Current Control by ECCD for W7-X Turkin, Yu; Maassberg, H.; Beidler, C. D. ...
Fusion science and technology,
10/2006, Volume:
50, Issue:
3
Journal Article
Peer reviewed
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.
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Electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) are used to heat the plasma, to tailor the current profiles and to achieve different operating regimes of ...tokamak plasmas. Plasmas with ECRH/ECCD are characterized by non-thermal electrons, which cannot be described by a Maxwellian distribution. Non-thermal electrons are also generated during MHD activity, like sawteeth crashes. Quantifying the non-thermal electron distribution is therefore a key for understanding EC heated fusion plasmas. For this purpose a vertical electron cyclotron emission (V-ECE) diagnostic is being installed at TCV. The diagnostic layout, the calibration, the analysis technique for data interpretation, the physics potentials and limitations are discussed.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
ECRH and ECCD scenarios for W7-X Marushchenko, N. B.; Beidler, C. D.; Erckmann, V. ...
EPJ Web of conferences,
01/2012, Volume:
32
Journal Article
Peer reviewed
Open access
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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The radial neoclassical fluxes of electrons in the 1/nu-regime are calculated with relativistic effects taken into account and compared with those in the non-relativistic approach. The treatment is ...based on the relativistic drift-kinetic equation with the thermodynamic equilibrium given by the relativistic Maxwell-Juttner distribution function. It is found that for the range of fusion temperatures, T sub(e) < 100 keV, the relativistic effects produce a reduction of the radial fluxes which does not exceed 10%. This rather small effect is a consequence of the non-monotonic temperature dependence of the relativistic correction caused by two counteracting factors: a reduction of the contribution from the bulk and a significant broadening with the temperature growth of the energy range of electrons contributing to transport. The relativistic formulation for the radial fluxes given in this paper is expressed in terms of a set of relativistic thermodynamic forces which is not identical to the canonical set since it contains an additional relativistic correction term dependent on the temperature. At the same time, this formulation allows application of the non-relativistic solvers currently used for calculation of mono-energetic transport coefficients.
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.
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The electron cyclotron current drive (ECCD) with extraordinary mode at the second cyclotron harmonic launched from the high-field-side (HFS) is investigated in W7-AS stellarator. Trapped particle ...effects and quasi-linear effects are taken into account in linear ray-tracing and quasi-linear bounce-averaged Fokker–Planck computations of the driven current. The results for both low-field-side (LFS) and HFS injection in W7-AS are compared with the experimental results. A slightly lower current drive efficiency is obtained, both theoretically and experimentally, in the case of HFS injection, confirming the LFS launch as the preferred mode of operation for ECCD.
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