J.L. Velasco

Janus: An FPGA-Based System for High-Performance Scientific Computing

Janus is a modular, massively parallel, and reconfigurable FPGA-based computing system. Each Janus module has one computational core and one host. Janus is tailored to, but not limited to, the needs of a class of hard scientific applications characterized by regular code structure, unconventional data-manipulation requirements, and a few Megabits database. The authors discuss this configurable systems architecture and focus on its use for Monte Carlo simulations of statistical mechanics, as Janus performs impressively on this class of application.

Ianus: an adaptive FPGA computer

With Ianus, a next-generation field-programmable gate array (FPGA)-based machine, the authors hope to build a system that can fully exploit the performance potential of FPGA devices. A software platform that simplifies Ianus programming will extend its intended application range to a wide class of interesting and computationally demanding problems.

Ion kinetic transport in the presence of collisions and electric field in TJ-II ECRH plasmas

The ion collisional transport is estimated for the TJ-II device without the assumption of radially narrow particle trajectories, which is usually made in neoclassical transport studies. One million particles are followed in a realistic TJ-II magnetic configuration, taking into account collisions and the electric field. Global features of transport, not present in the customary neoclassical models, appear. The main results reached by considering the actual particle geometries are the monotonic increasing of heat and particle fluxes with minor radius, the non-diffusive character of transport, the appearance of asymmetries on the magnetic surfaces and the non-Maxwellian character of the distribution function.

Flux-expansion divertor studies in TJ-II

The flux-expansion divertor concept is theoretically explored in TJ-II. This concept seems to be most suitable for this flexible stellarator in which the positions and the order of the resonances can be varied. As a first step, a family of configurations that exhibit flux expansion is identified. Then, a three-dimensional map of the collisional particle flux has been obtained for two different plasma regimes using the full-f Monte Carlo code ISDEP (Integrator of Stochastic Differential Equations in Plasmas), which computes the ion guiding-centre trajectories. The particle trajectories rather than the field lines must be considered since ion orbits can drift from the field lines in TJ-II and the plasma electric field and the collisionality must be considered in such calculations. We have checked that it is possible to strongly reduce the heat and particle fluxes by intercepting the trajectories at a given zone of the space. These calculations are done for thermal ions and for fast particles coming from the neutral beam injection heating. Future studies, possibly including the creation of an ergodic zone, will determine the best strategy for intercepting the trajectories.

Ion Heating in transitions to CERC in the Stellarator TJ-II.

In TJ-II stellarator plasmas, in the electron cyclotron heating regime, an increase in the ion temperature is observed, synchronized with that of the electron temperature, during the transition to the core electron-root confinement (CERC) regime. This rise in ion temperature should be attributed to the joint action of the electron–ion energy transfer (which changes slightly during the CERC formation) and an enhancement of the ion confinement. This improvement must be related to the increase in the positive electric field in the core region. In this paper, we confirm this hypothesis by estimating the ion collisional transport in TJ-II under the physical conditions established before and after the transition to CERC. We calculate a large number of ion orbits in the guiding-centre approximation considering the collisions with a background plasma composed of electrons and ions. The ion temperature profile and the thermal flux are calculated in a self-consistent way, so that the change in the ion heat transport can be assessed.

Finite orbit width effect in ion collisional transport in TJ-II

The validity of the traditional local diffusive approach and of the use of monoenergetic calculations has been studied for the stellarator TJ-II [Alejaldre et al., Fusion Technol. 17, 131 (1990)]: it is shown to be doubtful, under some circumstances, even in a purely collisional description of transport. The diffusion in physical space starting from Dirac-delta-like initial conditions has been studied using the code Integrator of Stochastic Differential Equations for Plasmas by Castejon et al. [Plasma Phys. Controlled Fusion 49, 753 (2007)]. Particles may experience large radial excursions from their original magnetic surfaces in a single collisional time. The contribution of these particles to the flux may make it nondiffusive; non-Gaussian density distributions, characterized by long tails, are observed. In the velocity space, there are important variations in the average particle kinetic energy after one collision time. We discuss the effect of this fact over the calculation of monoenergetic transport ...

Critical properties of the four-state commutative random permutation glassy Potts model in three and four dimensions

We investigate the critical properties of the four-state commutative random permutation glassy Potts model in three and four dimensions by means of Monte Carlo simulations and a finite-size scaling analysis. By using a field programmable gate array, we have been able to thermalize a large number of samples of systems with large volume. This has allowed us to observe a spin-glass ordered phase in d=4 and to study the critical properties of the transition. In d=3, our results are consistent with the presence of a Kosterlitz-Thouless transition, but also with different scenarios: transient effects due to a value of the lower critical dimension slightly below 3 could be very important.

Studies of the fast ion energy spectra in TJ-II

The dynamics of the neutral beam injection fast ions in the TJ-II stellarator is studied in this paper from both the theoretical and experimental points of view. The code Integrator of Stochastic Differential Equations for Plasmas (ISDEP) is used to estimate the fast ion distribution function in 3D:1D in real space and 2D in velocity space, considering the 3D structure of TJ-II, the electrostatic potential, non turbulent collisional transport, and charge exchange losses. The results of ISDEP are compared with the experimental data from the compact neutral particle analyzer, which measures the outgoing neutral flux spectra in the energy range E∈(1−45)u2009u2009keV.

Numerical study of the enlarged O(5) symmetry of the 3D antiferromagnetic RP2 spin model

Abstract We investigate by means of Monte Carlo simulation and finite-size scaling analysis the critical properties of the three-dimensional O ( 5 ) non-linear σ model and of the antiferromagnetic R P 2 model, both of them regularized on a lattice. High accuracy estimates are obtained for the critical exponents, universal dimensionless quantities and critical couplings. It is concluded that both models belong to the same universality class, provided that rather non-standard identifications are made for the momentum-space propagator of the R P 2 model. We have also investigated the phase diagram of the R P 2 model extended by a second-neighbor interaction. A rich phase diagram is found, where most of the phase transitions are of the first order.

The search for a flux‐expansion divertor in TJ‐II

The role of the divertor in a stellarator‐based fusion reactor is discussed, making emphasis on the flux‐expansion concept. In this context, the possibility of having a flux‐expansion divertor in TJ‐II is explored. As a first step, the three‐dimensional map of the particle flux has been estimated using the code ISDEP in two different plasma regimes. This full‐f Monte Carlo code computes the ion guiding‐centre trajectories. The particle trajectories rather than the field lines must be considered due to the fact that the typical ion orbits are pretty wide in comparison with the typical plasma lengths. Moreover the plasma electric field and the collisionality must be considered. We have chosen a configuration that presents flux expansion at given toroidal positions. We have estimated the three‐dimensional map of heat and particle fluxes and checked that it is possible to reduce them strongly by intersecting the trajectories at a given zone of the space. Future studies will include the development of ergodic ...