Negative-energy perturbations in cylindrical equilibria with a radial electric field

The impact of an equilibrium radial electric field $E$ on negative-energy perturbations (NEPs) (which are potentially dangerous because they can lead to either linear or nonlinear explosive instabilities) in cylindrical equilibria of magnetically confined plasmas is investigated within the framework of Maxwell-drift kinetic theory. It turns out that for wave vectors with a non-vanishing component parallel to the magnetic field the conditions for the existence of NEPs in equilibria with E=0 [G. N. Throumoulopoulos and D. Pfirsch, Phys. Rev. E 53, 2767 (1996)] remain valid, while the condition for the existence of perpendicular NEPs, which are found to be the most important perturbations, is modified. For $|e_i\phi|\approx T_i$ ($\phi$ is the electrostatic potential) and $T_i/T_e > \beta_c\approx P/(B^2/8\pi)$ ($P$ is the total plasma pressure), a case which is of operational interest in magnetic confinement systems, the existence of perpendicular NEPs depends on $e_\nu E$, where $e_\nu$ is the charge of the particle species $\nu$. In this case the electric field can reduce the NEPs activity in the edge region of tokamaklike and stellaratorlike equilibria with identical parabolic pressure profiles, the reduction of electron NEPs being more pronounced than that of ion NEPs.Comment: 30 pages, late

Negative-Energy Perturbations in Circularly Cylindrical Equilibria within the Framework of Maxwell-Drift Kinetic Theory

The conditions for the existence of negative-energy perturbations (which could be nonlinearly unstable and cause anomalous transport) are investigated in the framework of linearized collisionless Maxwell-drift kinetic theory for the case of equilibria of magnetically confined, circularly cylindrical plasmas and vanishing initial field perturbations. For wave vectors with a non-vanishing component parallel to the magnetic field, the plane equilibrium conditions (derived by Throumoulopoulos and Pfirsch [Phys Rev. E {\bf 49}, 3290 (1994)]) are shown to remain valid, while the condition for perpendicular perturbations (which are found to be the most important modes) is modified. Consequently, besides the tokamak equilibrium regime in which the existence of negative-energy perturbations is related to the threshold value of 2/3 of the quantity $\eta_\nu = \frac {\partial \ln T_\nu} {\partial \ln N_\nu}$, a new regime appears, not present in plane equilibria, in which negative-energy perturbations exist for {\em any} value of $\eta_\nu$. For various analytic cold-ion tokamak equilibria a substantial fraction of thermal electrons are associated with negative-energy perturbations (active particles). In particular, for linearly stable equilibria of a paramagnetic plasma with flat electron temperature profile ($\eta_e=0$), the entire velocity space is occupied by active electrons. The part of the velocity space occupied by active particles increases from the center to the plasma edge and is larger in a paramagnetic plasma than in a diamagnetic plasma with the same pressure profile. It is also shown that, unlike in plane equilibria, negative-energy perturbations exist in force-free reversed-field pinch equilibria with a substantial fraction of active particles.Comment: 31 pages, late

Modelo discreto 3d para mejoramiento del contraste térmico y estimación de profundidad de defectos en láminas de CFRP

Abstract Two finite difference discretization approaches of the Fourier’s 3D heat propagation model are introduced, from which a new technique is proposed to enhance the thermal contrast of infrared sequences of images acquired from pulsed active thermography experiment for non-destructive testing of CFRP slabs. The discrete models defined are easily adaptable to a spatial filter structure, which can be applied to each image of the infrared sequence to obtain a better contrast between possible internal flaws and sound regions of material, and hence, a better probability of flaws detection. The performance of the technique proposed is evaluated using artificial thermal sequences generated by ThermoCalc6L, software that is able to compute dynamic thermal distributions in anisotropic layered solids, simulating internal defects and different excitation sets. Results show that this technique offers a better contrast between defects and image background than other relevant techniques like modified-differential absolute contrast, and a potentially faster execution than techniques based on thermal distribution reconstruction like the 3D thermal filtering method. Resumen Se introducen dos aproximaciones por diferencias finitas al modelo clásico de Fourier de propagación del calor en 3D a partir de las cuales se propone una nueva técnica para mejorar el contraste térmico en secuencias de imágenes infrarrojas adquiridas a partir de experimentos de termografía activa pulsada para ensayo no destructivo de láminas delgadas de CFRP. Los modelos anteriores se adaptan fácilmente a una estructura de filtro espacial que puede aplicarse a cada imagen de la secuencia con el fin de obtener un mejor contraste entre posibles defectos internos y las regiones sanas del material, y por tanto, una mayor probabilidad de detección. El desempeño de la técnica propuesta se evalúa empleando secuencias artificiales sintetizadas con el software ThermoCalc6L, que permite computar las distribuciones de temperatura en láminas sólidas anisotrópicas, simulando defectos internos y diferentes esquemas de excitación. Los resultados muestran que la técnica propuesta ofrece un mejor contraste térmico que técnicas relevantes como el contraste absoluto diferencial modificado, y una velocidad potencialmente superior de ejecución sobre las técnicas basadas en la reconstrucción de la distribución térmica, como el caso del método de filtrado térmico 3D

A model for microinstability destabilization and enhanced transport in the presence of shielded 3-D magnetic perturbations

A mechanism is presented that suggests shielded 3-D magnetic perturbations can destabilize microinstabilities and enhance the associated anomalous transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with small 3-D deformations are constructed. In the vicinity of rational magnetic surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly perturbed by the 3-D modulations of the local magnetic shear associated with the presence of nearresonant Pfirsch-Schluter currents. These currents are driven by 3-D components of the magnetic field spectrum even when there is no resonant radial component. The infinite-n ideal ballooning stability boundary is often used as a proxy for the onset of virulent kinetic ballooning modes (KBM) and associated stiff transport. These results suggest that the achievable pressure gradient may be lowered in the vicinity of low order rational surfaces when 3-D magnetic perturbations are applied. This mechanism may provide an explanation for the observed reduction in the peak pressure gradient at the top of the edge pedestal during experiments where edge localized modes have been completely suppressed by applied 3-D magnetic fields