High-Specific Impulse Hall Thrusters, Part 2: Efficiency Analysis

Abstract

A laboratory-model Hall thruster with a magnetic circuit designed for high-specific impulse (2000-3000 s) was evaluated to determine how current density and magnetic field affect thruster operation. Results have shown for the first time that a minimum current density and optimum magnetic field shape exist at which efficiency will monotonically increase with specific impulse. At the nominal mass flow rate of 10 mg/s and between discharge voltages of 300 and 1000 V, total specific impulse and total efficiency ranged from 1600 to 3400 s and 51 to 61%, respectively. Comparison with a similar thruster showed how efficiency can be optimized for specific impulse by varying the shape of the magnetic field. Plume divergence decreased from a maximum of 48 deg at 400 V to a minimum of 35 deg at 1000 V, but increased between 300 and 400 V as the likely result of a large increase in discharge current oscillations. The breathing-mode frequency continuously increased with voltage, from 14.5 kHz at 300 V to 22 kHz at 1000 V, in contrast to other Hall thrusters where a sharp decrease of the breathing-mode frequency was found to coincide with increasing electron current and decreasing efficiency. These findings suggest that efficient, high-specific impulse operation was enabled through the regulation of the electron current with the applied magnetic field. η t = total efficiency θ = azimuthal coordinate direction ∇ z B r = axial gradient of the centerline radial magnetic field Nomenclature

Experimental Characterization of a Micro-Hall Thruster

“…The shape of the main peak is relatively broad in comparison to what is seen in larger, higher power thrusters [28][29][30], as shown in Fig. 13.…”

Cathode Effects in Cylindrical Hall Thrusters

“…4 and 5).Plume angles as high as 66 • were measured at low discharge currents. Current utilization [8,27] (η I = I ion /I d = I ion /(I electron + I ion )), however, decreased from 63% to 59% with discharge current (Fig. 8).…”

“…The half plume angle of a CHT can be as high as 60-80 • [3], compared to 45-50 • for the best annular Hall thrusters [8], where the plume angle is defined as the angle that contains at least 90% of the total ion current [8][9][10]. The resulting anode efficiency is typically 15-25% in the 100-200 W power range [11].…”

Cathode effects in cylindrical Hall thrusters

“…Current utilization [8,27] (η I = I ion /I d = I ion /(I electron + I ion )), however, decreased from 63% to 59% with discharge current (Fig. 8).…”

“…• for the best annular Hall thrusters [8], where the plume angle is defined as the angle that contains at least 90% of the total ion current [8][9][10]. The resulting anode efficiency is typically 15-25% in the 100-200 W power range [11].…”