Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session UI1: Theory and Applications of Low Temperature Plasmas |
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Chair: Scott Baalrud, University of Iowa Room: Acadia |
Thursday, October 30, 2014 2:00PM - 2:30PM |
UI1.00001: Rotating structures and vortices in low temperature plasmas Invited Speaker: Jean-Pierre Boeuf Rotating structures are present in a number of low temperature EXB devices such as Hall thrusters, magnetrons, Penning discharges etc\textellipsis Some aspects of the physics of these rotating instabilities are specific to low temperature plasmas because of the relatively large collisionality, the role of ionization, and the fact that ions are often non-magnetized. ~On the basis of fully kinetic simulations (Particle-In-Cell Monte Carlo Collisions) we describe the formation of a rotating instability associated with an ionization front (``rotating spoke'') [1] and driven by a cross-field current in a self-sustained cylindrical magnetron discharge at gas pressure on the order of 1 Pa. The rotating spoke is a strong double layer (electrostatic sheath) moving towards the higher potential region at a velocity close to the critical ionization velocity, a concept proposed by Alfv\'{e}n in the context of the formation of the solar system. The mechanisms of cross-field electron transport induced by this instability are analyzed. At lower pressure (\textless 0.01 Pa) the plasma of a magnetron discharge is non-neutral and the simulations predict the formation of electron vortices rotating in the azimuthal direction and resulting from the diocotron instability. The properties of these vortices are specific since they form in a self-sustained discharge where ionization (and losses at the ends of the plasma column) play an essential role in contrast with the electron vortices in pure electron plasmas [2]. We discuss and analyze the mechanisms leading to the generation, dynamics and merging of these self-sustained electron vortices, and to the periodic ejection of fast electrons at the column ends (consistent with previous experimental observations). \\[4pt] [1] J.P. Boeuf and B, Chaudhury, Phys. Rev. Lett. 111, 155005 (2013)\\[0pt] [2] K.S. Fine, C.F. Driscoll, J.H. Malmberg, T.B. Mitchell, Phys. Rev. Lett. 67, 588 (1991) [Preview Abstract] |
Thursday, October 30, 2014 2:30PM - 3:00PM |
UI1.00002: Effects of Anomalous Electron Cross-Field Transport in a Low Temperature Magnetized Plasma Invited Speaker: Yevgeny Raitses The application of the magnetic field in a low pressure plasma can cause a spatial separation of low and high energy electrons. This so-called magnetic filter effect is used for many plasma applications, including ion and neutral beam sources, plasma processing of semiconductors and nanomaterials, and plasma thrusters. In spite of successful practical applications, the magnetic filter effect is not well understood. In this work, we explore this effect by characterizing the electron and ion energy distribution functions in a plasma column with crossed electric and magnetic fields. Experimental results revealed a strong dependence of spatial variations of plasma properties on the gas pressure. For xenon and argon gases, below $\sim$ 1 mtorr, the increase of the magnetic field leads to a more uniform profile of the electron temperature. This surprising result is due to anomalously high electron cross-field transport that causes mixing of hot and cold electrons. High-speed imaging and probe measurements revealed a coherent structure rotating in E cross B direction with frequency of a few kHz. Theory and simulations describing this rotating structure has been developed and points to ionization and electrostatic instabilities as their possible cause [1,2]. Similar to spoke oscillations reported for Hall thrusters [2,3], this rotating structure conducts the large fraction of the cross-field current. The use of segmented electrodes with an electrical feedback control is shown to mitigate these oscillations [3]. Finally, a new feature of the spoke phenomenon that has been discovered, namely a sensitive dependence of the rotating oscillations on the gas pressure, can be important for many applications.\\[4pt] [1] W. Frias, A. I. Smolyakov, I. D. Kaganovich, Y. Raitses, Phys. Plasmas 20, 052108 (2013);\\[0pt] [2] D. Escobar, E. Ahedo, Phys. Plasmas 21, 043505 (2014);\\[0pt] [3] C. L. Ellison, Y. Raitses, N. J. Fisch, Phys. Plasmas 19, 013503 (2012). [Preview Abstract] |
Thursday, October 30, 2014 3:00PM - 3:30PM |
UI1.00003: Electric discharge microplasmas generated in highly fluctuating fluids: Characteristics and application to the synthesis of molecular diamond Invited Speaker: Sven Stauss Plasma-based fabrication of novel nanomaterials and nanostructures is paramount for the development of next-generation electronic devices and for green energy applications. In particular, controlling the interactions between plasmas and materials interfaces, and the plasma fluctuations are crucial for further development of plasma-based processes and bottom-up growth of nanomaterials. Discharge microplasmas generated in supercritical fluids represent a special class of high-pressure plasmas, where fluctuations on the molecular scale influence the discharge properties and the possible bottom-up growth of nanomaterials. In the first part of the talk, we will discuss an anomaly observed for microplasmas generated near the critical point, a local decrease in the breakdown voltage, which has been observed for both molecular and monoatomic gases. This anomalous behavior is suggested to be caused by the concomitant decrease of the ionization potential due to the formation of clusters near the critical point, and the formation of extended electron mean free paths induced by the high-density fluctuation near the critical point. We will also show that when generating microplasma discharges close to the critical point, that the high-density fluctuation of the supercritical fluid persists. In the second part of the presentation, we will first introduce the basic properties of diamondoids and their potential for application in many different fields - biotechnology, medicine, opto- and nanoelectronics - before discussing their synthesis by microplasmas generated inside both conventional batch-type and continuous flow reactors, using the smallest diamondoid, adamantane, as a precursor and seed. Finally we show that one possible growth mechanism of larger diamondoids from smaller ones consists in the repeated abstraction of hydrogen terminations and the addition of methyl radicals. [Preview Abstract] |
Thursday, October 30, 2014 3:30PM - 4:00PM |
UI1.00004: Theory and design of emission-driven microplasmas for plasma-assisted processes: Tiny devices for large outcomes Invited Speaker: Ayyaswamy Venkattraman With the growing emphasis on nano/microscale systems, sustaining and utilizing plasmas in the microscale has transformed into an exciting and novel research area in the last decade or so with several experimental, theoretical and numerical investigations. While initial efforts in trying to understand microdischarges focused on trying to prevent breakdown in electrostatic microscale devices, recent research has expanded to exploiting these versatile plasmas in several applications. The overarching goal of this talk is to describe the unique characteristics of microplasmas summarizing some of the recent contributions following which we will look at the future of microplasmas, potential applications and challenges. Specifically, microplasmas require emission mechanisms from the cathode other than secondary electron emission which could be provided either by field emission or a combination of thermionic and field emission. The first half of the talk will thus focus on theories for microscale breakdown and their relation to the traditional Townsend criterion, interplay of emission mechanisms and plasma number densities/operating modes of these plasmas. The second half of the talk will discuss proof-of-concept results for potential applications that can benefit from microplasma devices. The discussion will emphasize the potentially superior performance of emission driven microplasmas in comparison to existing alternatives such as dielectric barrier discharges. Finally, current challenges and the future research road map will be laid out. [Preview Abstract] |
Thursday, October 30, 2014 4:00PM - 4:30PM |
UI1.00005: Coupling effects of driving frequencies on the electron heating in electronegative capacitive dual-frequency plasmas Invited Speaker: Julian Schulze The coupling of the driving frequencies represents a serious limitation of the control of process relevant plasma parameters in low-pressure electropositive capacitive discharges excited by two significantly different frequencies. Here, we investigate the interaction of the low-frequency (LF) and high-frequency (HF) driving sources in electronegative capacitive radio frequency discharges by Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations. Such discharges can operate in the drift-ambipolar (DA) mode [1], where the ionization is dominated by electrons accelerated (i) by a strong drift field in the plasma bulk due to the low dc conductivity resulting from the depleted electron density and (ii) by an ambipolar field at the sheath edges caused by local maxima of the electron density in the electropositive edge region of the discharge. The PIC/MCC simulations reveal frequency coupling mechanisms, different from those characteristic of electropositive discharges, due to the presence of the DA electron heating mode [2]. These mechanisms affect the electron heating (i) in the plasma bulk due to constructive/destructive interaction of drift electric fields originating from the HF and LF sources and (ii) at the collapsing sheath edge due to ambipolar electric fields influenced by the LF voltage amplitude via a modification of the sheath width. We analyze the effect of these phenomena on the discharge operation and plasma parameters. \\[4pt] [1] J. Schulze et al., Phys. Rev. Lett. 107 (2011) 275001.\\[0pt] [2] A Derzsi et al., J. Phys. D: Appl. Phys. 46 (2013) 482001 [Preview Abstract] |
Thursday, October 30, 2014 4:30PM - 5:00PM |
UI1.00006: Controlling the dynamics of electrons and ions in large area capacitive radio frequency plasmas via the Electrical Asymmetry Effect Invited Speaker: Edmund Schuengel The processing of large area surfaces in capacitive radio-frequency plasmas is a crucial step in the manufacturing of various high-technological products. To optimize these discharges for applications, understanding and controlling the dynamics of electrons and ions is vitally important. A recently proposed method of controlling these dynamics is based on the Electrical Asymmetry Effect (EAE) [1]: By driving the capacitive discharge with a dual-frequency voltage waveform composed of two consecutive harmonics, the symmetry of the discharge can be varied by tuning the relative phase. In this experimental study, the EAE is tested in hydrogen diluted silane discharges. The electron dynamics visualized by Phase Resolved Optical Emission Spectroscopy depends on the electrical asymmetry, the heating mode, and the presence of dust particles agglomerating in the plasma volume [2,4]. In particular, a transition from the $\alpha$-mode (heating by sheath expansion and field reversal) to the $\Omega$-mode (heating by drift field in the bulk) is observed. The ion dynamics are strongly affected by the sheaths electric fields, which can be controlled via the EAE: Separate control of the flux and mean energy of ions onto the electrodes is possible via the EAE [1,3]. Furthermore, investigations of the spatially resolved ion flux in the electromagnetic regime, i.e. using higher driving frequencies, reveal that the ion flux profile is controllable via the phase, as well, allowing for a significant improvement of the uniformity [4]. Thus, it is demonstrated that the EAE is a powerful tool to control the properties of large area capacitive discharges in the volume and at the surfaces in various ways. \\[4pt] [1] B.G. Heil et al., J. Phys. D: Appl. Phys. 41 (2008) 165202\\[0pt] [2] E. Sch\"ungel et al., J. Phys. D: Appl. Phys. 46 (2013) 175205\\[0pt] [3] E. Sch\"ungel et al., Plasma Sources Sci. Technol. 23 (2014) 015001\\[0pt] [4] E. Sch\"ungel, PhD thesis, 2013, Ruhr-University Bochum, Germany, http://www-brs.ub.ruhr-uni-bochum.de/netahtml/HSS/Diss/SchuengelEdmund/diss.pdf [Preview Abstract] |
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