Analysis of sawtooth phenomena in Alcator C-Mod plasmas

The physics of sawtooth oscillations in plasmas produced by the Alcator C-M od machine is discussed by identifying the regimes that characterize these oscillations, which occur in the central value of the electron temperature, as measured by X ray and/or ECE diagnostics. Sawteeth are generally presen t in ohmic as well as in RF heated plasmas. It is shown by numerical analys is that these plasmas are stable to ideal internal, pressure driven modes. Hence, the modes responsible for the crash phase of the sawteeth have to be resistive in nature. This is confirmed by numerical computations using a r esistive code. The role of energetic ions, produced by ion cyclotron heatin g, is also discussed.