Common Features of Core Electron-Root Confinement in Helical Devices

Abstract

The characteristics of core electron-root confinement (CERC) in helical devices are illustrated using results from the four different experiments, CHS, LHD, TJ-II and W7-AS. Common features include strongly peaked electron temperature profiles and large positive radial electric fields, E r , in the core region for discharges with sufficient central electron cyclotron heating (ECH). Such observations are consistent with a transition to the "electron-root" solution of the ambipolarity condition for E r , a feature of neoclassical theory which is unique to non-axisymmetric configurations. The magnetic topology of the configuration plays a role in this transition and thresholds are found for the particle density and ECH power, in accordance with neoclassical expectations. Neoclassical theory alone cannot explain all observations, however, as CERC formation can also be influenced by ECH-driven convective fluxes of localized electrons and by the presence of magnetic islands in the core region. This is the first report describing collaborative activities within the framework of the International Stellarator Profile Data Base.2

Formation of Electron-Root Radial Electric Field and its Effect on Thermal Transport in LHD High <i>T</i>_ePlasma

“…The formation of the electron internal transport barrier (eITB) observed in many helical devices [1,2] is promising for the better confinement. High electron temperature (T e ) plasmas followed by the formation of the eITB are called CERC (Core Electron-Root Confinement) plasmas since they have a large positive (electron-root) radial electric field (E r ) at the plasma core.…”

“…A neoclassical transport code, FORTEC-3D, which was used in that study, has been developed with the aim to apply to such high T e plasmas. FORTEC-3D code has features below: (1) it involves the electron FOW effect in following the orbit of (marker) particles with the δ f Monte Carlo approach, (2) it can be applied to arbitrary magnetic field configurations such as tokamaks and helical/stellarator plasmas when the field is expressed in Boozer coordinates [8], (3) the collision operator for like-particles satisfies the elementary conservation laws for the particle number, the momentum, and the total energy, and (4) the steady-state ambipolar E r is determined self-consistently by the ambipolar condition of Eq. (4) shown later.…”

Momentum correction techniques for neoclassical transport in stellarators

“…These simulations are performed with 10 MW ECRH in the extra-ordinary mode (X2-mode) with high absorption [36]. For both scenarios, the highly localised central power deposition results in an "electron-root" feature [33,37] with positive radial electric fields, with steep electron temperature gradients and with flat ion temperature profiles (rather weak collisional coupling even at i r x s j mpt density). Due to the large ripple, the "electron-root" is more pronounced in the "high-mirror" configuration whereas the electron neoclassical confinement is degraded at larger radii, u k v w x y x i ÿ m. Also for this example, the ion parallel flow is very small with respect to the ion thermal velocity.…”

A New Simulation Method of Geodesic Acoustic Mode in Toroidal Plasmas by Using Band-Limited White Noise in a δf Neoclassical Transport Code

“…In the recent LHD experiments, however, core electron temperature becomes very high in Core Electron-Root Confinement (CERC) plasmas, 15) and positive ambipolar radial electric field is considered to play an important role to achieve good electron confinement. Since the T e gradient in CERC is very steep and trapped electrons in such high-T e plasmas is insensitive to Coulomb collisions and can drift widely in radial direction, it is concerned that the FOW effect on electron neoclassical transport in CERC plasmas would not be negligible.…”