Ion Jet Research Articles

Ag 5 is a planar trapezoidal molecule

The Raman spectrum of Ag5, mass-selected from a sputtered jet of silver cluster ions, neutralized and matrix isolated in solid argon, is reported. Analysis of the frequencies and intensities of the 7 vibrational modes recorded suggests unequivocally that the molecule has a planar trapezoidal structure. No other reasonable structure is consistent with the observations. Nor is there evidence for more that one structure coexisting with the planar trapezoid. The observation is in accord with the conclusions of most quantum computations on Ag5. The silver pentamer is found to photofragment into the dimer and trimer when irradiated with 458 or 466 nm laser light.

Preparation of oxide superconductor specimens for TEM examination

We have investigated a wide variety of oxide superconductors and report here on a number of techniques that can be effectively used to prepare transmission electron microscopy (TEM) specimens from these materials. Crushing, cleaving, ion milling, ultramicrotomy, and jet polishing all were successfully utilized, and details of each technique, as well as equipment used, are described. Selection among these methods depends both on the starting form of the material and the information required. Ion milling and crushing generally give the best results and have the widest applicability in our particular work, while crushing and cleaving involve the least equipment cost. In some cases, particularly with ion milling and jet polishing, small variations in the details of preparation have a dramatic effect on the success rate. We have found it to be a great advantage that the same techniques can be applied in a similar manner to a whole range of oxide materials, even (with some refinements and special precautions) to those that are extremely oxygen or moisture sensitive.

Formation of a plasma jet of multiply charged ions in the interaction of a laser plasma with an external pulsed magnetic field

A dense jet of a plasma consisting of multiply charged ions was generated in the interaction of a laser plasma with a strong external axial magnetic field. Images were formed by spectral lines and the soft x-ray spectrum range of the plasma jet was obtained with a large-aperture spectrograph containing a mica crystal bent to form a spherical surface with a radius of R = 10 cm. A tenfold increase in the density of the He-like Mg XI plasma, compared with a freely expanding plasma, was observed at a distance of 5 mm from the target.

Selective etching of AlAs for preparation of III-V semiconductor thin foils.

A new method of thin section preparation of III-V semiconductors and multilayers for transmission electron microscopy (TEM) is presented that exhibits considerable advantages over conventional methods such as ion beam milling and jet thinning. GaAs thin films and multilayers of GaAs/In chi Ga1-chi As/GaAs are grown over an etch release layer of AlAs on GaAs substrates by molecular beam epitaxy (MBE). Planar TEM sections prepared by selective etching from these samples show improved ability to image film morphology and dislocation arrangements, and the resulting large thin electron transparent areas facilitate dislocation density measurements and detection of spatial variations. Avoidance of radiation effects and wedge shaping, both common to ion milled samples, allows this method to be used to prepare uniform thickness standards of single layer GaAs films for EDS analysis or lattice imaging.

Angular and time resolved intensity distributions of laser-desorbed matrix ions

A new instrument for studying the behavior of laser-desorbed bio-organic molecules is described. The instrument consists of a large diameter vacuum chamber containing a tiltable sample holder in the center, a fixed beam path for laser irradiation and two individual time-of-flight systems, one of them being rotatable around the center of the chamber. Velocity and intensity distributions of ions can be obtained as a function of the angle of incidence of the laser beam and the ion ejection angle with respect to the sample surface. Velocity and kinetic energy distributions show strong differences for different ions ejected from single crystals of 2,5-dihydroxybenzoic acid (DHB). Distributions of the dehydrated molecular ion of DHB (the most intense signal in the spectrum) are strongly peaked forward in intensity and kinetic energy. The characteristics of this hypersonic jet of ions have been investigated as a function of the laser irradiance and the focus diameter. The lowest mean velocities have been observed for large focus diameters ( d ≈ 180 μm) at threshold irradiance. Under focused conditions ( d ≈ 10 μm) mean kinetic energies of ions rose by a factor of 4 to 25 for threshold to 6 times threshold irradiance, respectively.

Evidence for ion jets in the high‐speed solar wind

Shapes of proton and alpha particle velocity distribution functions in the high‐speed solar wind are examined for the time period from 1973 to 1987. Whereas enhanced high‐energy extensions are a near universal feature of the proton distributions, no such enhancements are observed in alpha particle distributions. Such distributions are examined and documented in detail. An interpretation of this observation is presented in terms of a two‐component model of coronal heating and acceleration. We suggest that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. This model predicts that the ambient flow of plasma from coronal holes is characterized by a low‐speed and a low‐mass flux. In particular, the proton flux is insufficient to drag alpha particles out of the solar gravitational potential well. This should lead to a considerable enhancement of the helium abundance close to the Sun. Plasma jets rich in helium and accelerated by reconnection close to the Sun buffet the ambient plasma from below. The slow‐moving ambient plasma is coupled to the faster‐moving jets by Coulomb friction close to the Sun, and then through microinstabilities far from the Sun. The net effect of this interaction is a high‐speed, double‐streaming proton population and a single component, but faster alpha particle flow at large heliocentric distances.

Non‐solar UV produced ions observed optically from the “CRIT I” Critical Velocity Ionization Experiment

A critical velocity ionization experiment was carried out with a heavily instrumented rocket launched from Wallops Island at dawn on May 13, 1986. Two neutral barium beams were created by explosive shaped charges released from the rocket and detonated at 48° to B at altitudes near 400 km and below the solar UV cutoff. Critical velocity ionization was expected to form a detectable ion jet along the release field line, but instead an ion cloud of fairly uniform intensity was observed stretching from the release field line across to where the neutral barium jet reached sunlight. The process creating these ions must have been present from the time of the release and the efficiency is estimated to be equivalent to an ionization time constant of 1800 s. This ionization is most likely from collisions between the neutral barium jet and the ambient atmospheric oxygen, and if so, the cross section for collisional ionization is 9×10−18 cm². A critical velocity ionization process may have been present during the first few tenths of a second after release, but its efficiency cannot have exceeded an equivalent ionization time constant of about 1800 s.

Propagation of an ion jet near a dielectric surface

Difficulties in determining experimentally the local electrical parameters of unipolar-charged jets are arousing interest in the theoretical investigation of electrogasdynamic (EGD) flows. Free EGD jets were examined, for example, in [1–3]. In order to control the charge on the dielectric parts of aircraft surfaces, which results from their static electrification and may have certain negative consequences [4], and, moreover, to influence the flow in the boundary layer use is being made of unipolar-charged jets propagating near the dielectric [5, 6]. In [6] the case of an ion jet near a dielectric surface possessing surface conductivity was investigated. In these circumstances it is possible to neglect charge diffusion, which considerably simplifies the problem. Space charge diffusion was taken into account in [7], but subject to certain very important simplifications. The author has calculated the electrical parameters of a unipolar-charged jet propagating in a viscous incompressible gas near an ideal dielectric plate, with allowance for surface and polarization charges and, moreover, the diffusion processes near the surface. An asymptotic solution is obtained for the equations of the ionic diffusion layer as the ratio of the thickness of the diffusion layer to the thickness of the hydrodynamic boundary layer tends to zero.

TEMPERATURE DEPENDENCE OF RETROGRADE VELOCITY OF VACUUM ARCS IN MAGNETIC FIELDS

Using optical method the retrograde velocities of vacuum arcs in magnetic fields were measured as a function of the cathode temperature for Cu, Al and 316 stainless steel used as the cathode material. It was found that with increasing the cathode temperature the arc velocity decreases. The experimentally measured temperature dependence of retrograde velocity of vacuum arcs can be explained by the ion jet model for retrograde motion of vacuum arcs.

Temperature Dependence of Retrograde Velocity of Vacuum Arcs in Magnetic Fields

Retrograde velocity of vacuum arcs in transverse magnetic fields is known to depend on, among other things, the magnetic induction, the arc current, the electrode spacing, the cathode material, and the cathode surface condition, and was also found to depend on the cathode temperature. Using the optical method, the retrograde velocity was measured as a function of the cathode temperature with copper, aluminum, and stainless steel as cathode materials. The optical measurement shows that by increasing the cathode temperature, the arc velocity decreases. It appears that with the increase in the cathode temperature, the decrease of the arc velocity is related to the increase of the cathode crater radius. The experimentally measured temperature dependence of the retrograde velocity of vacuum arcs can be explained by the ion jet model for retrograde motion of vacuum arcs [10]. The relative decrease of retrograde velocity as a function of the cathode temperature calculated according to this model agrees quantitatively with the reported measurements.

Electromagnetic effects in the analysis of prompt striations

In a number of barium injection experiments in the ionosphere, field‐aligned structures have been observed to develop on the ion‐cyclotron time scale. These structures, called prompt striations, have been attributed to plasma jetting across the ambient magnetic field and have previously been analyzed in the electrostatic limit. The electrostatic analysis is reasonable for experiments in which the directed ion kinetic energy per unit volume is much smaller than the magnetic field pressure; however, for higher altitude experiments in which the ambient magnetic field strength is weaker, justification for the electrostatic treatment fails and the full electromagnetic dispersion relation must be examined. It is demonstrated that electromagnetic effects inhibit the growth of the linear instability considered to be the source of prompt striations in the earlier experiments. Consistent with the electrostatic analysis, cold background plasma and thermal spread in the ion loss cone distribution function tend to stabilize the instability in the full electromagnetic treatment. The theory suggests that prompt striations in the magnetosphere can form from ion jets only if the plasma density is sufficiently tenuous so that electromagnetic stabilization is not complete.

On the velocity distribution of ion jets during substorm recovery

The velocity distribution of earthward jetting ions that are observed principally during substorm recovery by satellites at ∼15–35 RE in the magnetotail is compared quantitatively with two different theoretical models, the ‘adiabatic deformation’ of an initially flowing Maxwellian moving into higher magnetic field strength (model A) and the field‐aligned electrostatic acceleration of an initially nonflowing isotropic Maxwellian including adiabatic deformation effects (model B). It is assumed that the ions are protons or, more generally, consist of only one species. Both models can explain the often observed concave‐convex shape of isodensity contours of the distribution function. Model A, however, gives a somewhat better overall quantitative agreement with the observed distributions, leading us to conclude that field‐aligned electrostatic acceleration may not play the fundamental role in the energizing of the jetting ions that was suggested earlier (Frank et al., 1978; DeCoster and Frank, 1979). The two models imply different plasma conditions in the tailward source region. Model A requires a source in the far tail with plasma of kT ≈ 1 keV and density less than that at the observing satellite. Model B requires a source, probably closer to the satellite position, with plasma of kT ≈ 2–5 keV and density greater than that at the observing satellite. The conditions at the source region implied by model A are consistent with the plasma's having originated in the tail lobes and having been accelerated by a reconnection process in the far tail.

K10 Anomalous effects of a laser produced plasma — self-phase modulation and ion jet stream

K10 Anomalous effects of a laser produced plasma — self-phase modulation and ion jet stream

Plasma expansion as a cause of metal displacement in vacuum-arc cathode spots

The pressure on the molten surface of a vacuum-arc cathode due to the recoil from outward-directed ion jets is calculated for copper arcs. The calculation uses current density data and electrode force per unit current data from experiment. In addition, the force per unit current measured by Tanberg is shown to be consistent with a calculation of the same quantity based on the energy of outgoing ions measured by Davis and Miller and the so-called saturated positive-ion current measured by Kimblin. The ion recoil pressure is shown to be sufficient to remove molten metal from a cathode-spot crater in time of the order of 25–250 nsec with a velocity of 2×103−2×104 cm sec−1. It is shown that motion of molten metal in the cathode-spot crater must be considered as a first-order effect in rigorous calculations of surface temperature and heat flow in the metal in contact with the cathode-spot plasma. It is suggested that the rapid removal of metal by the plasma pressure causes molten droplets to be ejected, as has been observed experimentally, and causes a preference for the cathode to operate, after the liquid is ejected, on the rim of the crater or nearby on the surface where hotter metal may exist due to liquid-metal overflow and spatter.

Fluorescent ion jets for studying the ionosphere and magnetosphere

Fast ion jets, the fluorescent light of which can be recorded on ground, become of increasing importance in rocket-release studies of the structure and dynamics of the ionosphere and magnetosphere. In the following, the physical principles for the production of such jets of high intensity by means of shaped charges are outlined. Simple expressions are derived to describe the basic features during the expansion of such jets in the collision dominated and collisionless regime of the ionosphere.

Electric space propulsion

General characteristics of electric propulsion systems for space vehicles, the energy sources and conversion efficiencies available, projected powerplant weights, and some of the various schemes for generating the ion and plasma jets. Also included are results of experimental units and their feasible range of application.