Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas

In this work, we examine the validity of several common simplifying assumptions used in numerical neoclassical calculations for nonaxisymmetric plasmas, both by using a new continuum drift-kinetic code and by considering analytic properties of the kinetic equation. First, neoclassical phenomena are computed for the LHD and W7-X stellarators using several versions of the drift-kinetic equation, including the commonly used incompressible-ExB-drift approximation and two other variants, corresponding to different effective particle trajectories. It is found that for electric fields below roughly one third of the resonant value, the different formulations give nearly identical results, demonstrating the incompressible ExB-drift approximation is quite accurate in this regime. However, near the electric field resonance, the models yield substantially different results. We also compare results for various collision operators, including the full linearized Fokker-Planck operator. At low collisionality, the radial transport driven by radial gradients is nearly identical for the different operators, while in other cases it is found to be important that collisions conserve momentum

Transport of injected impurities in Heliotron E

No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in part by or for the United States govern-ment is permitted. By acceptance of this article, the publisher and/or recipient ac-knowledges the U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright covering this paper

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$^{19}$ m$^{-3}$ 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$^{20}$ m$^{-3}$. CW ECRH system is successfully operated to sustain 756 s discharge.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

Long-Pulse Heating and Plasma Production by Neutral Beam Injection in Large Helical Device

Abstract We have achieved long-pulse NBI heating in Large Helical Device (LHD). A quasi-steadystate plasma was sustained for 21 sec with an injection power of 0.6 MW, where the central plasma temperature was around 1 keV with a line-averaged electron density of 0.3 x 10 19 m -3 . At higher densities, relaxation oscillation phenomena were observed for 20 sec at a period of 1 -3 sec. We have also achieved plasma production by NBI alone. The density build-up time was several hundreds msec, and the produced plasma showed the same characteristics as an ECH-initiated plasma. These results indicate unique characteristics of LHD where only external superconducting coils generate the plasma confinement magnetic field. Introduction Negative-ion-based neutral beam injection (NBI) heating started in September, 1998, in Large Helical Device (LHD), which is the world largest superconducting helical system On the other hand, plasma initiation by ECH is a unique characteristic in the helical system producing currentless plasmas, and neutral beams are usually injected into the ECHproduced target plasmas as an additional heating. However, since the ECH utilizes a given EC resonance magnetic field, the magnetic field strength is restricted in experiments to the resonance field. We achieved NBI-initiated plasma production for the first time, which gives no experimental constraint on the magnetic field strength. In this paper, we focus on the results of both long-pulse NBI plasma heating and plasma production by NBI itself

Initial physics achievements of large helical device experiments

The Large Helical Device (LHD) experiments [O. Motojima, et al., Proceedings, 16th Conference on Fusion Energy, Montreal, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 3, p. 437] 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 (R = 3.9 m, a = 0.6 m), 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 (NBI) has produced plasmas with a fusion triple product of 8 X 10^18 keV m^3 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 = 0.7%, with neither unexpected confinement deterioration nor visible magnetohydrodynamics (MHD) 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