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6 COPPI ET AL.fore, in order to study truly "thermonuclear" plasmas as soon aspossible, it is important to develop such a device for deuterium-tritiumexperiments with the goal of achieving the ideal ignition temperature,with«r> 10 14 .In the case of a magnetically confined plasma in a toroidal configuration,confinement of the alpha particles requires that the radial amplitudeof their orbits (one type is shown in Fig. 3) be smaller than theminor radius of the plasma. Considerations based on analytical formulasapproximated for the representation of the alpha-particle orbits aswell as numerical calculations have allowed detailed descriptions of thegreat variety of orbits possible and have demonstrated that the conditionsfor confinement of more than 95% of the alpha particles are given,in a reasonably accurate manner, byI>9/A MA, (6)where / is the toroidal current and the "aspect ratio" isA = R/a,where R is the major radius and a is the minor radius of the torus.For values of A ~ 2.5-3, therefore, we must have/ > 3-3.5 MA. (7)For considerations of stability of the plasma column as well as forquestions of technical feasibility, on the other hand, the plasma currentcannot be increased arbitrarily. In particular, the so-called safety factorq, must be considered:q s - 5aB,/AI - 5a 2 B,/Rl, (8)where / is the plasma current given in amperes, B, is the toroidal fieldgiven in gauss, and a and R are given in centimeters. This factor mustbe >2 to avoid strong instabilities in the plasma.From Hq. (8) we see that to obtain high currents we must have largevalues of the quantity a 2 B,/R. It is interesting to note that using B, =IJ5R, where /, is the total current in the magnetic coil for the toroidalfield, and Eq. (8) we get/, - IA 2 q s . (9)