Results 1 - 10 of 5525
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[en] We study the effects of the X-point configuration on edge localized mode (ELM) triggering peeling and ballooning modes using fixed boundary equilibria and modifying the plasma shape to approach the limit of a true X-point. The current driven pure peeling modes are asymptotically stabilized by the X-point while the stabilizing effect on ballooning modes depends on the poloidal location of the X-point. The coupled peeling-ballooning modes experience the elimination of the peeling component as the X-point is introduced. This can significantly affect the edge stability diagrams used to analyse the ELM triggering mechanisms.
[en] An investigation of the 2D toroidal eigenmode problem reveals the possibility of a new consistent 2D structure, the dissipative BM-II mode. In contrast to the conventional ballooning mode, the new mode is poloidally localized at π/2 (or -π/2), and possesses significant radial asymmetry. The radial asymmetry, in turn, allows the dissipative BM-II to generate considerably larger Reynolds stress as compared to the standard slab drift type modes. It is also shown that a wide class of localized dissipative toroidal modes are likely to be of the dissipative BM-II nature, suggesting that at the tokamak edge, the fluctuation generated Reynolds stress (a possible source of poloidal flow) can be significant
[en] The GRENOUILLE traces of FLAME Probe line pulses (60 mJ, 10mJ after compression, 70 fs, 1 cm FWHM, 10 Hz) were acquired in the FLAME Front End Area (FFEA) at the Laboratori Nazionali di Frascati (LNF), Instituto Nazionale di Fisica Nucleare (INFN). The complete characterization of the laser pulse parameters was made using a new algorithm: GRenouille/FrOG (GROG). A characterization with a commercial algorithm, QUICKFrog, was also made. The temporal and spectral parameters came out to be in great agreement for the two kinds of algorithms. In this experimental campaign the Probe line of FLAME has been completely characterized and it has been showed how GROG, the developed algorithm, works as well as QuickFrog algorithm with this type of pulse class.
[en] Kink modes are investigated in pedestal for shaped tokamaks. An analytic combining criterion is presented. It lies on the middle of the sufficient criterion of Lortz and necessary criterion of Mercier giving a more restricted necessary criterion. Growth rates and mode structure are calculated. For large poloidal mode number, the modes are highly localized in both poloidal and radial directions. The modes increase rapidly when they approach to the resonant surface. They are typical of edge localized modes (ELMs). It is assumed that the modes vanish inside the next resonant surface, then, there seems to be a second stable region. Several mitigation methods for controlling ELMs are proposed
[en] Several novel theoretical results related to L → H transition physics, VH-mode evolution, Edge Localized Modes and active confinement control are presented. Critical issues are identified, results are discussed and important unresolved questions are listed. The basic physics is discussed in the contexts of current experiments and of ITER
[en] Characteristics of a complex-conjugate impedance antenna system for ion cyclotron resonance frequency (ICRF) heating and current drive (H and CD) are discussed in this paper. Large RF power is reflected in such transitions as that of ELMy H-L mode due to the large change in plasma resistance during the ICRF H and CD. In such a case the RF power injection to the plasma must be ceased in order to protect the tetrode vacuum tubes. The idea of a complex-conjugate impedance antenna system to mitigate the large reflected RF power, referred to as ELM tolerance, has recently been proposed. It has been proved that the reflected RF power fraction can be reduced even during a large change in plasma resistance. But it is not reduced to the level allowable for the tetrode tubes. Therefore, an improved system adding a single stub tuner is proposed. Using it, the reflected RF power fraction can be reduced to the allowable level. Numerical calculations and experiments are carried out for this system. It is found that the experimental data agree with numerical calculations.
[en] The impact of the sheath evolution during edge localized modes (ELMs) on the ion impinging energy on the divertor target is studied by particle-in-cell simulation. A significant increase of the ion impact energy is obtained at the beginning of ELM. The arrival of the upstream bulk plasma leads to a suppression of the potential drop in front of the divertor target. The peak value of the ion impact energy emerges after the maximum potential drop in front of the divertor target. It is found that the peak value of the ion impact energy is increased by about 30% compared to the ion initial energy, which is induced by the potential drop during ELMs. (letter)
[en] Experiments in a number of conventional aspect ratio tokamaks have been successful in pacing edge localized modes (ELMs) by rapid vertical jogging of the plasma. This paper demonstrates the first pacing of ELMs in a spherical torus plasma. Applied 30 Hz vertical jogs synchronized the ELMs with the upward motion of the plasma. 45 Hz jogs also lead to an increase in the ELM frequency, though the synchronization of the ELMs and jogs was unclear. A reduction in the ELM energy was observed at the higher driven ELM frequencies.
[en] The local E × B shear has been demonstrated to be correlated with control of the 3D structure of turbulence in the edge transport barrier (pedestal) on EAST with edge-localized modes (ELMs) suppressed by externally applied 3D magnetic fields. The toroidal variations in turbulence intensity, spectrum width and eddy tilting are found to be correlated with the local E × B shear, and may thus provide the potential mechanism for 3D-field-induced non-axisymmetric E × B shear in regulation of the turbulence. These new findings may significantly advance the understanding of pedestal transport and structure with applied 3D fields for ELM control, which is critical for next-step fusion development. (paper)