v. MAGNETOHYDRODYNAMIC EQUILIBRIUM AND STABILITY

The layout of the major components of the BPX poloidal field (PF) system is shown in Fig. 5.1. External to the toroidal field (TF) magnet and assembly are the seven updown symmetric pairs of primary PF-producing coils. These are called the external PF coils. They are labeled in pairs, with PFlU being the inner (small major radius) PF coil closest to and above the midplane and PFZU-PF7U proceeding clockwise around the machine. Similarly, PFlL is the reflection of PFlU about the midplane, and PF2L-PF7L proceeds counterclockwise around the machine to the outside. The external PF coils have three major functions during a BPX discharge. First, they are the induction coils that provide the flux linkage (or volt-seconds) required to build up and sustain the plasma current by carrying coil currents changing in the opposite direction. Second, they are the shaping coils that produce and control the plasma shape and details of the divertor sweep. Third, they provide control of both the radial and vertical plasma position on time scales greater than about 100 ms. Each of these functions is described in detail in later sections. In addition to the external PF coils, there are two smaller updown symmetric pairs of coils lo cated inside the TF magnet, but outside the vacuum vessel. These are called the internal control (IC) coils. The upper coils, IClU and ICZU, are numbered clockwise, and the lower coils, IClL and IC2L, are numbered counterclockwise in the same convention as for the external coils. The IC coils have three major functions. They provide the fast time scale vertical control of the wall stabilized axisymmetric instability as discussed in Sec. V.E. They also provide control of the radial plasma position on the fast time scale, enabling the plasma to recover, for example, from a minor disruption or to have its position adjusted