Heavy-ion fusion system assessment: final focus and transport model

The required small spot size on the pellet is met by low specified emittance, small momentum spread, and a set of other focal and reactor constraints that are only partially understood at present. The convergence cone half-angle, Theta, is set at a value that is determined by tradeoffs among economic and physical factors. Factors driving a low value of Theta are dispersion, aberrations, magnet costs, reactor economics, shielding and beam-line vacuum. Factors driving a large value of Theta are the emittance limit, space charge limits, and jitter control. The typical range of reactor values (Theta = 5-20 mr) is the result of compromise among these factors. Aside from the spot size condition, it is desired to make the normalized emittance relatively large (epsilon/sub n/ = β γ epsilon > 10-5 m-r) since transportable current is found to vary as epsilon/sub n//sup 2/3/, and too small a value of epsilon/sub n/ could result in an unreasonably large number of beamlets in the high energy portion of the linac (N increasing above ∼ 24)