MPD Thruster


Propellant consumption rate of rocket is determined by its exhaust velocity.
Deep space explorations or future space transport missions require the high exhaust velocity, and it can not be achieved by chemical fuel combustion energy. For these missions, it is necessary to use “Electric Propulsion” which can use electric energy.
Our laboratory intend to develop the electric propulsion technology with high efficiency and high exhaust velocity using high power.

The Advantage of Electric Propulsion

Fig.1 Chemical propulsion and Electric Propulsion

The main purpose of rocket engine is to accelerate the rocket by propellant injection and achieve the intended velocity. The requirement of velocity increment DV depends on the missions.
Electric propulsion enables high exhaust velocity so that it is use for missions required high DV. These engines are able to get high DV with small propellant consumption. In other words, the rate of burden (payload) is increased. (Fig. 1)
Furthermore, in the missions required extremely high DV such as deep space exploration, it is impossible to get to the destination by chemical propulsion. Ion thruster used for asteroid probe “Hayabusa” returned from Itokawa asteroid in 2010, is one of the electric propulsion systems.

Usefulness of MPD Thruster

Fig.2 Difference of Ion thruster and MPD thruster

In the electric propulsion systems, the propellant is heated electro thermally, or ionized and accelerated by electric field or magnetic field. Ion thruster can accelerate only ions by Coulomb force in electrostatic field, but it has a limit of current caused by the mutual repulsion of ions.
Therefore, it can’t increase its thrust density. On the other hand, MPD thruster can accelerate both ions and electrons by Lorentz force in the electromagnetic field, so that it can get large thrust density without repulsion. And more, MPD thruster has features compared to other electric propulsion,

  • It has simple configuration
  • Various propellant gases can be used
  • Improvement of thrust is expected when it is applied high-power

For these reasons, MPD thruster is one of the promising candidates for future high-power electric propulsion systems.

Current outcome

Coaxial MPD Thruster

We have developed a steady-state, co-axial MPD thruster. Varying propellant mass flow rate, applied magnetic field and discharge current, thruster performance test were conducted. As a result, the thrust efficiency increased under row mass flow rate and high magnetic field. We also find that using co-axial geometry, wear on the cathode is very severe and it influences the thruster operation characteristics.

Rectangular MPD Thruster
To prevent cathode erosion, we have developed a MPD thruster using hollow cathode which is durable against ion bombard. Using this cathode, prolonging the thruster life time is expected. Furthermore, we employed rectangular acceleration channel to direct the Lorentz force to the exhaust. To develop a thruster with high thrust efficiency and specific impulse, we are currently working on analyzing thrust efficiency and improving the thruster.
Rectangular MPD Thruster

One of our approaches for the problem of erosion of electrode, we developed electrodeless thruster. This thruster has no electrode exposed to the plasma. It can ionize the propellant by inductive heating using high frequency solenoidal coil current. We carefully tune the RF current phase to generate rotating magnetic and make azimuthal current. The interaction between this current and external magnetic field generate Lorentz force and accelerate the propellant.

Laser Propulsion


Laser ablation propulsion is a next generation spacecraft propulsion method. Thrust is generated by beaming laser to propellant from a distant position.
Our study had investigated thrust generation law by using a high speed camera and had demonstrated an original launch method.
Currently, the focus is on impulse characteristics of repetition pulse irradiation as repetitive irradiation is expected to be needed in practical use.

Laser Propulsion

Fig.1 Principle of Laser Propulsion

Laser ablation is a phenomenon that a solid material is changed to gas or plasma by laser irradiation. Laser ablation propulsion is a next generation propulsion method using the ablation jet as thrust (Fig. 1). Since energy is partly supplied by laser beaming from a distant position, there is a possibility of reducing loaded propellant. Because of these merits it has, laser propulsion is expected to be applied to space debris re-orbiting and attitude control of satellite.

Theory of thrust generation

Laser ablation is a phenomenon that a solid material is changed to gas or plasma by laser irradiation. Momentum generation theory has been investigated by nanosecond scale visualization using a high speed camera (fig. 2). As a result, thrust generation theory has been verified. This is a theory that ablation is generated just after absorption of laser light and successive impulse is generated by expansion. In addition, a lot of phenomenons have been unveiled. For example, laser energy efficiency is decreased in atmosphere by plasma shielding.

Fig.2 Schlieren Image(TEO CO2 Laser,polyacetal,1000kfps)

How to launch without mounted fuel


Fig.3 Flying test in Tube
oday, percentage of propellant in a spacecraft is extremely large and it is expected to decreased its ratio. As an ultimate model to pursue this object, propulsion method that propellant is completely apart from spacecraft is developed and launch demonstration has been conducted (fig. 3). The projectile is sat into the tube which is stood vertically from ground and ablator is placed at the side of the tube. Laser irradiated from the bottom is reflected at lower part of the projectile which has parabolic shape and focused on the ablator. The ablator wall is ablated by laser irradiation. By doing so, the projectile obtains thrust from pressure rise in the tube. As a result, the energy efficiency is extremely improved compare with a conventional method whose mount propellant is on the projectile.

How to get large thrust from limited energy source

The relation between laser irradiation method and impulse has yet to be clear. Therefore, laser is irradiated in various pulse conditions and impulse generated in these experiments are investigated(fig. 4). Heretofore, original impulse balance was developed in order to take accurate measurement of impulse generated by repeated irradiation of pulsed laser. This impulse balance is displaced by ablation when laser beam is irradiated to the target installed on the edge of the impulse balance. As long as impulse balance would not surpass quarter period, total impulse is able to be measured.

Fig.4 Experimental Setup

Helicon Electrostatic Thruster


Electric propulsion is the propulsion method that is expected for in future manned Mars exploration, farther space exploration, and so on. To use that way, the electric propulsion is needed to become a greater thrust, higher specific impulse, and higher efficiency thruster. In our study, We pay attention to a thruster using the helicon plasma as a large power thruster and push forward a study.

Helicon Plasma

Helicon plasma is the plasma that is made by a magnetic field and a high frequency electric discharge, and higher plasma density than like plasma made by high frequency discharge (e.g. Electron cyclotron resonant plasma(ECR), Inductively coupled plasma (ICP)). This plasma is regarded as the likely plasma source of  high power electric propulsion.

Plasma source Frequency range Plasma density [m³]
ECR GHz 1017 ~ 1019
~ 1019
Helicon 1019 ~

TABLE1. The Comparison of Other High Frequency Plasma


Fig1. Helicon Plasma  (Argon)

Helicon Electrostatic Thruster (HEST)

It is Helicon Electrostatic Thruster(HEST) which is the new acceleration method for Electric thruster which is proposed in Nagoya University to emit the above-mentioned helicon plasma using the accelerator of the shape such as figure 2.

In this way, an electric field between the anode and the cathode is maintained  by using the cusp magnetic field and accelerates the ion. After measuring the ion energy in the plume, I confirmed generation of the ion energy of 280eV in RF power 1,500W, acceleration voltage 300V. This was equivalent to 93% of acceleration voltage, and the effectiveness of the electrostatic acceleration was confirmed.

We are aiming at the performance enhancement of the thruster by elucidation of the acceleration mechanism and optimizing the thruster.


Fig2. HEST Overview


Fig3. HEST Operating Picture

Related Journal

  • S.Harada, T.Baba, A.Uchigashima, S.yokota, A.Iwakawa, A.Sasoh, T.Yamazaki, and H.Shimizu, Applied Physics Letters, Vol. 105, ArtID 194101, 2014.
  • A. Uchigashima, T. Baba, D. Ichihara, A. Iwakawa, A. Sasoh, T. Yamazaki, S. Harada, M. Sasahara, T. Iwasaki, IEEE Transactions on Plasma Science, Vol. PP(99), pp. 1-8,