381 research outputs found
Correlated electron systems periodically driven out of equilibrium: Floquet + DMFT formalism
We propose to combine the Floquet formalism for systems in ac fields with the
dynamical mean-field theory to study correlated electron systems periodically
driven out of equilibrium by external fields such as intense laser light. This
approach has a virtue that we can nonperturbatively include both the
correlation effects and nonlinear effects due to the driving field, which is
imperative in analyzing recent experiments for photoinduced phase transitions.
In solving the problem, we exploit a general theorem that the Hamiltonian in a
Floquet matrix form can be exactly diagonalized for single-band noninteracting
systems. As a demonstration, we have applied the method to the Falicov-Kimball
model in intense ac fields to calculate the spectral function. The result shows
that photoinduced midgap states emerge from strong ac fields, triggering an
insulator-metal transition.Comment: 19 pages, 12 figures; minor change
Extension and its characteristics of ECRH plasma in the LHD
One of the main objectives of the LHD is to extend the plasma confinement
database for helical systems and to demonstrate such extended plasma
confinement properties to be sustained in steady state. Among the various
plasma parameter regimes, the study of confinement properties in the
collisionless regime is of particular importance. Electron cyclotron resonance
heating (ECRH) has been extensively used for these confinement studies of the
LHD plasma from the initial operation. The system optimizations including the
modification of the transmission and antenna system are performed with the
special emphasis on the local heating properties. As the result, central
electron temperature of more than 10 keV with the electron density of 0.6 x
10 m is achieved near the magnetic axis. The electron temperature
profile is characterized by a steep gradient similar to those of an internal
transport barrier observed in tokamaks and stellarators. 168 GHz ECRH system
demonstrated efficient heating at over the density more than 1.0 x 10
m. CW ECRH system is successfully operated to sustain 756 s discharge.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Dynamics of the Density Matrix in Contact with a Thermal Bath and the Quantum Master Equation
We study the structure of the time evolution of the density matrix in contact
with a thermal bath in a standard projection operator sheme. The reduced
density matrix of the system in the steady state is obtained by tracing out the
degree of freedom of the thermal bath from the equilibrium density matrix of
the total system. This reduced density matrix is modified by the interaction,
and is different from that of the equilibrium of the system alone. We
explicitly calculate the contribution of each term in quantum master equation
to the realization of the steady state density matrix, and make clear roles of
each term. By making use of the role of each term, the properties of the
commonly used quantum master equation are examined.Comment: 17 pages, to appear in JPS
マイクロ フォーカス エックスセン シーティー ヲ モチイタ シカン ホテツ ソウチ ノ サンジゲンテキ テキゴウ ヒョウカホウ ノ カイハツ
Reversed-shear Alfv?n eigenmodes were observed for the first time in a helical plasma having negative q0′′ (the curvature of the safety factor q at the zero shear layer). The frequency is swept downward and upward sequentially via the time variation in the maximum of q. The eigenmodes calculated by ideal MHD theory are consistent with the experimental data. The frequency sweeping is mainly determined by the effects of energetic ions and the bulk pressure gradient. Coupling of reversed-shear Alfv?n eigenmodes with energetic ion driven geodesic acoustic modes generates a multitude of frequency-sweeping modes
Energy Confinement Time and Heat Transport in Initial Neutral Beam Heated Plasmas
The confinement characteristics of large net-current-free plasmas heated by neutral-beam injection have been investigated in the Large Helical Device (LHD). A systematic enhancement in energy-confinement times from the scaling derived from the medium-sized heliotron/torsatron experiments have been observed, which is attributed to the edge pedestal. The core confinement is scaled with the Bohm term divided by the square root of the gyro radii. The comparative analysis using a dimensionally similar discharge in the Compact Helical System indicates gyro-Bohm dependence in the core and transport improvement in the edge region of LHD plasmas
Characteristics of confinement and stability in large helical device edge plasmas
Recent progress in the heating capability in the large helical device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] has allowed the highest average beta value (4.1%) obtained in the helical devices, and enables exploration of magnetohydrodynamics (MHD) stability in this beta region. MHD activities in the periphery are found to become stable spontaneously from the inner region to the outer region when the averaged beta value exceeds a threshold, and then a flattening of the electron temperature profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m/n=1/1 magnetic island, and the complete stabilization of the m/n=1/1 mode is demonstrated by the moderate island width. In addition, attempts to control peripheral plasmas are also performed by using a limiter and a local island divertor utilizing the m/n=1/1 island, to improve plasma confinement and, especially, to stabilize pressure-driven modes in the present study. The stabilization of peripheral MHD modes is obtained with both approaches, and this indicates that these are available to the production of higher-beta plasmas without edge MHD activities
Formation of electron internal transport barrier and achievement of high ion temperature in Large Helical Device
An internal transport barrier (ITB) was observed in the electron temperature profile in the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] with a centrally focused intense electron cyclotron resonance microwave heating. Inside the ITB the core electron transport was improved, and a high electron temperature, exceeding 10 keV in a low density, was achieved in a collisionless regime. The formation of the electron-ITB is correlated with the neoclassical electron root with a strong radial electric field determined by the neoclassical ambipolar flux. The direction of the tangentially injected beam-driven current has an influence on the electron-ITB formation. For the counter-injected target plasma, a steeper temperature gradient, than that for the co-injected one, was observed. As for the ion temperature, high-power NBI (neutral beam injection) heating of 9 MW has realized a central ion temperature of 5 keV with neon injection. By introducing neon gas, the NBI absorption power was increased in low-density plasmas and the direct ion heating power was much enhanced with a reduced number of ions, compared with hydrogen plasmas
Characteristics of transport in electron internal transport barriers and in the vicinity of rational surfaces in the Large Helical Device
Characteristics of transport in electron internal transport barriers (ITB) and in the vicinity of a rational surface with a magnetic island are studied with transient transport analysis as well as with steady state transport analysis. Associated with the transition of the radial electric field from a small negative value (ion-root) to a large positive value (electron-root), an electron ITB appears in the Large Helical Device [M. Fujiwara et al., Nucl. Fusion 41, 1355 (2001)], when the heating power of the electron cyclotron heating exceeds a power threshold. Transport analysis shows that both the standard electron thermal diffusivity, chie, and the incremental electron thermal diffusivity, chieinc (the derivative of normalized heat flux to temperature gradient, equivalent to heat pulse chie), are reduced significantly (a factor 5 10) in the ITB. The chieinc is much lower than the chie by a factor of 3 just after the transition, while chieinc is comparable to or even higher than chie before the transition, which results in the improvement of electron transport with increasing power in the ITB, in contrast to its degradation outside the ITB. In other experiments without an ITB, a significant reduction (by one order of magnitude) of chieinc is observed at the O-point of the magnetic island produced near the plasma edge using error field coils. This observation gives significant insight into the mechanism of transport improvement near the rational surface and implies that the magnetic island serves as a poloidally asymmetric transport barrier. Therefore the radial heat flux near the rational surface is focused at the X-point region, and that may be the mechanism to induce an ITB near a rational surface
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