1,871 research outputs found

### The universal instability in general geometry

The "universal" instability has recently been revived by Landreman, Antonsen
and Dorland [1], who showed that it indeed exists in plasma geometries with
straight (but sheared) magnetic field lines. Here it is demonstrated
analytically that this instability can be present in more general sheared and
toroidal geometries. In a torus, the universal instability is shown to be
closely related to the trapped-electron mode, although the trapped-electron
drive is usually dominant. However, this drive can be weakened or eliminated,
as in the case in stellarators with the maximum-$J$ property, leaving the
parallel Landau resonance to drive a residual mode, which is identified as the
universal instability

### Distinct turbulence saturation regimes in stellarators

In the complex 3D magnetic fields of stellarators, ion-temperature-gradient
turbulence is shown to have two distinct saturation regimes, as revealed by
petascale numerical simulations, and explained by a simple turbulence theory.
The first regime is marked by strong zonal flows, and matches previous
observations in tokamaks. The newly observed second regime, in contrast,
exhibits small- scale quasi-two-dimensional turbulence, negligible zonal flows,
and, surprisingly, a weaker heat flux scaling. Our findings suggest that key
details of the magnetic geometry control turbulence in stellarators.Comment: Erratum added to en

### Impurity transport in temperature gradient driven turbulence

In the present paper the transport of impurities driven by trapped electron
(TE) mode turbulence is studied. Non-linear (NL) gyrokinetic simulations using
the code GENE are compared with results from quasilinear (QL) gyrokinetic
simulations and a computationally efficient fluid model. The main focus is on
model comparisons for electron temperature gra- dient driven turbulence
regarding the sign of the convective impurity velocity (pinch) and the impurity
density gradient R/LnZ (peaking factor) for zero impurity flux. In particular,
the scaling of the impurity peaking factors with impurity charge Z and with
driving temper- ature gradient is investigated and compared with the results
for Ion Temperature Gradient (ITG) driven turbulence. In addition, the impurity
peaking is compared to the main ion peaking obtained by a self-consistent fluid
calculation of the density gradients corresponding to zero particle fluxes.
For the scaling of the peaking factor with impurity charge Z, a weak
dependence is obtained from NL GENE and fluid simulations. The QL GENE results
show a stronger dependence for low Z impurities and overestimates the peaking
factor by up to a factor of two in this region. As in the case of ITG dominated
turbulence, the peaking factors saturate as Z increases, at a level much below
neoclassical predictions. However, the scaling with Z is weak or reversed as
compared to the ITG case.
The scaling of impurity peaking with the background temperature gradients is
found to be weak in the NL GENE and fluid simulations. The QL results are also
here found to significantly overestimate the peaking factor for low Z values.
For the parameters considered, the background density gradient for zero
particle flux is found to be slightly larger than the corresponding impurity
zero flux gradient.Comment: 23 pages, 13 figures. Submitted to AIP: Physics of Plasma

### Collisionless microinstabilities in stellarators II - numerical simulations

Microinstabilities exhibit a rich variety of behavior in stellarators due to
the many degrees of freedom in the magnetic geometry. It has recently been
found that certain stellarators (quasi-isodynamic ones with maximum-$J$
geometry) are partly resilient to trapped-particle instabilities, because
fast-bouncing particles tend to extract energy from these modes near marginal
stability. In reality, stellarators are never perfectly quasi-isodynamic, and
the question thus arises whether they still benefit from enhanced stability.
Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic
configuration, QIPC, are investigated numerically and compared with the
National Compact Stellarator Experiment (NCSX) and the DIII-D tokamak. In
gyrokinetic simulations, performed with the gyrokinetic code GENE in the
electrostatic and collisionless approximation, ion-temperature-gradient modes,
trapped-electron modes and mixed-type instabilities are studied. Wendelstein
7-X and QIPC exhibit significantly reduced growth rates for all simulations
that include kinetic electrons, and the latter are indeed found to be
stabilizing in the energy budget. These results suggest that imperfectly
optimized stellarators can retain most of the stabilizing properties predicted
for perfect maximum-$J$ configurations.Comment: 15 pages, 40 figure

### Magnetic compressibility and ion-temperature-gradient-driven microinstabilities in magnetically confined plasmas

The electromagnetic theory of the strongly driven ion-temperature-gradient
(ITG) instability in magnetically confined toroidal plasmas is developed.
Stabilizing and destabilizing effects are identified, and a critical
$\beta_{e}$ (the ratio of the electron to magnetic pressure) for stabilization
of the toroidal branch of the mode is calculated for magnetic equilibria
independent of the coordinate along the magnetic field. Its scaling is
$\beta_{e}\sim L_{Te}/R,$ where $L_{Te}$ is the characteristic electron
temperature gradient length, and $R$ the major radius of the torus. We
conjecture that a fast particle population can cause a similar stabilization
due to its contribution to the equilibrium pressure gradient. For sheared
equilibria, the boundary of marginal stability of the electromagnetic
correction to the electrostatic mode is also given. For a general magnetic
equilibrium, we find a critical length (for electromagnetic stabilization) of
the extent of the unfavourable curvature along the magnetic field. This is a
decreasing function of the local magnetic shear

### Constraints on dynamo action in plasmas

Upper bounds are derived on the amount of magnetic energy that can be
generated by dynamo action in collisional and collisionless plasmas with and
without external forcing. A hierarchy of mathematical descriptions is
considered for the plasma dynamics: ideal MHD, visco-resistive MHD, the
double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD, and
other kinetic models. It is found that dynamo action is greatly constrained in
models where the magnetic moment of any particle species is conserved. In the
absence of external forcing, the magnetic energy then remains small at all
times if it is small in the initial state. In other words, a small "seed"
magnetic field cannot be amplified significantly, regardless of the nature of
flow, as long as the collision frequency and gyroradius are small enough to be
negligible. A similar conclusion also holds if the system is subject to
external forcing as long as this forcing conserves the magnetic moment of at
least one plasma species and does not greatly increase the total energy of the
plasma (i.e., in practice, is subsonic). Dynamo action therefore always
requires collisions or some small-scale kinetic mechanism for breaking the
adiabatic invariance of the magnetic moment

### 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 $1/\nu$-collisionality regime
and need to be modified in the $\sqrt{\nu}$-regime. The correction due to
finite collisionality is also discussed and is found to scale as $\nu^{2/5}$

### Impurity flows and plateau-regime poloidal density variation in a tokamak pedestal

In the pedestal of a tokamak, the sharp radial gradients of density and
temperature can give rise to poloidal variation in the density of impurities.
At the same time, the flow of the impurity species is modified relative to the
conventional neoclassical result. In this paper, these changes to the density
and flow of a collisional impurity species are calculated for the case when the
main ions are in the plateau regime. In this regime it is found that the
impurity density can be higher at either the inboard or outboard side. This
finding differs from earlier results for banana- or Pfirsch-Schl\"uter-regime
main ions, in which case the impurity density is always higher at the inboard
side in the absence of rotation. Finally, the modifications to the impurity
flow are also given for the other regimes of main-ion collisionality.Comment: 15 pages, 5 figures, submitted to Physics of Plasma

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