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THERMONUCLEAR IGNITION 19increase of energy loss from instability tied to the behavior of the ionswas observed; but this could have been seen theoretically. The situationinstead is much more uncertain regarding the instability related toelectron transport in regimes that are barely collisional.There are theoretical indications that at low density and hightemperature the losses in the "electron channel" can be noticeablygreater than those encountered in plasmas obtained up to now. On theother hand, with some of the experiments performed on the PLT theelectron temperature has remained rather low (2 keV). For this reason,it is safer to operate at the highest densities possible and the lowesttemperatures possible, as proposed for the Ignitor.(2) The large devices require large investments and long-term decisions.Thus every variation in a line of thought that can be undertakenon the basis of new physical information requires a long period of timefor acceptance, incorporation into a project, and final realization. Thisis not true for compact devices, whose cost is of lower order than that ofthe large devices.In conclusion, we maintain that there still exists a state of uncertaintyin the understanding of many phenomena that could be important infuture fusion reactors. This uncertainty means that it is not wise toinvest "all or nothing" into the large devices, and not leave room for theparallel development of a line of experimental devices that, while notexcessively costly, will provide information of great interest for futuremagnetic-confinement fusion reactors.REFERENCES1. B. Coppi and J. Rem, Sci. American (July 1972).2. R. Pozzoli and E. Sindoni, Scientia 113, 659 (1978).3. M. Fontanesi and E. Eazzaro, Energ. Nucl. 25, 451 (1978).4. M. Murakami and H. Eubank, Phys. Today 32, 25 (1979), No. 2.5. H.P. Furth, Sci. American (July 1979).6. B. Coppi, Comm. Plasma Phys. Contr. Fusion 3, 47 (1977).7. B. Coppi and A. Taroni, Proc. Joint Varenna-Grenoble Intl. Symposium onHeating in Toroidal Plasmas, Grenoble 2, 113(1978).8. B. Coppi, Massachusetts Institute of Technology RLE Report PRR-75/18(1975).