A plasma is a fully or partially ionised gas where positive ions and free electrons coexist. Key parameters:

• Debye length $\lambda_D = \sqrt{\varepsilon_0 k_B T / n e^2}$ — electrostatic screening length
• Plasma frequency $\omega_p = \sqrt{ne^2/\varepsilon_0 m_e}$ — natural oscillation frequency
• Criterion (Langmuir): $n\lambda_D^3 \gg 1$

Plasmas conduct electricity but also exhibit complex wave modes (Alfvén waves, Langmuir waves).

Fusion fuel: deuterium ${}^2\text{H}$ + tritium ${}^3\text{H} \to {}^4\text{He} + n + 17.6\ \text{MeV}$. The Lawson criterion for ignition: $nT\tau_E > 3\times10^{21}\ \text{m}^{-3}\text{keV}\text{s}$ ($T \sim 100$ million K).

Tokamaks use nested toroidal magnetic fields to confine hot plasma. ITER (France) aims to deliver $Q = P_{fusion}/P_{input} > 10$.