Alfven eigenmode properties and dynamics in the TJ-II stellarator

Alfven eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density n(e), electric potential phi and poloidal magnetic field B-pol with frequencies 50 kHz < f(AE) < 300 kHz. The LP, MP and HIBP data showed a high level of coherency for specific branches of AEs. Poloidal mode wave-vectors k(theta), mode numbers m (m < 8) and propagation velocities V-theta similar to 30 km s(-1) were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfven law f(AE) similar to n(e)(-1/2). During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector k(theta) and mode number m remained constant, (ii) the cross-phases between the oscillations in B-pol, n(e) and electric potential remained constant, having an individual value for each AE branch, (iii) V-theta decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in E-pol and n(e), which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

Magnetic topology and current channels in plasmas with toroidal current density inversions

The equilibrium magnetic field inside axisymmetric plasmas with inversions on the toroidal current density is studied. Structurally stable non-nested magnetic surfaces are considered. For any inversion in the internal current density the magnetic families define several positive current channels about a central negative one. A general expression relating the positive and negative currents is derived in terms of a topological anisotropy parameter. Next, an analytical local solution for the poloidal magnetic flux is derived and shown compatible with current hollow magnetic pitch measurements shown in the literature. Finally, the analytical solution exhibits non-nested magnetic families with positive anisotropy, indicating that the current inside the positive channels have at least twice the magnitude of the central one.

Radar observations of non-thermal plasmas at different aspect angles

Data are presented from the EISCAT CP-3-E experiment which show the presence of non-thermal plasma over a range of latitudes. The O+ ion-velocity distribution function is almost toroidal when the electric field reaches values of 125 mV m−1. The ion temperature derived from such data assuming a Maxwellian distribution function will overestimate the true ion temperature when the observing angle is large with respect to the magnetic field, and underestimate the temperature when the aspect angle is small. When the expressions for the distribution function are extended to include mixed ion composition, an improvement is sometimes found in fitting the observed data, and estimates of the composition can be made. Such an analysis suggests that N2+ can occasionally form a significant part of the total ion density in a narrow height region centred at 275 km.

Temporal behaviour of toroidal rotation velocity in the TCABR tokamak

A new method for determining the temporal evolution of plasma rotation is reported in this work. The method is based upon the detection of two different portions of the spectral profile of a plasma impurity line, using a monochromator with two photomultipliers installed at the exit slits. The plasma rotation velocity is determined by the ratio of the two detected signals. The measured toroidal rotation velocities of C III (4647.4 angstrom) and C VI (5290.6 angstrom), at different radial positions in TCABR discharges, show good agreement, within experimental uncertainty, with previous results (Severo et al 2003 Nucl. Fusion 43 1047). In particular, they confirm that the plasma core rotates in the direction opposite to the plasma current, while near the plasma edge (r/a > 0.9) the rotation is in the same direction. This technique was also used to investigate the dependence of toroidal rotation on the poloidal position of gas puffing. The results show that there is no dependence for the plasma core, while for plasma edge (r/a > 0.9) some dependence is observed.

Mean Motion in Stochastic Plasmas With a Space-Dependent Diffusion Coefficient

Radial transport in the tokamap, which has been proposed as a simple model for the motion in a stochastic plasma, is investigated. A theory for previous numerical findings is presented. The new results are stimulated by the fact that the radial diffusion coefficients is space-dependent. The space-dependence of the transport coefficient has several interesting effects which have not been elucidated so far. Among the new findings are the analytical predictions for the scaling of the mean radial displacement with time and the relation between the Fokker-Planck diffusion coefficient and the diffusion coefficient from the mean square displacement. The applicability to other systems is also discussed. (c) 2009 WILEY-VCH GmbH & Co. KGaA, Weinheim

Self-consistent equilibrium calculation through a direct variational technique in tokamak plasmas

A self-consistent equilibrium calculation, valid for arbitrary aspect ratio tokamaks, is obtained through a direct variational technique that reduces the equilibrium solution, in general obtained from the 2D Grad-Shafranov equation, to a 1D problem in the radial flux coordinate rho. The plasma current profile is supposed to have contributions of the diamagnetic, Pfirsch-Schluter and the neoclassical ohmic and bootstrap currents. An iterative procedure is introduced into our code until the flux surface averaged toroidal current density (J(T)), converges to within a specified tolerance for a given pressure profile and prescribed boundary conditions. The convergence criterion is applied between the (J(T)) profile used to calculate the equilibrium through the variational procedure and the one that results from the equilibrium and given by the sum of all current components. The ohmic contribution is calculated from the neoclassical conductivity and from the self-consistently determined loop voltage in order to give the prescribed value of the total plasma current. The bootstrap current is estimated through the full matrix Hirshman-Sigmar model with the viscosity coefficients as proposed by Shaing, which are valid in all plasma collisionality regimes and arbitrary aspect ratios. The results of the self-consistent calculation are presented for the low aspect ratio tokamak Experimento Tokamak Esferico. A comparison among different models for the bootstrap current estimate is also performed and their possible Limitations to the self-consistent calculation is analysed.

Shearless transport barriers in magnetically confined plasmas

Shearless transport barriers appear in confined plasmas due to non-monotonic radial profiles and cause localized reduction of transport even after they have been broken. In this paper we summarize our recent theoretical and experimental research on shearless transport barriers in plasmas confined in toroidal devices. In particular, we discuss shearless barriers in Lagrangian magnetic field line transport caused by non-monotonic safety factor profiles. We also discuss evidence of particle transport barriers found in the TCABR Tokamak (University of Sao Paulo) and the Texas Helimak (University of Texas at Austin) in biased discharges with non-monotonic plasma flows.

Equilibrium topology for plasmas with reversed current density.

Actually, transition from positive to negative plasma current and quasi-steady-state alternated current (AC) operation have been achieved experimentally without loss of ionization. The large transition times suggest the use of MHD equilibrium to model the intermediate magnetic field configurations for corresponding current density reversals. In the present work we show, by means of Maxwell equations, that the most robust equilibrium for any axisymmetric configuration with reversed current density requires the existence of several nonested families of magnetic surfaces inside the plasma. We also show that the currents inside the nonested families satisfy additive rules restricting the geometry and sizes of the axisymmetric magnetic islands; this is done without restricting the equilibrium through arbitrary functions. Finally, we introduce a local successive approximations method to describe the equilibrium about an arbitrary reversed current density minimum and, consequently, the transition between different nonested topologies is understood in terms of the eccentricity of the toroidal current density level sets.

Dense plasmas in magnetic traps : generation of focused ion beams with controlled ion-to-neutral flux ratios

Customized magnetic traps were developed to produce a domain of dense plasmas with a narrow ion beam directed to a particular area of the processed substrate. A planar magnetron coupled with an arc discharge source created the magnetic traps to confine the plasma electrons and generate the ion beam with the controlled ratio of ion-to-neutral fluxes. Images of the plasma jet patterns and numerical vizualizations help explaining the observed phenomena.

Effect of atmospheric gas plasmas on cancer cell signaling

Cancer is one of the most life-threatening diseases with many forms still regarded as incurable. The conventional cancer treatments have unwanted side effects such as the death of normal cells. A therapy that can accurately target and effectively kill tumor cells could address the inadequacies of the available therapies. Atmospheric gas plasmas (AGP) that are able to specifically kill cancerous cells offer a promising alternative approach compared to conventional therapies. AGP have been shown to exploit tumor-specific genetic defects and a recent trial in mice has confirmed its antitumor effects. The mechanism by which the AGP act on tumor cells but not normal cells is not fully understood. A review of the current literature suggests that reactive oxygen species (ROS) generated by AGP induce death of cancer cells by impairing the function of intracellular regulatory factors. The majority of cancer cells are defective in tumor suppressors that interfere normal cell growth pathways. It appears that pro-oncogene or tumor suppressor-dependent regulation of antioxidant/or ROS signaling pathways may be involved in AGP-induced cancer cell death. The toxic effects of ROS are mitigated by normal cells by adjustment of their metabolic pathways. On the other hand, tumor cells are mostly defective in several regulatory signaling pathways which lead to the loss of metabolic balance within the cells and consequently, the regulation of cell growth. This review article evaluates the impact of AGP on the activation of cellular signaling and its importance for exploring mechanisms for safe and efficient anticancer therapies.

Atmospheric pressure plasmas : infection control and bacterial responses

Cold atmospheric pressure plasma (APP) is a recent, cutting-edge antimicrobial treatment. It has the potential to be used as an alternative to traditional treatments such as antibiotics and as a promoter of wound healing, making it a promising tool in a range of biomedical applications with particular importance for combating infections. A number of studies show very promising results for APP-mediated killing of bacteria, including removal of biofilms of pathogenic bacteria such as Pseudomonas aeruginosa. However, the mode of action of APP and the resulting bacterial response are not fully understood. Use of a variety of different plasma-generating devices, different types of plasma gases and different treatment modes makes it challenging to show reproducibility and transferability of results. This review considers some important studies in which APP was used as an antibacterial agent, and specifically those that elucidate its mode of action, with the aim of identifying common bacterial responses to APP exposure. The review has a particular emphasis on mechanisms of interactions of bacterial biofilms with APP.

Plasma nanoscience : from nano-solids in plasmas to nano-plasmas in solids

The unique plasma-specific features and physical phenomena in the organization of nanoscale soild-state systems in a broad range of elemental composition, structure, and dimensionality are critically reviewed. These effects lead to the possibility to localize and control energy and matter at nanoscales and to produce self-organized nano-solids with highly unusual and superior properties. A unifying conceptual framework based on the control of production, transport, and self-organization of precursor species is introduced and a variety of plasma-specific non-equilibrium and kinetics-driven phenomena across the many temporal and spatial scales is explained. When the plasma is localized to micrometer and nanometer dimensions, new emergent phenomena arise. The examples range from semiconducting quantum dots and nanowires, chirality control of single-walled carbon nanotubes, ultra-fine manipulation of graphenes, nano-diamond, and organic matter to nano-plasma effects and nano-plasmas of different states of matter. © 2013 Taylor and Francis Group, LLC.

Energy efficiency in nanoscale synthesis using nanosecond plasmas

We report a nanoscale synthesis technique using nanosecond-duration plasma discharges. Voltage pulses 12.5 kV in amplitude and 40 ns in duration were applied repetitively at 30 kHz across molybdenum electrodes in open ambient air, generating a nanosecond spark discharge that synthesized well-defined MoO 3 nanoscale architectures (i.e. flakes, dots, walls, porous networks) upon polyamide and copper substrates. No nitrides were formed. The energy cost was as low as 75 eV per atom incorporated into a nanostructure, suggesting a dramatic reduction compared to other techniques using atmospheric pressure plasmas. These findings show that highly efficient synthesis at atmospheric pressure without catalysts or external substrate heating can be achieved in a simple fashion using nanosecond discharges.

Low-temperature plasmas in carbon nanostructure synthesis

Plasma-based techniques offer many unique possibilities for the synthesis of various nanostructures both on the surface and in the plasma bulk. In contrast to the conventional chemical vapor deposition and some other techniques, plasma-based processes ensure high level of controllability, good quality of the produced nanomaterials, and reduced environmental risk. In this work, the authors briefly review the unique features of the plasma-enhanced chemical vapor deposition approaches, namely, the techniques based on inductively coupled, microwave, and arc discharges. Specifically, the authors consider the plasmas with the ion/electron density ranging from 10^10 to 10^14 cm−3, electron energy in the discharge up to ∼10 eV, and the operating pressure ranging from 1 to 10^4 Pa (up to 105 Pa for the atmospheric-pressure arc discharges). The operating frequencies of the discharges considered range from 460 kHz for the inductively coupled plasmas, and up to 2.45 GHz for the microwave plasmas. The features of the direct-current arc discharges are also examined. The authors also discuss the principles of operation of these systems, as well as the effects of the key plasma parameters on the conditions of nucleation and growth of the carbon nanostructures, mainly carbon nanotubes and graphene. Advantages and disadvantages of these plasma systems are considered. Future trends in the development of these plasma-based systems are also discussed.

Silica nanoparticles treated by cold atmospheric-pressure plasmas improve the dielectric performance of organic–inorganic nanocomposites

We report on the application of cold atmospheric-pressure plasmas to modify silica nanoparticles to enhance their compatibility with polymer matrices. Thermally nonequilibrium atmospheric-pressure plasma is generated by a high-voltage radio frequency power source operated in the capacitively coupled mode with helium as the working gas. Compared to the pure polymer and the polymer nanocomposites with untreated SiO2, the plasma-treated SiO2–polymer nanocomposites show higher dielectric breakdown strength and extended endurance under a constant electrical stress. These improvements are attributed to the stronger interactions between the SiO2 nanoparticles and the surrounding polymer matrix after the plasma treatment. Our method is generic and can be used in the production of high-performance organic–inorganic functional nanocomposites.