Manuel Martinez-Sanchez

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this months Special Feature. We particularly hope that younger scientists will find value in this months Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.

A two-dimensional hybrid model of the Hall thruster discharge

Particle-in-cell methods are used for ions and neutrals. Probabilistic methods are implemented for ionization, charge-exchange collisions, gas injection, and particle-wall interaction. A diffusive macroscopic model is proposed for the strongly magnetized electron population. Cross-field electron transport includes wall collisionality and Bohm-type diffusion, the last one dominating in most of the discharge. Plasma quasineutrality applies except for space-charge sheaths, which are modeled taking into consideration secondary-electron-emission and space-charge saturation. Specific weighting algorithms are developed in order to fulfil the Bohm condition on the ion flow at the boundaries of the quasineutral domain. The consequence is the full development of the radial plasma structure and correct values for ion losses at lateral walls. The model gains in insight and physical consistency over a previous version, but thrust efficiency is lower than in experiments, indicating that further model refinement of some...

One-dimensional model of the plasma flow in a Hall thruster

A macroscopic model which accounts for the complex interactions between electrostatic, thermal, and kinetic effects in a Hall thruster is presented. The analysis establishes the one-dimensional steady structure of the flow as consisting of an anode sheath, a long electron free-diffusion region, with reverse ion flow, a thin ionization layer, and the acceleration region, which extends into the plume. The ion flow presents a forward sonic point around the exit of the ionization layer, which can be either internal, with a smooth sonic transition, or localized at the channel exit. The supersonic plume is included via a simple expansion model, allowing closure of the formulation and calculation of thruster performance. The results indicate good agreement with experimental data for the case of an internal sonic point, and they delineate the existence and nonexistence regions in the space of externally controllable parameters. They also unveil the importance of the electron pressure, the reverse flow of ions, an...

A numerical study of low-frequency discharge oscillations in Hall thrusters

A two-dimensional numerical model has been constructed for use in modeling Hall thruster plasma dynamics. An important feature is a detailed electron-insulator interaction model which has yielded improved predictions of electron temperature. The complete simulation is used here as a tool for analyzing the dynamics of low-frequency discharge oscillations in Hall thrusters. A simulation of SPT-lOO-type geometry and operating conditions leads to a pronounced \\kHz discharge oscillation with peak-to-peak amplitude of 65% of the DC value. The dynamics seem to be related to an ionization fluctuation which can be modeled in a predator-prey fashion. The oscillation frequency is found to be proportional to the square root of the ion velocity change across the acceleration zone, and inversely proportional to the acceleration zone length.

Effects of the radial plasma-wall interaction on the Hall thruster discharge

The interaction of the plasma discharge with the ceramic walls of a Hall thruster leads to plasma recombination, energy losses, and extra electron collisionality. These three phenomena are included in a one-dimensional axial model of the discharge through source terms obtained from an auxiliary model of the radial dynamics. Spatial solutions are presented for different discharge voltages and wall materials, and agree satisfactorily with experimental data. The parameters related to wall effects are investigated extensively. The energy balance among Joule heating, wall-losses cooling, and heat conduction shapes the temperature profile; three different profile types are identified depending on the wall material and the discharge voltage. For long chambers, the main source of energy losses is the plasma interaction with the walls, even for zero secondary electron emission. By contrast, wall collisionality due to primary/secondary exchanges of electrons is negligible always. The current utilization is related directly to the total energy losses. The propellant utilization is set by the balance between gas ionization and wall recombination in the acceleration region. The rate of wall recombination suggested by the axial solution is much lower than the values given by radial models based on a Maxwellian electron distribution function.

A Micro-Fabricated Linear Array of Electrospray Emitters for Thruster Applications

This paper reports the design, fabrication, and experimental characterization of an internally fed linear array of electrospray emitters intended for space propulsion applications. The engine uses doped formamide as propellant and operates in the single-Taylor-cone droplet emission regime. The engine implements the concept of hydraulic and electrodynamic flow rate matching to achieve electrical control. The engine uses a set of meso-scaled silicon deflection springs to assemble the hydraulics to the electrodes, allowing to decouple the corresponding process flows. The micro-fabrication of the engine is described and novel technologies that were developed are reported. Experimental results that demonstrate cumulative uniform and steady operation are provided. Current-flowrate characteristics of the engine are in agreement with a reduced-order model. Experimental data demonstrating the low divergence of electrospray emitter arrays operated in the single Taylor Cone is in qualitative agreement with a reduced-order mode that assumes the absence of a thermalized tail in the plume

Electrodynamic Tether Applications and Constraints

Propulsion and power generation by bare electrodynamic tethers are revisited in a unified way and issues and constraints are addressed. In comparing electrodynamic tethers, which do not use propellant, with other propellantconsuming systems, mission duration is a discriminator that defines crossover points for systems with equal initial masses. Bare tethers operating in low Earth orbit can be more competitive than optimum ion thrusters in missions exceeding two-three days for orbital deboost and three weeks for boosting operations. If the tether produces useful onboard power during deboost, the crossover point reaches to about 10 days. Power generation by means of a bare electrodynamic tether in combination with chemical propulsion to maintain orbital altitude of the system is more efficient than use of the same chemicals (liquid hydrogen and liquid oxygen) in a fuel cell to produce power for missions longer than one week. Issues associated with tether temperature, bowing, deployment, and arcing are also discussed. Heating/cooling rates reach about 4 K/s for a 0.05-mm-thick tape and a fraction of Kelvin/second for the ProSEDS (0.6-mm-radius) wire; under dominant ohmic effects, temperatures areover200K (night) and 380 K (day) for the tape and 320 and 415 K for that wire. Tether applications other than propulsion and power are briefly discussed.

Model of the plasma discharge in a Hall thruster with heat conduction

The inclusion of heat conduction into a one-dimensional, macroscopic model of the plasma inside a Hall thruster and in the near plume is found to smooth the temperature profile of previous solutions with a nonconductive model. The spatial structure still consists of reverse-flow, ionization, and acceleration regions. Conductive energy flow, being of the same order of convective flow, has significant effects on the rear part of the channel where it can make impossible the establishment of a steady anode sheath. As a result, there is an upper bound on the plasma reverse flow for the existence of stationary solutions. The analysis of inertial effects on the electron dynamics concludes that the main contribution is the azimuthal electron motion, which can produce extra collisionality, mainly in the near plume. The different contributions to the effective axial diffusion of electrons and the ion temperature are evaluated. A parametric investigation yields the basic scaling laws of the thruster stationary perfo...

Hybrid Particle-in-Cell Erosion Modeling of Two Hall Thrusters

An axisymmetric hybrid particle-in-cell model of the Hall thruster plasma discharge has been upgraded to simulate the erosion of the thruster acceleration channel, the degradation of which is the main life-limiting factor of the propulsion system. Evolution of the thruster geometry as a result of material removal due to sputtering is modeled by calculating wall erosion rates, stepping the grid boundary by a chosen time step and altering the computational mesh between simulation runs. The code is first tuned to predict the nose cone erosion of a 200-W Busek Hall thruster, the BHT-200. Simulated erosion profiles from the first 500 h of operation compare favorably with experimental data. The thruster is then subjected to a virtual life test that predicts a lifetime of 1330 h, well within the empirically determined range of 1287-1519 h. The model is then applied to the BHT-600, a higher-power thruster, to reproduce wear of its exit ring configuration over 932 h of firing. Though some optimized code features remain the same, others need adjustment to achieve comparable erosion results. Better understanding of the physics of anomalous plasma transport and low-energy sputtering are identified as the most pressing needs for improved lifetime models.

Efficiency Estimation of EMI-BF4 Ionic Liquid Electrospray Thrusters

Ionic liquid-based Electrospray Thrusters offer an attractive alternative for flexible, high specific impulse micropropulsion systems. Among the most promising attributes of these thrusters is their high efficiency as previously estimated by mass spectrometric and time-of-flight techniques. The energetic structure was indirectly revealed by these studies, suggesting that charged particles are emitted with a narrow energy spread centered close to the applied potential with just a small differential probably due to the energy invested in extracting charged species directly from the liquid surface. A direct characterization of the beam energy distribution from a single tungsten emitter externally wetted with the ionic liquid EMI-BF 4 using a retarding potential analyzer complements original estimations allowing a more reliable determination of the thruster overall efficiency.