Hall Effect Thrusters (HET) are the technology of choice for large GEO telecoms and other medium/high ΔV missions, quickly gaining acceptance for LEO applications as well. Thanks to their high specific impulse and proven reliability, HETs are also employed for space exploration (ESA SMART-1). These thrusters can work with satisfactory efficiencies over a power range from around 100 W up to tens of kW.
In HETs the propellant gas, usually xenon, is ionized and then accelerated electrostatically. Electrons emitted from the cathode are trapped in a region of strong magnetic field, causing the electric potential to drop locally. This axial electric field associated with this potential drop accelerates the ions towards the thruster exhaust section. Specific impulses up to 3000 s can be obtained, together with efficiency over 60% and thrust levels ranging from mN (mini-Hall) to a few N (High Power HETs).
HT100C Hall Effect Thruster, ignition, during characterization test in IV4
Initially developed in the former Soviet Union, flight proven since 1972, the HET technology became available in Europe only after 1990. While most western HET's are based on Russian know-how, Alta's approach has been to develop its own HET technological capability. The first Italian HET prototype was succesfully tested in 1994. Today, Alta is one of the few companies worldwide with full in-house HET design, development and testing capability.
Current and past ESA-funded Hall thrusters activities at Alta include:
The 100 W Hall Thruster HT-100 has currently reached over 600 hrs of firing between characterization and endurance test campaigns. A detailed performance characterization including direct thrust measurements was carried out in 2007 on the EM 1 unit. The operational envelope ranges between 75 and 400 W for power, 3 and 12.5 mN for thrust, 800 and 1650 s for Isp and an efficiency between 16 and 29%.
The endurance test was performed in Alta IV-4 test bench to assess the long term performance and erosion rate of the discharge chamber. A 1600 hr accelerated endurance thermal test was carried out to validate the permanent magnets by a thermal cycling between room temperature and 400°C.
In 2007 a characterization test was carried out on a twin engine cluster configuration to assess the possible interactions between the two unit operating simultaneously and to assess the effects of cross or coupled neutralization (using a single cathode for the neutralization of the two thrusters). A power supply breadboard, developed by Alta and Skytech s.r.l. under ESA SME founding, was also tested succesfully.
The near term future activities on the HT-100 supported by ASI and ESA will be focused on development at thruster and system level, including: