Bulletin of the American Physical Society
68th Annual Gaseous Electronics Conference/9th International Conference on Reactive Plasmas/33rd Symposium on Plasma Processing
Volume 60, Number 9
Monday–Friday, October 12–16, 2015; Honolulu, Hawaii
Session FT3: Microplasmas |
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Chair: Laxminarayan Raja Room: 305 AB |
Tuesday, October 13, 2015 1:30PM - 2:00PM |
FT3.00001: Spots and patterns on electrodes of gas discharges Invited Speaker: Mikhail Benilov Concentration of electrical current onto the surface of electrodes of gas discharges in well-defined regions, or current spots, is often the rule rather than the exception. These spots occur on otherwise uniform electrode surfaces, a regime where one might expect a uniform distribution of current over the surface. In many cases, multiple spots may appear, forming beautiful patterns and surprising the observer. Important advances have been attained in the last 15 years in experimental investigation, understanding, and modelling of spots and patterns in discharges of different types, in particular, high-pressure arc discharges, dc glow discharges, and barrier discharges. It became clear that in many, if not most, cases there is no need to look for special physical mechanisms responsible for the formation of spots or patterns on uniform electrode surfaces: the spots or patterns originate in self-organization caused by (nonlinear) interaction of well-known mechanisms. In particular, standard mechanisms of near-cathode space-charge sheath are sufficient to produce self-organization, and it is this kind of self-organization that gives rise to cathode spots in low-current high-pressure arcs and normal spots and patterns of spots on cathodes of dc glow discharges. It was shown that spots and patterns on electrodes of gas discharges, being self-organization phenomena, are inherently related to multiple solutions, with one of the solutions describing a mode with a uniform distribution of current over the electrode surface and the others describing regimes with different spot patterns. These multiple solutions exist even in the most basic self-consistent models of gas discharges. In particular, multiple solutions have been found for dc glow discharges; the fact rather surprising by itself, given that such discharges have been under intensive theoretical investigation for many years. A concise review of the above-described advances is given in this talk. [Preview Abstract] |
Tuesday, October 13, 2015 2:00PM - 2:15PM |
FT3.00002: Modeling of Microplasmas with Nano-Engineered Electrodes Sergey Macheret, Siva Shashank Tholeti, Alina Alexeenko Microplasmas can potentially be used as unique tunable dielectrics for reconfigurable radio-frequency systems, if electron densities of 10$^{10}$-10$^{12}$ cm$^{-3}$ can be sustained in cavities smaller than 100 micron. However, for low loss tangent, gas pressures below 10 mTorr would be required, whereas the physics of electron impact ionization dictates the pd scaling so that microplasmas must operate at high gas pressures, hundreds of Torr, and also high voltages. We analyze a new principle of plasma generation that goes well beyond the pd scaling by eliminating electron impact ionization. In the new concept, electrons are generated at the cathode by field emission from nanotubes, and ions are independently produced in field ionization at atomically-sharp tips on the anode. The electrons and ions then move in the opposite directions, mix, and create a plasma. The low pressure results in collisionless motion with no electron-impact ionization. One-dimensional PIC/MCC calculations show that emitters such as carbon nanotubes placed sparsely on the cathode, combined with field ionization nanorods at the anode, can indeed ensure steady-state electron densities of up to 10$^{12}$ cm$^{-3}$ at gas pressure lower than 10 mTorr with only 50-100 Volts applied cross a 40-50 $\mu $m gap. [Preview Abstract] |
Tuesday, October 13, 2015 2:15PM - 2:30PM |
FT3.00003: Breakdown phenomenon across micrometer-scale surface gap under positive impulse voltage Hiroyuki Iwabuchi, Shigeyasu Matsuoka, Akiko Kumada, Kunihiko Hidaka With the miniaturization of electronic devices, insulation width between electrodes have been accordingly reduced. Consequently, electrical breakdown phenomenon across micrometer-scale gap is of great practical interest for insulation designing of miniaturized devices. In this research, breakdown characteristics across micrometer-scale surface gap were observed under the application of positive impulse voltage. As a result, breakdown voltage under positive impulse voltage across surface gap was independent of gap width. The results indicate that initial electrons emitted from the surface of the insulator in the vicinity of anode. In order to investigate the breakdown process, particle-in-cell simulation based on monte-carlo method was also conducted. Considering electron emission from the surface of the insulator, electrons emitted from the insulator surface can collide to the neutral particles and positive ions are generated. Generated ions move into the insulator surface and the secondary electrons are emitted. Consequently, discharge path along the surface of the insulator is formed. The results show that electron emission from the surface of the insulator plays an important role in breakdown across micrometer-scale surface gap. [Preview Abstract] |
Tuesday, October 13, 2015 2:30PM - 2:45PM |
FT3.00004: Operating modes of field emission assisted microplasmas in the microwave regime Arghavan Alamatsaz, Ayyaswamy Venkattraman Field-induced electron emission from the cathode and its interaction with microdischarges has gained significant attention in the last few years particularly in the context of microscale gas breakdown. Recent advances in nanofabrication have led to the development of novel cathodes that demonstrate impressive field emission properties with turn-on fields as low as 1 V/$\mu $m and field enhancement factors as high as 1000 implying that field emission could play an important role in microplasmas as large as 500 $\mu $m. Recent studies on direct current microplasmas have shown that field emission triggers the transition from an abnormal glow mode to an arc-like mode with negative differential resistance. This talk will extend the results obtained for DC field emission assisted (FEA) microplasmas to the high frequency regime with specific emphasis on radio frequency and microwave excitations. Particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations are used to determine the current-voltage characteristics and microplasma properties including number density, electron temperature, electron energy distribution function and power density. Apart from quantifying the influence of excitation frequency, the role of field emission on transition to $\gamma $-modewill also be discussed. [Preview Abstract] |
Tuesday, October 13, 2015 2:45PM - 3:00PM |
FT3.00005: Optical emission spectroscopy of nanosecond repetitively pulsed microplasmas generated in air at atmospheric pressure Thomas Orriere, Eric Moreau, Nicolas Benard, David Pai Nanosecond repetitively pulsed (NRP) microplasmas are generated in room temperature air at atmospheric pressure, in order to investigate the enhanced control of discharge properties via the combined effects of spatial confinement and nanosecond repetitive pulsing. Discharges were generated using high-voltage pulses of 15-ns duration applied to a tungsten pin-to-pin reactor, with inter-electrode gap distances ($d)$ from 2 mm down to 0.2 mm. Optical emission spectroscopy and electrical characterization performed on the discharge indicate that heat transfer and plasma chemistry are influenced by the microplasma geometry. Ultrafast gas heating is observed upon deducing the rotational temperature of N$_{\mathrm{2}}$ from the measured emission spectrum of the N$_{\mathrm{2}}$ (C$\to $B) (0, 2) and (1, 3) transition bands, but use of the microplasma geometry ($d \quad =$ 0.2 mm) results in lower gas temperatures than in larger discharge gaps ($d \quad =$ 2 mm), including at high pulse repetition frequency (30 kHz) where substantial steady-state gas heating can occur. The measured Stark broadening of the H$_{\mathrm{\alpha }}$ transition is significantly greater than for previously studied NRP discharges in air at atmospheric pressure, indicating that the maximum electron number density may be correspondingly much greater, up to 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$. Furthermore, for NRP microplasmas, the intensities of emission from excited atomic ions (O$^{\mathrm{+}}$ and N$^{\mathrm{+}})$ are much higher than those of excited neutral atoms (O and N), in contrast to NRP discharges generated in larger discharge gaps. [Preview Abstract] |
Tuesday, October 13, 2015 3:00PM - 3:15PM |
FT3.00006: Suppression of Instability of High Pressure DC Microplasma Operating in the Negative Differential Resistance (NDR) Regime Rajib Mahamud, Tanvir I. Farouk Microplasma devices have been the subject of considerable interest and research during the last decade. In a DC system most of the operation regime of the plasma discharges studied fall in the ``abnormal,'' ``normal'' and ``corona'' modes - where a quasi-steady state is achieved. It is well known that even in a DC system the negative differential resistance (NDR) regime can trigger self pulsing discharges. These pulsations are initiated by the parasitic capacitance of the system hence governed by the response time of the power circuit. The circuit response time is required to be larger than the ion transit time to initiate the oscillations. In this present study a suppressor circuit element in the form of an inductor is used to restrain the plasma from switching to a self pulsing mode. It has been identified that the combined response time of the inductor and the plasma discharge (L/Rplasma) has to be larger than the power circuit time constant (RC) to achieve suppression. Inhibition of oscillation has been observed in both experiments and numerical simulations. The obtained voltage-current characteristics show that the inductor element extends the normal glow regime to lower current. Additional parametric simulations are conducted to map out a ``stable'' operation regime. [Preview Abstract] |
Tuesday, October 13, 2015 3:15PM - 3:30PM |
FT3.00007: Plasma Tunable LC Resonator for High-Power Electromagnetic Applications Abbas Semnani, Sergey Macheret, Dimitrios Peroulis High-power tunable filters are in high demand in transmitters found in radars and many communication systems such as satellite and broadcasting stations. Limited power handling renders most semiconductor technologies inherently suboptimal options for these systems. Therefore, mechanically-tunable cavity-based filters are often employed in such cases, resulting in bulky, slow, and heavy systems. In this work, we study the application of plasma as an alternative frequency tuning mechanism for high-power applications even in environmentally and/or mechanically harsh conditions. For a given gas type and pressure, the real and imaginary parts of the dielectric permittivity of a plasma can be varied by changing the electron density, which, depending on the discharge regime, can be implemented by changing the discharge current, voltage, or the magnitude of an auxiliary electric field. In this work, a simple LC resonator tuned to several hundred MHz was fabricated and tested. The tunable capacitor of the resonator was implemented by a commercially available gas discharge tube (GDT), a mm-scale plasma device with gas pressure of 100s of mTorr. Measurement results reveal a continuous tuning range of more than 50\% when the applied discharge current is increased from zero to 90 mA. [Preview Abstract] |
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