experiments, as above, gas is ionized and an axial magnetic field is produced. A rotating magnetic field is produced by external magnetic coils perpendicular to the axis of the machine, and the direction of this field is rotated about the axis. When the rotation frequency is between the ion and electron gyro-frequencies, the electrons in the plasma co-rotate with the magnetic field (are "dragged"), producing current and reversing the magnetic field. More recently, so-called odd parity rotating magnetic fields have been used to preserve the closed topology of the FRC. Single particle orbits FRCs contain an important and uncommon feature: a "magnetic null," or circular line on which the magnetic field is zero. This is necessarily the case, as inside the null the magnetic field points one direction and outside the null the magnetic field points the opposite direction. Particles far from the null trace closed cyclotron orbits as in other magnetic fusion geometries. Particles which cross the null, however, trace not cyclotron or circular orbits but betatron or figure-eight-like orbits, as the orbit's curvature changes direction when it crosses the magnetic null. Because the particle's orbits are not cyclotron, models of plasma behavior based on cyclotron motion like magnetohydrodynamics (MHD) are inapplicable in the region around the null. The size of this region is related to the s-parameter, or the ratio of the distance between the null and separatrix, and the thermal ion gyroradius. At high-s, most particles do not cross the null and this effect is negligible. At low-s, ~2, this effect dominates and the FRC is said to be "kinetic" rather than "MHD." Plasma stability At low s-parameter, most ions inside an FRC follow large betatron orbits (their average gyroradius is about half the size of the plasma) which are typical in accelerator physics rather than plasma physics. These FRCs are very stable because the plasma is not dominated by usual small gyroradius particles like other thermodynamic equilibrium or nonthermal plasmas. Its behavior is not described by classical magnetohydrodynamics, hence there are no Alfvén waves and almost no MHD instabilities despite their theoretical prediction, and it avoids the typical "anomalous transport", i.e. processes in which excess loss of particles or energy occurs. , several remaining instabilities are being studied: The tilt and shift modes. Those instabilities can be mitigated by either including a passive stabilizing conductor, or by forming very oblate plasmas (i.e. very elongated plasmas), or