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
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.
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