Neutral Beam Ion Research Articles

Computational study of the first stage of hypersonic ion beam neutralization: The cross neutralization stage

A study is made of the first stage of evolution of a hypersonic ion beam in which thermoelectrons are emitted by a heated grid, known as the neutralizer. Downstream from the neutralizer there appears successively as a sheath a range of periodic and quasi-stationary electric field and a front where the electric field oscillates with the plasma frequency. The sheath is self-regulated. Some electrons are trapped in the periodic and stationary electric field. The characteristics of the periodic, quasi-stationary range correspond to those of both an experimental study and one-dimensional time-independent macroscopic theory. This quasi-stationary regime builds up in a time smaller than ωpe−1 and is studied for a few periods ωPe−1. The subsequent evolution of this state of nonequilibrium is not investigated. The experimental study has shown that, as for the neutralization of a subsonic ion beam, it leads to a field-free, homogeneous medium: a ’’synthesized plasma.’’ The importance of the first stage described herein, which may be called the gross neutralization stage, is due to the properties of mean neutrality in the current and in the charge insured by the regulating sheath, properties which will be preseved downstream.

Forced neutralisation of a positive ion beam by negative ions

A stationary one-dimensional macroscopic description of the forced neutralization of a positive ion beam by negative ions is proposed. From the discussion, three types of solutions are forthcoming, involving a mean or fine neutralization, depending on the injection parameters: periodic, periodic with sheath and aperiodic solutions. The domains of neutralization corresponding to these solutions are analysed in two simple cases.

Production of a Synthesized Plasma

Neutralization of a helium ion beam extracted from a stationarily operated duoplasmatron is studied. It was confirmed that the neutralization of ion space charge occurred mainly at the deceleration stage of the einzel lens, and not in the downstream of the neutralizer. The magnetic field and the arc current in the duoplasmatron affected substantially the beam divergence. The energy specturm of the extracted ions was measured with an energy analyzing system consisting of a pre-decelerator and an electrostatic energy analyzer with 90° deflection. The energy spectrum of ions in the synthesized plasma was essentially defined by the operating conditions of the ion source; the gas pressure, magnetic field and arc current in the arc discharge. A small difference between the space potential and the potential of the emitter changed the characteristics of the synthesized plasma.

A supersonic secondary beam neutralizer for low energy ion beams

A compact, cryogenically cooled, supersonic molecular beam unit is described, which is suited to the neutralization of ion beams having energies as low as 2 eV. Three criteria govern the secondary beam design: it must have sufficient divergence to encompass the primary ion beam, must be sufficiently intense to give the required degree of neutralization, and must be well enough directed that a negligible number of secondary beam neutral molecules leave the crossed beam chamber, or are adsorbed on its entrance and exit holes. These criteria are formulated into four generalized design equations. A solution of these equations for a particular experimental situation is illustrated, and a supersonic beam neutralizer based on this solution is described. Characterization of this instrument shows good agreement with the design requirements.

Experimental study of the forced neutralization of a hypersonic ion beam by thermoelectrons

Neutralization of a hypersonic ion beam by the injection of thermal electrons is studied. With the correct adjustment of parameters, neutralization of charge and current is possible. Downstream from the neutralizer, a stationary periodic electric field appears with characteristics which can be deduced from macroscopic theory but with an amplitude which decreases rapidly with distance.

Transient ion neutralization by electrons

The nonlinear initial-boundary-value problems describing the lateral neutralization of ion beams for the cases that (i) an auxiliary electric field accelerates the electrons into the ion space, and (ii) the electrons are injected into the ion space at a prescribed current density are treated. Analytical solutions are derived which give the position and speed of the neutralization front as a function of time, and the temporal development of the electron density, velocity, and electric fields during the neutralization process.

Low Energy Ion Bombardment Effects in SiO2

Evidence has been presented for the presence of mobile ions in ion bombarded SiO2, these ions being possibly protons or sodium ions. A model invoking neutralization of the incident ion beam at the surface, followed by impurity ion transport to the interface under the influence of the field building up across the SiO2 film during bombardment has been constructed to explain the experimental data.

PLANETARY EXPLORATION WITH ELECTRICALLY PROPELLED VEHICLES

The characteristics of propulsion systems required for carrying out flight missions within the solar system, as desired by planetary physicists and astronomers, are reviewed. It is shown that an encouraging answer to these requirements is available in the form of electrostatic or ion propulsion systems. The design and performance characteristics of an electrostatic thrustor employing an ion source, accelerating electrode, beam neutralizer, and power source are discussed, together with those of the Kaufmann engine (electrostatic thrustor employing bombardment type ionization). More demanding missions which will become feasible with the advent of nuclear-electric power sources (such as the incore thermionic reactor) may include close orbiters around all the planets, and asteroid and cometary missions.

On the possibility of producing 0.1 GeV ions by focussing ultrashort laser pulses on thin foils

The possibility of creating a high energy neutralized ion beam (energy ≈ 1 MeV per nucleon), by a laser pulse, is considered.

Study of a synthesized plasma

Neutralization of a mercury ion beam ejected from an electron bombardment source is studied. The electrons are emitted from a hot filament located close to the ion source. It is shown that if the temperature of the neutralizer is high enough a complete charge and current neutralization occurs. The neutralizer is surrounded by a plasma sheath which is calculated. The stability of the synthesized plasma resulting from the neutralized beam is investigated.

Harmonics of ion oscillations in a neutralized ion beam plasma

Harmonics of ion plasma oscillations are observed in a helium plasma produced by a back diffusion source.

Magnetic field effect on space-charge neutralization in electric propulsion.

Some basic aspects of ion beam neutralization in the presence of magnetic fields are investigated. The problem leads to an integrodifferential equation for the electrostatic potential. This equation is solved numerically for several values of the electron-ion mass ratio. Of more interest for practical applications is the limit as the mass ratio tends to zero. In this case, the governing equation becomes linear and the problem reduces to finding the inverse of a Hilbert transform. The solution can be expressed in terms of elementary functions. Among the results obtained are the distribution of the charge densities for electrons and ions in the beam, and the electron trajectories. The limiting condition for beam reversal, which applies when the magnetic field is sufficiently strong to prevent the electrons from being drawn into the beam, is determined. Also computed is the loss in propulsive efficiency due to electron-swirl. Finally, it is pointed out that this loss can be entirely avoided if a magnetic field is employed whose transverse component changes direction.

Charge-changing collisions involving fast helium beams - the effect of a metastable atom component

Cross sections ?10 and ?01 for one-electron capture and loss have been determined in the energy range 60-400 kev by studying, respectively, the charge neutralization and stripping of helium ion and atom beams during passage through `thin' targets of hydrogen and helium In the studies of electron loss, measurements made with helium atom beams prepared by charge-transfer neutralization of He+ ions during passage through various `thick' gaseous targets were found to yield apparent cross sections ?01 which were dependent on the particular neutralizer gas. For example, in the energy range investigated, values of ?01 for helium atoms in hydrogen were found to increase by between 18% and 40% when hydrogen was substituted for helium as the neutralizer gas, an effect believed to be due to the formation of an increased fraction of metastable helium atoms in the fast beam. The apparently strong influence of a helium metastable atom component in this case suggests that a satisfactory description of the charge equilibriation of fast helium beams during passage through thick gaseous targets cannot always be given in terms of a simple three-component system Previous data on electron capture and loss obtained by the use of such a treatment must therefore be interpreted with extreme care

Untersuchungen �ber die Erzeugung und den Nachweis von Alkaliatomstrahlen im Energiebereich zwischen 6 und 4000 eV

An alkali metal atomic beam source for the 6–4000 eV energy range is described. The fast atomic beam is obtained by the neutralization of an alkali ion beam and is detected by surface ionization on an unheated filament.

Computer studies of plasma boundaries and lens effects created by immersed and withdrawn neutralizers.

Neutralizer configurations, immersed and withdrawn filaments, for given ion gun based on two- dimensional computer simulation of ion-beam neutralization

Cesium ion-beam neutralization with energetic electrons

The neutralization of the positive space charge of cesium ion beams of energies up to 2500 ev by means of energetic electrons (of the order of 100 ev) is investigated experimentally. It is found that neutralization occurs provided that electrons are supplied at the same rate as the ions. The results are independent of any other parameter of the electron beam. An attempt is made to justify the observed experimental results by means of a two-stream collective scattering interaction between the energetic electrons and the ion-beam plasma. The onedimensional dispersion relation for this interaction indicates the existence of growing longitudinal oscillations under the conditions prevailing in the experiments. These oscillations may be the mechanism by which the electron drift energy is thermalized, thus leading to the observed neutralization. The results of this work suggest that neutralizing electrons can be introduced into the ion beam of an ion-propelled spaceship by means of an electron gun instead of being made available by placing a hot electron source in the vicinity of the ion beam. Through use of an electron gun, the electron current can be controlled continuously, and the electron emitter can be well protected.

The production of intense neutral and negative ion beams

The basic concepts underlying the production of ion beams are reviewed. The properties of the plasma from which the ions are extracted are discussed with respect to their effect on the ion beam. The influence of the positive ion beam emittance and electron attachment on the production of fast neutral particle beams, and beams of negative ions is then examined. It is shown that the production of negative ions is best achieved by direct extraction from a plasma, whereas neutral particle beams must be formed by neutralization of energetic primary positive ion beams.

Computer experiments on ion-beam neutralization with initially cold electrons

BS>Trajectories of electrons and ions shot into an initially field-free space, as in the ion-propulsion problem, were obtained using a one-dimensional model. The results of a set of computer experiments are presented, in which the ratios of election-to-ion injection velocities and currents are varied for large but finite ratios of ion-to-electron mass. For electron-to-ion velocity ratios less than about 2, static theories of neutralization are confirmed. For unneutralized and partially neutralized beams, oscillations, and in some cases, randomization of electron trajectories are observed. For electron-to-ion velocity ratios greater than 2 and electron-to-ion current ratios greater than a critical value, neutralization is obtained by means of an oscillating electron sheath that feeds electron current to the ion beam in a self-compensating manner. (auth)

Investigation of RF Noise Generation from Space Vehicles

Analytical and experimental studies have been made on RF noise generated by two types of ion propulsion engines, namely the cesium contact ion engine and the Penning discharge engine. An analytical determination of noise generated by neutralized ion beams is made in which the following mechanisms are considered: electron-ion collisions, acceleration of electrons at the plasma boundary, ion-atom collisions, cathode shot noise, ion acceleration, black body radiation from ion emitter surface, and noise growth radiation. No noise was experimentally observed from the cesium contact engine over the frequency spectrum of 38 to 2200 Mc, but the Penning engine exhibited electron cyclotron radiation. Preliminary conclusions of this study are that reliable communications systems with a vehicle involving a Penning discharge propulsion engine should probably be operated above the cyclotron and plasma frequencies.

Production of a High Energy, Quasi-Thermal Plasma

The production of a high-temperature, high-density plasma by the transmission of a high-energy space-charge neutralized ion beam through a cusped magnetic field was studied. The high-energy beam is produced by using an electrostatic gradient in a space-charge neutral plasma to accelerate the ions to high velocity. The function of the cusped magnetic field is to transform a significant portion of the streaming energy of the particles into the energy of rotation about the particle guiding center. In this way a velocity randomization, or quasi-thermalization, of the beam is achieved. An analysis of electrostatic gradients in nonthermal plasmas is presented and compared to experiment. A simplified model of the energy transfer mechanism in the cusp was used to predict the fraction of streaming energy converted to rotational energy and was verified by the computation of actual particle trajectories using an analog computer. Experiments were performed in which a plasma beam has been injected into a cusped magnetic field and which verify that a significant portion of the streaming energy has been converted to rotational motion and that quasi- thermalization at a high temperature has been achieved. (auth)