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Available energy and ground states of collisionless plasmas

Part of: Focus on Fusion Frontiers in Plasma Physics Conference

Published online by Cambridge University Press:  06 July 2017

Per Helander
Per Helander*
Affiliation:
Max-Planck-Institut für Plasmaphysik, D-17491 Greifswald, Germany
*
†Email address for correspondence: per.helander@ipp.mpg.de
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Abstract

The energy budget of a collisionless plasma subject to electrostatic fluctuations is considered, and the excess of thermal energy over the minimum accessible to it under various constraints that limit the possible forms of plasma motion is calculated. This excess measures how much thermal energy is ‘available’ for conversion into plasma instabilities, and therefore constitutes a nonlinear measure of plasma stability. A distribution function with zero available energy defines a ‘ground state’ in the sense that its energy cannot decrease by any linear or nonlinear plasma motion. In a Vlasov plasma with small density and temperature fluctuations, the available energy is proportional to the mean square of these quantities, and exceeds the corresponding energy in ideal or resistive magnetohydrodynamics. If the first or second adiabatic invariant is conserved, ground states generally have inhomogeneous density and temperature. Magnetically confined plasmas are usually not in any ground state, but certain types of stellarator plasmas are so with respect to fluctuations that conserve both these adiabatic invariants, making the plasma linearly and nonlinearly stable to such fluctuations. Similar stability properties can also be enjoyed by plasmas confined by a dipole magnetic field.

Keywords

plasma confinementplasma energy balancemagnetized plasmas
Type
Research Article
Information
Journal of Plasma Physics , Volume 83 , Issue 4 , August 2017 , 715830401
Copyright
© Cambridge University Press 2017 

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