1,976 research outputs found
Non-linear Simulations of MHD Instabilities in Tokamaks Including Eddy Current Effects and Perspectives for the Extension to Halo Currents
The dynamics of large scale plasma instabilities can strongly be influenced
by the mutual interaction with currents flowing in conducting vessel
structures. Especially eddy currents caused by time-varying magnetic
perturbations and halo currents flowing directly from the plasma into the walls
are important. The relevance of a resistive wall model is directly evident for
Resistive Wall Modes (RWMs) or Vertical Displacement Events (VDEs). However,
also the linear and non-linear properties of most other large-scale
instabilities may be influenced significantly by the interaction with currents
in conducting structures near the plasma. The understanding of halo currents
arising during disruptions and VDEs, which are a serious concern for ITER as
they may lead to strong asymmetric forces on vessel structures, could also
benefit strongly from these non-linear modeling capabilities. Modeling the
plasma dynamics and its interaction with wall currents requires solving the
magneto-hydrodynamic (MHD) equations in realistic toroidal X-point geometry
consistently coupled with a model for the vacuum region and the resistive
conducting structures. With this in mind, the non-linear finite element MHD
code JOREK has been coupled with the resistive wall code STARWALL, which allows
to include the effects of eddy currents in 3D conducting structures in
non-linear MHD simulations. This article summarizes the capabilities of the
coupled JOREK-STARWALL system and presents benchmark results as well as first
applications to non-linear simulations of RWMs, VDEs, disruptions triggered by
massive gas injection, and Quiescent H-Mode. As an outlook, the perspectives
for extending the model to halo currents are described.Comment: Proceeding paper for Theory of Fusion Plasmas (Joint Varenna-Lausanne
International Workshop), Varenna, Italy (September 1-5, 2014); accepted for
publication in: to Journal of Physics: Conference Serie
Numerical study of tearing mode seeding in tokamak X-point plasma
A detailed understanding of island seeding is crucial to avoid (N)TMs and
their negative consequences like confinement degradation and disruptions. In
the present work, we investigate the growth of 2/1 islands in response to
magnetic perturbations. Although we use externally applied perturbations
produced by resonant magnetic perturbation (RMP) coils for this study, results
are directly transferable to island seeding by other MHD instabilities creating
a resonant magnetic field component at the rational surface. Experimental
results for 2/1 island penetration from ASDEX Upgrade are presented extending
previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge
with low collisionality and active RMP coils. Our numerical studies are
performed with the 3D, two fluid, non-linear MHD code JOREK. All three phases
of mode seeding observed in the experiment are also seen in the simulations:
first a weak response phase characterized by large perpendicular electron flow
velocities followed by a fast growth of the magnetic island size accompanied by
a reduction of the perpendicular electron velocity, and finally the saturation
to a fully formed island state with perpendicular electron velocity close to
zero. Thresholds for mode penetration are observed in the plasma rotation as
well as in the RMP coil current. A hysteresis of the island size and electron
perpendicular velocity is observed between the ramping up and down of the RMP
amplitude consistent with an analytically predicted bifurcation. The transition
from dominant kink/bending to tearing parity during the penetration is
investigated
Green’s-function solution for a special class of master equations
We consider a one-dimensional stochastic process described by a master equation and calculate the time-dependent distribution function. The time evolution of the system is given by the conditional probability h(x,t‖x0), where x and x0 are continuous variables. Assuming a quite general dependence of the transition probability WΩ(x→x’) on a parameter Ω, we show that the backward form of the master equation can be used to calculate arbitrary conditional averages 〈f(x)‖x0⟩t up to any given power in Ω−1. This general expansion procedure will be used to construct h(x,t‖x0) itself. We show that—introducing a new stochastic variable y—the conditional probability h(x,t‖x0) can be expanded into a series of Hermite functions. The coefficients of this expansion, bn(t), which depend on the expansion parameter Ω, can be uniquely determined via a recursion relation. We show that in the limit Ω→∞ all coefficients bn(t) vanish, except b0, which is time independent. In this limit a Gaussian distribution for the conditional probability is obtained, which is in agreement with the so-called linear noise approximation
The Search for Stable, Massive, Elementary Particles
In this paper we review the experimental and observational searches for
stable, massive, elementary particles other than the electron and proton. The
particles may be neutral, may have unit charge or may have fractional charge.
They may interact through the strong, electromagnetic, weak or gravitational
forces or through some unknown force. The purpose of this review is to provide
a guide for future searches - what is known, what is not known, and what appear
to be the most fruitful areas for new searches. A variety of experimental and
observational methods such as accelerator experiments, cosmic ray studies,
searches for exotic particles in bulk matter and searches using astrophysical
observations is included in this review.Comment: 34 pages, 8 eps figure
Coupling JOREK and STARWALL for Non-linear Resistive-wall Simulations
The implementation of a resistive-wall extension to the non-linear MHD-code
JOREK via a coupling to the vacuum-field code STARWALL is presented along with
first applications and benchmark results. Also, non-linear saturation in the
presence of a resistive wall is demonstrated. After completion of the ongoing
verification process, this code extension will allow to perform non-linear
simulations of MHD instabilities in the presence of three-dimensional resistive
walls with holes for limited and X-point plasmas.Comment: Contribution for "Theory Of Fusion Plasmas, Joint Varenna - Lausanne
International Workshop, Villa Monastero, Varenna, Italy (27.-31.8.2012)",
accepted for publication in Journal of Physics Conference Serie
A New Method for Searching for Free Fractional Charge Particles in Bulk Matter
We present a new experimental method for searching for free fractional charge
in bulk matter; this new method derives from the traditional Millikan liquid
drop method, but allows the use of much larger drops, 20 to 100 mm in diameter,
compared to the traditional method that uses drops less than 15 mm in diameter.
These larger drops provide the substantial advantage that it is then much
easier to consistently generate drops containing liquid suspensions of powdered
meteorites and other special minerals. These materials are of great importance
in bulk searches for fractional charge particles that may have been produced in
the early universe.Comment: 17 pages, 5 figures in a singl PDF file (created from WORD Doc.).
Submitted to Review of Scientific Instrument
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