There are mainly two types of propulsion system namely chemical & plasma propulsion system that is used to lift the payload from earth's surface to space. Chemical rockets burn tremendous amount of fuel, are very costly & only suitable for short range trip like lifting the payload from earth's surface to an orbit round the earth. Plasma propulsion system is most suitable for long range voyage to deep space in outer solar system because they are cost effective. Rockets using plasma as propellant , generate much less thrust, but their extremely frugal consumption of propellant allows them to operate for much longer periods & can reach greater speeds or breaking necessary to achieve the desired trajectory.
Plasma is a near ideal gas of electrons with an almost equal charge density of ions. A plasma is produced by adding energy to a gas by radiating it with lasers, microwaves or by subjecting it to a very strong electric field, when electrons are liberated from atoms of gas leaving the latter with a positive charge & the electrons move freely in the gas.
Plasmas contain charged particles, whose motion is strongly affected by electric & magnetic field which can be utilised to regulate the flow of plasma. Application of electric or electromagnetic fields to a plasma can accelerate its constituents & send them out at the exhaust of the engine as thrust generating exhaust. The necessary fields can be generated by passing currents through the electrodes or external wire coils or by driving electric currents through the plasma or by using magnets. Due to presence of free electrons, the ionized gas in plasma is a far better electrical conductor than copper.
So long the deep space probe is within the vicinity of sun's strong rays the plasma engine can be powered by solar panels that collect energy from the sun. But when it goes past Mars, it must rely on nuclear power source because sun's rays are too weak there. Small deep space probe can rely on thermoelectric devices heated by the decay of a nuclear isotope. But bigger missions would need nuclear reactor.
The first generation plasma engines like "Ion Drive", "Hall Thruster" etc. can generate limited thrust density. In case of Ion Drive there is a positive charge build up at the exhaust & in case of Hall Thruster electrons collide more frequently with atoms & ions, when the thrust density of plasma drive is increased beyond the thresh hold limit, thereby limiting the capacity drastically. They also face substantial plasma erosion on the inner wall during operation.
Magneto Plasma Dynamic Thruster (MPDT)
An alternative system known as "Magneto plasma dynamic Thruster" has subsequently developed that allows for a denser plasma utilizing the current component that is mostly aligned with the electric field & is far less prone to disruption by atomic collision.
As shown in the figure given below, an MPDT consists of a central cathode sitting within a larger cylindrical anode. Hot liquid lithium propellant is pumped into the annular space between the cathode & anode. There it is ionized by an electric current flowing radically from the cathode to the anode. This current induces an azimuthally magnetic field (one that encircles the central cathode), which interacts with the same current that induced it to generate thrust producing Lorentz force. Due to the action of this force ions are accelerated rearwards & come out of the exhaust plume generating thrust.
A single MPD engine ,the size of a household pail can process about a million watts of electric power from a source of a solar or nuclear power plant into thrust which is substantially larger than the maximum power limits of any other plasma engine of the same size. It can generate 15 to 100 km/sec exhaust velocity, thrust 2.5 to 25 newtons & has efficiency of 40 to 60%.
This design also offers the advantage of throttling: its exhaust speed & thrust can be easily adjusted by varying the electric current level or the flow rate of the propellant. The throttling allows a mission planner to alter a spacecraft's engine thrust & exhaust velocity as needed to optimize its trajectory. In order to tackle various operational problems like electrode erosion, plasma instability & power dissipation in the plasma certain design improvements have been adopted. One such concept is lithium & barium vapors as propellant. These two elements in combination ionize easily, yield lower internal energy losses in the plasma & help to cool the cathode. The adoption of this propellant combination along with a new type of cathode design that contains channels has resulted in substantially less erosion of the cathode. Further lining of the engine's internal wall with synthetic polycrystalline diamond substantially boosts resistance to plasma erosion. Another variant of this engine called variable specific impulse magneto plasma rocket (VASIMIR) fires within a vacuum chamber resulting in less consumption of fuel, but generates enough thrust & predicted to shorten the travel time from earth to Mars from 6 months required by other plasma engine to just 39 days.