Electron Pressure Profiles in High-Density Neutral Beam Heated Plasmas in the Large Helical Device

Abstract

In the Large Helical Device (LHD), electron pressure profiles in gas-fueled high-density discharges tend to have a similar shape, as if these were frozen. This frozen profile is insensitive to variations in the magnetic field strength and moderate changes in the neutral beam heat deposition profile. At the same time, however, the absolute value of the electron pressure itself increases with the heating power, the electron density, and the magnetic field strength. In this study, a reference model for the electron pressure is proposed which consists of the frozen profile and parametric dependences derived from experimental observations. It is possible to define an operational regime where this typical profile appears by comparing the electron pressure profiles with this model. In the standard configuration, at which the maximum plasma stored energy in LHD has been obtained, the frozen profile appears in the plateau to the Pfirsh-Schluter regimes. As the collisionality decreases to the collisionless regime, the electron pressure becomes smaller than the prediction of the model and the deterioration is significant in the plasma core region. This tendency is enhanced in the configuration with the outward-shifted magnetic axis. The global energy confinement time, τE, in the high-collisionality regime has a weaker density dependence together with the mitigated power degradation, scaling as τE∝nebar0.28P-0.43 (nebar and P are the line-averaged density and the heating power, respectively), compared with the International Stellarator Scaling 95, where τE∝nebar0.51P-0.59.