| Current
Researches: |
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A model for the H-mode pedestal and type I ELMs, which is based on the
magnetic and ExB shear reduction of anomalous transport, is introduced. The
model takes into account the fact that different modes are suppressed at
different rate. The model is used in an integrated transport code to follow the
time evolution of tokamak discharges from the Ohmic stage through the transition
from L-mode to H-mode to the formation of the H-mode pedestal and the triggering
of ELMs. The model predicts the width and height of the H-mode pedestal and the
frequency of ELM. Two mechanisms for triggering ELMs are considered. The ELMs
can be triggered by the ballooning modes if the pressure gradient exceeds the
ballooning limit, ac,
or by peeling modes if the edge current density exceeds the peeling condition.
Suppression of the anomalous transport within the pedestal region enhances the
role on neoclassical transport in this region. Several models for the
neoclassical transport are tested together with the combined model for ELMs and
H-mode pedestal. The dependencies of pedestal temperature, pedestal width, and
frequency of ELMs as a function heating power, magnetic field, and pedestal
density are discussed (more details are
here). |
This page is constantly
under constructions.
Please contact me by:
E-mail |
Evaluation of Models for L-H Transition
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A model for the transition from L-mode to
H-mode is required for use in predictive integrated modeling
codes in order to simulate the time dependence of tokamak
discharges that include the L-H transition. In order to
develop and test a model for the L-H transition, two
theories for the transition are analyzed and the resulting
criteria for the transition are compared. One theory
considered is based on the stabilization of a strongly
ballooning resistive mode by the ion diamagnetic drift [J.
Weiland et al., in Proc. of the 17th
IAEA Int. Conference (Yokohama, Japan, October 1998)
4 1537]. This theory provides the condition for the L-H
transition in terms of hi=Ln/LT
(where Ln is the density scale length and
LT is the temperature scale length)
and indicates that the transition to H-mode can occur
without toroidal rotation, when the ion temperature
gradients are sufficiently steep at the edge of the plasma.
The second L-H theory is based on the idea of the generation
of flow shear by finite β drift waves [P. N. Guzdar et al.,
Phys. Rev. Lett. 89 265004 (2002)]. In this case, the
criterion for L-H transition is expressed in the terms of
electron temperature. This criterion depends on the electron
density gradients, but does not depend on the ion or
electron temperature gradients. To test the latter theory,
an analytical expression for the critical value of electron
temperature is employed in the BALDUR code. The critical
electron temperature depends on the global plasma parameters
such as major radius, toroidal magnetic field, and effective
impurity content. Discharges with a variety of values of
these parameters are considered in order to form systematic
scans and to test the model in the different operational
regimes. Also implemented in the BALDUR code is an empirical
model in which the condition for
the L-H transition is expressed in terms of a critical value
for the heating power [Y. Shimomura et al. Nuclear
Fusion 41 309 (2001)]. The theory model based on
flow shear and the empirical model are tested against each
other and against experimental data from the Alcator C-Mod,
DIII-D, and JET tokamaks. Simulation results obtained with
the theory-based model and some experimental observations
suggest that
edge plasma profile gradients are responsible for the L-H
transition. |
NUBEAM module for Neutral Beam
Injection heating
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| The NUBEAM module is a comprehensive
computational model for Neutral Beam Injection (NBI) in
tokamaks. It is used to compute power deposition, driven
current, momentum transfer, fueling, and other profiles in
tokamak plasmas due to NBI. NUBEAM computes the
time-dependent deposition and slowing down of the fast ions
produced by NBI, taking into consideration beam geometry and
composition, ion-neutral interactions (atomic physics),
anomalous diffusion of fast ions, the effects of large scale
instabilities, the effect of magnetic ripple, and finite
Larmor radius effects. The NUBEAM module can also treat
fusion product ions that contribute to alpha heating and ash
accumulation, whether or not NBI is present. These physical
phenomena are important in simulations of present day
tokamaks and projections to future devices such as ITER. The
NUBEAM module was extracted from the TRANSP integrated
modeling code, using standards of the National Transport
Code Collaboration (NTCC), and was submitted to the NTCC
module library. Numerical techniques that are used for
computing the Neutral Beam Injection (NBI) physics used in
several integrated modeling codes are compared. The
Monte-Carlo NUBEAM module and the Fokker-Planck NBI ASTRA,
DBEAMS, FPP, and NBEAMS neutral beam injection modules are
considered. Physics included in these modules is discussed.
Resulting electron and ion power heating profiles and
particle source profiles for the TFTR discharge 66887 and
the JET discharge 52009 are compared when computed with the
NUBEAM, NBI ASTRA, DBEAMS and FPP modules. |
Toroidal ETG mode structure in the presence of
nonuniform background flows
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| The influence of nonuniform poloidal and
toroidal background plasma flows on the spatial structure
and growth rate of the electrostatic Electron Temperature
Gradient (ETG) mode is investigated in the linear
approximation. This derivation includes the ballooning mode
formalism and a more recently developed version of the
direct method by Taylor and Wilson [Plasma Physics
and Control. Fusion 38 1999 (1996)]. It is shown
that the growth rate of the ETG mode is not changed
significantly by flow shear. However, it is found that the
spatial structure of the ETG mode depends crucially on the
derivative of the flow shear rate with respect to the minor
radius of the tokamak cross section and also depends
crucially on the magnetic shear. For moderate magnetic
shear, the unstable ETG mode is strongly localized in the
poloidal direction and is elongated along the radial
direction, with a characteristic radial width much larger
than the electron Larmor radius. This may explain the
formation of streamer structures above the threshold of ETG
mode instability. Streamers are believed to enhance electron
thermal transport beyond the values provided by simple
mixing length estimates. For very low values of magnetic
shear, the ETG mode structure becomes extended in the
poloidal direction, and the ballooning formalism does not
apply. In this case, the direct method is used and it is
shown that the ETG mode is strongly localized in the radial
direction. The small radial extent of these modes may
considerably reduce electron heat transport, which would
enhance the formation of an electron thermal transport
barrier. |
Simulation of Alcator C-Mod discharges
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| Predictive simulations for the Alcator
C-mod tokamak [I. Hutchinson et
al., Phys. Plasmas 1 1511 (1994)] are
carried out using the BALDUR
integrated modeling code [C.E. Singer
et al., Comp.
Phys. Comm.
49 275 (1988)]. The results are obtained for
temperature and density profiles
using the Multi-Mode transport model [G.
Bateman et al.,
Phys. Plasmas 5 1793 (1998)] as well as the
mixed-Bohm/gyro-Bohm transport model [M.~Erba et
al., Plasma Phys. Control.
Fusion 39 261 (1997)]. The simulated discharges
are characterized by very high plasma
density in both low and high modes of
confinement. The predicted profiles for each of the
transport models match the
experimental data about equally well in spite of the fact
that the two models have different
dimensionless scalings. Average relative rms
deviations are less than 8\% for the electron density
profiles and 16 % for
the electron and ion temperature profiles. |
ITG
fluid model near boundary of marginal stability
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| The iontemperature-gradient driven modes (hi-modes)
along with the trapped electron mode and pressure gradient
ballooning modes are dominating instabilities for the most
realistic tokamak parameters (low b and weak collisionality) and in many cases are
responsible for anomalous ion transport in tokamak
plasmas. As a graduate student of Prof. Davydova I took
part in the development of the hi-mode
fluid model in slab and toroidal geometry near the
boundary of marginal stability. This approach allowed to
elucidate the problem of subcritical turbulence of strong
reactive unstable modes such as the
ion-temperature-gradient driven modes are. Our particular
attention has been focused on the possibility of soliton
and vortex formation, which are the structure elements of
turbulence. The turbulent state arises from interactions
(collisions) of these structures. Besides, formation of
coherent long-lived large-scale structures of drift modes
is sometimes considered to be one of the main reasons of
transition to the regime of improved confinement in
tokamaks (L-H transition) and sometimes as the reason for
anomalous transport. |
TAE
instability
|
| One of the major issues in a-particle
physics of tokamaks is the low n toroidicity-induced
Alfven eigenmode (TAE) instability driven by a-particle pressure gradient and the resultant a-particle
transport. The main results of the previous studies of
this problem are that the volume averaged a-particle beta threshold for TAE instability is small
and is on the order of 10-4, and that for
relatively low fluctuation level with (dBr/B0)³10-4
the a-particle loss time is comparable and even shorter than
the a-particle slowing-down time. That is why the problem of
efficient suppression of this instability seems to be
important. We have considered an alternative damping
mechanism, which should arise in the stationary tokamak
reactor, when the plasma current is driven by lower hybrid
(LH) waves, which is the most perspective current drive
scheme at present. |
Precessional
current in ECR heated plasma
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| Electron cyclotron resonance (ECR) heating in a
tokamak can produce superthermal runaway electrons having
their turning points located at the cyclotron resonance
surface (so called ''sloshing'' electrons). The drift
motion causes these trapped hot electrons to precess
around the torus, thus forming a hot electron current. A
semi-analytic model has been used to calculate the
direction, magnitude and profile of this current. For
typical high power central ECR heating, a hot electron
current density of j||~140A/cm2
is found which is peaked at a radius ~ RF power deposition
where it enhances the local current density. The effect of
the hot electron current is to flatten the q-profile
within the RF power deposition region. If this radius is
comparable with q=1 radius, this results in
sawtooth stabilization. Moreover, low magnetic shear is
favourable for hot current filaments formation, which was
observed in experiment. |
Explosive
Instability
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| Non-equilibrium media can relax toward
equilibrium by a nonlinear instability, which involves the
interaction of many waves either with each other or with
particles. One of the most interesting examples of this is
the explosive instability. The instability develops if at
least one of resonantly interacting waves is a
“negative” or “zero-energy” wave. The last case
gives an especially efficient channel of realignment or
relaxation of the energy in plasma, fluid, and electronic
beam systems. We found that account for the boundedness of
the wave interaction region due either to the
inhomogeneity of the media, leading to detuning of the
wave phases, or to the boundedness of the system may
affect the explosive instability dynamics. In this case
the modified explosive instability is saturated at
sufficiently lower wave amplitudes if unstable
perturbations escape from the interaction region in a
smaller time than the “explosion time”. This
conclusion has been predicted theoretically and then
proved numerically. |
Diffusion
process in plasma arcs
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| Plasma arc is a commonly used tool of modern
technologies. Transport processes determine substantially
the characteristics of the arcs. The commonly used
assumption that gives a basis for the most recent models
of free burning arc is the local thermal equilibrium (LTE).
The problem of applicability of the assumption is
typically out of consideration, while it is known a number
of cases where LTE does not work. We have elaborated an
analytical model of free burning arc plasma, which takes
into account two factors to be considered as major reasons
of the deviation from the LTE: diffusion and radiation
transfer. We have analyzed both factors and built a
numerical model of an electric arc with melting copper
electrodes. It has been shown that prevalent mechanism in
many cases is resonant radiation transfer. The developed
model allows determining the spatial distributions of
partial concentrations in multi-component arc plasmas and
could be used for optimization of welding processes. The
results have been reported on the national and
international conferences, in technical reports, and in
articles [1, 12]. I took active part in different stages
of the studies. In particular, I have developed analytical
and numerical models of the diffusive processes in the
free-burning arc with melted copper electrodes. |
Monte-Carlo
modeling of abnormal glow discharges
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| Abnormal glow discharges are a commonly used tool
to achieve the technological objectives, such as ion
implantation or fast etching by intensive ion flows. For
applied physics aspects the energy characteristics of ion
fluxes attacking the electrode surface is of great
interest. We have investigated experimentally the energy
spectra of ions striking cathode in abnormal glow
discharges for discharge current densities in the range of
10-100 mA/cm2 and gas pressure above 1 Torr
in molecular (H2 and D2) and in
noble gases (He and Ar). We have built up a Monte-Carlo
model of the discharge and applied it to abnormal glow
discharges in helium and hydrogen. The studies aimed to
improve discharge facility, which used for ion
implantation of first wall metal patterns in fusion
devices. I took part in all parts of the studies and,
especially, in analytical and numerical model development. |
Wave
propagation in a plane heterogeneous dielectric waveguide
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| My thesis work for B.Sc. was dedicated to the
investigation of wave propagation in a plane heterogeneous
dielectric waveguide (scientific supervisor Prof. E.
Belokolos). We extended a very narrow class of the profile
of dielectric permeability º(z) that allows
exact analytical solutions. In addition to the known
linear, parabolic and Epshtain profiles we found that
profile º(z) in the form of the Weierstrass
P-function allows rigorous solutions. The set of Maxwell
equations was reduced to Shröedinger equation and
later on was presented in the form of the Lame equation.
The latter equation was analyzed for arbitrary boundary
conditions and rigorous solutions have been found. At the
same time we elaborated principles of the approach on the
base of inverse spectral problem, which allowed
determining the profiles of dielectric permeability by
known (or required) spectral characteristics. |
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