811 research outputs found
Stellarator bootstrap current and plasma flow velocity at low collisionality
The bootstrap current and flow velocity of a low-collisionality stellarator
plasma are calculated. As far as possible, the analysis is carried out in a
uniform way across all low-collisionality regimes in general stellarator
geometry, assuming only that the confinement is good enough that the plasma is
approximately in local thermodynamic equilibrium. It is found that conventional
expressions for the ion flow speed and bootstrap current in the
low-collisionality limit are accurate only in the -collisionality regime
and need to be modified in the -regime. The correction due to
finite collisionality is also discussed and is found to scale as
Less constrained omnigeneous stellarators
A stellarator is said to be omnigeneous if all particles have vanishing
average radial drifts. In omnigeneous stellarators, particles are perfectly
confined in the absence of turbulence and collisions, whereas in
non-omnigeneous configurations, particle can drift large radial distances. One
of the consequences of omnigeneity is that the unfavorable inverse scaling with
collisionality of the stellarator neoclassical fluxes disappears. In the
pioneering and influential article [Cary~J~R and Shasharina~S~G 1997 {\it Phys.
Plasmas} {\bf 4} 3323], the conditions that the magnetic field of a stellarator
must satisfy to be omnigeneous are derived. However, reference [Cary~J~R and
Shasharina~S~G 1997 {\it Phys. Plasmas} {\bf 4} 3323] only considered
omnigeneous stellarators in which all the minima of the magnetic field strength
on a flux surface must have the same value. The same is assumed for the maxima.
We show that omnigenenous magnetic fields can have local minima and maxima with
different values. Thus, the parameter space in which omnigeneous stellarators
are possible is larger than previously expected. The analysis presented in this
article is only valid for orbits with vanishing radial width, and in principle
it is not applicable to energetic particles. However, one would expect that
improving neoclassical confinement would improve energetic particle
confinement.Comment: 10 pages, 5 figure
Collisional transport across the magnetic field in drift-fluid models
Drift ordered fluid models are widely applied in studies of low-frequency
turbulence in the edge and scrape-off layer regions of magnetically confined
plasmas. Here, we show how collisional transport across the magnetic field is
self-consistently incorporated into drift-fluid models without altering the
drift-fluid energy integral. We demonstrate that the inclusion of collisional
transport in drift-fluid models gives rise to diffusion of particle density,
momentum and pressures in drift-fluid turbulence models and thereby obviate the
customary use of artificial diffusion in turbulence simulations. We further
derive a computationally efficient, two-dimensional model which can be time
integrated for several turbulence de-correlation times using only limited
computational resources. The model describes interchange turbulence in a
two-dimensional plane perpendicular to the magnetic field located at the
outboard midplane of a tokamak. The model domain has two regions modeling open
and closed field lines. The model employs a computational expedient model for
collisional transport. Numerical simulations show good agreement between the
full and the simplified model for collisional transport
Long-wavelength limit of gyrokinetics in a turbulent tokamak and its intrinsic ambipolarity
Recently, the electrostatic gyrokinetic Hamiltonian and change of coordinates
have been computed to order in general magnetic geometry. Here
is the gyrokinetic expansion parameter, the gyroradius over the
macroscopic scale length. Starting from these results, the long-wavelength
limit of the gyrokinetic Fokker-Planck and quasineutrality equations is taken
for tokamak geometry. Employing the set of equations derived in the present
article, it is possible to calculate the long-wavelength components of the
distribution functions and of the poloidal electric field to order
. These higher-order pieces contain both neoclassical and turbulent
contributions, and constitute one of the necessary ingredients (the other is
given by the short-wavelength components up to second order) that will
eventually enter a complete model for the radial transport of toroidal angular
momentum in a tokamak in the low flow ordering. Finally, we provide an explicit
and detailed proof that the system consisting of second-order gyrokinetic
Fokker-Planck and quasineutrality equations leaves the long-wavelength radial
electric field undetermined; that is, the turbulent tokamak is intrinsically
ambipolar.Comment: 70 pages. Typos in equations (63), (90), (91), (92) and (129)
correcte
Drift of ablated material after pellet injection in a tokamak
Pellet injection is used for fuelling and controlling discharges in tokamaks,
and it is foreseen in ITER. During pellet injection, a movement of the ablated
material towards the low-field side (or outward major radius direction) occurs
because of the inhomogeneity of the magnetic field. Due to the complexity of
the theoretical models, computer codes developed to simulate the cross-field
drift are computationally expensive. Here, we present a one-dimensional
semi-analytical model for the radial displacement of ablated material after
pellet injection, taking into account both the Alfv\'en and ohmic currents
which short-circuit the charge separation creating the drift. The model is
suitable for rapid calculation of the radial drift displacement, and can be
useful for e.g. modelling of disruption mitigation via pellet injection.Comment: 22 pages, 4 figures. Submitted to Journal of Plasma Physic
Systemic fungal endophytes and ploidy level in Festuca vivipara populations in North European Islands
Exploring the regional pattern of variation in traits driven by symbiotic interactions may provide insights to understand the evolutionary processes that operate over plant populations. Polyploidy, which is associated with fitness improvement, is expected to increase with latitude and altitude. However, it has never been explored in relation with the occurrence of epichloid fungal endophytes in plants. Both, variation in ploidy level and in the incidence of fungal endophytes, are known to occur in species of fine fescues. Here, we surveyed the occurrence of systemic fungal endophytes in natural Festuca vivipara populations in North European islands. In addition, we identified the fungal species associated with this grass and determined the predominant ploidy level for each population. Endophytes were found in four of six, two of three, and one of three populations for Faroe Islands, Iceland and Great Britain, respectively. With an average low incidence level of 15 % in infected populations, there was no relationship between infection level and either latitude or altitude. The phylogenetic analysis based on sequences ITS and the tub2 genes, supports that the endophytic species is EpichloA << festucae, the same as in other fine fescues. We found no variation in ploidy level as all the plants were tetraploid (4X) with 28 chromosomes, a pattern which contrasts with the variation reported in previous antecedents. Our results suggest that apart from low and variable benefits of the endophyte to the plants, there would be a complex dynamics between epichloid endophytes and species of the fine fescue complex which merits further studies.</p
Sources of intrinsic rotation in the low flow ordering
A low flow, gyrokinetic formulation to obtain the intrinsic
rotation profiles is presented. The momentum conservation equation in the low
flow ordering contains new terms, neglected in previous first principles
formulations, that may explain the intrinsic rotation observed in tokamaks in
the absence of external sources of momentum. The intrinsic rotation profile
depends on the density and temperature profiles and on the up-down asymmetry.Comment: 20 page
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