Samuel Marini

Breakdown of the ponderomotive approximation as an acceleration mechanism in wave-particle nonlinear dynamics

In the present analysis, we study the dynamics of charged particles submitted to the action of slowly modulated electromagnetic carrier waves. While the velocity of the particles remains smaller than the carriers phase-velocity, their dynamics is well described by a refined ponderomotive approach. The ponderomotive approach has its own validity limits well established, beyond which particles are resonantly trapped by the carrier waves. We show that under adequate conditions, the trapping mechanism places particles at an optimal relative phase with respect to the carrier for maximum acceleration. In addition to the analytical approach involved in the ponderomotive description, we use numerical simulations to validate the corresponding dynamics as well as to explore various features of the resonant trapping and acceleration.

Stationary to nonstationary transition in crossed-field devices

The previous results based on numerical simulations showed that a cold electron beam injected in a crossed field gap does not reach a time independent stationary state in the space charge limited regime [P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725 (1994)]. In this work, the effect of finite injection temperature in the transition from stationary to nonstationary states is investigated. A fully kinetic model for the electron flow is derived and used to determine the possible stationary states of the system. It is found that although there is always a stationary solution for any set of parameters, depending on the injection temperature the electron flow becomes very sensitive to fluctuations and the stationary state is never reached. By investigating the nonlinear dynamics of a characteristic electron, a theory based on a single free parameter is constructed to predict when the transition between stationary and nonstationary states occurs. In agreement with the previous numerical results, the the...

Thermal effects and space-charge limited transition in crossed-field devices

A fully kinetic model for the electron flow in a crossed field device is derived and used to determine the system stationary states. It is found that for low injection temperatures, there is a simultaneous presence of distinct stationary solutions and an abrupt transition between accelerating and space-charge limited regimes. On the other hand, for high injection temperatures, there is only a single stationary solution branch and the change between the regimes becomes continuous. For intermediate temperatures, it is then identified a critical point that separates the abrupt and continuous behaviors. It is also investigated how intrinsic space-charge oscillations may drive stationary states unstable in certain parameter regimes. The results are verified with N-particle self-consistent simulations.

Single electron dynamics in a Hall thruster electromagnetic field profile

In this work, the single electron dynamics in a simplified three dimensional Hall thruster model is studied. Using Hamiltonian formalism and the concept of limiting curves, one is able to determine confinement conditions for the electron in the acceleration channel. It is shown that as a given parameter of the electromagnetic field is changed, the particle trajectory may transit from regular to chaotic without affecting the confinement, which allows one to make a detailed analysis of the role played by the chaos. The ionization volume is also computed, which measures the probability of an electron to ionize background gas atoms. It is found that there is a great correlation between chaos and increased effective ionization volume. This indicates that a complex dynamical behavior may improve the device efficiency by augmenting the ionization capability of each electron, requiring an overall lower electron current.

Space-charge and thermal effects in relativistic crossed-field devices

In this paper, a fully kinetic theory for the relativistic electron flow in a crossed-field device is developed and analyzed. The theory takes into account self-electric, self-magnetic, and thermal effects and allows determining the final stationary state achieved by the electrons in phase-space. A number of different possible stationary modes are identified and described in detail. Particular attention is given to the study of how space charge and thermal effects affect the magnetic insulation when the external magnetic field exceeds the Hull cutoff field. In the nonrelativistic limit, it is found that there is only a single mode transition that leads to the loss of the magnetic insulation. This transition is completely independent of the electron density and occurs for relatively large injection temperatures. On the other hand, in a moderate relativistic regime a much richer scenario is found with the onset of a series of stationary state mode transitions as both electron density and injection temperature are varied. In particular, it is found that the transitions and the consequent loss of magnetic insulation may occur even at very low injection temperatures. Self-consistent numerical simulation results are also presented and used to verify the theoretical findings.In this paper, a fully kinetic theory for the relativistic electron flow in a crossed-field device is developed and analyzed. The theory takes into account self-electric, self-magnetic, and thermal effects and allows determining the final stationary state achieved by the electrons in phase-space. A number of different possible stationary modes are identified and described in detail. Particular attention is given to the study of how space charge and thermal effects affect the magnetic insulation when the external magnetic field exceeds the Hull cutoff field. In the nonrelativistic limit, it is found that there is only a single mode transition that leads to the loss of the magnetic insulation. This transition is completely independent of the electron density and occurs for relatively large injection temperatures. On the other hand, in a moderate relativistic regime a much richer scenario is found with the onset of a series of stationary state mode transitions as both electron density and injection temperatu...

Triplet and beam interaction in a plasma

The interaction of three waves requires wavelength and frequency matching conditions. Without the presence of a particle beam, if the conditions are satisfied and if the frequency of the envelope is lower than the lowest frequency of the waves, they exchange energy and the evolution of the envelope of each wave is given by a constant plus a sinusoidal function. On the other hand, if a particle beam propagates within electrostatic and electromagnetic fields with no wavelength and frequency match, the energy exchange between the modes is done due to the particles. One of the modes could be amplified in this scheme. In the present work, we propose a model where a non-relativistic particle beam propagates in a plasma within two electromagnetic modes and one electrostatic mode with wavelength and frequency matching conditions. Then, the waves are allowed to exchange energy between themselves and with the particle beam as well. We present new features in comparison to the isolated triplet interaction and to the...

QoS aware schedulers for multi-users on OFDMA downlink: Optimal and heuristic

Orthogonal Frequency Division Multiple Access (OFDMA) has been adopted in the newest fifth generation radio networks (5G) such as Cloud Radio Access Networks (C-RANs) and Heterogeneous Cloud Radio Access Networks (H-CRANs). Therefore, new scheduling strategies should be projected. In 5G radio networks, restricted requirements must be considered to implement an effective service to users such as throughput, delay, packet loss, and fairness. Despite efforts to solve the downlink (DL) scheduling problem on wireless networks, we are not aware of previous attempts that have addressed the above requirements together, and more than that, in overbooked scenarios. In this paper, we address the radio DL resource scheduling problem for multiple users using LTE networks as a case study. A new optimal scheduler is modeled regarding Quality of Service (QoS) provisioning through delay. In addition, a parameterized heuristic based on user channel quality and service delay is proposed to reach scheduling solutions for overbooked scenarios. Results demonstrate that the proposed scheduling approaches led to a throughput of 7.5% lower than the optimal ones and 25% lower packet losses in overbooked scenarios. Our model also ensure that the resultant scheduling be as fair as 0.91 (according to Jain fairness index). Additionally, the obtained results show a reasonable trade-off between spectral efficiency and QoS metrics.