Field-emission Electric Propulsion - The Slit Emitter

The Slit Emitter

Liquid Metal Ion Sources (LMIS) based on field ionization or field evaporation have been introduced in the late ‘60s and have quickly become widespread as simple, cheap ion sources for a number of applications. In particular, the use of LMIS operated on Ga, In, alkali metals or alloys is standard practice in Secondary Ion Mass Spectrometry (SIMS) since the ‘70s.

While there exist different field emitter configurations, such as the needle, the capillary and slit emitter types, the principle of operation is the same in all cases. In the slit emitter, for example, a liquid metal propellant is fed by capillary forces through a narrow channel. The emitter consists of two identical halves made from stainless steel, and clamped or screwed together. A nickel layer, sputter deposited onto one of the emitter halves, outlines the desired channel contour and determines channel height (a.k.a. slit height, typically 1 - 2 μm) and channel width (a.k.a. slit length, ranging from 1 mm up to about 7 cm).

The channel ends at the emitter tip, formed by sharp edges that are located opposite a negative, or accelerator, electrode, and separated by a small gap (about 0.6 mm) from the emitter tip. An extraction voltage is applied between the two electrodes. The emitter carries a positive potential while the accelerator is at negative potential. The electric field being generated between the emitter and accelerator now acts on the liquid metal propellant.

The narrow slit width not only enables the capillary feed, but, when combined with the sharp channel edges directly opposite the accelerator, also ensures that a high electric field strength is obtained near the slit exit. The liquid metal column, when subjected to this electric field, begins to deform, forming cusps (Taylor cones), which protrude from the surface of the liquid. As the liquid cusps form ever sharper cones due to the action of the electric field, the local electric field strength near these cusps intensifies. Once a local electric field strength of about 109 V/m is reached, electrons are ripped off the metal atoms. These electrons are collected through the liquid metal column by the channel walls, and the positive ions are accelerated away from the liquid through a gap in the negative accelerator electrode by the same electric field that created them.

Slit emitters had been developed to increase the emitting area of the thruster in order to yield higher thrust levels and to avoid the irregular behaviour observed for single emitters. The substantial advantage of slit emitters over stacked needles is in the self-adjusting mechanism governing the formation and redistribution of emission sites on the liquid metal surface according to the operating parameters; in a stacked-needle array, on the contrary, the Taylor cones can only exist on the fixed tips, which pre-configure a geometrical arrangement that can only be consistent with one particular operating condition.

Slit emitters with a wide variety of slit widths have been fabricated; currently, devices with slit widths between 2 mm and 7 cm are available. These devices, spanning a thrust range from 0.1 μN to 2 mN, are operated with cesium or rubidium.