Impact of heat deposition profile on global confinement of NBI heated plasmas in the LHD

Abstract

Energy confinement and heat transport of net-current-free NBI heated plasmas in the large helical device (LHD) are discussed with emphasis on density and power deposition profile dependences. Although the apparent density dependence of the energy confinement time has been demonstrated in a wide parameter range in LHD, the loss of this dependence has been observed in the high density regime under specific conditions. Broad heat deposition due to off-axis alignment and shallow penetration of neutral beams degrades the global energy confinement while the local heat transport maintains a clear temperature dependence, lying between Bohm and gyro-Bohm characteristics. The central heat deposition tends towards an intrinsic density dependence like τE∝(n̄e/P)0.6 from the state where density dependence is lost. The broadening of the temperature profile due to the broad heat deposition profile contrasts with the invariant property that has been observed widely as profile resilience or stiffness in tokamak experiments. The confinement improvement as a result of the inward shift of the magnetic axis is obvious in the core region, which emphasizes the improvement of transport because of the geometry being unfavourable for the central heating of NBI in this configuration. The edge pressure, clearly, does not depend on the magnetic axis position. Unlike a tokamak H-mode, the edge pressure is determined by transport and can be increased by increasing the heating power.

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

“…It is also observed, however, that the strong positive density dependence is lost in the high-density region during the density ramp-up by gas puffing. The energy confinement time in such a case is lower than the prediction of scalings with significant density dependence [3,4].…”

“…2(b). Apparently, a higher p e profile is achieved in the central heating cases [3], although the profile shape, or the L p profile, is frozen. Such an example is shown in Fig.…”

“…On the other hand, many examples of confinement degradation in the high-density region have been reported from both of tokamak and helical plasma experiments [1][2][3][4]. Use of empirical scaling laws for the energy confinement time, τ E , is effective to identify the data deviating from the typical expectation, possibly due to a degradation or improvement of confinement.…”

Parameter Regime of Ion Internal Transport Barrier Formation in the Large Helical Device

“…The property of gyro-Bohm diffusion is observed in LHD plasmas [9]. In the most simple transport model, the heat transport is given by…”

New evidence and impact of electron transport non-linearities based on new perturbative inter-modulation analysis

“…Current literature related to LHD points into the direction of a power dependence of transport via the turbulence level [30], which has also been extensively studied at W7-AS [31,32,33]. These and other non-linear transport properties have been analyzed using perturbations (often via the heat pulse diffusivity χ HP ) at different operating points and comparing them to each other and to steadystate results (power balance diffusivity χ P B ) [34,2]. Alternatively, model codes based on the physics are used to fit parameters of the non-linear models, e.g., [20,35].…”