Plasma thruster advances bring deep space exploration closer

The way electric propulsion works is by employing electromagnetic fields to accelerate a gas propellant, creating the thrust necessary to propel a spacecraft. The thrusters use solar power to ionize a propellant inert gas (turning it into plasma) and generate thrust energy. As Dr. Takahashi writes in the study paper: “As electric power obtained by solar panels is converted into the kinetic energy of the propellant, the EP [electronic propulsion] devices can provide a high specific impulse, which corresponds to a thrust per unit mass of the propellant, compared with chemical propulsion devices.”

Operating almost nonstop for years

Several space missions have already employed tech like gridded ion thrusters and Hall thrusters, including NASA’s upcoming Psyche mission, which uses xenon as the propellant gas. One big advantage of these thrusters from NASA’s point of view is that they could operate almost nonstop for years without running out of fuel. 

Another innovation in the burgeoning field has been the move towards using electrodeless plasma thrusters, which do not depend on electrodes that can be harmed by the plasma, extending the longevity of the devices. As Dr. Takahashi explains in the study, "When DC electric power is coupled with the plasmas, the electrodes in the thruster and the neutralizer have to be exposed to the plasmas and are often damaged by ion sputtering and thermal load." As such, "the lifetime of the thruster has been a critical issue."

One new type of thruster under Dr. Takahashi's consideration -- the MN rf plasma thrusters, or helicon thrusters -- take an electrodeless approach to generate plasma by using radio frequencies. When an antenna emits radio waves into a cylindrical chamber to create plasma, a magnetic nozzle there controls and accelerates the plasma to create thrust. 

These types of thrusters are more flexible to operate and have high thrust-to-power ratios, as explained in the press release from Tohoku University. But the process of converting the power to thrust energy can be affected by conversion efficiency, which has been in single digits during earlier experiments but was recently improved to 20 percent. 

A conversion efficiency of 30 percent

In his new study, Professor Takahashi shared that he actually achieved a conversion efficiency of 30 percent. What’s more, he used argon propellant instead of the commonly-employed xenon, which tends to be quite expensive and difficult to supply in large quantities. 

Interesting Engineering (IE) reached out to Professor Takahashi for more details on his work. The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: What are the implications of the improved efficiency you've achieved for the development of new space propulsion systems?

Professor Takahashi: The development of the new space propulsion system is still an undergoing issue. The efficiency is now improved step by step. We still need to develop the system by incorporating the generator, the gas controller, thermal design, etc...  When we achieve the development, it will enable the massive transportation system in space.

IE: What level of efficiency needs to be achieved for the thrusters to truly revolutionize space travel?

In general, Hall thruster has already achieved about 50-60 percent efficiency with xenon, but it is lowered down to 20-30 percent for argon. If we achieve 40-50 percent with argon and an operation power level above 10 kW, it will open the new transportation technology. The required efficiency depends on what we will do in space. Since we can now get better efficiency than before, we are just starting to think about what we can do.

IE: Are you looking to try propellants other than argon to increase efficiency further? What's next for your research?

Argon is already an alternative propellant, which is very cheap and enough quantities can be supplied from the Earth. This is a significant difference with xenon. But, we guess that the thrust would be operational with other propellants, while I have no clear idea about what happens in the efficiency.

The next step is further improvement and system development. Also, I'm very interested in the plasma physics relating to the thruster performance.

Read the full study here. 

Abstract:

Innovations for terrestrial transportation technologies, e.g., cars, aircraft, and so on, have driven historical industries so far, and a similar breakthrough is now occurring in space owing to the successful development of electric propulsion devices such as gridded ion and Hall effect thrusters, where solar power is converted into the momentum of the propellant via acceleration of the ionized gases, resulting in a high specific impulse. A magnetic nozzle (MN) radiofrequency (rf) plasma thruster consisting of a low-pressure rf plasma source and a MN is an attractive candidate for a high-power electric propulsion device for spacecraft, as it will provide a long lifetime operation at a high-power level due to the absence of an electrode exposed to the plasma and a high thrust density. The high-density plasma produced in the source is transported along the magnetic field lines toward the open-source exit and the plasma is then spontaneously accelerated in the MN. By ejecting the plasma flow from the system, the reaction forces are exerted to the thruster structure including the source and the MN, and the spacecraft is resultantly propelled. The thruster will open the next door for space technologies, while the performance of the MN rf plasma thruster has been lower than those of the mature electric propulsion devices due to the energy loss to the physical walls. Here the thruster efficiency of about 30%, being the highest to date in this type of thruster, is successfully obtained in the MN rf plasma thruster by locating a cusp magnetic field inside the source, which acts as a virtual magnetic wall isolating the plasma from the source wall. The increase in the thrust by the cusp can be explained by considering the reductions of the loss area and the plasma volume in a thrust analysis combining a global source model and a one-dimensional MN model.