927yields ^1/^4=0.45, and the Blatt-Weisskopf level densities 3 give pi/p±= 0.40. Thus, the emission of alpha particles is not impeded, so that the preformation factor, /, is near unity (note that f=pi/pi).At higher excitation energies, / decreases; for example, with £/=100 Mev and N= 100, p 1 /p4=0.63. It is thus apparent that the reason why alphas are frequently emitted in nuclear reactions although ice crystals never evaporate from water droplets is that nuclei are highly degenerate Fermi systems, whereas a water droplet is a nondegener-

Experimental results are presented which show that the Alfvtn wave is strongly guided by magnetic fields. The experiment was conducted in a Tokamak plasma using a small dipole loop antenna to generate a localised Alfven ray. The ray was observed, with magnetic probes, to propagate as a localised disturbance along the curved lines of the steady magnetic field without significant refraction due to the effects of finite frequency, resistivity or magnetic field gradients. These results agree with theoretical predictions and demonstrate that a localised Alfven wave may be excited, and may propagate, independently of the fast wave, as expected. The implication of these results for the Alfven wave heating scheme is discussed.

The development of a high-frequency, step-tunable gyrotron operating at submillimeter wavelengths is described. The gyrotron design was optimized for operation at the second harmonic of the electron cyclotron frequency in the TE261 cavity mode, whose resonant frequency is 384 GHz. Experimental results show that second harmonic operation can occur without mode competition as long as the beam current is low (Ib ≲0.8 A), but as the current is increased, the fundamental TE231 cavity mode increases and eventually (Ib ≳1 A) suppresses the second harmonic. The competition between the two modes is studied in detail. The starting current for second harmonic operation is also studied experimentally and compared with calculated results. Other resonances have also been examined. With the present superconducting magnet, the maximum frequency achieved is 402 GHz (second harmonic operation in the TE551 cavity mode) at several kilowatts.

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