Helicon (physics)
Encyclopedia
A helicon is a low frequency electromagnetic wave that can exist in plasmas
in the presence of a magnetic field. The first helicons observed were atmospheric whistlers
, but they also exist in solid conductors or any other electromagnetic plasma.
Helicons have the special ability to propagate through pure metals, given conditions of low temperature and high magnetic fields. Most electromagnetic waves in a normal conductor are not able to do this, since the high conductivity of metals (due to their free electrons) acts to screen out the electromagnetic field. Indeed, normally an electromagnetic wave would experience a very thin skin depth
in a metal: the electric or magnetic fields are quickly reflected upon trying to enter the metal. (Hence the shine of metals.) However, skin depth depends on an inverse proportionality to the square root of angular frequency. Thus a low frequency electromagnetic wave may be able to overcome the skin depth problem, and thereby propagate throughout the material.
A helicon discharge is an excitation of plasma by helicon waves induced through radio frequency heating. The difference between a helicon plasma source and an inductively coupled plasma
is the presence of a magnetic field directed along the axis of the antenna. The presence of this magnetic field creates a helicon mode of operation with higher ionization efficiency and greater electron density than a typical ICP. The Australian National University, in Canberra, Australia, is currently researching applications for this technology. A commercially developed magnetoplasmadynamic engine called VASIMR also uses helicon discharge for generation of plasma in its engine. Potentially, Helicon Double Layer Thruster
plasma based rockets are suitable for interplanetary travel.
A metal such as 99.999% pure indium
is commonly used: it is cooled using liquid helium
to reach the conditions of low temperature, while the high magnetic field is accomplished using a superconducting solenoid. Ultimately, the experiment characterizes the resonance frequency and resonance width of the helicon standing waves. It can also be used to measure the magnetoresistance
and Hall coefficients of the pure metal.
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
in the presence of a magnetic field. The first helicons observed were atmospheric whistlers
Whistler (radio)
A whistler is a very low frequency electromagnetic wave which can be generated, for example, by lightning. Frequencies of terrestrial whistlers are 1 to 30 kHz, with maximum usually at 3 to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be...
, but they also exist in solid conductors or any other electromagnetic plasma.
Helicons have the special ability to propagate through pure metals, given conditions of low temperature and high magnetic fields. Most electromagnetic waves in a normal conductor are not able to do this, since the high conductivity of metals (due to their free electrons) acts to screen out the electromagnetic field. Indeed, normally an electromagnetic wave would experience a very thin skin depth
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
in a metal: the electric or magnetic fields are quickly reflected upon trying to enter the metal. (Hence the shine of metals.) However, skin depth depends on an inverse proportionality to the square root of angular frequency. Thus a low frequency electromagnetic wave may be able to overcome the skin depth problem, and thereby propagate throughout the material.
A helicon discharge is an excitation of plasma by helicon waves induced through radio frequency heating. The difference between a helicon plasma source and an inductively coupled plasma
Inductively coupled plasma
An inductively coupled plasma is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.-Operation:...
is the presence of a magnetic field directed along the axis of the antenna. The presence of this magnetic field creates a helicon mode of operation with higher ionization efficiency and greater electron density than a typical ICP. The Australian National University, in Canberra, Australia, is currently researching applications for this technology. A commercially developed magnetoplasmadynamic engine called VASIMR also uses helicon discharge for generation of plasma in its engine. Potentially, Helicon Double Layer Thruster
Helicon Double Layer Thruster
The Helicon Double Layer Thruster is a prototype spacecraft propulsion engine. It was created by Australian scientist Dr. Christine Charles, based on a technology invented by Professor Rod Boswell, both of the Australian National University....
plasma based rockets are suitable for interplanetary travel.
Experiments
This experiment can be conducted with fairly affordable equipment, and may be found in university-level undergraduate advanced physics laboratory courses.A metal such as 99.999% pure indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
is commonly used: it is cooled using liquid helium
Liquid helium
Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...
to reach the conditions of low temperature, while the high magnetic field is accomplished using a superconducting solenoid. Ultimately, the experiment characterizes the resonance frequency and resonance width of the helicon standing waves. It can also be used to measure the magnetoresistance
Magnetoresistance
Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. The effect was first discovered by William Thomson in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This...
and Hall coefficients of the pure metal.
See also
- Helicon Double Layer ThrusterHelicon Double Layer ThrusterThe Helicon Double Layer Thruster is a prototype spacecraft propulsion engine. It was created by Australian scientist Dr. Christine Charles, based on a technology invented by Professor Rod Boswell, both of the Australian National University....
- Variable Specific Impulse Magnetoplasma RocketVariable specific impulse magnetoplasma rocketThe Variable Specific Impulse Magnetoplasma Rocket is an electro-magnetic thruster for spacecraft propulsion. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust...