Experimental study of ion optics with square apertures for high-power ion thrusters

Abstract

Large-diameter ion-optical systems are widely used in gridded ion thrusters for missions in near-earth space and for deep-space research. Ion sputtering of the accelerator grid is the main factor that limits the thruster’s life. The use of carbon-based materials with a lower sputtering rate compared to metal grids allows the thruster to increase its life more than four times. The most resistant to mechanical loads are grids made of carbon-carbon composite materials. Traditionally, ion optics grids use circular-shaped apertures with the centers located in the nodes of the hexagonal pattern. However, with this arrangement of holes, all carbon fibers in the carbon-carbon composite are cut, and this weakens the mechanical strength of the grid. Previously, the Keldysh Research Centre carried out computational studies of a new form of apertures, in which it is possible to preserve uncut fibers and thereby increase the mechanical strength of grids made of carbon-carbon composites. This paper presents the results of a comparative experimental study of the operational characteristics of ion optics with the traditional round shape of apertures and with the new shape of apertures, which are squares with rounded corners. Measurements of the accelerator grid currents depending on the accelerating voltage have shown that both traditional and new ion-optical systems have close perveance limits in which there is no direct ion impingement on the accelerator grid.

Formation and neutralization of electric charge and current of an ion thruster plume

“…As already indicated, an optimal perveance value exists that minimizes the divergence angle of the plume, and hence maximizes the thrust efficiency. The simulations have also shown that, in the plume formation process, the ion beamlets become circularly shaped very soon after the grid exit, in spite of the square shape of the considered apertures, supporting recent numerical and experimental works [26] that have found no significant differences in the emitted plasma plume between grid assemblies with square and circular holes. Therefore, the manufacturing of ion optics with new composite materials which benefit from square holes designs may be employed in future thrusters without incurring in large performance losses.…”

“…Observe that their shape has become circular, in spite of the apertures' shape being square in the present simulation, thus having short memory of its upstream extraction shape. This observation supports recent numerical and experimental works that have found no significant difference in the extracted beam using square and circular holes grid assemblies [26,37], thus justifying new designs with composite materials that face important difficulties in manufacturing circular apertures. At z = 20 mm, the beamlets have partially merged and finally, at z = 28 mm, the beamlets mixing process is almost complete and a single-peaked beam is emerging.…”

“…A c e p t e d M a n u s c r i p t comparison presented here is complementary to other existing works in the literature [18], and with the main difference that the simulated apertures are squared-shaped (not circular as in Ref. [18]), a geometry that is being considered in new designs with materials like composites [26]. The 3D hybrid model is completed with a 1D semi-analytical model of the plasma plume allowing to characterize the far-plume expansion.…”

“…Several aspects of the formation and neutralization of a GIT are explored with the simulations and contrasted with existing data: (i) the relation between perveance and divergence angle, (ii) the effect of the ion beamlet coalescence on nearplume properties and beam divergence, (iii) the comparison between infinite-apertures and finite-apertures simulations, (iv) the influence of electron inertia on the electron current, and (v) the influence of the neutralizer position on the electric current and charge neutralization phenomena. Regarding (iii), the comparison presented here is complementary to other existing works in the literature [18], and with the main difference that the simulated apertures are squared-shaped (not circular as in reference [18]), a geometry that is being considered in new designs with materials like composites [26]. The 3D hybrid model is completed with a 1D semi-analytical model of the plasma plume allowing to characterize the far-plume expansion.…”

Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer

“…A major issue of spacecraft EP interaction is related to the interaction of the neutralized ion plume with parts of the thruster itself, 129,[344][345][346][347][348][349][350][351][352] other components of the spacecraft, [353][354][355] other spacecrafts in the case of debris removal employing EP for momentum transfer, 92 or with parts of the test facility in the case of terrestrial testing. 265,356 One has to distinguish between the damage induced by material sputtering by the ion beam itself and effects due to the deposition of either the propellant itself or its sputter products on surfaces of the spacecraft.…”