J. A. Jimenez

Electron cyclotron heating and current drive in the TJ-II stellarator

Electron cyclotron resonance heating (ECRH) has become an attractive technique in todays fusion experiments not only as a heating mechanism but also as a diagnostic tool and a way to induce controllable currents through electron cyclotron current drive (ECCD). In this paper we show, using ray tracing simulations, how one can address all these issues with the ECRH system designed for the TJ-II stellarator. This ECRH set up consists of two 500 kW gyrotrons at f = 53.2 Ghz coupled to the plasma through two quasi-optical transmission lines, placed at two stellarator symmetric positions, and equipped with an internal steerable mirror. Using the three-dimensional (3D) Hamiltonian ray tracing code, TRECE, almost full single-pass absorption with localized power deposition is found for both on- and off-axis heating for any magnetic configuration. The simulations also show that it is possible to induce an appreciable ECCD despite the small oblique incidence angles attainable with the movable mirrors because of the rapid variation of the magnetic field direction and strength with the toroidal angle in TJ-II.

Comments on "Accelerated learning algorithm for multilayer perceptrons: optimization layer by layer"

This letter analyzes the performance of the neural network training method known as optimization layer by layer. We show, from theoretical considerations, that the amount of work required with OLL-Learning scales as the third power of the network size, compared with the square of the network size for commonly used conjugate gradient training algorithms. This theoretical estimate is confirmed through a practical example. Thus, although OLL is shown to function very well for small neural networks (less than about 500 weights per layer), it is slower than CG for large neural networks. Second, we show that OLL does not always improve on the accuracy that can be obtained with CG. It seems that the final accuracy that can be obtained depends strongly on the initial network weights.

Diagnostics for measuring equilibrium plasma β in stellarators

Abstract In stellarators, average equilibrium β can be derived from measurements of the magnetic flux produced by the Pfirsch-Schluter current. This method has been compared with the conventional diamagnetic method. The important advantages of using this method are much higher relative signal ratio and easier installation of the diagnostic in any stellarator devices.

Measurements of plasma beta in stellarators

Measurements are reported of the average plasma beta of ECRH plasmas in the TJ-IU torsatron using external flux loops. The method is based on measurements of the magnetic flux produced by the Pfirsch-Schluter current due to the plasma pressure. The most important advantages of using this method over the conventional diamagnetic method are much higher relative signal ratio (useful signal owing to plasma beta over the total background signal) and easy installation of the diagnostic in any type of stellarator device. A general description of the technique and its applications to more complicated stellarator devices like TJ-II is also presented

Vandellos: A European site for ITER

On April 17/sup th/ 2002, Spanish Government formally offered the Vandellos site as a possible European ITER site. In this work main technical site specific characteristics of the site are compared with generic requirements. It is shown that Vandellos site offers a number of technical advantages over the reference design mainly in relation with the geotechnical and seismic assumptions as well as in relation with heat sink, electrical supply and heavy transport topics.