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).

A new type of electrospray technology that could be used for space propulsion was developed at Michigan Technological University. This thruster utilized an ionic liquid ferrofluid that was synthesized by suspending magnetic nanoparticles in an ionic liquid carrier solution so that the resulting fluid is superparamagnetic. The magnetic properties of the fluid were exploited to create self-assembling static arrays of surface peaks which were then amplified with an applied electric field until ion current was emitted from the array. The current and voltage profile of the emitting array was measured and its ability to self-heal after a damaging event was observed. I. IntroductionTypical electrospray thruster concepts utilize linear "blade like" emitters, 2-D arrays of tips, or 2-D arrays of micro-capillaries to generate large electric fields and ion/droplet emission from an electrostatic Taylor cone. Linear arrays are manufactured from thin sheets of metal, such as porous tungsten, and sharp peaks are etched into the surface. The arrays are aligned with a slit in the extraction electrode and the propellant (ionic liquid) is passively pumped through the porous tungsten. 1, 2 This type of electrospray emitter can have good packing density along the length of the array, but leaves a sizeable gap between consecutive arrays. Legge and Lozano were able to demonstrate 2 tips per mm packing density along the length of the array. 2 A similar strategy is the tungsten crown emitter, which is a porous tungsten structure shaped like a king's crown with a circular array of 28 emitters. [3][4][5] The crown emitter used indium as its propellant.Two-dimensional arrays are typically etched from silicon with regularly spaced discrete needles. 6 For this type of arrangement, the propellant is typically externally applied to the surface of the silicon where capillary flow causes a uniform layer of propellant to coat the substrate and emitter tips. In order for the propellant to easily wet to the silicon surface, a surface treatment is usually necessary, such as making black silicon. 7 These arrays are then assembled with an extraction electrode and alignment/retention mechanisms. 8 The third type of electrospray thruster is the capillary thruster. These types of thrusters generally contain the fluid in a hollow tube or needle and the electric field is created at the exit of the capillary. 9 One such device is the capillary array of 19 emitters built by Krpoun and Shea. 10 In this device each of the capillaries are etched out of silicon and then filled with silica spheres to provide a more uniform hydraulic resistance within each emitter. A micromanufactured extraction electrode was placed and aligned using ruby balls for alignment and electrical isolation. Another version of a capillary thruster was developed by Busek. 11 Each of Busek's thruster heads contained an array of 9 individually manufactured emitters assembled into an array. A third type of thruster array which does not cleanly fit into any of these groups is the porous tungst...

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.

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Non-invasive measurements of electron temperature and density in the near-field plume of a Hall-effect thruster were performed using a laser Thomson scattering diagnostic. Laser measurements were processed using a maximum likelihood estimation method and results were compared to electrostatic double probe measurements. Electron temperature ranged from approximately 1 -40 eV and density ranged from 1.0 × 10 17 m −3 to 1.3 × 10 18 m −3 over discharge voltages from 250 to 450 V and mass flow rates of 40 to 80 SCCM. NomenclatureA p = probe area e = fundamental charge I = probe current I Bohm = Bohm ion current I R = Rayleigh intensity I sat = ion saturation current I T = Thomson intensity k B = Boltzmann's constant m e = electron mass m i = ion mass n = particle number density n e = electron number density n i = ion number density n R = particle number density during Rayleigh calibration p = gas pressure P R = laser power during Rayleigh calibration P T = laser power during Thomson scattering measurement r p = probe radius T = gas temperature T e = electron temperature T i = ion temperature v i,th = ion thermal speed β = mean detector gain ∆λ 1/e = 1/e half-width λ D = Debye length