The Tokamak sawtooth refers to a recurrent magnetohydrodynamic (MHD) instability observed in the core of tokamak plasmas, characterized by a periodic rise and sudden collapse of the central plasma temperature and density. When plotted against time, this cycle produces a distinctive waveform that resembles the teeth of a saw, hence its name.
This phenomenon is a common and fundamental feature in tokamak operations, representing an internal redistribution of energy and particles within the plasma core. The cycle typically begins with a slow, gradual increase in the central plasma temperature and density, primarily driven by ohmic heating and, in reactor-relevant plasmas, by alpha particle heating from fusion reactions. As the current density in the plasma core becomes increasingly peaked, the safety factor ($q$) on the magnetic axis ($q_0$) often drops below unity. This condition is a key trigger for an internal kink mode, specifically a $m=1, n=1$ helical instability, where $m$ and $n$ are the poloidal and toroidal mode numbers, respectively.
Once this instability is triggered, it leads to a rapid "sawtooth crash." During the crash, the central plasma temperature and density abruptly flatten across a region defined by the $q=1$ magnetic surface. This process effectively expels energy and particles from the core to the outer regions, causing a sharp drop in central values. Following the crash, the core begins to reheat and refill, initiating the next slow rise phase and completing the sawtooth cycle.
While sawteeth can contribute to a periodic loss of central confinement and act as a mechanism for flushing impurities from the core, large or infrequent sawtooth crashes can be detrimental to overall plasma performance. Strong sawtooth crashes have the potential to trigger other, more dangerous MHD instabilities, such as neoclassical tearing modes (NTMs), which can significantly degrade overall plasma confinement and, in severe cases, lead to plasma disruptions. Consequently, understanding and actively controlling sawtooth oscillations are crucial for achieving stable and high-performance operation in tokamaks, particularly for future fusion reactors like ITER. Various control methods, such as electron cyclotron current drive (ECCD), ion cyclotron resonance heating (ICRH), or neutral beam injection (NBI), are employed to modify the current profile and thereby stabilize or tailor the sawtooth activity to avoid adverse effects.