c) Traveling wave
A third type of plasma accelerator, sometimes called the magnetic-induction plasma motor, offers potential advantages over both the foregoing accelerators. It requires neither magnets nor electrodes, and relies on currents being induced in the plasma by a traveling magnetic wave. If the current in a conductor surrounding a tube containing plasma increases, the magnetic field strength in the plane of the conductor will increase. Then an electromotive force will be induced in any loop in this plane. If the conductor current increases rapidly enough, the induced electric field will establish substantial plasma current. The induced magnetic field and plasma current then interact to cause a body force normal to both, which tends to compress the plasma toward the axis of the tube and expel it axially. A traveling-wave accelerator makes use of a number of sequentially energized external conductors along the tube. As the switches are fired in turn, the magnetic field lines move axially along the tube, interacting with induced currents and imparting axial motion to the plasma.
The inward radial force on the plasma this accelerator appears to offer an advantage in keeping the high temperature plasma away from the solid walls of the tube. The fact that no electrodes are needed is also an attractive feature
Laser propulsion is a form of beam-powered propulsion where the energy source is a remote (usually ground -based) laser system and separate from the reaction mass. This form of propulsion differs from a conventional chemical rocket where both energy and reaction mass come from the solid or liquid propellants carried on board the vehicle. Types of laser propulsion are:
1) Ablative Laser Propulsion
Ablative Laser Propulsion (ALP) is a form of beam-powered propulsion in which an external pulsed laser is used to burn off a plasma plume from a solid metal propellant, thus producing thrust. The measured specific impulse of small ALP setups is very high at about 5000 s (49 kN·s/kg). Material is directly removed from a solid or liquid surface at high velocities by laser ablation by a pulsed laser. Depending on the laser flux and pulse duration, the material can be simply heated and evaporated, or converted to plasma. Ablative propulsion will work in air or vacuum. Specific impulse values from 200 seconds to several thousand seconds are possible by choosing the propellant and laser pulse characteristics. Variations of ablative propulsion include double-pulse propulsion in which one laser pulse ablates material and a second laser pulse further heats the ablated gas,
laser micro propulsion in which a small laser onboard a spacecraft ablates very small amounts of propellant for attitude control or maneuvering, and space debris removal, in which the laser ablates material from debris particles in low Earth orbit, changing their orbits and causing them to reenter.
2) Pulsed Plasma Propulsion
A high energy pulse focused in a gas or on a solid surface surrounded by gas produces breakdown of the gas (usually air). This causes an expanding shock wave which absorbs laser energy at the shock front expansion of the hot plasma behind the shock front during and after the pulse transmits momentum to the craft. Pulsed plasma propulsion using air as the working fluid is the simplest form of air-breathing laser propulsion.
3) Laser electric propulsion
A general class of propulsion techniques in which the laser beam power is converted to electricity, which then powers some type of electric propulsion thruster. Usually, laser electric propulsion is considered as a competitor to solar electric or nuclear electric propulsion for low-thrust propulsion in space. However, Leik Myrabo has proposed high-thrust laser electric propulsion, using magneto hydrodynamics to convert laser energy to electricity and to electrically accelerate air around a vehicle for thrust.
4) Laser accelerated plasma propulsion system
Recently conducted experiments at the University of Michigan have shown that ultra short pulse (ultra fast) lasers could accelerate charged particles to relativistic speeds. Current achievable laser peak power of about 10 ^15 Watts has been utilized in the study of relativistic nonlinear optics in plasmas, and it is expected that laser power values will be reached in the near future that will accelerate protons to energies equal to their rest mass energy. That readily means that is such particles are ejected from a propulsion system at 0.866 (the speed of light), they will produce a specific impulse of 26 million seconds. Present day experiments have also demonstrated that a beam of MeV protons containing more than 10^10 particles at 100 MeV energy will indeed be achieved in the not too distant future. Propulsion systems based on such concepts will indeed make distant planets in the solar system, and some interstellar missions, achievable in relatively short times.