Current Control by ECCD for W7-X Turkin, Yu; Maassberg, H.; Beidler, C. D. ...
Fusion science and technology,
10/2006, Letnik:
50, Številka:
3
Journal Article
Recenzirano
The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is optimized following a set of criteria including a rotational transform profile with low shear and minimized bootstrap current ...that must be controlled for proper functioning of the island divertor. This paper studies the compensation of residual bootstrap current by using electron cyclotron current drive (ECCD). The modeling shows that the loop voltage induced by ECCD leads to a redistribution of the current density with a diffusion time of ~2 s. The relaxation time of the total current is much longer, however - for W7-X plasma parameters the total toroidal current reaches steady state after several L/R times requiring hundreds of seconds. In order to keep the toroidal current and its profile in the acceptable range, a feed-forward or predictive control method using ECCD as actuator is proposed. The main steps are as follows: (a) calculate the bootstrap current distribution using plasma parameters measured in the online transport analysis and (b) determine and apply ECCD as needed. For the current control to work properly and to avoid long relaxation times, the reaction time of the control loop must be less than the current skin time.
The advanced adjoint approach for arbitrary collisionalities with momentum conservation in the like-particle collision is considered. The results are generally applicable for the parallel ...conductivity as well as for current drive calculations. In addition, the weakly relativistic extension of the variational principle for the Spitzer function with momentum conservation in the electron-electron collision is described. The models developed are well suited to ray-tracing calculations.
Electron-Bernstein-wave (EBW) current drive in an overdense plasma was demonstrated at the Wendelstein 7-AS stellarator for the first time. The EBWs were generated by O-X-B mode conversion. The ...relatively high current drive efficiency was consistent with theoretical predictions. The experiments provided first investigations of EBW phase space interaction for wave refractive indices much larger than unity.
The characteristics of core electron-root confinement (CERC) in helical devices are illustrated using results from four different experiments: the Compact Helical System, Large Helical Device, TJ-II, ...and Wendelstein 7-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 that is unique to nonaxisymmetric 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.
Accuracy and abilities of the electron cyclotron emission (ECE) diagnostic system planned to be installed in the Wendelstein 7-X stellarator are analysed with help of ray-tracing simulations. For the ...expected plasma parameters, the spatial resolution of the standard low-field-side (lfs) X2-mode observation scheme is estimated to be sufficiently high, ~5%. Apart from the lfs scheme, the applicability of other complementary schemes is analyzed, in particular, high-field-side (hfs) X2-mode observation. It is shown that in combination with the standard lfs scheme, the hfs scheme can be very informative for the problem of distinguishing thermal and nonthermal contributions in the ECE spectrum.
ECRH and ECCD scenarios for W7-X Marushchenko, N. B.; Beidler, C. D.; Erckmann, V. ...
EPJ Web of conferences,
01/2012, Letnik:
32
Journal Article
Recenzirano
Odprti dostop
The main ECRH scenarios for the W7-X Stellarator are described. Both X2 (low and moderate densities) and O2 scenarios (high density) have been studied. Since O2 scenario cannot be realized without ...pre-heating, transition from X2 to O2 scenarios has been discussed. Due to a lack of Ohmic transformer, only ECCD is available for compensating the bootstrap current and for controlling the edge rotational transform value. The efficiency of ECCD for all main scenarios has been estimated. All simulations have been performed by a 1D transport code coupled self-consistently with ray-tracing code.
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