Institute of Fundamental Technological Research

The underlying mechanism of low-frequency oscillations in Hall accelerators is investigated theoretically. It is shown that relaxation oscillations arise from a competition between avalancheionization and the advective transport of the working gas. The model derived recovers the slow progression and fast recession of the ionization front. Analytical approximations of the shape of current pulses and of the oscillation frequency are provided for the case of large amplitude oscillations.

Electrospinning is based on so-called bending instability which results in an erratic spiralling motion of the liquid jet as it proceeds towards a collecting electrode, where it is eventually de posited as a mat of micro/nanosized fibres. Most electrospinning models formulated within the slender approximation rely, however, on an inconsistent description of electrostatic interactions which renders them grossly inappropriate whenever the discretization is either too coarse or too fine. The present work aims at proposing a discrete slender model which is numerically consistent (allowing use of arbitrary fine meshes) and remains accurate even for coarse meshes. At the same time, efficient numerical techniques based on hierarchical charge clustering are introduced that drastically decrease computational times. Finally, a versatile boundary value method is implemented to enforce fixed-potential boundary conditions, allowing realistic electrode configurations to be investigated.

Electrospinning numerical model, boundary value method, bending instability

Very thin liquid jets can be obtained using electric field, whereas an electrically-driven bending instability occurs that enormously increases the jet path and effectively leads to its thinning by very large ratios, enabling the production of nanometre size fibres. This mechanism, although it was discovered almost one century ago, is not yet fully understood. In the following study, experimental data are collected, with the dual goal of characterizing the electro-spinning of different liquids and evaluating the pertinence of a theoretical model.

nanofibres, electrospinning, polymer fibres, electrified liquid jet

A fast discrete model for the simulations of thin charged jets produced during the electrospinning process is derived, based on an efficient implementation of the boundary element method for the computation of electrostatic interactions of the jet with itself and with the electrodes. Short-range electrostatic forces are evaluated with slender-body analytical approximations, whereas a hierarchical force evaluation algorithm is used for long-range interactions. Qualitative comparisons with experiments is discussed.

modelling electrospinning, electrostatic interactions, BEM

Using the fluid equations of Hall thruster plasma we analyze the influence of the electron energy balance on the stability of ion sound modes. For frequencies lower than ωc = 107 s−1 the gains and losses in the source term are approximately equal, thus the temperature can be in principle determined in terms of other dependent variables. This permits to reduce the number of equations. It appears however, that the new system can have complex characteristics in some regions. This in turn implies instability of certain modes with frequencies lower than ωc.

Energy balance, Acoustic analysis, Fluid equations, Instability analysis, Plasma instabilities