Inferring test models from user bug reports using multi-objective search.

Bug reports are used by software testers to identify abnormal software behaviour. In this paper, we propose a multi-objective evolutionary approach to automatically generate finite state machines (FSMs) based on bug reports written in natural language, to automatically capture incorrect software behaviour. These FSMs can then be used by testers to both exercise the reported bugs and create tests that can potentially reveal new bugs. The FSM generation is guided by a Multi-Objective Evolutionary Algorithm (MOEA) that simultaneously minimises three objectives: size of the models, number of unrealistic states (over-generalisation), and number of states not covered by the models (under-generalisation). We assess the feasibility of our approach for 10 real-world software programs by exploiting three different MOEAs (NSGA-II, NSGA-III and MOEA/D) and benchmarking them with the baseline tool KLFA. Our results show that KLFA is not practical to be used with real-world software, because it generates models that over generalise software behaviour. Among the three MOEAs, NSGA-II obtained significantly better results than the other two for all 10 programs, detecting a greater number of bugs for 90% of the programs. We also studied the differences in quality and model performance when MOEAs are guided by only two objectives rather than three during the evolution. We found that the use of under-approximation (or over-approximation) and size as objectives generates infeasible solutions. On the other hand, using as objectives over-approximation and under-approximation generates feasible solutions yet still worse than those obtained using all three objectives for 100% of the cases. The size objective acts as a diversity factor. As a consequence, an algorithm guided by all three objectives avoids local optima, controls the size of the models, and makes the results more diverse and closer to the optimal Pareto set.

Turbulent dynamo in a collisionless plasma.

Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.

New ion-wave path in the energy cascade.

We present the results of kinetic numerical simulations that demonstrate the existence of a novel branch of electrostatic nonlinear waves driven by particle trapping processes. These waves have an acoustic-type dispersion with phase speed comparable to the ion thermal speed and would thus be heavily Landau damped in the linear regime. At variance with the ion-acoustic waves, this novel electrostatic branch can exist at a small but finite amplitude even for low values of the electron to ion temperature ratio. Our results provide a new interpretation of observations in space plasmas, where a significant level of electrostatic activity is observed in the high frequency region of the solar-wind turbulent spectra.

Exploring the thermodynamic limit of Hamiltonian models: convergence to the Vlasov equation.

We here discuss the emergence of quasistationary states (QSS), a universal feature of systems with long-range interactions. With reference to the Hamiltonian mean-field model, numerical simulations are performed based on both the original N-body setting and the continuum Vlasov model which is supposed to hold in the thermodynamic limit. A detailed comparison unambiguously demonstrates that the Vlasov-wave system provides the correct framework to address the study of QSS. Further, analytical calculations based on Lynden-Bell's theory of violent relaxation are shown to result in accurate predictions. Finally, in specific regions of parameters space, Vlasov numerical solutions are shown to be affected by small scale fluctuations, a finding that points to the need for novel schemes able to account for particle correlations.

Vlasov-Maxwell simulations of high-frequency longitudinal waves in a magnetized plasma.

The plasma response to the injection of a propagating purely electrostatic wave of finite amplitude is investigated by means of a kinetic code which solves the Vlasov equations for electrons and ions in the three-dimensional (one spatial and two in velocity, 1D2V) phase space, self-consistently coupled to the Maxwell equations. The plasma is uniformly magnetized, and the wave frequency close to the cold upper-hybrid resonance omega(0)=sqrt[omega(2)(pe)+omega(2)(ce)] is considered. Coherent structures are formed in the phase space that would be completely missed by a hydrodynamic analysis. In particular, in the early stage of the interaction, the initially unperturbed equilibrium electron distribution is strongly affected as a whole by the pump, taking a ringlike shape in the velocity plane transverse to the magnetic field. Then, a sort of instability occurs, leading to the broadening and flattening of the electron distribution.

Vlasov-Maxwell kinetic simulations of radio-frequency-driven ion flows in magnetized plasmas.

The generation of a coherent ion flow due to the injection in a plasma of a purely electrostatic wave of finite amplitude, propagating at right angle with the ambient uniform magnetic field, is investigated making use of a kinetic code which solves the fully nonlinear Vlasov equations for electrons and ions, coupled with the Maxwell equations, in one spatial and two velocity dimensions. A uniformly magnetized slab plasma is considered. The wave frequency is assumed in the range of the fourth harmonic of the ion cyclotron frequency, and the wave vector is chosen in order to model the propagation of an ion Bernstein wave. The computation of the first-order moment of the ion distribution function shows that indeed a quasistationary transverse average ion drift velocity is produced. The time evolution of the ion distribution function undergoes a "resonant" interaction of Cherenkov type, even if the plasma ions are magnetized (omega(ci)/omega(pi) approximately 0.5). During the wave-plasma interaction, the electron distribution function remains Gaussian-like, while increasing its energy content.