Current Density Measurements of an Annular-Geometry Ion Engine

Abstract

The concept of the annular-geometry ion engine, or AGI-Engine, has been shown to have many potential benefits when scaling electric propulsion technologies to higher power. However, the necessary asymmetric location of the discharge cathode away from thruster centerline could potentially lead to non-uniformities in the discharge not present in conventional geometry ion thrusters. In an effort to characterize the degree of this potential non-uniformity, a number of current density measurements were taken on a breadboard AGI-Engine. Fourteen button probes were used to measure the ion current density of the discharge along a perforated electrode that replaced the ion optics during conditions of simulated beam extraction. Three Faraday probes spaced apart in the vertical direction were also used in a separate test to interrogate the plume of the AGI-Engine during true beam extraction. It was determined that both the discharge and the plume of the AGIEngine are highly uniform, with variations under most conditions limited to ±10% of the average current density in the discharge and ±5% of the average current density in the plume. Beam flatness parameter measured 30 mm from the ion optics ranged from 0.85 -0.95, and overall uniformity was shown to generally increase with increasing discharge and beam currents. These measurements indicate that the plasma is highly uniform despite the asymmetric location of the discharge cathode.

Development Status of High-Thrust Density Electrostatic Engines

“…During these tests with beam extraction, low discharge losses were demonstrated along with extremely-high beam current flatness values (~0.85-0.95). 5 As reported in 2013 for the GEN1 engine: 40 cm diameter flat annular geometry electrodes of high-perveance design were successfully manufactured from PG; the PG electrodes were integrated with the annular discharge chamber, and the optics demonstrated relatively high perveance, with very-high collimation (β ≈ 0.997) and high beam flatness at multiple NEXT thruster throttle levels. 4 The tests were impeded however by the lack of a mechanical mounting assembly for the ion optics electrodes, which resulted in occurrences of shorting, and potentially excessive electrostatic deflection of the electrodes.…”

“…2 is the maximum thrust density vs. specific impulse that would be anticipated for the NEXT thruster ion optics ('NEXT Optics Capability') as predicted from the beam current density from Eq. (5). In this instance the maximum total voltage, ion optics geometry, and thrust loss correction factors are per NEXT.…”

“…For xenon propellant the expression for ion optics beam current density reduces to: (5) Where the current density is maximized as the total voltage is increased to the highest practical value (while staying below the threshold which would result in interelectrode breakdown), and l g , t s , and d s electrode geometric parameters are reduced to practical limits.…”

“…5 In this instance the discharge losses are assumed to be equal to 80 W/A. The value of 80 W/A is a 'push goal.'…”

Advancements in the Demonstration of High Thrust to Power Ion Thruster Technology

“…The annular engine incorporates an annular discharge chamber, a magnetic circuit designed to circumvent ion source limiting, and high perveance flat graphite optics and is described in detail in Refs. [17][18][19][20]. The "conventional" engine pathway involves modification to a cylindical geometry such as the NEXT engine.…”