A mass spectrometer system which is capable of sampling ions directly from an atmospheric pressure source has been constructed and used to investigate ions which are evaporated from the surface of highly charged water droplets. Solutions of electrolytes are sprayed into the air, and a large charge imparted to the droplets by induction; an electric field applied across the plume of evaporating spray extracts small ions, probably already clustered with solvent molecules, which are drawn through a 25 μm orifice into the vacuum chamber. Differential pumping is applied and the ions are focused into a quadrupole mass filter. Approximately 30 univalent anions and cations of various sizes and structures have been tested, and almost all have been observed to evaporate; significant exceptions are Ag+, Tl+, and Cu+. Furthermore, certain molecules such as urea and acetanilide, when present as neutrals in the sprayed water, are observed to remain attached to the ions when these evaporate. The experimental results are ...

Field induced ion evaporation from liquid surfaces at atmospheric pressure

Electrospraying utilises electrical forces for liquid atomisation. Droplets obtained by this method are highly charged to a fraction of the Rayleigh limit. The advantage of electrospraying is that the droplets can be extremely small, down to the order of 10’s nanometres, and the charge and size of the droplets can be controlled to some extent be electrical means. Motion of the charged droplets can be controlled by electric field. The deposition efficiency of the charged spray on an object is usually higher than that for uncharged droplets. Electrospray is, or potentially can be applied to many processes in industry and in scientific instruments manufacturing. The paper reviews electrospray methods and devices, including liquid metal ion sources, used for thin film deposition. This technique is applied in modern material technologies, microelectronics, micromachining, and nanotechnology.

Electrospray droplet sources for thin film deposition

The beam from an EHD ion source using liquid gallium has been shown to have a brigtness of 0.9×105 A cm−2 sr−1 at 21 kV and an energy spread of 12 eV at 10‐μA total current. The effective source diameter was 0.2 μm as imaged by a 2‐cm‐long Einzel lens with a 0.12‐mm aperture. Cesium was significantly poorer and mercury much poorer than gallium.

Ion source of high brightness using liquid metal

The mechanisms of electron and ion generation from Taylor cones of liquid metals are discussed. In the case of electron emission the vacuum arcing mechanism of Swanson and Schwind, which accounts for the observed high current repetitive pulsing is briefly reviewed. For ion emission mechanisms from onset to the high current regime are proposed. It is concluded that at onset ions are generated exclusively by field desorption. A theory to account for the observed emitter heating is advanced, and it is concluded that high currents result from field ionization of thermally evaporated atoms. It is shown that space charge becomes important even at very low ion currents and'is instrumental in providing stabilization in all regimes of ion emission.

On the mechanism of liquid metal electron and ion sources

Publisher Summary This chapter discusses several experimental studies conducted on cluster ions. Cluster ions comprise a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state. Research on the properties of cluster ions is valuable in obtaining a complete understanding of the forces between ions and neutral atoms or molecules, where, for example, data on energetics provide a direct measure of the depth of the potential well of interaction between the ion and the collection of neutral molecules. There are three types of experimental approaches to produce and study these cluster ions depending on different degrees of physical isolation, that is, clusters moving freely in space (gas phase studies), clusters supported upon a substrate surface, and clusters existing in solids and liquids. Moreover, the use of cluster ions for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.

Experimental studies on cluster ions

An electrohydrodynamic (EHD) technique is used to generate ions from liquid metals. Liquid metal is fed to the tip of a capillary needle emitter with a voltage difference applied between the emitter and an extractor electrode to produce an intense electric field at the liquid surface. Electrostatic forces overcome surface tension forces to produce ions by field emission. When using liquid cesium, time‐of‐flight mass analysis showed the ion current to be primarily Cs+ with a small percentage of Cs2+ and Cs3+. Electron currents of over 1 mA have been produced by operating the emitter at 2 kV negative. Besides cesium, alkali ion beams have been generated using NaK/cesium alloy and sodium. Calculations show that liquid metals of low work functions appear more suitable for production of atomic ions while higher work functions metals may produce multiatomic ions and charged droplets in addition to atomic ions.

Electrohydrodynamic Ion Source

The desorption times for K+, Na+, and Li+ from continuously oxygenated tungsten have been measured in the 1000–1500°K temperature range using an ac modulation technique. Under conditions of low alkali‐metal coverage on the surface, the experimentally derived values of desorption energy (l) and zero intercept (τ0) in the Arrhenius expression τ = τ0exp(l / kT) sec are K+: l = 1.5 eV; τ0 = 2 × 10−10sec, Na+: l = 1.6 eV; τ0 = 7 × 10−10sec, Li+: l = 2.2 eV; τ0 = 2 × 10−10sec. Alteractions of the surface using an oxygen spray produced noticeable effects, particularly for K+.

K+, Na+, and Li+ Desorption from Oxygenated Tungsten

The operational characteristics of colloid thrusters were determined using experimental testing of several annular emitters and an analytical study based on empirical data. The analytical study was initiated following the exploration of the previously unknown linear region of operation at low-mass-flow rates where the current and thrust vary linearly with extraction voltage and mass-flow rate. This analytical description of colloid operation has reduced the extent of performance-mapping data required to characterize the performance of an emitter-propellant combination. Typical performance mapping includes determinations of thrust, specific impulse, emitter current, and charge-to-mass ratio as functions of emitter voltage and mass flow rate. Emitter performance was upgraded by obtaining stable operation with very low extractor drain currents and with no glow discharge. The relationship between performance level with beam divergence and emitter temperature was also examined. The performance indices and operational curves of a thruster designed to satisfy specific performance goals are derived using the analytical techniques. Methods of achieving improved performance are discussed.

Operational Characteristics of Colloid Thrusters

: Research was performed on the generation of charged particles by subjecting a liquid, at the tip of an emitter, to a high electric field. The research was aimed at the ultimate development of a highly efficient microthrustor capable of emitting equal currents of positive and negative particles. Several emitter geometries and materials were tested including capillary needles and slits with dimensions ranging to a few mils. Performance was determined by time-of-flight spectroscopy. Propellants utilized to generate charged particles include organic solvents with conductive dopants and liquid cesium. A specific charge range from 50 to 70,000 C/kg has been obtained. Investigations of energy loss and particle formation mechanisms were made using retarding potential techniques. Results indicated that heating effects cause droplet evaporation in the accelerating and/or drift region. A 50-hour life test of a bipolar thrustor was successfully completed. Capillary emitter potentials were +4.4 kV and -5.8 kV with average currents of +150 microamps and -154 microamps. Thrustor power was 1.55W with a power-to-thrust ratio of about 20W/mlb at measured thrust levels greater than 70 micro-lb. (Author)

Electrodeless particle thrustor.

: Research and development was performed on the generation and acceleration of charged particles toward the ultimate development of a colloid thruster. Standard and specialized laboratory techniques were used to construct and test thruster components. Particle emitter material and geometry studies were made which include capillary needles, rows of needles, and an annular geometry. Both liquid metals and organic solvents containing conductive dopants were investigated as potential propellants. A device which was developed to measure mass flowrates as low as a few micrograms/sec was used to test the bipolar thruster. Thrust vectoring techniques were investigated including deflection of positive and negative particle beams using a minimum of electrodes. A 10 positive and 10 negative capillary needle bipolar thruster having vectoring capability was tested. Initial measurements showed the feasibility of using measureable continuous operational parameters to determine operational levels which cannot be measured directly. (Author)

Arthur Y. Yahiku

Papers

Electrohydrodynamic Ion Source

K+, Na+, and Li+ Desorption from Oxygenated Tungsten

Operational Characteristics of Colloid Thrusters

Electrodeless particle thrustor.

Research on Charged Particle Electrostatic Thrusters.