Limits for the feasibility of ignition experiments

Summary form only given. Ignition has been recognized as the most important objective of fusion research, stressing that "the initial model (experiment) need not resemble the one that is later commercialized". In fact, there are difficult conditions to be met such as the macroscopic stability of the plasma column, rate of thermal energy transport, degree of plasma purity, thermal loading on the first wall, ability to survive major disruptions. For reasons of confinement, the plasma current must be the maximum compatible with reasonable low values of the safety factor, (e.g., q/sub a//spl sime/), for monotonic shear profiles, or he relatively low (e.g., q/sub a//spl sime/6) for a reversed shear configuration. When q/sub a/ low, the MHD stability against m/sup 0/=1 and coupled m/sup 0/=2 modes requires that /spl beta/-poloidal be sufficiently low, such as /spl beta//sub p/<0.3. Since the central plasma pressure is prescribed within a rather well defined range, relatively high poloidal field, /spl beta//sub p/, are required. Where q/sub a/ is high, the toroidal field should be sufficiently high, since there is a limitation on the plasma size and on the minimum value of B that is P acceptable to avoid ballooning modes. In all cases, the fields required are well above those permitted by present superconducting magnet technology. Thus ignition over several (>10) energy replacement times can be pursued by high field compact experiments igniting at peak densities and n/sub eo//spl ap/10/sup 21/ m/sup 3/ relatively low temperature (T/sub o//spl ap/12 keV). Considering the collisional current equilibration time /spl tau//sub |//spl prop/a/sup 2/T/sup 3/2/ as another important reference time of (a is the plasma minor radius), it is clear that relatively small dimensions and low ignition temperatures are compatible with relatively short current pulses. In the range of high field machines that have been studied, from Ignitor (R/sub o//spl ap/1.32 m) to Candor (R/sub o//spl ap/2- 5 m), the set of parameters chosen for construction is based on assessing cost versus benefits.