Session CO7: Plasma Sources & Technologies

Observation of warm, higher energy electrons transiting a double layer in a helicon plasma

Experimental observations in MadiHeX indicate that fast electrons with substantial density fractions can be created at low helicon operating pressure. Two-temperature electron distributions including a fast (\textgreater 80 eV) tail are observed in an inductive RF helicon argon plasma double layer at 0.17 mTorr Ar pressure. The fast, untrapped electrons measured downstream of the double layer have a higher temperature of 13 eV than the trapped, upstream electrons with a temperature of 4 eV [1]. The reduction of plasma potential and density observed in the double layer region would require an upstream temperature ten times the measured 4 eV if occurring via Boltzmann ambipolar expansion. Upstream fluctuations of $\pm$ 30{\%} are also observed in the emissive probe measured plasma potential. Sideband frequencies have been observed at $\pm$ 2 kHz of the driven RF frequency of 13.56 MHz, implying a beam instability effect dominantly upstream of the double layer. This can affect ion acceleration and electron temperature distribution in the region. The mechanism behind this has been explored via several plasma diagnostics tools. An RF-compensated Langmuir probe has been used to measure the electron temperatures and densities, which are cross-checked with ADAS, OES and millimeter wave IF. The EEDF in the plasma has also been profiled to understand the acceleration mechanism. A four-grid RPA and an emissive probe have been used to measure the IEDF and plasma potential. The measured IEDF has also been checked with LIF techniques.\\[4pt] [1] Y.-T. Sung, Y. Li, J. E. Scharer, Phys. Plasmas 22, 034503 (2015)   

Direct identification of axial plasma momentum in a magnetic nozzle helicon plasma

Axial components of the plasma momentum arising from plasma production, plasma loss to a wall, and plasma expansion in a magnetic nozzle are experimentally identified by the direct measurement of the force exerted to the helicon source structures, which consist of the axial and radial source boundary and the magnetic nozzle. The axial momentum corresponding to the source electron pressure is exerted to the source back plate, while some of the axial momentum is lost to the radial wall by the radially lost ions when they are already accelerated in the plasma core along the axis. Once the plasma is ejected into the magnetic nozzle, their radial momentum is converted into the axial one by the Lorentz force due to the radial magnetic field and the azimuthal plasma current.   

Diamagnetic Effect in a Partially-Ionized High-Beta Plasma

Balance between magnetic pressure and plasma pressure is expected in fully ionized plasmas confined by a magnetic field. The magnetic force on the plasma is due to a current carried by the plasma which is diamagnetic. The magnetic field inside the plasma is then lowered by that current. In a partially-ionized plasma, however, the plasma pressure is balanced not only by the magnetic field pressure but also by neural-gas pressure. In that case the diamagnetic effect of the plasma, even if high beta, is expected to be lower. We calculate the steady-state of a cylindrical low temperature magnetized partially-ionized plasma (such as rf plasma source). We solve for the radial dependencies of the plasma density, the neutral density, and the magnetic field profile. Neutral pressure gradient is established by neutral depletion under the plasma pressure. We demonstrate how neutral depletion affects the diamagnetic effect of a high beta plasma.   

Design Considerations in Capacitively Coupled Plasmas

Microelectronics industry has driven transistor feature size scaling from 10$^{-6}$ m to 10$^{-9}$ m during the past 50 years, which is often referred to as Moore's law. It cannot be overstated that today's information technology would not have been so successful without plasma material processing. One of the major plasma sources for the microelectronics fabrication is capacitively coupled plasmas (CCPs). The CCP reactor has been intensively studied and developed for the deposition and etching of different films on the silicon wafer. As the feature size gets to around 10 nm, the requirement for the process uniformity is less than 1-2 nm across the wafer (300 mm). In order to achieve the desired uniformity, the hardware design should be as precise as possible before the fine tuning of process condition is applied to make it even better. In doing this procedure, the computer simulation can save a significant amount of resources such as time and money which are critical in the semiconductor business. In this presentation, we compare plasma properties using a 2-dimensional plasma hydrodynamics model for different kinds of design factors that can affect the plasma uniformity. The parameters studied in this presentation include chamber accessing port, pumping port, focus ring around wafer substrate, and the geometry of electrodes of CCP.   

Modeling of a plasma vacuum window for high power beam applications

A major new facility for the Department of Energy (DOE) Office of Nuclear Physics is the Facility for Rare Isotope Beams (FRIB). FRIB will accelerate heavy ion beams (up to uranium) to energies as high as 200 MeV/u and with powers as high as 400 kW in a few mm diameter. Due to the limited lifetime at these high powers of solid foil strippers, FRIB researchers are pursuing gas jet strippers as a new approach. By exciting an arc discharge across the gas jet, the resulting plasma can act as a vacuum window. We are developing models of these plasma windows, including the complex geometry of the nozzle, including viscosity effects, and including a temperature dependent air conductivity. We present here results for the flow velocity as a function of the pressure drop, and for the temperature as a function of discharge current. We compare these results with recent experiments performed at FRIB.   

Field Emission Microplasma Actuated Microchannel Flow

Flow actuation by dielectric barrier discharges (DBD) involve no moving parts and provide high power density for flow enhancement, heating and mixing applications in microthrusters, micropumps and microcombustors. Conventional micro-DBDs require voltages $\sim$ kV for flow enhancement of a few m/s for 500 $\mu $m high channel. However for gaps \textless 10 microns, field emission lowers the breakdown voltage following modified Paschen curve. We consider a micropump concept that takes advantage of the field emission from a micro-DBD with dielectric thickness of 3 $\mu $m and a peak voltage of -325 V at 10 MHz. At 760 Torr, for electrode thickness of 1 $\mu $m, Knudsen number with respect to the e-nitrogen collisions is 0.1. So, kinetic approach of particle-in-cell method with Monte Carlo collisions is applied in nitrogen at 300 K to resolve electron (n$_{\mathrm{e}})$ and ion (n$_{\mathrm{i}})$ number densities. Body force, \textbf{f}$_{\mathrm{\mathbf{b}}}=$ e\textbf{E}(n$_{\mathrm{i}}$-n$_{\mathrm{e}})$, where, $e $is electron charge and \textbf{E}is electric field. The major source of heating from plasma is Joule heating, \textbf{J.E}, where \textbf{J }is current density. At 760 Torr, for f$_{\mathrm{b,avg}}=$ 1 mN/cubic mm and \textbf{J.E}$=$ 8 W/cubic mm, micro-DBD induced a flow with a velocity of 4.1 m/s for a 64 mW/m power input for a channel height of 500 $\mu $m. The PIC/MCC plasma simulations are coupled to a CFD solver for analysis of the resulting flow actuation in microchannels at various Reynolds numbers.   

Generation and dynamics of single-electrode nanosecond pulsed microplasma jets

Several millimeter long, 160 - 260 micrometer-in-width, helium plasma jets were generated in ambient atmosphere when a needle electrode was excited with nanosecond high voltage pulses at single shot or up to 500 Hz. This single-electrode system does not require the use of ground electrode for plasma generation, and thus has advantages in simplicity and small-dimension for a variety of biomedical applications. Dynamics of the microplasma jet powered by high voltage pulses with two different nanosecond pulses -- 5 ns and 164 ns, was studied with high speed imaging, and spatiotemporally resolved optical emission spectroscopy. Whereas the plasma jet exhibits three different modes including a positive-streamer mode, a stochastic transition, and a negative streamer-like mode when it was excited with 164 ns kilovolt pulses, such modes and transitions in the plasma development were not observed for the 5 ns pulsed excitation. Shorter pulses with shorter rise times allowed higher energy deposition into the plasma and promote rapid acceleration of the plasma wavefronts; 5 ns pulsed excitation resulted in 4 times increase in the wavefront velocity compared with the 164 ns pulsed excitation. Importantly, the production of excited atomic oxygen increased by a factor of 2 for the 5 ns pulsed plasma jet when compared with that for a 164 ns pulsed plasma jet, whereas the other excited species including He, O, H, OH, N$_{2}$(C-B) and N$_{2}^{+}$ (B-X) were produced at comparable rates for the two plasma jets.   

A Nanosecond Pulsed Plasma Brush for Surface Decontamination

This work optimizes a non-thermal, atmospheric pressure plasma brush for surface decontamination. The generated plasma plumes with a maximum length of 2 cm are arranged in a 5 cm long, brush-like array. The plasma was generated in ambient air with $\le $ 10 kV, 200 ns pulses at a repetition rate of 1.5 kHz. The energy per pulse and average power are in the range of 1-3 mJ and 0.5-1.5 W, respectively. Helium containing varying concentrations of water vapor was evaluated as the carrier gas and was fed into the plasma chamber at a rate varying between 1 to 7 SLPM. Optimization of the cold plasma brush for surface decontamination was tested in a study of the plasma inactivation of two common pathogens, Staphylococcus aureus and Acinetobacter baumannii. Laminate surfaces inoculated with over-night cultured bacteria were subject to the plasma treatment for varying water concentrations in He, flow rates and discharge voltages. It was found that increasing the water content of the feed gas greatly enhanced the bactericidal effect. Emission spectroscopy was performed to identify the reactive plasma species that contribute to this variation.   

Power Efficient Plasma Technique for Rapid Water Sterilization

Water especially good quality drinking water is a dwindling resource for significant segments of the world population. The BBC quoted this article (http://www.ft.com/cms/s/2/8e42bdc8-0838-11e4-9afc-00144feab7de.html) for a claim that water shortage is a bigger problem than climate change. One option for increasing the water supply is to recycle waste and polluted water by inexpensive, environmentally friendly methods. First steps involve filtrations while the last step is water disinfection. Presently disinfection is done chemically and/or UV radiation. Some chemicals cannot be used in large quantity due to residual toxicity, while UV disinfection systems consume a great deal electricity. Plasmas in water are very attractive for water sterilization due to UV radiation, ozone, etc. generation inside the water volume. Commercially available devices like NK-03 Blue Ballast System are used aboard ships for water purification. But, presently utilized plasmas: glow, pulsed arcs are not power efficient. Vortex stabilized plasmas, which are power efficient, can even degrade medications (antibiotics) advancing the state-of-the-art by orders of magnitude, especially when combined with electron beams. Disinfection scheme will be presented.   

Measurement of electron density transients in pulsed RF discharges using a frequency boxcar hairpin probe

Time resolved electron density measurements in pulsed RF discharges are shown using a hairpin resonance probe using low cost electronics, on par with normal Langmuir probe boxcar mode operation. Time resolution of 10 microseconds has been demonstrated. A signal generator produces the applied microwave frequency; the reflected waveform is passed through a directional coupler and filtered to remove the RF component. The signal is heterodyned with a frequency mixer and rectified to produce a DC signal read by an oscilloscope. At certain points during the pulse, the plasma density is such that the applied frequency is the same as the resonance frequency of the probe/plasma system, creating reflected signal dips. The applied microwave frequency is shifted in small increments in a frequency boxcar routine to determine the density as a function of time. A dc sheath correction is applied for the grounded probe, producing low cost, high fidelity, and highly reproducible electron density measurements. The measurements are made in both inductively and capacitively coupled systems, the latter driven by multiple frequencies where a subset of these frequencies are pulsed. Measurements are compared to previous published results, time resolved OES, and in-line measurement of plasma impedance.   

Nonlinear pattern formation in Dielectric barrier discharge

Dielectric barrier discharge (DBD) has proven to be a fascinating system for the study of nonlinear pattern formation, which presents an extraordinary variety and richness of patterns with the prominent convenience and practicality of experimental setups. In recent years, by using the special designed DBD system with two water electrodes, we have obtained a rich variety of patterns through nonlinear self-organization of the filaments [Phys. Rev. E 87, 042914 (2013), Phys. Rev. E 85, 066403 (2012), Phys. Rev. E 86, 036211 (2012)]. The spatio-temporal dynamics of these patterns have been studied systematically, and furthermore, the detailed plasma diagnostics have been carried out. These results are of great significance to give deep insight into the nature of nonlinear pattern formation. Based on our previous studies, here we will present the first report on a new complex superlattice pattern, as so called concentric superlattice. It evolves from hexagon pattern and transits to homogenous glow discharge with an increasing of the applied voltage. The spatio-temporal dynamics of the patterns have been investigated by a high speed camera. Results show that the concentric superlattice is an interleaving of three different sub-lattices, which are concentric-ring, concentric-framework, and concentric-dot embedded in the concentric-framework. Based on the experimental measurements, the involved intrinsic physical mechanism will be demonstrated.   

Control and measurement of ion-energy distributions in a beam-plasma system

A double plasma device is divided by two grids separating source and experimental chambers. Ions are accelerated by the voltage drop between two grids. To study the ion-energy in the experimental region, the ion distribution function (IDF) was probed using a retarding field energy analyzer, also the correlation between the IDF and discharge parameters was studied. It is shown that the IDF in the experimental region exhibits a double-peak structure containing a background and a high-energy streaming-ion groups. The proportion of the ion groups can be controlled by the filament current.   

Azimuthal Doppler Effect in Optical Vortex Spectroscopy

Optical vortices (OV) are a set of solutions of the paraxial Helmholtz equation in the cylindrical coordinates, and its wave front has a spiral shape. Since the Doppler shift is caused by the phase change by the movement in a wave field, the observer in the OV, which has the three-dimensional structured wave front, feels a three-dimensional Doppler effect. Since the multi-dimensional Doppler components are mixed into a single Doppler spectrum, development of a decomposition method is required. We performed a modified saturated absorption spectroscopy to separate the components. The OV and plane wave are used as a probe beam and pump beam, respectively. Although the plane-wave pump laser cancels the z-direction Doppler shift, the azimuthal Doppler shift remains in the saturated dip. The spatial variation of the dip width gives the information of the azimuthal Doppler shift. The some results of optical vortex spectroscopy will be presented.   

Design of magnetic field configuration in Space Plasma Environment Research Facility (SPERF)

The Space Plasma Environment Research Facility (SPERF) for geospace plasma environment simulation, as a component of Space Environment Simulation Research Infrastructure (SESRI), is designed to investigate fundamental space plasma phenomenon such as magnetic reconnection at magnetopause and magnetotail, as well as energetic particles transport and interaction with waves in magnetosphere, etc. To achieve the scientific and experimental goals, it is essential to realize the magnetic field configuration. In this report, the magnetic field coils, including four flux cores for simulating the magnetosheath field and plasma, a dipole coil for simulating the inner magnetosphere a disturbance coil for simulating magnetic storm distortion, and a group of magnetotail coils for simulating the magnetotail and the near earth neutral line, are designed to imitate the large-scale space structures based on the numerical simulations and the scaling relation of hydromagnetism between the laboratory and the magnetosphere. Three scenarios with operations of various coils to simulate specified processes in space plasmas will also be presented.