**2.2 Electromagnetic thrusters**

After simplification, we get

*Selected Topics in Plasma Physics*

mass fraction, the exhaust velocity *U*

**2. Thrust, impulse and efficiency**

is the specific impulse *Isp*, defined below

and the specific impulse simplifies to

Finally, the rocket equation turned into

gets converted into kinetic energy and produce a thrust.

into three main categories.

**24**

**2.1 Electrothermal thrusters**

a higher *dυ*

ln *mf mi*  ¼ � *<sup>υ</sup><sup>f</sup>* � *<sup>υ</sup><sup>i</sup> U* ! *ex*

The above rocket equation provides the relationship between the mission velocity and the mass of propellant *mp* ¼ *mi* � *mf* required for a given mission. It is clear that

! demands more propellant. Unfortunately, the mass ratio cannot be increased so much to avoid payloads problems in space mission, therefore for a given

The performance of thrusters is usually characterized by a number of parameters. A first quantity relevant to thruster performance is the thrust *T*, which is the total force undergone by the rocket. The specific impulse is used to compare the efficiencies of different type of propulsion systems [2]. The performance parameter

> *Isp* <sup>¼</sup> *<sup>T</sup> m*\_ *pg*

Here *m*\_ *<sup>p</sup>* is the mass flow rate and *g* is the acceleration due to gravity. The specific impulse has the dimension of time and is a measure for the effective lifetime of the thruster, when lifting its own propellant from the earth's surface. For the case of a constant mass flow rate the thrust is also constant as

> *T* ¼ *m*\_ *pU* !

> > *Isp* <sup>¼</sup> *<sup>U</sup>* ! *ex g*

*mf mi* ¼ *e* �Δ *υ* !

The rocket equation is equally applicable to all type of propulsion systems. Therefore high specific impulse related to better efficiency for a propellant. Based on the acceleration of gases for propulsion, electrical thrusters have been classified

In electrothermal thrusters, the hot gas is expanded through a nozzle without ionizing it. When it is being passed through a thin nozzle, the thermal energy of gas

!

and the higher the propellant exit velocity, the less propellant mass is required.

¼ � <sup>Δ</sup>*<sup>υ</sup>* !

*U* ! *ex*

*ex* of the propellant needs to be the order of *dυ*

*ex*, (5)

*gIsp* (7)

(3)

!

(4)

(6)

In electromagnetic thrusters an inert gas is used as a propellant and it is ionized by heating to produce plasma. Then these ionized gas (charged particles) are acceleration by electromagnetic force to generate thrust.
