Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GI2: Micro and Low-temperature Plasmas |
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Chair: Martin Lampe, Naval Research Laboratory Room: Centennial I |
Tuesday, November 3, 2009 9:30AM - 10:00AM |
GI2.00001: Microplasma assembly for novel electromagnetic media Invited Speaker: Microplasmas whose sizes are smaller than a few millimeters can perform functionality for chemical micro-reactors, conversion fields for biomaterials, and interactive media for photons. In particular, as far as interaction with photons or electromagnetic waves is concerned, in addition to microplasma generation and photon emission in an intensified electric field of waves, microplasmas can also play a number of potential roles of controllers for propagating waves. This report focuses on novel physics of microplasma assembly for electromagnetic media [1]. When we make an assembly composed of microplasmas, novel functions are expected due to its complex dielectric function arising from dielectric and lossy properties. The dielectric property creates photonic band gaps (PBGs), and the lossy property drastically changes transmittance around the PBGs. As a result, a ``complex'' dispersion relation or band diagram in the three-dimensional space of real and imaginary wavenumbers and wave frequency will open new possibilities to control electromagnetic waves by complex-value filters composed of microplasma assembly.\\[4pt] [1] O. Sakai et al., Plasma Physics and Controlled Fusion, vol. 49 (2007), pp. B453-B463. [Preview Abstract] |
Tuesday, November 3, 2009 10:00AM - 10:30AM |
GI2.00002: Kinetic effects in low-pressure discharges with secondary electron emission Invited Speaker: Secondary electron emission (SEE) from material surfaces can result in efficient plasma cooling and large energy losses from the plasma. This is relevant to Hall thrusters and divertors, where the electron temperature is high enough to provide strong SEE. The SEE reduces negative charge of a wall immersed into a plasma and increases the flux of plasma electrons to the wall. If the plasma is hot enough to produce intense SEE, the plasma potential relative to the wall can be several times lower than that without the SEE. Then the electron heat flux to the wall will be extremely high provided the plasma electron velocity distribution function (EVDF) is Maxwellian, with a large number of electrons flying toward the walls and capable to penetrate the sheath potential barrier. This is the case if the electron mean free path is small compared to the plasma dimensions. Further increase of the electron temperature saturates the wall losses once the space charge limited (SCL) regime of the sheath establishes. In low-pressure plasmas, however, amplification of the electron heat flux to the wall due to the SEE is much weaker because the electron mean free path is large compared to the plasma dimensions. In such plasmas, the EVDF is depleted of the energetic electrons flying toward the walls. This considerably reduces the flux of hot electrons to the walls. The secondary electrons propagate freely between the walls forming electron beams that do not mix with the plasma electrons. The presence of these secondary electron beams with intense current and relatively low energy prevents the occurrence of a steady SCL sheath. Instead, relaxation oscillations of the sheath between the SCL and non-SCL states may occur under conditions with intense anisotropic heating. [Preview Abstract] |
Tuesday, November 3, 2009 10:30AM - 11:00AM |
GI2.00003: Kinetic Theory of Instability-Enhanced Collisional Effects Invited Speaker: A generalization of the Lenard-Balescu collision operator is derived which accounts for the scattering of particles by instability amplified fluctuations that originate from the thermal motion of discrete particles (in contrast to evoking a fluctuation level externally, as is done in quasilinear kinetic theory) [1]. Emphasis is placed on plasmas with convective instabilities. It is shown that an instability-enhanced collective response results which can be the primary mechanism for scattering particles, being orders of magnitude more frequent than conventional Coulomb collisions, even though the fluctuations are in a linear growth phase. The resulting collision operator is shown to obey conservation laws (energy, momentum, and density), Galilean invariance, and the Boltzmann ${\mathcal{H}}$-theorem. It has the property that Maxwellian is the unique equilibrium distribution function; again in contrast to weak turbulence or quasilinear theories. Instability-enhanced collisional effects can dominate particle scattering and cause strong frictional forces. For example, this theory has been applied to two outstanding problems: Langmuir's paradox [2] and determining Bohm's criterion for plasmas with multiple ion species [3]. Langmuir's paradox is a measurement of anomalous electron scattering rapidly establishing a Maxwellian distribution in gas discharges with low temperature and pressure. This may be explained by instability-enhanced scattering in the plasma-boundary transition region (presheath) where convective ion-acoustic instabilities are excited. Bohm's criterion for multiple ion species is a single condition that the ion fluid speeds must obey at the sheath edge; but it is insufficient to determine the speed of individual species. It is shown that an instability-enhanced collisional friction, due to streaming instabilities in the presheath, determines this criterion.\\[4pt] [1] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Plasmas {\bf 15}, 092111 (2008).\\[0pt] [2] S.D. Baalrud, J.D. Callen, and C.C. Hegna, Phys. Rev. Lett. {\bf 102}, 245005 (2009).\\[0pt] [3] S.D. Baalrud, C.C. Hegna, and J.D. Callen (submitted July 2009); preprint UW-CPTC 09-5 at www.cptc.wisc.edu. [Preview Abstract] |
Tuesday, November 3, 2009 11:00AM - 11:30AM |
GI2.00004: Cold microplasmas at one atmosphere: Simulation and characterization Invited Speaker: Cold atmospheric pressure plasma offers many of the same technical advantages as conventional low pressure glow discharges, but without the need for a vacuum system. The cold atmospheric plasma is distinct from most high-pressure plasmas, such as arcs and sparks, because the low gas temperature allows for the treatment of temperature sensitive materials. This talk focuses on the generation of microwave-frequency microplasmas of air and inert gases. These plasmas exhibit gas temperatures of 300-600 K, but electron temperatures of 1-2x10$^{4}$ K. The electron density is greater than 10$^{14}$ cm$^{-3}$. Microplasma is generated in a 200 micron-wide gap in a ring-shaped microstrip transmission line. When operated at electrical resonance, a microwave potential forms across the discharge gap and generates a microplasma. Microplasma generation becomes more efficient at higher frequencies. Inert gas microplasmas are characterized using excitation frequencies of 450 MHz, 900 MHz, and 1.8 GHz at both 1 atm and 0.5 mbar. The microplasma resistance decreases with increasing frequency. Simultaneously, the reactive sheath impedance and the microwave electrode voltage also decrease. At higher microwave frequency, the decreased electrode voltage reduces both the plasma potential and the ion kinetic energy losses, thus increasing the electron density. A three-dimensional fluid model confirms these experimental measurements. [Preview Abstract] |
Tuesday, November 3, 2009 11:30AM - 12:00PM |
GI2.00005: Tailored Positron Beams from Trapped Single-component Plasmas Invited Speaker: There are a number of important uses of antiparticles (e.g., positrons and antiprotons) including the creation of antihydrogen, modeling astrophysical processes, and the characterization of materials and material surfaces. Much of this progress has been driven by the development of new plasma techniques to accumulate, manipulate and store antiparticles. This talk focuses on recent work\footnote{T. R. Weber, J. R. Danielson and C. M. Surko, Phys. Plasmas {\bf 15}, 012106 (2008).}$^,$\footnote{T. R. Weber, J. R. Danielson and C. M. Surko, Phys. Plasmas {\bf 16}, 057105 (2009).} to create specially tailored positron beams with small transverse spatial extent $\rho_b$, narrow energy spreads $\Delta E$, and high brightness by pulsed extraction from plasmas in a Penning-Malmberg trap. Experiments are presented using electron plasmas for increased data rate. By briefly lowering the exit-gate potential, beam pulses ($\Delta t < 10$ $\mu$sec) from near the plasma center are created with $\rho_b = 2 \lambda_D$ (HW $1/e$) and $\Delta E \approx T$, where $\lambda_D$ is the plasma Debye length, and $T$ is the plasma temperature. Specifically, by tailoring the plasma temperature to $T \approx 25$ meV and density to $n_0 \approx 10^{10}$ cm$^{-3}$, beams are created with $\Delta E < 35$ meV and $\rho_b < 50$ $\mu$m. A nonlinear model for beam extraction is used to derive expressions for the beam amplitude $N_b$, transverse spatial profile $\sigma_b(r)$, and single particle energy distribution as a function of the exit-gate potential $V_E$, trap wall radius $R_W$, and plasma parameters.$^3$ All predictions are verified for a wide range of plasmas. Protocols to optimize $\rho_b$ and $\Delta E$ for various applications will be discussed. Prospects for cryogenic beams and pulsed extraction from the confining $B$ field (to $B = 0$, for brightness enhancement and electrostatic focusing) will be discussed along with selected applications. [Preview Abstract] |
Tuesday, November 3, 2009 12:00PM - 12:30PM |
GI2.00006: Exploring Plasma Mechanisms of Carbon Nanostructures Synthesis in Arc Discharge Invited Speaker: Plasma enhanced approached are widely used for synthesis of carbon nanostructures. Among several methods for synthesis carbon nanostructures (single wall carbon nanotubes (SWNT), multi-wall carbon nanotubes (MWNT), graphene) arc discharge is the most practical one for scientific and technological purposes due to the number of advantages in comparison with other techniques. Firstly, arc discharge method yields highly graphitized nanostructures with very small defects, because the synthesis occurs at a very high temperature. As results, arc-grown SWNTs demonstrate the highest time of emission capability degradation than those produced by other techniques. Secondly, nanotubes produced in arc usually demonstrate a high flexibility, thus eventually demonstrating higher strength characteristics. The primary focus of this presentation is to review state of the art understanding of SWNT synthesis mechanism in arc discharge, methods and approaches to control parameters of arc discharge. Fundamental issues related to synthesis of SWNTs, which is relationship between plasma parameters and SWNT characteristics will be considered. It is believed that characteristics of synthesized SWNTs can be controlled by means of plasma parameters and arc discharge conditions. Effects of electrical and magnetic fields applied during SWNT synthesis in arc plasma will be explored. For instance, our recent experiments suggest that magnetic field has very strong effect on the arc discharge and the carbon nanostructures synthesis. It is also demonstrated that the magnetic field has a profound effect on the length of a SWNT synthesized in the arc discharge. An average length of SWNT increases by a factor of 2 in discharge with magnetic field as compared with the discharge without magnetic field, and an amount of long nanotubes with the length above 5 micron also increases. Electric and magnetic fields allow effective SWNT transport to the collection area, in-situ SWNT filtration from the soot and SWNT separation by their characteristics (e.g. by chirality, length, structural properties). [Preview Abstract] |
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