Particle Motion

The motion of charged particles in a plasma depends on a number of physical quantities. The motion is fairly easy to describe mathematically when the following approximations are satisfied:

The magnetic field is time-independent
The magnetic field varies only weakly in space
The electric field is both constant in time and in space
Other forces (e.g., gravitation) are weak and constant

Under the above circumstances, the particle motion is a superposition of three parts:
- the particles move in a circle around the magnetic field lines
- they move along the field lines with a speed that may vary slowly
- they drift across the field lines due to magnetic field line curvature, magnetic field gradients, or the presence of an electric field or other forces
- the total particle energy is constant, except when the electric field or other force has a component parallel to the magnetic field
- the "magnetic moment" of the particle (i.e., the ratio of the energy associated with its motion perpendicular to the field, divided by the magnetic field strength) is constant

The above drifts depend on the sizes of the electric and magnetic field, the sizes of the magnetic field gradient and curvature, and the charge of the particle. Also, the sense of gyration of the particle around the magnetic field lines depends on whether it has positive charge (e.g., a proton), or negative charge (e.g., an electron: clockwise motion when viewed along the magnetic field direction).

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