ABSTRACT
In reduced recycling discharges in the Large Helical Device, a super dense core plasma develops when a series of pellets are injected. A core region with density as high as 4.5 x 10(20) m(-3) and temperature of 0.85 keV is maintained by an internal diffusion barrier with very high-density gradient. These results may extrapolate to a scenario for fusion ignition at very high density and relatively low temperature in helical devices.
ABSTRACT
A stochastic magnetic boundary, produced by an applied edge resonant magnetic perturbation, is used to suppress most large edge-localized modes (ELMs) in high confinement (H-mode) plasmas. The resulting H mode displays rapid, small oscillations with a bursty character modulated by a coherent 130 Hz envelope. The H mode transport barrier and core confinement are unaffected by the stochastic boundary, despite a threefold drop in the toroidal rotation. These results demonstrate that stochastic boundaries are compatible with H modes and may be attractive for ELM control in next-step fusion tokamaks.
ABSTRACT
Associated with the transition from ion root to electron root, an electron internal transport barrier (ITB) appears in the large helical device, when the heating power of electron cyclotron resonance heating exceeds the threshold power. The incremental thermal diffusivity of electron heat transport chi(inc)(e) in the ITB plasma is much lower than that in the plasma with the heating power below the threshold, and the thermal diffusivity chi(e) decreases with increasing of heating power [dchi(e)/d(P/n(e))<0] in helical ITB plasmas.
ABSTRACT
Sawtooth oscillations have been observed in current-carrying helical plasmas by using electron-cyclotron-emission diagnostics in the Large Helical Device. The plasma current, which is driven by neutral beam injection, reduces the beta threshold of the sawtooth oscillation. When the central q value is increased due to the plasma current, the core region crashes, and, when it is decreased, the edge region crashes annularly. Observed rapid mixture of the plasma in the limited region suggests that these sawtooth crashes are reconnection phenomena. Unlike previous experiments, no precursor oscillation has been observed.
ABSTRACT
In the Large Helical Device plasma discharges, the size of an externally imposed island with mode number ( n/m = 1/1) decreases substantially when the plasma is collisionless ( nu(*)< approximately 1) and the beta is finite ( > approximately 0.1%) at the island location. For the collisional plasmas with finite beta, on the other hand, the size of the island increases. However, there is a threshold in terms of the vacuum island size below which the island enlargement is not seen.
ABSTRACT
Radial profiles of ion temperature and plasma flow are measured at the n/m = 1/1 magnetic island produced by external perturbation coils in the Large Helical Device. The sheared poloidal flows and sheared radial electric field are observed at the boundaries of the magnetic island, because the poloidal flow vanishes inside the static magnetic island. When the width of the magnetic island becomes large, the flow along the magnetic flux surface inside the magnetic island appears around the O point in the direction which reduces the shear of the poloidal flow at the boundary of the magnetic island.
ABSTRACT
It was observed that the vacuum magnetic island produced by an external error magnetic field in the large helical device shrank in the presence of plasma. This was evidenced by the disappearance of flat regions in the electron temperature profile obtained by Thomson scattering. This island behavior depended on the magnetic configuration in which the plasmas were produced.
ABSTRACT
Recent large helical device experiments revealed that the transition from ion root to electron root occurred for the first time in neutral-beam-heated discharges, where no nonthermal electrons exist. The measured values of the radial electric field were found to be in qualitative agreement with those estimated by neoclassical theory. A clear reduction of ion thermal diffusivity was observed after the mode transition from ion root to electron root as predicted by neoclassical theory when the neoclassical ion loss is more dominant than the anomalous ion loss.
ABSTRACT
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.
ABSTRACT
In LHD discharges a significant enhancement of the global energy confinement has been achieved for the first time in a helical device with an edge thermal barrier, which exhibits a sharp gradient at the edge of the temperature profile. Key features associated with the barrier are quite different from those seen in tokamaks: (i) almost no change in particle (including impurity) transport, (ii) a gradual formation of the barrier, (iii) a very high ratio of the edge temperature to the average temperature, and (iv) no edge relaxation phenomenon. These features are very attractive in applying the thermal barrier to future reactor grade devices.
ABSTRACT
New derivatives of an intriguing marine natural product are now accessible. The first asymmetric synthesis of the simple tetrodotoxin analogue, 5,11-dideoxytetrodotoxin (3), was achieved. Hydroxylation at position C8 of the key intermediate 1 relied on the neighboring trichloroacetamide group, and stereoselective elaboration of the vinyl group gave alpha-hydroxylactone 2, which was transformed into the title compound through a new guanidylation method.
ABSTRACT
Arenastatin A, isolated from the Okinawan marine sponge Dysidea arenaria, is an antimitotic depsipeptide containing a 16-membered ring. Interaction of the compound with tubulin was investigated by the use of [3H]arenastatin A and other microtubule disruptors. Scatchard analysis indicated the presence of one binding site for arenastatin A per tubulin heterodimer with a dissociation constant (Kd) of 1.8 x 10(-6) M. Rhizoxin was a competitive inhibitor of arenastatin A binding, and vinblastine also inhibited arenastatin A binding in a partially competitive manner. Arenastatin A had no inhibitory effect on colchicine binding to tubulin.
Subject(s)
Brain/metabolism , Depsipeptides , Peptides, Cyclic/metabolism , Tubulin/metabolism , Animals , Binding, Competitive , Colchicine/metabolism , Lactones/metabolism , Macrolides , Protein Binding , Swine , Vinblastine/metabolismABSTRACT
Inhibition of microtubule assembly by arenastatin A (1) and five synthetic analogs (3-7) was examined. Arenastatin A and the triamide 6 showed potent and moderately strong inhibitory activities, respectively (IC50; 2.3 microM for 1, 7.8 microM for 6) and also depolymerized preformed microtubules. The other analogs tested showed no activity.
Subject(s)
Antineoplastic Agents/pharmacology , Depsipeptides , Microtubules/drug effects , Peptides, Cyclic/pharmacology , Tubulin/metabolism , Animals , Biopolymers , Cell Division/drug effects , Humans , KB Cells , Microscopy, Electron , Microtubules/ultrastructure , Peptides, Cyclic/chemistry , Structure-Activity Relationship , SwineABSTRACT
An efficient asymmetric synthesis of a cyclic depsipeptide arenastatis A (1) is described. 1, isolated from the marine sponge Dysidea arenaria, exhibited extremely potent cytotoxicity with IC50 of pg/ml for KB cells, and in this context the structure-activity relationship among several stereoisomers of 1 and allied compounds has also been examined.
