Efficient plasma heating was accomplished by use of high-power (3 MW) radiofrequency waves in the ion-cyclotron range of frequencies (ICRF) even when the power deposition layer was placed half way out in minor radius (Figure 1). Highly localized electron heating was achieved, and control of the power deposition location was demonstrated (Figure 2) utilizing the mode conversion process in which the externally excited fast wave converts to the ion Bernstein wave, which is strongly absorbed by electrons. This technique can be used to drive off-axis plasma current in future experiments.
An improved confinement mode called H-mode was observed above a threshold heating power. A transport barrier is formed at the plasma edge in H-mode plasmas. This transition was shown to be critically dependent on the edge temperature (Figure 3). A substantial spontaneous acceleration of the plasma in the toroidal direction was found in H-mode plasmas. A new type of H-mode called ``enhanced Da H-mode'', was discovered (Figure 4) on Alcator C-Mod. This mode is very favorable because of absence of edge localized modes (ELMs) which can cause severe erosion of plasma facing components, and because a steady state condition can be reached due to a reduced particle confinement time while maintaining good energy confinement. When a centrally peaked density profile was produced by use of pellet injection fuelling, subsequent central heating of the plasma resulted in formation of a transport barrier in the plasma core region (Figure 5). The fusion reactivity was highly enhanced because a very high central pressure could be achieved. Effective plasma heating during the plasma current ramp-up phase was demonstrated, and it was confirmed that a negative magnetic shear region forms in the plasma core.
A complete list of publications.