Studies on nonconventional high-gain target design for ICF

This paper reports some of the studies on nonconventional ICF approaches pe rformed at the ICF Physics and Technology Laboratory of the AEEF in Frascat i, Italy. Having as reference potential difficulties associated to the conv entional central spark ignition (fuel mixing) and to the usual approach to fast ignition by laser (transfer and coupling of the energy pulse, fast ele ctrons energy tuning), we have made explorative work on possible alternativ es. The performances of targets ignited near stagnation by pulses of heavy ion beams (HIB) or by macroparticle impact were previously studied. The nee ded driver energy, the power, and the beam quality requirements, as well as the level of synchronization the implosion and the igniting pulse have bee n found. More recently, to relax some requirements on the HIB beam paramete rs set by the previous approach, the injected entropy approach (IE) has bee n introduced. In this method, the conditions for spark formation are set in the final stages of the implosion, when the spark fuel size is a few times the final size at stagnation (volume a few tens of the final). Energy is i njected at this time to set the spark fuel on a high adiabat. In this paper , for illustration and comparison purposes, some relevant results we previo usly obtained for near-stagnation ignition are first introduced and critica lly reviewed. The new IE method, after a short analytical introduction, is presented and illustrated by the results of extensive 2-D numerical simulat ions. The considered cases refer to imploding cylinders of finite length. A s required by this approach, one or two opposing beams axially injected add itional energy, whereas the acceleration stage of the cylindrical low-entro py implosion was assumed driven by a different driver. Heavy ion beams, sof t X-rays (SXR), and laser generated light ion beams were considered as vect ors for the entropy injection. Issues related to the feasibility of these g enerators are discussed. The study was made for various initial conditions leading to different ignition modes and burn propagation. The, most recent results on the injected entropy method to the ignition of high gain targets are included.