Inelectromagnetism, ahelicon is a low-frequencyelectromagnetic wave that can exist in boundedplasmas in the presence of amagnetic field. The first helicons observed were atmosphericwhistlers,^[1][2] but they also exist in solid conductors^[3][4] or any other electromagnetic plasma. Theelectric field in the waves is dominated by theHall effect, and is nearly at right angles to theelectric current (rather than parallel as it would be without the magnetic field); so that the propagating component of the waves is corkscrew-shaped (helical) – hence the term “helicon,” coined byAigrain.^[5]

Helicons have the special ability to propagate through puremetals, given conditions of low temperature and high magnetic fields. Most electromagnetic waves in a normalconductor are not able to do this, since the high conductivity of metals (due to theirfree electrons) acts to screen out the electromagnetic field. Indeed, normally an electromagnetic wave would experience a very thinskin 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 ofangular frequency. Thus a low-frequency electromagnetic wave may be able to overcome the skin depth problem, and thereby propagate throughout the material.

One property of the helicon waves (readily demonstrated by a rudimentary calculation, using only the Hall effect terms and aresistivity term) is that at places where the sample surface runs parallel to the magnetic field, one of the modes contains electric currents that “go to infinity" in the limit of perfect conductivity; so that theJoule heating loss in such surface regions tends to a non-zero limit.^[6][7][8] The surface mode is especially prevalent in cylindrical samples parallel to the magnetic field, a configuration for which an exact solution has been found for the equations,^[6][9] and which figures importantly in subsequent experiments.

The practical significance of the surface mode, and its ultra-high current density, was not recognized in the original papers, but came to prominence a few years later when Boswell^[10][11] discovered the superior plasma generating ability of helicons – achieving plasma charge densities 10 times higher than had been achieved with earlier methods, without a magnetic field.^[12]

Since then, helicons found use in a variety of scientific and industrial applications – wherever highly efficient plasma generation was required,^[13] as innuclear fusion reactors^[14] and inspace propulsion (where thehelicon double-layer thruster^[15] and theVariable Specific Impulse Magnetoplasma Rocket^[16] both make use of helicons in their plasma heating phase). Helicons are also utilized in the procedure ofplasma etching,^[17] used in the manufacture of computer microcircuits.^[18]

A helicon discharge is an excitation of plasma by helicon waves induced throughradio frequency heating. The difference between a helicon plasma source and aninductively coupled plasma (ICP) 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 calledVASIMR also uses helicon discharge for generation of plasma in its engine. Potentially,helicon double-layer thruster plasma-based rockets are suitable for interplanetary travel.

• Helicon double-layer thruster
• Variable Specific Impulse Magnetoplasma Rocket

• Electromagnetic radiation

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