Physics basis for compact ignition experiments

Compact ignition experiments can be designed to attain peak densities n₀ ≃ 10²¹ m^-3 reliably and to achieve "low temperature" D-T ignition under conditions where the heating power of the α-particles does not exceed twice the ohmic heating power. An average current density near 1 kA/cm² with a maximum plasma current I_p ≃ 12 MA are the reference design parameters of the representative Ignitor Ult machine. The peak density value is consistent with that of B/R₀, as R₀ ≃ 1.3m and B_Υ ≃ 13 T, when compared to the Alcator, FT, and TFTR machines. The vacuum toroidal field B_Υ is reinforced by the contribution of a large paramagnetic poloidal current (I_θ < 10 MA). The high values of the poloidal field B_p (≃3.9T) produce a strong rate of ohmic heating, while the correspoinding high values of I_p ensure that most of the fusion α-particles can be confined to deposit their energy in the central part of the plasma column. The confinement parameter n₀ τ_E is expected to exceed ≃4 x 10²⁰sec/m³ by a considerable margin on the basis of the confinement properties of plasmas with prevalent ohmic heating. The necessary degree of plasma purity (Z_eff ≲ 1.6) is expected to be ensured by the high particle densities, the high magnetic fields and the relatively low values of the thermal loading on the first wall. Since ignition is achieved through transient conditions rather than, as is frequently assumed, a sequence of steady state conditions, numerical simulations have shown that the ohmic power can remain considerable up to ignition, when programming the rise of the plasma current and the particle density while gradually increasing the cross section of the plasma. The volume where the magnetic parameter q ⩽ 1 and macroscopic m⁰ = 1 plasma modes may be excited, has been shown to maintain quite low values until ignition is attained. The stability margin against these modes is made uniquely favorable by the low values of β-poloidal that are characteristic of Ignitor. An injected heating source of 16MW of ICRF power is included to broaden the scenarios under which ignition is achieved.