Radial energy channels may account for unexpected plasma losses

Magnetically confined plasmas, like those found in experimental tokamak reactors, roil with energy fluxes. The coupling between fast-moving ions and waves injected by experimentalists sustain a plasma’s hot core, and diffusion and thermal conduction can carry energy away — a process typically enhanced by electromagnetic fluctuations in the plasma. A precise understanding of these energy fluxes is necessary for the development of any future fusion reactor.

Recently, scientists have suggested that a particular family of plasma oscillations known as Alfvén waves might explain the unusually high energy losses suffered in some plasma experiments. In 1942, Hannes Alfvén proposed the existence of these waves as a mechanism to transport heat within the outer magnetized regions of the sun. Alfvén waves exist throughout the universe, and Hannes Alfvén was awarded the 1970 Nobel Prize in recognition of the importance of these waves to magnetized plasma in stars and in the laboratory.

While the magnetic field emerges outward from the sun’s surface, in a tokamak, the magnetic field is wrapped into a torus, and additional considerations are needed to explain how Alfvén waves could account for energy leaving the hot regions of tokamak plasma.

In a paper by Kolesnichenko et al., researchers resolved this apparent paradox by showing that the outward radial velocity of Alfvén energy transfer could be nonzero in tokamaks. The calculation — a more realistic result than previous calculations that assumed a homogenous and infinite plasma — suggests that these modes cause a radial compression of the plasma and lead to a coupling between traveling Alfvén waves and fast magnetoacoustic waves that permeate the plasma.

The researchers showed that although Alfvén modes are standing waves, they can transfer energy as long as there are regions that act as sources and sinks of energy, often caused by plasma instabilities. In this way, the amplitude of an Alfvén mode gets damped away from the plasma core, suggesting unequal contributions from radially inward- and outward-traveling waves and a net energy flux. For parameter regimes relevant to plasma experiments, the authors calculate that this flux can be quite large — even rivaling the size of the energy flux due to thermal conduction.

Source: “Mechanisms of the energy transfer across the magnetic field by Alfvén waves in toroidal plasmas,” by Ya. I. Kolesnichenko, Yu. V. Yakovenko, and M. H. Tyshchenko, Physics of Plasmas (2018). The article can be accessed at https://doi.org/10.1063/1.5049543 .