Helicon (physics)

A helicon is a low frequency electromagnetic wave.

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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 be 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 reduced upon trying to enter the metal. (Hence the shine of metals: the electromagnetic waves of light are screened out, leading to a high reflectivity.) 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. This includes plasma based rockets for interplanetary travel. ^[1]

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 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.

References

• J.R. Merrill et al (1970). "A Helicon Solid-State Plasma Experiment for the Advanced Laboratory". American Journal of Physics 38 (1): 417–420.
• B.W. Maxfield (1969). "Helicon Waves in Solids". American Journal of Physics 37 (3): 241–269.