13,059 research outputs found
Radial convection of finite ion temperature, high amplitude plasma blobs
We present results from simulations of seeded blob convection in the
scrape-off-layer of magnetically confined fusion plasmas. We consistently
incorporate high fluctuation amplitude levels and finite Larmor radius (FLR)
effects using a fully nonlinear global gyrofluid model. This is in line with
conditions found in tokamak scrape-off-layers (SOL) regions.
Varying the ion temperature, the initial blob width, and the initial
amplitude, we found an FLR dominated regime where the blob behavior is
significantly different from what is predicted by cold-ion models. The
transition to this regime is very well described by the ratio of the ion
gyroradius to the characteristic gradient scale length of the blob.
We compare the global gyrofluid model with a partly linearized local model.
For low ion temperatures we find that simulations of the global model show more
coherent blobs with an increased cross-field transport compared to blobs
simulated with the local model. The maximal blob amplitude is significantly
higher in the global simulations than in the local ones. When the ion
temperature is comparable to the electron temperature, global blob simulations
show a reduced blob coherence and a decreased cross-field transport in
comparison with local blob simulations
The influence of temperature dynamics and dynamic finite ion Larmor radius effects on seeded high amplitude plasma blobs
Thermal effects on the perpendicular convection of seeded pressure blobs in
the scrape-off layer of magnetised fusion plasmas are investigated. Our
numerical study is based on a four field full-F gyrofluid model, which entails
the consistent description of high fluctuation amplitudes and dynamic finite
Larmor radius effects. We find that the maximal radial blob velocity increases
with the square root of the initial pressure perturbation and that a finite
Larmor radius contributes to highly compact blob structures that propagate in
the poloidal direction. An extensive parameter study reveals that a smooth
transition to this compact blob regime occurs when the finite Larmor radius
effect strength, defined by the ratio of the magnetic field aligned component
of the ion diamagnetic to the vorticity, exceeds unity.
The maximal radial blob velocities agree excellently with the inertial velocity
scaling law over more than an order of magnitude. We show that the finite
Larmor radius effect strength affects the poloidal and total particle transport
and present an empirical scaling law for the poloidal and total blob
velocities. Distinctions to the blob behaviour in the isothermal limit with
constant finite Larmor radius effects are highlighted
Topological equivalence of crystal and quasicrystal band structures
A number of recent articles have reported the existence of topologically
non-trivial states and associated end states in one-dimensional incommensurate
lattice models that would usually only be expected in higher dimensions. Using
an explicit construction, we here argue that the end states have precisely the
same origin as their counterparts in commensurate models and that
incommensurability does not in fact provide a meaningful connection to the
topological classification of systems in higher dimensions. In particular, we
show that it is possible to smoothly interpolate between states with
commensurate and incommensurate modulation parameters without closing the band
gap and without states crossing the band gap.Comment: 7 pages, 9 figures. Editors' Suggestio
Planar Ion Trap Geometry for Microfabrication
We describe a novel high aspect ratio radiofrequency linear ion trap geometry
that is amenable to modern microfabrication techniques. The ion trap electrode
structure consists of a pair of stacked conducting cantilevers resulting in
confining fields that take the form of fringe fields from parallel plate
capacitors. The confining potentials are modeled both analytically and
numerically. This ion trap geometry may form the basis for large scale quantum
computers or parallel quadrupole mass spectrometers.
PACS: 39.25.+k, 03.67.Lx, 07.75.+h, 07.10+CmComment: 14 pages, 16 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
Colour-singlet strangelets at finite temperature
Considering massless and quarks, and massive (150 MeV) quarks in
a bag with the bag pressure constant MeV, a colour-singlet
grand canonical partition function is constructed for temperatures
MeV. Then the stability of finite size strangelets is studied minimizing the
free energy as a function of the radius of the bag. The colour-singlet
restriction has several profound effects when compared to colour unprojected
case: (1) Now bulk energy per baryon is increased by about MeV making the
strange quark matter unbound. (2) The shell structures are more pronounced
(deeper). (3) Positions of the shell closure are shifted to lower -values,
the first deepest one occuring at , famous -particle ! (4) The shell
structure at vanishes only at MeV, though for higher
-values it happens so at MeV.Comment: Revtex file(8 pages)+6 figures(ps files) available on request from
first Autho
Magnetoresistence engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot
device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The
wire segment can be electrically tuned to a single dot or to a double dot
regime using the FSGs and a backgate. In both regimes we find a strong MR and a
sharp MR switching of up to 25\% at the field at which the magnetizations of
the FSGs are inverted by the external field. The sign and amplitude of the MR
and the MR switching can both be tuned electrically by the FSGs. In a double
dot regime close to pinch-off we find {\it two} sharp transitions in the
conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic
contacts, with one transition near zero and one at the FSG switching fields.
These surprisingly rich characteristics we explain in several simple resonant
tunneling models. For example, the TMR-like MR can be understood as a
stray-field controlled transition between singlet and a triplet double dot
states. Such local magnetic fields are the key elements in various proposals to
engineer novel states of matter and may be used for testing electron spin-based
Bell inequalities.Comment: 7 pages, 6 figure
Very Small Strangelets
We study the stability of small strangelets by employing a simple model of
strange matter as a gas of non-interacting fermions confined in a bag. We solve
the Dirac equation and populate the energy levels of the bag one quark at a
time. Our results show that for system parameters such that strange matter is
unbound in bulk, there may still exist strangelets with that are stable
and/or metastable. The lifetime of these strangelets may be too small to detect
in current accelerator experiments, however.Comment: 13 pages, MIT CTP#217
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