Machine design and electron beam control of a single-pass linac for free electron laser : the FERMI@Elettra case study

In classical electromagnetism, a charged particle radiates energy in the form of electromagnetic radiation when it is subject to a force. This effect is the principle behind many useful sources of radiation such as electron synchrotrons and linear accelerators. The main figures of merit of synchrotron radiation sources are (narrow) spectral bandwidth, photon wavelength tunability and brilliance. The periodic motion of particles in a synchrotron makes these machines well-suited for a stable emission at high repetition rate. However, in addition to the synchrotron radiation and complementary to that, a strong need has emerged over the last few years for a source of radiation with extremely high brilliance, close to full coherence, a bandwidth approaching the Fourier limit and a stable and well characterized temporal structure in the femtosecond time domain. Such a source is the single-pass Free Electron Laser (FEL) that, due to Doppler frequency upshifting of emitted radiation by relativistic electrons, is particularly well-suited to generate short wavelength X-ray pulses with peak brilliance many orders of magnitude higher than that generated in modern synchrotrons and with subpicosecond pulse lengths. There are currently no alternative sources that have such high pulse energies and short durations. The investigation domain opened by the new FEL sources covers essentially all basic science fields giving access to explorations of matter in practically unexplored regimes. The scientific opportunities will in fact impact studies of a large number of disciplines encompassing material and biomaterial science, nanoscience, plasma physics, molecular and cluster femto- and nano- physics and chemistry, as well as having various connections to life, environmental, astrophysical and earth science. The FEL high brilliance, high intensity and shot-to-shot stability strongly depends on the electron beam source. Delivering a high quality electron beam and machine flexibility to serve a broad range of potential applications imposes severe requirements on the final electron beam parameters and the machine design. To meet these requirements, the need of a linac design based on extensive studies of possible perturbations that may affect the electron beam dynamics, of means to correct them and of parameter optimization has emerged.

The Direction of Technical Change in Capital-Resource Economies

We analyze a multi-sector growth model with directed technical change where man-made capital and exhaustible resources are essential for production. The relative profitability of factor-specific innovations endogenously determines whether technical progress will be capital- or resource-augmenting. We show that convergence to balanced growth implies zero capital-augmenting innovations: in the long run, the economy exhibits purely resource-augmenting technical change. This result provides sound microfoundations for the broad class of models of exogenous/endogenous growth where resource-augmenting progress is required to sustain consumption in the long run, contradicting the view that these models are conceptually biased in favor of sustainability.Endogenous Growth; Directed Technical Change; Exhaustible Resources; Sustainability

On algebraic TVD-VOF methods for tracking material interfaces

We revisit simple algebraic VOF methods for advection of material interfaces based of the well established TVD paradigm. We show that greatly improved representation of contact discontinuities is obtained through use of a novel CFL-dependent limiter whereby the classical TVD bounds are exceeded. Perfectly crisp numerical interfaces are obtained with very limited numerical atomization (flotsam and jetsam) as compared to previous SLIC schemes. Comparison of the algorithm with accurate geometrical VOF shows larger error at given mesh resolution, but comparable efficiency when the reduced computational cost is accounted for

A Hybrid Drift Diffusion Model: Derivation, Weak Steady State Solutions and Simulations

In this paper we derive a new hybrid model for drift di usion equations. This model provides a description of the quantum phenomena in the parts of the device where they are relevant, and degenerates to a semiclassical model where quantum e ects are negligible, so that the system can be considered classically. The study of quantum correction to the equation of state of an electron gas in a semiconductor with the assumption of localized quantum e ects leads to a further condition on the classical-quantum interface. Moreover, we prove the existence of weak solutions for our hybrid model. Finally, we present numerical results for di erent devices, by means of Colsys software