Francisco Castejón

Distributed and asynchronous solver for large CPU intensive problems

Solving large-scale scientific problems represents a challenging and large area in numerical optimisation. Devoted techniques may improve the results achieved for these problems. We aimed to design an specific optimisation technique for these problems. In this case, a new swarm-based algorithm based on bees foraging behaviour is presented. This system must rely on large computing infrastructures that present specific characteristics. We designed this algorithm for being executed on the grid. The resulting algorithm improves the results obtained for the large-scale problem described in the paper by other algorithms. It also delivers an optimal usage of the computational resources. This work represents one of the few evidences for solving real large-scale scientific problems with a devoted algorithm using large and complex computing infrastructures. We show the capabilities of this approach when solving these problems.

Grid Computing for Fusion Research

This work deals with the problems solved in fusion research by means of grid computing. The computing necessities for fusion are discussed and the applications that have been ported to grid as well as their main physical results are described. The range of plasma physics research covered by this set of tools is analysed and the future of grid computing for fusion research is discussed. The possibility of establishing complex workflows between grid and high performance computers (HPC) applications has been also explored.

Stellarator optimization under several criteria using metaheuristics

A new algorithm based on metaheuristics has been developed to perform stellarator optimization. This algorithm, which is inspired by the behaviour of bees and is called distributed asynchronous bees, has been used for the optimization under three criteria: minimization of B × grad(B) drift, Mercier and ballooning stability. This algorithm is tested by partially optimizing TJ-II and, afterwards, a three-period optimized configuration is found by performing a full optimization that starts from a three-period heliac. (Some figures may appear in colour only in the online journal)

Simulations of fast ions distribution in stellarators based on coupled Monte Carlo fuelling and orbit codes

The numerical simulation of the dynamics of fast ions coming from neutral beam injection (NBI) heating is an important task in fusion devices, since these particles are used as sources to heat and fuel the plasma and their uncontrolled losses can damage the walls of the reactor. This paper shows a new application that simulates these dynamics on the grid: FastDEP. FastDEP plugs together two Monte Carlo codes used in fusion science, namely FAFNER2 and ISDEP, and add new functionalities. Physically, FAFNER2 provides the fast ion initial state in the device while ISDEP calculates their evolution in time; as a result, the fast ion distribution function in TJ-II stellerator has been estimated, but the code can be used on any other device. In this paper a comparison between the physics of the two NBI injectors in TJ-II is presented, together with the differences between fast ion confinement and the driven momentum in the two cases. The simulations have been obtained using Montera, a framework developed for achieving grid efficient executions of Monte Carlo applications.

Performance improvements for the neoclassical transport calculation on Grid by means of pilot jobs

The neoclassical transport is a lower limit of the whole transport in plasmas confined in fusion devices, either stellarators or tokamaks. Even more, the determination of a vast database compiling monoenergetic and transport coefficients is very useful for coupling different codes, which can use those values as input data. The DKEsG application is able to obtain such parameters on Grid infrastructures. Since a large number of regular jobs are needed for filling the aforementioned database, a fast and robust execution scheme is necessary. For this purpose, a new DRMAA-enabled DKEsG version that makes use of a new generic pilot-job platform is used, avoiding the most significant overheads related to standard Grid middleware. This new developed mechanism is suitable for many other scientific applications involving high-throughput calculations.

FAFNER2: A comparison between the Grid and the MPI versions of the code

FAFNER2 is a 3D code that simulates by Monte Carlo methods the Neutral Beam Injection (NBI) technology. The original version was implemented for shared memory computers with MIPS proccesors, so an update to be executed by means of MPI on standard Linux clusters has been carried out as well as a new version to be run on Grid. To do the latest, a serial version has also been developed, together with a Java DRMAA program that is submitted by the GridWay metascheduler. As a result, two new improved versions of the code (HPC and Grid) are available.

Influence of magnetic well on electromagnetic turbulence in the TJ-II stellarator

A magnetic well scan has been performed in the TJ-II stellarator to investigate the confinement properties with different values of the well, or even of the hill, and to explore the properties of electromagnetic turbulence. Stable plasmas have been obtained in theoretically Mercier-unstable configurations, and the electrostatic turbulence levels in the edge are increased. Three families of modes appear during the experiments: (1) a family of modes of Alfvenic nature with high frequencies; (2) a second set of modes of middle frequencies (tens of kHz) and (3) an oscillation at f ≈ 10–20 kHz happens in several cases. In spite of the fact that the vacuum rotational transform is very similar in all of the cases, the Alfvenic mode family changes drastically when decreasing the magnetic well, showing a non-monotonic behaviour of the amplitude, and a decrease of the typical frequencies. This behaviour cannot be explained only by current or density variations, so the effect of the modification of the configuration is playing a key role. Regarding the intermediate frequencies, a coherent mode appears with decreasing frequency as the magnetic well decreases. This mode is a candidate for a GAM, which can survive in these TJ-II plasmas, despite of the strong damping these modes should suffer in this device.

A New Numerical Technique for the Inverse Problem in Thomson Scattering and Its Application to Experimental and Synthetic Spectra

Abstract The inverse problem for Thomson scattering (TS), that is, finding the electron distribution function (EDF), not restricted to be Maxwellian or isotropic, from the observation of the scattered spectrum, is addressed. Based on previous results by the authors, a new parallel FORTRAN code, INVERT, has been developed that allows to estimate the free parameters of a wide class of distribution functions by fitting experimental or numerical (synthetic) spectra using a variant of the simplex method. The application of these techniques to the extraction of non-Maxwellian or anisotropic features in the electron distribution function is analyzed in detail. The performance of the new code on noisy synthetic spectra and its capabilities to quantitatively discriminate among several competing EDFs modeling data are discussed. The issues of uniqueness (or nonuniqueness) of the inverse problem in case of multiparameter distribution functions are discussed. In such cases, the prospects of multiple diagnostics synthesis, or having several simultaneous scattering chords to remove the ambiguity in the reconstruction of the EDF, are also discussed. Some comments on the requirements of a TS system able to detect nonthermal or anisotropic effects are also included.

Optimization of 3D Fusion Devices

Optimization and design of nuclear fusion devices is a complex task with large computational requirements. The complexity is defined by the number of parameters involved in every single possible optimization function that focuses on the different aspects of plasma confinement. This paper presents a possible optimization of an existing nuclear fusion device. The optimization process is carried out by a parallel algorithm specifically designed to work with large scale problems. While the focus of the paper is fusion, the approach used can be applied to any other large scale problem. We have run our experiments on an HPC cluster. The results show the validity of our approach and how complex scientific problems can benefit from the outcomes of this work.

Neoclassical Transport and Iota Scaling in the TJ-II Stellarator

Abstract Neoclassical transport properties are studied in the TJ-II stellarator, taking effective ripple and plateau factor as the figures of merit. Using the DKES code run by grid computing techniques, these two quantities have been estimated as functions of rotational transform and plasma volume. The effective helical ripple increases with plasma volume and rotational transform. These findings suggest the degradation of confinement with iota or volume, which contradicts the scaling laws of energy confinement and the TJ-II experimental results. The plateau factor is almost constant with volume, but it increases following an almost quadratic law with rotational transform. This indicates that the improvement in confinement with iota cannot be explained by neoclassical transport in TJ-II.