Electrical drives are not new technology. They have been available for many years. However, lack of a suitable mission and, in particular, lack of an adequate power source has hampered their development. Although electrical propulsion is not very powerful, it is exceptionally efficient and can be applied for very long periods. So despite tiny accelerations, a vehicle with ion thrusters can eventually reach very high speeds.
Electrothermal Propulsion Systems
An arcjet heats a propellant using an electric arc rather than a chemical reaction. It is therefore a thermal engine. The ISP from an arcjet can be higher than for a chemical rocket, but remains at around 500s, one order of magnitude less than what is required for systems more efficient that standard chemical rockets.
Microwave & ECR thrusters
Electromagnetic Propulsion Systems
Magnetoplasmadynamic (MPD) Thrusters
In a MPD thruster a gas is ionised, turned into a plasma and fed into a acceleration chamber, where the interaction between an electrical current in the plasma and the magnetic field produced by electromagnets pushes the plasma up to high speeds. Vasimir is an application of this principle.
This is one of the best candidates for the Interplanetary propulsion. However, since it is more efficient at larger sizes, the lack of a suitable power source to test the principle in space has hampered the development of this technology. Thrusters with thrust up to 500 N and more are possible. SUPREME, an advanced MPD thruster utilizing high temperature superconductors, has been proposed for use in Earth/Luna missions and may also improve payload capacity for Earth/Mars missions.
The Vasimr (Variable Specific Impulse Magnetoplasma rocket) engine, as per 2011, has an optimum specific impulse of 5000s. The required power is 200 kW, with an efficiency of 60%, for a thrust of 6 N. The fuel is argon, but other gases can be used. The concept should be scalable up to 500N per unit.
A material is transformed into a burst of plasma by a short lived electric arc (think of a spark plug), and the plasma is accelerated by the electric field between an anode and a cathode. This is a simple but inefficient type of thruster that, at 10% efficiency, is not suitable for Mars transportation.
However, in  a proposal is made for a much more powerful version, that, although still only 50% efficient, might be further upgraded to provide the required thrust. The proposed fuel would be lithium. The design is very simple, and might be very light.
Electrostatic Propulsion Systems
The Hall effect thruster is a (mostly) Russian technology. Over 200 units have been flown. Engine performances are comparable to ion grid. Hall effect thrusters are physically smaller than ion grid thrusters. This is a distinct advantage for some configurations. Wikipedia cites efficiencies up to 75%
The ion grid thruster is a mature technology that has performances very close to the Interplanetary transportation mission requirements.
The current NASA model is the NEXT thruster. The engine thrust is very small, at 0,2 N per motor, with 6,9 kW and 70% efficiency. The fuel is Xenon gas. With a size of about 600mm wide per unit, they are physically large.
The Hipep Ion engine has an efficiency of 80% and similar characteristics than the NEXT. The HIPEP is rectangular and can be assembled in tight grids. The model tested was 600mm x 1200mm (approx, to be confirmed).
Colloidal Accelerators & FEEP
This is a technology in the very early stages of development. The fuel is composed of tiny droplets of semi conductors, encased in a shell of protein. The propulsion method uses electric fields to accelerate the particles. Efficiencies may be very high. The envisioned market is micro satellites, but it might be possible to 'print out' large boards of these micro thrusters using micropressor production technologies and eventually reach the required thrust (with millions of thrusters).
Fusion ion systems
Scaled radioisotope positron propulsion
An experimental system described in  showcased in  and animated in  from the company Positron Dynamics builds upon earlier work on coupling the energy produced from electron/positron annihilation into Deuterium , forming a possible design for a high performance rocket engine. In this design 79Kr is frozen on a cold plate, forming a positron source as it naturally decays. Those positrons are then moderated, formed into a pulse train and then focused through a set of ion beam optics on to deuterium contained on a metal tape. Those positrons then annihilate electrons in the deuterium, rapidly releasing energy and igniting fusion. The fusing deuterium then forms an expanding ball of hot plasma that exits through a magnetic nozzle providing thrust. It also releases neutrons, breeding 79Kr from 78Kr stored in tanks around the reaction chamber. The 79Kr is then enriched from the Kr tanks via an enrichment process proposed by Mills et al.
Pulsed Fission Fusion (PuFF)
PuFF is an experimental spacecraft propulsion system, utilizing high energy plasma produced through controlled microfusion detonations, similar to a micro scale Orion drive with a predicted Isp of 30,000 seconds, a thrust of 29KN and an in space system weight of 240 Tons, allowing for theorized mission profiles of Earth to Mars in 39 days. Electric power is collected in a linear transformer driver, producing a 2MA/2MV pulse which is discharged through a target consisting of a Deuterium/Tritium core wrapped in a 235U liner encased in a lithium tamper. This forms a Z pinch as the target is vaporized into a plasma and then crushed through lorenz forces. This crushing then increases the density of the 235U plasma past supercriticality, causing it to start to rapidly fission and further heat the plasma. Once that plasma is hot enough it begins D/T fusion, producing neutrons that amplify the fission chain reaction. The hot plasma eventually eventually exits through a magnetic nozzle producing thrust. The system then cycles for the next reaction, repeating ~100 times per second. The chain reaction is overall:
- + + + ~
- + + 2.5n + ~
With a side D/D fusion stage of:
- + + + ~
- + + + ~
- + + + ~
This side reaction is important as although D/D fusion requires more heat to ignite, it leaves considerably more energy in the reaction products instead of high energy neutrons like in D/T fusion. Tritium is produced in situ via neutron capture of 6Li in a lithium blanket around the reaction chamber or alternatively using a solid target core of 6LiD. Lithium, Deuterium and 233U (via Thorium) are all reasonably common on Mars and so this system could possibly be built or refueled on Mars.
The Dipole Drive by Robert Zubrin utilizes charged particles in free space to produce thrust via a set of external electrostatic grids. These grids extend outwards like a solar sail, and accelerate charged particles that pass through them via the electric field between them.
Xenon is an inert gas that is relatively easy to ionise and denser that other inert gases. It is the best choice for the fuel of most types of ion engines and may be used as well as an ionizing agent in the coolant system, to provide the required plasma for the MHD generator. Xenon is quite expensive, at about $20 per liter (6g). There are about 45 billions tons of Xenon in Earth's atmosphere. Due to cost and availability concerns, argon may be a better choice that Xenon.
Argon is a inert gas that composes almost 1% of Earths atmosphere. It can be used instead of Xenon as propellant for electric propulsion. It should also be possible to use it as the ionising agent in the cooling system for a MHD generator operation. The Vasimir engine can use argon as propellant. Argon is available in the Martian atmosphere and might be an in-situ resource for space electric propulsion.
Krypton, another inert gas, is used by SpaceX for the thrusters in its Constellation project.
Water has been proposed for some kinds of thermal rockets. The high temperatures and the need for ionisation in electrical engines would probably dissociate water its components.
Helium is fairly easy to ionize.
Sodium, Lithium, lead, lead bismuth, mercury have all been proposed for electric propulsion. Concerns with toxicity during testing has led to the the abandonment of a number of these metals.
Nanoparticles in suspension in a carrier fluid can have interesting propulsive properties.