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PC368 · Introduction to Plasma Physics

A knowledge wiki for PC368 — Thammasat University, Department of Physics. Fifteen weeks from "what is the fourth state of matter?" to reconnection in a solar flare, rebuilt here as a cross-linked knowledge system rather than a transcription of the notes.

The course in one paragraph

A plasma is an ionised gas that has stopped behaving like a gas. Because the Coulomb force is long-ranged, every particle feels every other one, and the collective response dominates: charge imbalances are screened over a Debye length, and disturbances ring at the plasma frequency. Those two scales define the subject. From there the course climbs a ladder of descriptions, each buying tractability with detail: single-particle orbits and the adiabatic invariants that trap them; the kinetic Vlasov equation, whose analysis yields the strange, collisionless Landau damping; two-fluid theory and its zoo of waves; and finally MHD, where the plasma becomes a conducting fluid that carries the magnetic field frozen into it. The last third asks the two questions that matter for fusion: can you build an equilibrium, and will it survive? The answers are "yes, but only with external coils" and "not without a great deal of care".

Learning path

Stage Topics
1 · What is a plasma Plasma → Ideal plasma → Debye shielding → Quasi-neutrality → Plasma frequency
2 · Why we care Fusion energy → Lawson criterion → Magnetic confinement
3 · Collisions & kinetics Coulomb collisions → Vlasov equation → Landau damping
4 · Boundaries Plasma sheath → Langmuir probe
5 · Single-particle motion Larmor radius → Drift motions → Adiabatic invariants → Magnetic mirror → Loss cone
6 · Waves Plasma waves → Langmuir & ion-acoustic → EM waves in plasma → Dielectric tensor → Magnetised waves → Resonance cones
7 · Instabilities Streaming instability → Landau damping
8 · MHD & equilibrium MHD → Frozen-in theorem → Magnetic stress tensor → MHD equilibrium → Pinch equilibria → Virial theorem
9 · MHD stability MHD instability → Interchange → Sausage & kink → Energy principle
10 · Reconnection Magnetic islands → Reconnection → Sweet–Parker

Jump in

How to study this subject

Plasma physics is a hierarchy of approximations, not a list of topics, and the usual way to get lost is to forget which rung you are standing on. Before any derivation, ask: are particles being tracked individually, as a distribution, as two fluids, or as one? Almost every confusing result in the course is a quantity from one description being compared with a quantity from another.

The computational companion

Nearly everything here is simulated in PHY653: drifts by direct orbit integration, waves and instabilities by particle-in-cell. The contour deformation behind Landau damping is developed properly in PHY622, and the fluid instabilities have direct analogues in PC316.