to a separate grating spectrometer (1200 lines/mm, with an optical resolution of 0.3 nm), which counts photons using a linear CCD array (2048 pixels). Samples are taken over the following wavelength ranges; 200-500 nm, 400-700 nm, and 600-850 nm. The results show that the laser ionization of the TMAE seed gas and transition to argon plasma is readily accomplished with our system. The spectroscopic measurements can also provide highly localized measurements of the plasma conditions in regions that are less accessible to interferometer measurements. We also present initial spectroscopic measurements of a high power UV laser focused in air near a dielectric microwave window

Diagnostics and simulation of highpressure argon and nitrogen plasma

Summary form only given. Experiments are performed using 193 nm ultraviolet (UV) laser pre-ionization of a seed gas in atmospheric pressure range argon or nitrogen to initiate a discharge that is sustained by 13.56 MHz radiofrequency (RF) power using efficient inductive wave coupling. High-density (1012-1013/cm3), large-volume (~500 cm 3) 760 torr argon plasma is initiated and maintained for 300 ms with less than 3 kW of net RF power. Using the same technique, a 50 torr nitrogen plasma with average electron densities greater than 1011/cm3 is obtained. The nitrogen plasma volume of 1000 cm3 is initiated by the laser and maintained by a net RF power of 5.5 kW for 300 ms. Measurements of the time-varying plasma load impedance and optimization of the RF matching for the transition from laser-initiated to RF-sustained plasma are carried out. Millimeter wave interferometry is used to determine the electron density and effective electron collision frequency. A new diagnostic technique based on interferometry is developed to measure the electron temperature in high pressure plasmas. Broadband plasma emission spectroscopy is used to illustrate the changes in the broad ionized species character immediately after the laser pulse and later during the RF pulse. Advanced RF impedance measurement and RF tuning techniques are developed to enhance RF matching of the inductively coupled plasma source to the RF system. Computer simulation of the interferometry diagnostics of plasma density and collision rate is performed. Results of a new focused-laser generated plasma near a microwave dielectric window are also presented

Efficient Creation of Laser Initiated, RF Sustained Atmospheric Pressure Range Plasmas

Summary form only given. We examine 193 nm (6.4 eV) UV photon ionization of low ionization level (6.1 eV) seed (15 mtorr) gas to reduce the power levels for radio frequency (RF) sustainment of large volume (500-2500 cc), high density (1012-13/cc), atmospheric pressure range (50-760 torr) plasmas. These plasmas have potential applications in the areas of biological decontamination, radar absorption, thin films and processing of industrial materials. We have demonstrated the use of UV seeded plasma as a load for efficient inductive coupling at lower RF powers in argon. The tetrakis (dimethylamino) ethylene (TMAE), laser-initiated seed plasma recombination rates and observation of substantial delay in addition to direct ionization processes have been characterized in air and other gases. Current research is focused on the temporal measurement and matching of the laser and RF plasma load to the inductive helical antenna to produce high density, large volume plasmas. Time resolved antenna impedance and RF power measurements are performed to improve matching and determine the characteristics of the laser-ionized and radio frequency sustained plasma. In addition, millimeter-wave interferometry and optical emission spectroscopy diagnostics are used to determine the spatial and temporal plasma constituents. The addition of small concentrations (~2%) of hydrogen and measurements of line broadening are used together with the other diagnostics to determine the spatial and temporal plasma temperature, density, and recombination rates immediately after the rapid shut off of RF power

RF Matchings Time Resolved Impedance, Power and Interferometer Measurements of Laser Initiated, RF Sustained Atmospheric Pressure Plasmas

Summary form only given. The feasibility of laser ionization and radio frequency sustainment of high-pressure seeded plasmas in argon has been demonstrated by our group in previous work. The tetrakis (dimethylamino) ethylene (TMAE) laser-initiated seed plasma recombination rates have also been characterized. Time resolved impedance and RF power measurements are performed on the plasma system in order to optimize and further characterize this laser-ionized and radio frequency sustained plasma. An atmospheric pressure range (40-760 torr), large volume (~2000 cc) air and air constituent (N2 and O2) plasma of high density (>1012 cm-3 ) is initiated by a 193 nm excimer UV laser and sustained by 13.56 MHz RF power through a helix coil antenna and matching system. The plasma loading impedance in the steady-state regime was characterized using high voltage probes and found to be of order 4.0 + j100.0 Omega, which varies dependent on the gas pressure and flow rate. The temporal variation of the plasma impedance and net coupled RF power are measured. Interferometer and spectroscopic analysis of plasma density, temperature, collision rate and recombination rates are discussed

Optical Emission Measurements of Laser Initiated, RF Sustained High Pressure Seeded Plasmas

Summary form only given. High density ~1012-1014 cm-3, large (500-2500 cc) volume, high-pressure air and air constituent atmospheric (50-760 torr) plasmas are of great interest in variety of scientific and industrial applications such as material processing, biological decontamination, radar and stealth antennas. We present a technique for producing these plasmas and measuring their temperature, density and recombination rates by means of optical emission and millimeter wave interferometry. The plasma is produced by utilizing a seed (15 mtorr) organic gas, tetrakis (dimethyl-amino) ethylene (TMAE), in high-pressure air and air component gases is pre-ionized by a 393 nm excimer laser (300 mJ), 20 ns pulse width. The seed plasma is then sustained by the efficient absorption of the pulsed 13.56 MHz radio frequency (RF) power (1-25 kW) through inductive coupling of the wave fields, which reduces the RF power requirement for plasma initiation to a great extent. High-resolution optical emission spectroscopic (OES) diagnostics are carried out to measure the density and temperature profiles of the plasma. We use a 0.5-m monochromator with a 1200-groove/mm grating with entrance and exit slits of 10 mum to obtain high resolution. The monochromator is calibrated for spectral intensities between 200-800 nm with a Hg-Ar light source. Hbeta (486.132 nm) and Halpha (656.28 nm) line broadening techniques have been used to measure the FWHM, which in turn used to measure the density and temperature of the plasma. A small amount of H2 (2% mole fraction) is mixed with the working gas. The background emission spectrum, due to the N2 first positive system B3Pig-A3Sigmau + is also measured by turning off hydrogen flow. For the temperature (0.4-0.6 eV) and density (1012-1014 cm -3) range of the plasma generated, the line width is dominated by Stark broadening. The spectroscopic measurements are used to obtain density and collisional rates are compared with those using a 105 GHz interferometer. The spectroscopic measurements can also provide highly localized measurements of the plasma conditions in regions that are less accessible to interferometer measurements