text: Nico van Kampen showed that a linearized set of equations describing these substances exhibits a continuous spectrum of normal modes.
full_question: Nico van Kampen showed that a linearized set of equations describing these substances exhibits a continuous spectrum of normal modes. These materials can be compressed by the "Z" or "theta" versions of the Bennett pinch, and when mobility in these substances is low, a characteristic distance measure is proportional to the square root of the electron temperature divided by the electron density. Stellarators provide alternatives to devices that use both toroidal and poloidal magnetic field lines in order to confine these substances into torus shapes. These substances, carry very fast oscillations called Langmuir waves, are used in tokamaks, and form atmospheric phenomena like St. Elmo's Fire. For 10 points, name these substances made of ions and electrons, the fourth phase of matter.
first_sentence: Nico van Kampen showed that a linearized set of equations describing these substances exhibits a continuous spectrum of normal modes.
raw_answer: plasmas