The authors have developed AGORA (Advanced Grid Observation Reliable Algorithms), a deterministic Load Flow solution that has enabled the development of automatic blackout recovery software for control centers in transmission operators. This restoration system is based on the deterministic, no iterative solution of the Load Flow equations. The no iterative nature of this Load Flow does not require an initial point and will execute full computation every time, even at the verge of voltage collapse. Based on this Load Flow solution, the authors have developed breakthrough technologies for the operation of control centers. The most important breakthrough is the automatic blackout recovery system. This system provides the user with a list of maneuvers (restoration plan) that ensure that the blackout situation will be corrected, and the system brought back to normal operation conditions in the optimal time. This plan guarantees that every maneuver is feasible and will improve the system conditions. This restoration plan is dynamic. The plan will be recalculated if the system is changed or the operator decides to take a different path, both cases invalidating the actual plan.

Design of a dedicated parallel computer for spin models featuring 150 ps per spin update range and lattice configuration of 224 spins is described. This computer can process various classes of Spin Glass models up to third order neighbours and three dimensions. Processing elements of the simplified prototype version are based on Programmable Logic Device (PLD) technology while the final machine will benefit by custom devices. Prototype machine is expected to work in Summer 1996, the final computer one year later. The computer is designed in collaboration of Spanish and Italian universities.

We present antiferromagnetism as a mechanism capable of modifying substantially the phase diagram and the critical behaviour of statistical mechanical models. This is particularly relevant in four dimensions, due to the connection between second order transition points and the continuum limit as a quantum field theory. We study three models with an antiferromagnetic interaction: the Ising and the 0(4) Models with a second neighbour negative coupling, and the RP 2 Model. Different conclusions are obtained depending on the model.

The use of last generation Programmable Electronic Components makes possible the construction of very powerful and competitive special purpose computers. We have designed, constructed and tested a three-dimensional Spin Glass model dedicated machine, which consists of 12 identical boards. Each single board can simulate 8 different systems, updating all the systems at every clock cycle. The update speed of the whole machine is 217ps/spin with 48 MHz clock frequency. A device devoted to fast random number generation has been developed and included in every board. The on-board reprogrammability permits us to change easily the lattice size, or even the update algorithm or the action. We present here a detailed description of the machine and the first runs using the Heat Bath algorithm.

We have simulated, using parallel tempering, the three-dimensional Ising spin glass model with binary couplings in a helicoidal geometry. The largest lattice $(L=20)$ has been studied using a dedicated computer (the SUE machine). We have obtained, measuring the correlation length in the critical region, strong evidence for a second-order finite-temperature phase transition, ruling out other possible scenarios like a Kosterlitz-Thouless phase transition. Precise values for the \ensuremath{\nu} and \ensuremath{\eta} critical exponents are also presented.

We consider a microcanonical local algorithm to be applied on the ±J spin glass model. We have compared the results coming from a microcanonical Monte Carlo simulation with those from a canonical one: Thermalization times, spin glass susceptibilities and Binder parameters. For a fixed lattice size we found different results between the two thermodynamic ensembles, which tend to vanish at bigger volumes. Moreover, microcanonical thermalization times are longer than the canonical ones. Finally we have checked that one of the Guerra relations is satisfied with good precision for the two largest lattices.

We present here a new MC study of ISB at finite temperature in a Z2 × Z2 λφ4 model in four dimensions. The results of our simulations, even if not conclusive, are favourable to ISB. Detection of the effect required measuring some critical couplings with six-digit precision, a level of accuracy that could be achieved only by a careful use of FSS techniques. The gap equations for the Debye masses, resulting from the resummation of the ring diagrams, seem to provide a qualitatively correct description of the data, while the simple one-loop formulae appear to be inadequate.

Is it always true that more heat means more disorder? The intuitive answer to this question would be yes and indeed this is what happens in the majority of physi- cal systems. Nevertheless, over 20 years ago, S. Wein- berg (1), quoting an unpublished remark by S. Coleman, observed that there may be exceptions to the general rule: in models with a sufficiently rich scalar sector, some of the scalars may acquire a negative Debye mass, with the result that the symmetric vacuum becomes neces- sarily unstable at high temperatures. This remarkable phenomenon is called Inverse-Symmetry-Breaking (ISB) or Symmetry-Non-Restoration (SNR) depending on the symmetry of the ground state at zero temperature. The possibility of ISB and SNR in realistic particle models and their cosmological consequences have been explored in a number of papers (2). Weinberg's analysis of ISB and SNR was based on a simple one-loop approximation and some authors have questioned its reliability. Subse- quent studies, using different approximations with vari- ous amounts of non-perturbative content, produced con- tradictory results. While some concluded that ISB and SNR cannot occur (3), others found that they do occur (4), even though (in the majority of the cases) in a region of the parameter space significantly reduced with respect to the one-loop result . In this paper we present the first Monte Carlo study of ISB in 4 dimensions in a two-scalar model, with a global Z2×Z2 symmetry. The job was mainly carried out on our RTNN computer, which holds 32 PentiumPro processors, for a total CPU time of approximately two months of the whole machine. The results were analogous to those found in 2+1 dimensions (5) and seem to show that ISB is absent for certain values of the renormalized couplings in the region for which perturbation theory predicts it, in accordance with the theorem proven in (6). The model we simulated is described by the bare (eu- clidean) action:

IEEE 1355 HS-Links and their support devices have been investigated as part of the ESPRIT projects Macramé and Arches. A description of the HS-Link technology and initial experience with the RCube 8-way packet router and the Bullit HS-Link interface device are presented. A 64 node HS-Link switching network based using these devices is being constructed at CERN. We report on the the design and construction of the network testbed.

We carry a Monte Carlo study of the coupled two-scalar lambda phi(1)(2) phi(2)(2) model in three dimensions. We End no trace of inverse symmetry breaking in the region of negative A's for which the one-loop effective potential predicts this phenomenon. Moreover, for lambda's negative enough, but still in the stability region for the potential, one of the transitions turns out to be of first order, both for zero and finite temperature. (C) 1998 Elsevier Science B.V.

We present antiferromagnetism as a mechanism capable of modifying substantially the phase diagram and the critical behaviour of statistical mechanical models. This is particularly relevant in four dimensions, due to the connection between second order transition points and the continuum limit as a quantum field theory.We study three models with an antiferromagnetic interaction: the Ising and the O(4) Models with a second neighbour negative coupling, and the RP(2) Model. Different conclusions are obtained depending on the model.

The φ34 model at finite temperature is simulated on the lattice. For fixed Nt we compute the transition line for Ns → ∞ by means of finite size scaling techniques. The crossings of a renormalization group trajectory with the transition lines of increasing Nt give a well-defined limit for the critical temperature in the continuum. By considering different RG trajectories, we compute Tc/g as a function of the renormalized parameters.

We present a parallel machine, based on programmable devices, dedicated to simulate spin glass models with Z2 variables and short-range interaction. A working prototype is described for two lattices containing 312 × 312 spins each with an update time of 50 ns per spin. The final version of the three-dimensional parallel machine is discussed with a spin update time up to 312 ps.

We study the phase diagram of the four dimensional Ising model with first and second neighbour couplings, especially in the antiferromagnetic region, by using Mean Field and Monte Carlo methods. From the latter, all the transition lines seem to be first order except that between ferromagnetic and disordered phases in a region including the first-neighbour Ising transition point.

An interface board capable to connect the PCI bus to two serial links at 1 Gbit/s each is presented. That PCI-HSLink board has been developed at CERN as a component of a testbed for the distribution and analysis of high energy data from new colliders. By using those boards and appropriate cross-link switches in our cluster of 16 Pentium Pro dual nodes (RTNN), a high performance, low cost full parallel machine is achieved.