Non-linear effects in electron cyclotron current drive applied for the stabilization of neoclassical tearing modes

Due to the smallness of the volumes associated with the flux surfaces around the O-point of a magnetic island, the electron cyclotron power density applied inside the island for the stabilization of neoclassical tearing modes (NTMs) can exceed the threshold for non-linear effects as derived previously by Harvey et al, Phys. Rev. Lett. 62 (1989) 426. We study the non-linear electron cyclotron current drive (ECCD) efficiency through bounce-averaged, quasi-linear Fokker-Planck calculations in the magnetic geometry as created by the islands. The calculations are performed for the parameters of a typical NTM stabilization experiment on ASDEX Upgrade. A particular feature of these experiments is that the rays of the EC wave beam propagate tangential to the flux surfaces in the power deposition region. The calculations show significant non-linear effects on the ECCD efficiency, when the ECCD power is increased from its experimental value of 1 MW to a larger value of 4 MW. The nonlinear effects are largest in case of locked islands or when the magnetic island rotation period is longer than the collisional time scale. The non-linear effects result in an overall reduction of the current drive efficiency for this case with absorption of the EC power on the low field side of the electron cyclotron resonance layer. As a consequence of the non-linear effects, also the stabilizing effect of the ECCD on the island is reduced from linear expectations

Non-existence of normal tokamak equilibria with negative central current

Recent tokamak experiments employing off-axis, non-inductive current drive have found that a large central current hole can be produced. The current density is measured to be approximately zero in this region, though in principle there was sufficient current drive power for the central current density to have gone significantly negative. Recent papers have used a large aspect-ratio expansion to show that normal MHD equilibria (with axisymmetric nested flux surfaces, non-singular fields, and monotonic peaked pressure profiles) can not exist with negative central current. We extend that proof here to arbitrary aspect ratio, using a variant of the virial theorem to derive a relatively simple integral constraint on the equilibrium. However, this constraint does not, by itself, exclude equilibria with non-nested flux surfaces, or equilibria with singular fields and/or hollow pressure profiles that may be spontaneously generated.Comment: 5 pages, 3 figures. Submitted to Physics of Plasmas, Feb. 14, 2003. Revised Feb. 24, 2003. Vers. 2: revised May 29 to clarify points raised by referee, add references to recent work. July 18, accepted for publicatio

Early evolution of electron cyclotron driven current during suppression of tearing modes in a circular tokamak

When electron cyclotron (EC) driven current is first applied to the inside of a magnetic island, the current spreads throughout the island and after a short period achieves a steady level. Using a two equation fluid model for the EC current that allows us to examine this early evolution in detail, we analyze high-resolution simulations of a 2/1 classical tearing mode in a low-beta large aspect-ratio circular tokamak. These simulations use a nonlinear 3D reduced-MHD fluid model and the JOREK code. During the initial period where the EC driven current grows and spreads throughout the magnetic island, it is not a function of the magnetic flux. However, once it has reached a steady-state, it should be a flux function. We demonstrate numerically that if sufficiently resolved toroidally, the steady-state EC driven current becomes approximately a flux function. We discuss the physics of this early period of EC evolution and its impact on the size of the magnetic island.Comment: 12 pages, 7 figure

The sele sentence completion questionnaire: a new instrument for the assessment of personal meaning in research on aging

In this paper, the SELE-instrumenits presented. The SELE-instrument is a sentence completion test and an accompanying coding scheme, which is designed to study cognitions about self and life. The related theoretical construct is the personal meaning system, which encompasses different meaning domains, like "self", "activities" and "social relations", as well as evaluations and time perspectives. It is argued that especially in aging research. it is necessarv to use more oven instruments in studying self-and-life-cognitions. A short history of sentence completion instruments in psychology, is presented in order to better understand the specific contents of the SELE-instrument. The SELE provides sentence stems in the first form singular ("I") combination with verbal factors, which express different combinations of evaluation and time perspective, to which the respondents have to react in their own words. The SELE-instrumenits thus a semi-structured questionnaire, which can be used in large-scale research. The sentence completions are coded with an extensive coding scheme, which is hierarchially and dimensionally organized. Some figures on the coding reliability as well as on the validity and stability of the instrument are presented. The different uses to which the SELE-instrument as been put are described shortly.En este articulo se presenta el instrumento SELE. Este instrumento, que consta de un cuestionario de frases incompletas y un esquema de codificación de respuestas, está diseñado para el estudio de las cogniciones sobre el self y la vida. El constructo teórico al que se aproxima es el sistema de significado personal, sistema que comprende tanto diferentes dominios de significado (por ejemplo el "self", "actividades" o "relaciones sociales") como evaluaciones y perspectivas temporales. Se argumenta que, especialmente en la investigación en envejecimiento, es necesario utilizar instrurnentos de carácter más abierto para el estudio de cogniciones relacionadas con el self y la vida. Por otra parte, para entender mejor los contenidos específicos del SELE, se presenta una breve historia de los instrumentos de frases incompletas en Psicologia. El SELE proporciona estimulos en forma de frases incompletas a los que la persona tiene que contestar en sus propias palabras. Estos estímulos comienzan con un sujeto en primera persona del singular ("Yo") seguido por una forma verbal que expresa diferentes combinaciones de perspectiva temporal y evaluación. Debido a este formato, el SELE es un cuestionario semiestructurado que puede ser utilizado en investigaciones a gran escala. Las respuestas se categorizan mediante un exhaustivo sistema de codificación que está organizado tanto jerárquica como dimensionalmente. En el articulo se presentan algunos resultados en relación con la fiabilidad, la validez y la estabilidad del cuestionario, asi como también se describen brevemente los diferentes usos a los que ha estado dirigido el SELE

The construction of knowledge-based economies versus knowledge societies: The cases of Germany and Singapore

In the past decades, terms such as knowledge-based economy (KBE)\u27, and \u27information/knowledge society\u27 have been adopted by governments worldwide in order to underline their interest in developing their economies and societies further and assure future growth. Many governments used these catchwords as labels for government programs and action plans aiming at economic and social prosperity. This aim of national governments to construct knowledge-based economies, information/knowledge societies, the actions taken and especially the ability or disability to do so, is the topic of this paper. As two cases of comparison act Singapore and Germany. (DIPF/Orig.

From time-series to complex networks: Application to the cerebrovascular flow patterns in atrial fibrillation

A network-based approach is presented to investigate the cerebrovascular flow patterns during atrial fibrillation (AF) with respect to normal sinus rhythm (NSR). AF, the most common cardiac arrhythmia with faster and irregular beating, has been recently and independently associated with the increased risk of dementia. However, the underlying hemodynamic mechanisms relating the two pathologies remain mainly undetermined so far; thus the contribution of modeling and refined statistical tools is valuable. Pressure and flow rate temporal series in NSR and AF are here evaluated along representative cerebral sites (from carotid arteries to capillary brain circulation), exploiting reliable artificially built signals recently obtained from an in silico approach. The complex network analysis evidences, in a synthetic and original way, a dramatic signal variation towards the distal/capillary cerebral regions during AF, which has no counterpart in NSR conditions. At the large artery level, networks obtained from both AF and NSR hemodynamic signals exhibit elongated and chained features, which are typical of pseudo-periodic series. These aspects are almost completely lost towards the microcirculation during AF, where the networks are topologically more circular and present random-like characteristics. As a consequence, all the physiological phenomena at microcerebral level ruled by periodicity - such as regular perfusion, mean pressure per beat, and average nutrient supply at cellular level - can be strongly compromised, since the AF hemodynamic signals assume irregular behaviour and random-like features. Through a powerful approach which is complementary to the classical statistical tools, the present findings further strengthen the potential link between AF hemodynamic and cognitive decline.Comment: 12 pages, 10 figure

Strong Scattering of High Power Millimeter Waves in Tokamak Plasmas with Tearing Modes

In tokamak plasmas with a tearing mode, strong scattering of high power millimeter waves, as used for heating and noninductive current drive, is shown to occur. This new wave scattering phenomenon is shown to be related to the passage of the O point of a magnetic island through the high power heating beam. The density determines the detailed phasing of the scattered radiation relative to the O-point passage. The scattering power depends strongly nonlinearly on the heating beam power