Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session PI2: Plasma Wall Interactions, Disruptions, and Plasma Technology |
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Chair: Michael Jaworski, Princeton Plasma Physics Laboratory Room: Ballroom DE |
Wednesday, October 31, 2012 2:00PM - 2:30PM |
PI2.00001: Plasma-Wall Interaction in Presence of Intense Electron Emission from Walls Invited Speaker: Igor Kaganovich The plasma-surface interaction in presence of strong thermionic or secondary electron emission has been studied theoretically and experimentally both as a basic phenomenon and in relation to numerous plasma applications such as, for example, cathodes, emissive probes, divertor plasma, surface discharges, dusty plasmas, plasma thrusters and plasma processing. The electron flux to the wall is determined by the electron velocity distribution function (EVDF) and by the sheath potential, which is set by ambipolar condition consistent with the EVDF and the wall emitting properties. Nonlinear coupling between EVDF and sheath potential is responsible for a number of unusual phenomena. For example, we observed relaxation sheath oscillations [1]. We have shown that the criterion for instability is that the secondary electron emission coefficient of electrons with energy normal to the wall bordering the wall potential becomes larger than unity [1]. We observed new regime where all plasma electrons leave and are substituted by secondary electrons. In this regime, there is practically no electric field in plasma and sheath, so that ions are not drawn to the wall, plasma electrons are not confined and the plasma potential is negative. We also observed disappearance of plasma and sheath potential in case of collisionless plasma decay. Emitted electrons excite electron plasma waves due to the two-stream instability, which is, consequently, followed by the parametric instability and excitation of ion sound waves. Implications of these instabilities on collisionless electron heating are being studied for RF-DC combined system. Finally, methods to control plasma profiles with an auxiliary electrode in dc discharges are studied experimentally and making use of particle-in-cell simulations. \\[4pt] [1] M. D. Campanell et. al. PRL 108, 235001 (2012) and 255001 (2012). [Preview Abstract] |
Wednesday, October 31, 2012 2:30PM - 3:00PM |
PI2.00002: Disruption mitigation experiments with multiple gas jets on Alcator C-Mod Invited Speaker: Geoff Olynyk Experiments have been conducted on the Alcator C-Mod tokamak to determine the effectiveness of disruption mitigation using multiple, toroidally separated massive gas injections (MGI). This represents the first study of MGI using more than one gas jet simultaneously. The implications are important for ITER, as a toroidal radiation peaking factor greater than approximately 2 could lead to localized melting of the first wall, even in the case of a successful mitigation. A new diagnostic system consisting of a toroidal array of six AXUV photodiodes with wide poloidal view, but narrowly collimated toroidal view, has been fielded, allowing for time-resolved measurements of the toroidal radiation asymmetry during disruptions. Synchronization has been achieved between the two gas jets, allowing for a scan of the ``stagger time'' $\Delta t$, defined as the difference in time between the gas from the two jets arriving at the plasma. In the limit of long stagger time ($\Delta t > 1$~ms), the two-jet system behaves like a single jet, dominated by the jet the fires first. It is found that in the pre-thermal quench phase of the disruption, the radiation is toroidally peaked, and is localized near the gas jet which fires first. In the thermal quench (when most of the plasma stored energy is radiated), the pattern is more complicated, and depends sensitively on physical differences between the gas jets. In the current quench phase, the radiation pattern is toroidally symmetric, as expected from previous single-jet experiments. Disruption parameters such as current decay time, and divertor deposited energy (diagnosed by infrared camera and surface thermocouple thermography) are investigated and compared between unmitigated and single- and two-jet mitigated disruptions. The role of low-n MHD modes in the mitigated disruption sequence is explored through a scan of plasma elongation and safety factor, as well as through simulation. [Preview Abstract] |
Wednesday, October 31, 2012 3:00PM - 3:30PM |
PI2.00003: Impurity Mixing, Radiation Asymmetry, and Runaway Electron Confinement in MGI Simulations of DIII-D and ITER Invited Speaker: V.A. Izzo Massive gas injection (MGI) is one candidate for the ITER disruption mitigation system (DMS), due to its proven effectiveness for mitigating heat loads and halo currents. The number and locations of the DMS ports will soon be chosen with the goals of maximizing assimilation of impurities into the core and keeping radiated power asymmetry low. Macroscopic MHD instabilities help redistribute impurities from the edge to the core during an MGI shutdown [1]. We present DIII-D and ITER simulations performed with the extended-MHD code NIMROD, in which the effects of an impurity species--including radiation cooling and mixing with the main ion species--are incorporated. The impurity source is localized to the edge, based on the observation that MGI doesn't penetrate deeply into tokamak plasmas. We find that $n>1$ MHD modes moderately increase impurity transport, while the $n=1/m=1$ mode dramatically enhances mixing. Because $n>0$ modes are responsible for impurity transport, we find that even with a toroidally symmetric source at the edge, radiated power from the core can vary by a factor of two toroidally; this may imply a hard lower bound in achievable radiation symmetry. Impurity assimilation for localized sources is also compared, such as high-field- vs low-field-side injection. Successful MGI suppression of runaway electrons (REs) in ITER will require additional loss mechanisms beyond the collisional suppression assumed by the theoretical (but hard to reach) ``Rosenbluth density,'' such as MHD-induced losses. A test-particle RE confinement model in NIMROD has been benchmarked against a range of Ar-pellet injection shots in DIII-D. MGI simulations find higher amplitude and longer lived fluctuations that can increase RE losses compared with deeply penetrating impurities.\par \vskip6pt \noindent [1] V.A.\ Izzo, {\em et al.}, Nucl.\ Fusion {\bf 51}, 063032 (2011). [Preview Abstract] |
Wednesday, October 31, 2012 3:30PM - 4:00PM |
PI2.00004: New insights into the experimental behavior of magnetized gas discharges Invited Speaker: Francis F. Chen Helicon discharges have been extensively researched for over 25 years, and over 700 papers have been published on this subject in that time. Helicons are different from other gas discharges because they exist in a dc magnetic field and depend on energy deposition from waves driven by an external radiofrequency (rf) antenna. They produce higher plasma densities than other rf plasmas, but the physics of how they do that turns out to be very complicated. This research has been like peeling an onion. Each layer reveals another layer deeper down. Though the properties of coupled helicon and Trivelpiece-Gould waves have been known for a long time, there has been no theory of the equilibrium profiles of density, electron temperature $T_{e}$, and neutral density. In tackling this problem, we found that the sheaths on the endplates are important. They allow electrons to cross the magnetic field via the Simon short-circuit effect. A radial electric field is then set up which drives the ions radially outward at a speed scaled to $T_{e}$. For fixed $T_{e}$, the density profile follows a ``universal'' profile which is independent of discharge radius and pressure. A physical reason is given for this universality. From this point forward, the theory goes into many details which give insights to the physics of all cylindrical gas discharges, with or without a magnetic field. [Preview Abstract] |
Wednesday, October 31, 2012 4:00PM - 4:30PM |
PI2.00005: Cold Atmosphere Plasma in Cancer Therapy Invited Speaker: Michael Keidar Plasma is an ionized gas that is typically generated in high-temperature laboratory conditions. Recent progress in atmospheric plasmas led to the creation of cold plasmas with ion temperature close to room temperature. Areas of potential application of cold atmospheric plasmas (CAP) include dentistry, drug delivery, dermatology, cosmetics, wound healing, cellular modifications, and cancer treatment. Various diagnostic tools have been developed for characterization of CAP including intensified charge-coupled device cameras, optical emission spectroscopy and electrical measurements of the discharge propertied. Recently a new method for temporally resolved measurements of absolute values of plasma density in the plasma column of small-size atmospheric plasma jet utilizing Rayleigh microwave scattering was proposed [1,2]. In this talk we overview state of the art of CAP diagnostics and understanding of the mechanism of plasma action of biological objects. The efficacy of cold plasma in a pre-clinical model of various cancer types (long, bladder, and skin) was recently demonstrated [3]. Both in-vitro and in-vivo studies revealed that cold plasmas selectively kill cancer cells. We showed that: (a) cold plasma application selectively eradicates cancer cells in vitro without damaging normal cells. For instance a strong selective effect was observed; the resulting 60--70{\%} of lung cancer cells were detached from the plate in the zone treated with plasma, whereas no detachment was observed in the treated zone for the normal lung cells under the same treatment conditions. (b) Significantly reduced tumor size in vivo. Cold plasma treatment led to tumor ablation with neighbouring tumors unaffected. These experiments were performed on more than 10 mice with the same outcome. We found that tumors of about 5mm in diameter were ablated after 2 min of single time plasma treatment. The two best known cold plasma effects, plasma-induced apoptosis and the decrease of cell migration velocity can have important implications in cancer treatment by localizing the affected area of the tissue and by decreasing metastasic development. In addition, cold plasma treatment has affected the cell cycle of cancer cells. In particular, cold plasma induces a 2-fold increase in cells at the G2/M-checkpoint in both papilloma and carcinoma cells at about 24 hours after treatment, while normal epithelial cells (WTK) did not show significant differences. It was shown that reactive oxygen species metabolism and oxidative stress responsive genes are deregulated. We investigated the production of reactive oxygen species (ROS) with cold plasma treatment as a potential mechanism for the tumor ablation observed. \\[4pt] [1] Shashurin A., Shneider M.N., Dogariu A., Miles R.B. and Keidar M. Appl. Phys. Lett. (2010) \textbf{96}, 171502.\\[0pt] [2] Shashurin A., Shneider M.N., Keidar M. Plasma Sources Sci. Technol. 21 (2012) 034006.\\[0pt] [3]. M. Keidar, R. Walk, A. Shashurin, P. Srinivasan, A. Sandler, S. Dasgupta , R. Ravi, R. Guerrero-Preston, B. Trink, British Journal of Cancer, 105, 1295-1301, 2011 [Preview Abstract] |
Wednesday, October 31, 2012 4:30PM - 5:00PM |
PI2.00006: Novel functional composites of plasmas and metamaterials Invited Speaker: Osamu Sakai Plasmas, which are fairly frequency-dispersive in their dielectric properties, have tunable and nonlinear features that cannot be achieved using other solids and liquids. Such features on variable complex permittivity can be activated in metamaterial structure; when we combine plasmas with metamaterials which have functional micro-structures leading to designable permeability, we can expect a quite broad range of negative refractive index on its complex plane for electromagnetic waves. Furthermore, if a given electromagnetic wave has sufficient wave amplitude to modulate electron density, such a composite work as a strong nonlinear medium with adjustability through the metamaterial features. Such kinds of arguments are reviewed in our recent reports [1,2]. One of the specific physical properties emerging in plasma metamaterials is an exchange phenomenon between attenuation and phase shift via regulated permeability. Conventional collisional plasmas work simply as attenuators for electromagnetic waves, but superposition of a negative permeability state induces significant phase shift of propagating waves with less attenuation. Another example is simultaneous generation of a high-density plasma with a negative-refractive-index state; we predicted quite strong nonlinear processes with double saddle-node bifurcations during this phenomenon, and verified them in our recent experiments. Such composites of plasmas and metamaterials will provide new scientific opportunities as well as industrial applications.\\[4pt] [1] O. Sakai et al., Physics of Plasmas, vol. 17 (2010), 123504.\\[0pt] [2] O. Sakai et al., Plasma Sources Sci. Technol., vol. 21 (2012), 013001. [Preview Abstract] |
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