No abstract

An artificial neural network, combining signals from a large number of plasma diagnostics, was used to estimate the high- beta disruption boundary in the DIII-D tokamak. It is shown that inclusion of many diagnostic measurements results in a much more accurate prediction of the disruption boundary than that provided by the traditional Troyon limit. A trained neural network constitutes a non-linear, non-parametric model of the disruption boundary. Through the analysis of the input-output sensitivities, the relative statistical significance of various diagnostic measurements (plasma parameters) for the determination of the disruption boundary is directly assessed and the number of diagnostics used by the neural network model is reduced to the necessary minimum. The neural network is trained to map the disruption boundary throughout most of the discharge. As a result, it can predict the high- beta disruption boundary on a time-scale of the order of 100 ms (much longer than the precursor growth time), which makes this approach ideally suitable for real time application in a disruption avoidance scheme. Owing to the relative simplicity of the required computations, the neural network is easily implemented in a real time system. A prototype of the neural network disruption alarm was installed within the DIII-D digital plasma control system, and its real time operation, with a typical time resolution of 10 ms, was demonstrated

The noninductive part of the measured current profile has been determined for DIII-D plasmas. A technique for determining the flux surface average of the quantity K~B and a model for the resistivity separates the current profile into inductive and noninductive portions. Analysis (1) where the cylindrical coordinate system (R, Z, @) is used, P = -fo BzR'dR' is the total poloidal flux per radian inside a major radius R, and F(p) = RB~. Furthermore, it is assumed that toroidally symmetric, nested fiux surfaces exist and can be labeled by either i/I or the en- gives a measurement at a fixed point.In a general toroidal geometry, the equation relating the current density to the electric field is (i B) = n '(E B) + (iNi B), (2) where (A) is the flux surface average of A, 71 is the parallel resistivity, and jNi represents any sources of noninductive current drive (including both bootstrap current and auxiliary driven currents). The flux surface average of the quantity (E B) can be shown to be [6] (E B) =

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Articles you may be interested inMeasurements of the internal magnetic field using the B-Stark motional Stark effect diagnostic on DIII-D (invited)a) Rev. Sci. Instrum. 81, 10D729 (2010); 10.1063/1.3491209Motional Stark effect diagnostic expansion on DIII-D for enhanced current and E r profile measurements Rev.Motional Stark effect polarimetry for a current profile diagnostic in DIIID (abstract) Rev. Sci.The motional electric field E= v x B, where v is the velocity and B is the tokamak magnetic field, produces a strong Stark effect in spectral lines emitted by hydrogenic neutral beams. The tilt angle of the magnetic field line, a quantity related directly to the distribution of the plasma toroidal current, is deduced from a measurement of the direction of polarization of the Stark components. In the DIII-D tokamak, the Balmer-cY line of deuterium emitted by one of the high-power heating beams is analyzed. A multichord polarimeter measures the magnetic field pitch angle at eight spatial locations covering -0.6 of the nominal plasma diameter at the midplane outboard side. The diagnostic offers 2-g-cm resolution in the major radius and 1-ms integration time. The accuracy of the measurement of the polarization direction necessary for an adequate reconstruction of the current profiles is obtained with the use of active polarizing elements which produce high-frequency intensity modulation with an amplitude related to the direction of linear polarization of the plasma radiation. The current profiles in highly shaped (noncircular) plasmas cannot be determined solely from the tilt angle measurements because they do not provide any information about the shape of magnetic surfaces. Thus, the polarization measurements are used in conjunction with a large set of external magnetic measurements (magnetic field and flux probes, diamagnetic loops, and Rogowski coils) by the magnetic field equilibrium code EFIT, and provide a constraint on the possible solutions for the current profile.