RESUMO
An extreme electric field on the order of 10(10) V m(-1) was applied to the free surface of an ionic liquid to cause electric-field-induced evaporation of molecular ions from the liquid. The point of ion emission was observed in situ using a TEM. The resulting electrospray emission process was observed to create nanoscale high-aspect-ratio dendritic features that were aligned with the direction of the electric field. Upon removal of the stressing field the features were seen to remain, indicating that the ionic liquid residue was solidified or gelled. Similar electrospray experiments performed in a field-emission scanning electron microscope revealed that the features are created when the high-energy electron beam damages the molecular structure of the ionic liquid. While the electric field does not play a direct role in the fluid modification, the electric stress was critical in detecting the liquid property change. It is only because the electric stress mechanically elongated the fluid during the electrospray process and these obviously non-liquid structures persisted when the field was removed that the damage was evident. This evidence of ionic liquid radiation damage may have significant bearing on electrospray devices where it is possible to produce high-energy secondary electrons through surface impacts of emitted ions downstream of the emitter. Any such impacts that are in close proximity could see reflected secondary electrons impact the emitter causing gelling of the ionic liquid.
RESUMO
Electrospray is a versatile technology used, for example, to ionize biomolecules for mass spectrometry, create nanofibers and nanowires, and propel spacecraft in orbit. Traditionally, electrospray is achieved via microfabricated capillary needle electrodes that are used to create the fluid jets. Here we report on multiple parallel jetting instabilities realized through the application of simultaneous electric and magnetic fields to the surface of a superparamagnetic electrically conducting ionic liquid with no needle electrodes. The ionic liquid ferrofluid is synthesized by suspending magnetic nanoparticles in a room-temperature molten salt carrier liquid. Two ILFFs are reported: one based on ethylammonium nitrate (EAN) and the other based on EMIM-NTf2. The ILFFs display an electrical conductivity of 0.63 S/m and a relative magnetic permeability as high as 10. When coincident electric and magnetic fields are applied to these liquids, the result is a self-assembling array of emitters that are composed entirely of the colloidal fluid. An analysis of the magnetic surface stress induced on the ILFF shows that the electric field required for transition to spray can be reduced by as much as 4.5 × 10(7) V/m compared to purely electrostatic spray. Ferrofluid mode studies in nonuniform magnetic fields show that it is feasible to realize arrays with up to 16 emitters/mm(2).
RESUMO
Laser Thomson scattering (LTS) is an established plasma diagnostic technique that has seen recent application to low density plasmas. It is difficult to perform LTS measurements when the scattered signal is weak as a result of low electron number density, poor optical access to the plasma, or both. Photon counting methods are often implemented in order to perform measurements in these low signal conditions. However, photon counting measurements performed with photo-multiplier tubes are time consuming and multi-photon arrivals are incorrectly recorded. In order to overcome these shortcomings a new data analysis method based on maximum likelihood estimation was developed. The key feature of this new data processing method is the inclusion of non-arrival events in determining the scattered Thomson signal. Maximum likelihood estimation and its application to Thomson scattering at low signal levels is presented and application of the new processing method to LTS measurements performed in the plume of a 2-kW Hall-effect thruster is discussed.
