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Plasma physics

Cosmic waves in the lab

An Alfvén-wave maser, a feature of atmospheric and astrophysical science, has been created in a laboratory, and opens the way for further Earth-bound investigations of cosmic phenomena.

In general, to be an observational astronomer is to be a spectroscopist, unravelling the workings of the cosmos through the varying wavelengths of radiation detected. Observations that began at optical wavelengths now extend to wavelengths ranging from hundreds of kilometres down to tens of nanometres, and satellites have also proved immensely useful in refining our image of the Universe. But, despite the assiduous measurements, the radiation is often produced, particularly at radio and X-ray wavelengths, by very complex processes that we struggle to understand. Nevertheless, a positive step has now been taken: in Physical Review Letters, J. E. Maggs and G. J. Morales report the resonant amplification of a typical astrophysical wave in their laboratory — an Alfvén-wave maser (Phys. Rev. Lett. 91, 035004; 2003).

For some years, the aim of these authors has been a laboratory study of magnetohydrodynamic turbulence — that is, turbulence in the interactions between a plasma and a magnetic field. The first step on the way is to create the basic element of the phenomenon, an Alfvén wave. Postulated in 1942 by Hannes Alfvén (Nature 150, 405–406; 1942), an Alfvén wave is a low-frequency electromagnetic wave that can be generated in magnetized plasmas throughout the Universe. These waves are thought to be intimately involved in diverse phenomena, such as the precipitation of electrons through the atmosphere in the auroral regions that are connected with the dazzling northern and southern lights — and that can take out power distribution systems. Alfvén waves are also implicated in the heating of electrons in the solar corona and in the dynamics of solar flares.

In many situations, an Alfvén wave might not propagate freely but instead be trapped between reflective surfaces, for example between the Sun's photosphere and corona, in the intervening layer known as the chromosphere. This is analogous to light waves trapped between the mirrors of a resonant cavity of a laser. Laser action at microwave frequencies — a 'maser' — is a known astrophysical phenomenon. So Maggs and Morales set out to demonstrate a laboratory-based maser for Alfvén waves.

In general, laboratory experiments attempting to mimic the behaviour of the aurora, the solar chromosphere or more exotic cosmic objects have met with scepticism, if not outright hostility, in the astrophysical community. Ever since William Gilbert, in about 1600, used a magnetized sphere — a terrella — to explain the magnetic compass to Queen Elizabeth I, many brave attempts have been made in terrestrial laboratories to pin down exotic cosmic phenomena. Designing a terrestrial experiment to study Alfvén waves is certainly not a simple exercise — several conflicting scalings need to be satisfied if the sceptics are to be convinced. For example, it is extremely difficult to make a laboratory experiment long enough and wide enough to allow the wave to propagate in a natural way and not have it cramped up and its motion constricted. Also, the laboratory plasma needs to be created from a background gas, which will collide with the plasma and damp out waves. This really does not exist in the vacuum of space.

Maggs and Morales' experiment was performed using the Large Plasma Device (at the Basic Plasma Science Facility at the University of California, Los Angeles), a unique facility with 90 magnetic-field coils surrounding a column of helium plasma 19 m long (Fig. 1). At one end of the column, a pulsed voltage draws electrons out of a thermionic cathode towards an anode made of copper mesh, located 55 cm away. The accelerated electrons drift into the 19-m vacuum chamber and strike neutral, low-pressure helium gas, generating a plasma that is more than 50% ionized.

Figure 1: Unique facility — the Large Plasma Device at the University of California, Los Angeles.
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W. GEKELMAN

In a helium plasma inside this 19-metre-long column of magnets, Maggs and Morales have created an Alfvén-wave maser.

The resonant cavity is formed by the space between the cathode and the semi-transparent anode: inside, Alfvén waves are spontaneously amplified, but they seep through the anode into the main plasma region, in which they can be detected. The magnetic field, the plasma density and the degree of ionization need to be manipulated so that the ion cyclotron frequency (the frequency with which the positively charged ions rotate around the magnetic field) is much greater than the ion neutral collision frequency (the rate at which plasma collides with the background gas). This means that the Alfvén waves are not damped in the plasma column. The Large Plasma Device is one of the very few experimental devices in the world in which such a happy combination of operational parameters can be achieved.

Using small magnetic loop antennae inserted into the plasma column, Maggs and Morales measured a broadband noise spectrum with a peak at 60% of the ion cyclotron frequency, and an upper limit of 70% of that frequency defined by the length of their resonant cavity. This noise was identified as a type of Alfvén wave called a shear wave. The pièce de résistance of the experiment was their observation of spontaneous amplification of the resonant mode (at 60% of the ion cyclotron frequency) for selected values of the confining magnetic field and plasma current. Maggs and Morales describe the phenomenon as "spectacular flares of extremely coherent signals [that] develop at low magnetic fields and extremely high plasma currents" — a little reminiscent of sci-fi writer E. E. 'Doc' Smith perhaps, but nevertheless explosively descriptive of the growth and subsequent decay of their Alfvén maser.

Maggs and Morales' experiment demonstrates the great potential for carrying out experiments of cosmic importance in the laboratory. It will no doubt be a boon for the many theoreticians wishing to test their theories of magnetohydrodynamic turbulence and of Alfvén-wave-induced heating of stellar atmospheres.

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