“…In this case, the pellet injection does not give rise to a peaked density profile although the trigger of inward pinch has been observed in other devices. 6 The decay time of the density after the pellet injection is the same as that before the injection, which may be consistent with the absence of an inward pinch. A full solid pellet penetrates to the center and forms a peaked density profile.…”
Section: Discharge Characteristics Of Currentless Plasmassupporting confidence: 69%
“…Clear evidence of further confinement improvement has also been observed. [4][5][6] However, the enhancement factor of confinement has been smaller than in large tokamaks. The reasons can be attributed to connection of the boundary where neutrals and atomic processes play an essential role and the core where high-temperature plasmas are contained.…”
Section: 2mentioning confidence: 99%
See 1 more Smart Citation
Motojima
1
,
Yamada
2
,
Komori
3
et al. 1999
Physics of Plasmas Self Cite
have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas ͑Rϭ3.9 m, aϭ0.6 m͒, which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection ͑NBI͒ has produced plasmas with a fusion triple product of 8ϫ10 18 keV m Ϫ3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to ͗͘ϭ0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics ͑MHD͒ instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.
“…In this case, the pellet injection does not give rise to a peaked density profile although the trigger of inward pinch has been observed in other devices. 6 The decay time of the density after the pellet injection is the same as that before the injection, which may be consistent with the absence of an inward pinch. A full solid pellet penetrates to the center and forms a peaked density profile.…”
Section: Discharge Characteristics Of Currentless Plasmassupporting confidence: 69%
“…Clear evidence of further confinement improvement has also been observed. [4][5][6] However, the enhancement factor of confinement has been smaller than in large tokamaks. The reasons can be attributed to connection of the boundary where neutrals and atomic processes play an essential role and the core where high-temperature plasmas are contained.…”
Section: 2mentioning confidence: 99%
Motojima
1
,
Yamada
2
,
Komori
3
et al. 1999
Physics of Plasmas Self Cite
have started this year after a successful eight-year construction and test period of the fully superconducting facility. LHD investigates a variety of physics issues on large scale heliotron plasmas ͑Rϭ3.9 m, aϭ0.6 m͒, which stimulates efforts to explore currentless and disruption-free steady plasmas under an optimized configuration. A magnetic field mapping has demonstrated the nested and healthy structure of magnetic surfaces, which indicates the successful completion of the physical design and the effectiveness of engineering quality control during the fabrication. Heating by 3 MW of neutral beam injection ͑NBI͒ has produced plasmas with a fusion triple product of 8ϫ10 18 keV m Ϫ3 s at a magnetic field of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.4 keV have been achieved. The maximum stored energy has reached 0.22 MJ, which corresponds to ͗͘ϭ0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics ͑MHD͒ instabilities. Energy confinement times, reaching 0.17 s at the maximum, have shown a trend similar to the present scaling law derived from the existing medium sized helical devices, but enhanced by 50%. The knowledge on transport, MHD, divertor, and long pulse operation, etc., are now rapidly increasing, which implies the successful progress of physics experiments on helical currentless-toroidal plasmas.
“…In contrast to the electron thermal transport barriers in tokamak plasmas, in a stellarator where the magnetic shear is negative, the electron internal transport barrier ͑ITB͒ has been observed associated with the transition from ion root ͑large neoclassical flux with a small E r ) to the electron root ͑small neoclassical flux with a large positive E r ), when the collisionality becomes low enough for the transition. [9][10][11][12] Although the mechanism of ITB formation associated with the transition from ion root to electron root has been studied, 10,11 the quantitative study of the incremental electron thermal diffusivity, e inc ͓ϭd(Q/n e )/d(ٌT e )͔ has been scarce in ITB plasmas in helical devices in spite of its importance in understanding transport in toroidal devices. 13 In the L-mode plasma, an instability, such as the electron temperature gradient mode ͑ETG͒, 14,15 often results in the sharp increase of the thermal diffusivity above the critical electron temperature gradients and determines the upper limit of the electron temperature for the available heating power.…”
Section: Introductionmentioning confidence: 99%
Ida
1
,
Inagaki
2
,
Shimozuma
3
et al. 2004
Physics of Plasmas Self Cite
Characteristics of transport in electron internal transport barriers ͑ITB͒ 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-root͒ to a large positive value ͑electron-root͒, an electron ITB appears in the Large Helical Device ͓M. Fujiwara et al., Nucl. Fusion 41, 1355 ͑2001͔͒, when the heating power of the electron cyclotron heating exceeds a power threshold. Transport analysis shows that both the standard electron thermal diffusivity, e , and the incremental electron thermal diffusivity, e inc ͑the derivative of normalized heat flux to temperature gradient, equivalent to heat pulse e ), are reduced significantly ͑a factor 5-10͒ in the ITB. The e inc is much lower than the e by a factor of 3 just after the transition, while e inc is comparable to or even higher than e 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 magnitude͒ of e inc 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.
“…13 Therefore, the radial electric field is mainly determined by the poloidal rotation. [14][15][16] The poloidal rotation (v ) as well as ion temperature (T i ) of fully ionized carbon are measured with CXS using heating neutral beam injection ͑NBI͒ in a large helical device ͑LHD͒. Because of the negative ion source of the NBI, the operating energy range of heating neutral beam is relatively high (E NBI ϭ100-180 keV/amu͒.…”
Section: Introductionmentioning confidence: 99%
Ida
1
,
Kado
2
,
Liang
3
2000
Review of Scientific Instruments Self Cite
Absolute measurements of poloidal rotation velocity with the accuracy up to 1 km/s ͑2 pm in wavelength͒ were done using charge exchange spectroscopy in a large helical device. Radial profiles of the absolute Doppler shift of charge exchange emission with a beam are obtained from spectra measured with four sets of optical fiber arrays that view downward and upward at the poloidal cross section with and without neutral beam injection. By arranging the optical fiber from four arrays close to each other at the entrance slit, the apparent Doppler shift due to aberrations of the spectrometer and due to interference of the cold component ͑the charge exchange between He-like oxygen and thermal neutrals 8 pm from the charge exchange emission with a beam͒ can be eliminated from the measurements. The measured poloidal rotation velocity is 1-3 km/s in the electron diamagnetic direction at half of the plasma minor radius.
