In this review we will focus on a topic of fundamental importance for both plasma physics and astrophysics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Ulysses’ high latitude observations and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.

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The Solar Wind as a Turbulence Laboratory

Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap.

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The 2012 Plasma Roadmap

Preface Acknowledgements Units Part I. Fusion Power: 1. Fusion and world energy 2. The fusion reaction 3. Fusion power generation 4. Power balance in a fusion reactor 5. Design of a simple magnetic fusion reactor Part II. The Plasma Physics of Fusion Energy: 6. Overview of magnetic fusion 7. Definition of fusion plasma 8. Single particle motion in a plasma - guiding center theory 9. Single particle motion - Coulomb collisions 10. A self consistent two-fluid model 11. MHD - macroscopic equilibrium 12. MHD - macroscopic stability 13. Magnetic fusion concepts 14. Transport 15. Heating and current drive 16. The future of fusion research Appendix A. Analytical derivation of Appendix B. Radiation from an accelerating charge Appendix C. Derivation of Boozer coordinates Appendix D. Poynting's theorem Index.

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Plasma Physics and Fusion Energy

Hall thrusters are very efficient and competitive electric propulsion devices for satellites and are currently in use in a number of telecommunications and government spacecraft. Their power spans from 100 W to 20 kW, with thrust between a few mN and 1 N and specific impulse values between 1000 and 3000 s. The basic idea of Hall thrusters consists in generating a large local electric field in a plasma by using a transverse magnetic field to reduce the electron conductivity. This electric field can extract positive ions from the plasma and accelerate them to high velocity without extracting grids, providing the thrust. These principles are simple in appearance but the physics of Hall thrusters is very intricate and non-linear because of the complex electron transport across the magnetic field and its coupling with the electric field and the neutral atom density. This paper describes the basic physics of Hall thrusters and gives a (non-exhaustive) summary of the research efforts that have been devoted to th...

Tutorial: Physics and modeling of Hall thrusters

Magnetic fluctuations in the solar wind are distributed according to Kolmogorov’s power law f −5/3 below the ion cyclotron frequency fci. Above this frequency, the observed steeper power law is usually interpreted in two different ways: a dissipative range of the solar wind turbulence or another turbulent cascade, the nature of which is still an open question. Using the Cluster magnetic data we show that after the spectral break the intermittency increases toward higher frequencies, indicating the presence of non-linear interactions inherent to a new inertial range and not to the dissipative range. At the same time the level of compressible fluctuations raises. We show that the energy transfer rate and intermittency are sensitive to the level of compressibility of the magnetic fluctuations within the small scale inertial range. We conjecture that the time needed to establish this inertial range is shorter than the eddy-turnover time, and is related to dispersive effects. A simple phenomenological model, based on the compressible Hall MHD, predicts the magnetic spectrum ∼ k −7/3+2α , which

https://iopscience.iop.org/article/10.1086/524056/pdf

Small-Scale Energy Cascade of the Solar Wind Turbulence

The Physics of Plasmas provides a comprehensive introduction to the subject, illustrating the basic theory with examples drawn from fusion, space and astrophysical plasmas. A particular strength of the book is its discussion of the various models used to describe plasma physics and the relationships between them. These include particle orbit theory, fluid equations, ideal and resistive magnetohydrodynamics, wave equations and kinetic theory. The reader will gain a firm grounding in the fundamentals, and develop this into an understanding of some of the more specialised topics. Throughout the text, there is an emphasis on the physical interpretation of plasma phenomena. Exercises are provided throughout. Advanced undergraduate and graduate students of physics, applied mathematics, astronomy and engineering will find a clear but rigorous explanation of the fundamental properties of plasmas with minimal mathematical formality. This book will also appeal to research physicists, nuclear and electrical engineers.

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The Physics of Plasmas

This paper considers electrostatic waves in a Vlasov plasma of unmagnetized ions and magnetized, Maxwellian electrons. The linear dispersion relation is derived for waves in a perpendicular shock such that the most important sources of instability are the E × B and ∇B electron drifts. For the case of cold ions, propagation perpendicular to the applied magnetic field, and the E × B drift alone, a numerical analysis of frequency vs. wave-number is presented. The effects of the ∇B drift are also considered, and it is shown that the maximum growth rate can be larger than the maximum growth rate for the zero magnetic field ion acoustic instabifity under comparable conditions.

Longitudinal waves in a perpendicular collisionless plasma shock: I. Cold ions

The linear Vlasov dispersion relation for electrostatic instabilities driven by a cross‐field current due to density and temperature gradients in a unidirectional magnetic field is examined in the local approximation. Both electrons and ions are taken as magnetized, and propagation is in the plane defined by the drift velocities and the magnetic field. The effects of temperature gradients on three different instabilities are studied and compared. Electron (ion) temperature gradients parallel (antiparallel) to the density gradient enhance the maximum growth rate of the universal drift instability; but, an antiparallel ∇T for either species is necessary to enhance the growth of the lower hybrid drift or to lower the threshold of the ion cyclotron ion drift instability. Oblique propagation of the ion cyclotron ion drift instability corresponds to lower thresholds than at perpendicular propagation.

Electrostatic temperature gradient drift instabilities

The linear Vlasov dispersion relation for electrostatic instabilities driven by a cross field current due to a density gradient in a unidirectional magnetic field is examined in the local approximation. Both electrons and ions are taken as magnetized, and propagation is in the plane defined by the drift velocity and the magnetic field. Three instabilities are studied and compared: the lower hybrid drift, ion cyclotron drift, and the universal drift instability. Thresholds and properties at oblique propagation are studied for a wide range of drift speeds and Te/Ti ratios. Each of these modes propagates at sufficiently oblique angles to prevent stabilization due to finite length in typical linear theta pinches greater than several hundred ion gyroradii in length.

Density gradient drift instabilities: Oblique propagation at zero beta

Energy transport from weak electrostatic microfluctuations, driven unstable by density and temperature gradients perpendicular to a uniform magnetic field in a Vlasov plasma, is calculated. Using the local approximation and assuming resonance broadening as the saturation mechanism, a consistent procedure is used to evaluate and compare the cross‐field energy transport due to enhanced microfluctuations from the universal, lower hybrid, and ion‐acoustic density drift, and the ion, electron, and lower hybrid temperature drift instabilities. If a temperature gradient is the primary source of free energy, the resultant enhanced fluctuations cause a cross‐field thermal energy loss rate which is much greater than the energy loss rate associated with cross‐field particle transport.

J. J. Sanderson

Papers

The Physics of Plasmas

Longitudinal waves in a perpendicular collisionless plasma shock: I. Cold ions

Electrostatic temperature gradient drift instabilities

Density gradient drift instabilities: Oblique propagation at zero beta

Energy transport by weak electrostatic drift fluctuations