120 research outputs found
Electrostatic stability of electron-positron plasmas in dipole geometry
The electrostatic stability of electron-positron plasmas is investigated in
the point-dipole and Z-pinch limits of dipole geometry. The kinetic dispersion
relation for sub-bounce-frequency instabilities is derived and solved. For the
zero-Debye-length case, the stability diagram is found to exhibit singular
behavior. However, when the Debye length is non-zero, a fluid mode appears,
which resolves the observed singularity, and also demonstrates that both the
temperature and density gradients can drive instability. It is concluded that a
finite Debye length is necessary to determine the stability boundaries in
parameter space. Landau damping is investigated at scales sufficiently smaller
than the Debye length, where instability is absent
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
Quasilinear particle transport from gyrokinetic instabilities in general magnetic geometry
The quasilinear particle flux arising from gyrokinetic instabilities is
calculated in the electrostatic and collisionless approximation, keeping the
geometry of the magnetic field arbitrary. In particular, the flux of electrons
and heavy impurity ions is studied in the limit where the former move quickly,
and the latter slowly, along the field compared with the mode frequency.
Conclusions are drawn about how the particle fluxes of these species depend on
the magnetic-field geometry, mode structure and frequency of the instability.
Under some conditions, such as everywhere favourable or unfavourable magnetic
curvature and modest temperature gradients, it is possible to make general
statements about the fluxes independently of the details of the instability. In
quasi-isodynamic stellarators with favourable bounce-averaged curvature for
most particles, the particle flux is always outward if the temperature gradient
is not too large, suggesting that it might be difficult to fuel such devices
with gas puffing from the wall. In devices with predominantly unfavourable
magnetic curvature, the particle flux can be inward, resulting in spontaneous
density peaking in the centre of the plasma. In the limit of highly charged
impurities, ordinary diffusion (proportional to the density gradient) dominates
over other transport channels and the diffusion coefficient becomes independent
of mass and charge. An estimate for the level of transport caused by
magnetic-field fluctuations arising from ion-temperature-gradient instabilities
is also given and is shown to be small compared with the electrostatic
component
Coulomb collisions in strongly anisotropic plasmas II. Cyclotron cooling in laboratory pair plasmas
The behaviour of a strongly-magnetized collisional electron-positron plasma
which is optically thin to cyclotron radiation is considered, and the
distribution functions accessible to it on the various timescales in the system
are calculated. Particular attention is paid to the limit in which the
collision time exceeds the radiation emission time, making the electron
distribution function strongly anisotropic. Indeed, these are the exact
conditions likely to be attained in the first laboratory electron-positron
plasma experiments currently being developed, which will typically have very
low densities and be confined in very strong magnetic fields. The constraint of
strong-magnetization adds an additional complication in that long-range Coulomb
collisions, which are usually negligible, must now be considered. A rigorous
collision operator for these long-range collisions has never been written down.
Nevertheless, we show that the collisional scattering can be accounted for
without knowing the explicit form of this collision operator. The rate of
radiation emission is calculated and it is found that the loss of energy from
the plasma is proportional to the parallel collision frequency multiplied by a
factor that only depends logarithmically on plasma parameters. That is, this is
a self-accelerating process, meaning that the bulk of the energy will be lost
in a few collision times. We show that in a simple case, that of straight
field-line geometry, there are no unstable drift waves in such plasmas, despite
being far from Maxwellian
Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma
The accumulation of impurities in the core of magnetically confined plasmas,
resulting from standard collisional transport mechanisms, is a known threat to
their performance as fusion energy sources. Whilst the axisymmetric tokamak
systems have been shown to benefit from the effect of temperature screening,
that is an outward flux of impurities driven by the temperature gradient,
impurity accumulation in stellarators was thought to be inevitable, driven
robustly by the inward pointing electric field characteristic of hot fusion
plasmas. We have shown in Helander et. al. (2017b) that such screening can in
principle also appear in stellarators, in the experimentally relevant mixed
collisionality regime, where a highly collisional impurity species is present
in a low collisionality bulk plasma. Details of the analytic calculation are
presented here, along with the effect of the impurity on the bulk ion flow,
which will ultimately affect the bulk contribution to the bootstrap current
Impurity transport in a mixed-collisionality stellarator plasma
A potential threat to the performance of magnetically confined fusion plasmas
is the problem of impurity accumulation, which causes the concentration of
highly charged impurity ions to rise uncontrollably in the center of the plasma
and spoil the energy confinement by excessive radiation. It has long been
thought that the collisional transport of impurities in stellarators always
leads to such accumulation (if the electric field points inwards, which is
usually the case), whereas tokamaks, being axisymmetric, can benefit from
"temperature screening", i.e., an outward flux of impurities driven by the
temperature gradient. Here it is shown, using analytical techniques supported
by results from a new numerical code, that such screening can arise in
stellarator plasmas too, and indeed does so in one of the most relevant
operating regimes, where the impurities are highly collisional whilst the bulk
plasma is in any of the low-collisionality regimes.Comment: 11 pages, 3 figure
Impurities in a non-axisymmetric plasma: transport and effect on bootstrap current
Impurities cause radiation losses and plasma dilution, and in stellarator
plasmas the neoclassical ambipolar radial electric field is often unfavorable
for avoiding strong impurity peaking. In this work we use a new continuum
drift-kinetic solver, the SFINCS code (the Stellarator Fokker-Planck Iterative
Neoclassical Conservative Solver) [M. Landreman et al., Phys. Plasmas 21 (2014)
042503] which employs the full linearized Fokker-Planck-Landau operator, to
calculate neoclassical impurity transport coefficients for a Wendelstein 7-X
(W7-X) magnetic configuration. We compare SFINCS calculations with theoretical
asymptotes in the high collisionality limit. We observe and explain a
1/nu-scaling of the inter-species radial transport coefficient at low
collisionality, arising due to the field term in the inter-species collision
operator, and which is not found with simplified collision models even when
momentum correction is applied. However, this type of scaling disappears if a
radial electric field is present. We also use SFINCS to analyze how the
impurity content affects the neoclassical impurity dynamics and the bootstrap
current. We show that a change in plasma effective charge Zeff of order unity
can affect the bootstrap current enough to cause a deviation in the divertor
strike point locations.Comment: 36 pages, 13 figure
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