Control of Sawtooth Oscillation Dynamics using Externally Applied Stellarator Transform

Abstract

The control of sawtooth oscillations is an active area of tokamak research. The sawtooth oscillation is driven by ohmic heating of the core plasma until the safety factor drops below one triggering the growth of an m=n=1 kink-tearing mode. Large sawtooth oscillations need to be avoided in ITER, since they can trigger neoclassical tearing modes and edge localized modes resulting in loss of plasma confinement in some cases. However, small sawtooth oscillations may be beneficial in preventing impurity and helium ash accumulation in the plasma core.

Sawtooth oscillations are observed in the Compact Toroidal Hybrid (CTH), a current-carrying stellarator/tokamak hybrid device. CTH has the unique ability to change the relative amount of applied vacuum rotational transform from stellarator coils to the rotational transform generated by the plasma current. The vacuum rotational transform is systematically varied from 0.02 to 0.13 to observe changes in the sawtooth oscillation. Three two-color soft x-ray cameras were constructed and installed on CTH. Each two-color camera employs two 20-channel diode arrays to detect the signatures of sawtooth instabilities. The diagnostic primarily measures bremsstrahlung radiation which is dependent on temperature, or thermal kinetic energy, of electrons within the plasma. The sawtooth instability is a periodic rearrangement of the core plasma thermal energy. Therefore, the bremsstrahlung radiation is strongly tied to the dynamics of the sawtooth oscillation.

Sawteeth observed within CTH are tokamak-like despite employing a three-dimensional confining field because: (1) the presence of the m=n=1 kink-tearing mode, (2) the monotonically decreasing rotational transform profile is dominated by the plasma current not the vacuum rotational transform, and (3) the measured scaling of the normalized inversion surface radius with total rotational edge transform.

The measured sawtooth period decreases by a factor of two over a vacuum rotational transform from 0.02 to 0.13. The sawtooth amplitude is observed to decrease with increasing levels of 3D field, as quantified by the amount of vacuum transform imposed. The measured crash time of the sawtooth oscillation does not appear to depend of the amount of vacuum transform applied, indicating that the final reconnection dynamics of the m=1 and n=1 mode are not significantly affected by the 3D stellarator fields.

Previous numerical simulations show that the internal kink mode is significantly destabilized with increasing flux surface elongation of the q=1 surface. The experimental results indicate that the decrease in sawtooth period and amplitude is correlated to the mean elongation of the non-axisymmetric plasmas within CTH. This dissertation describes the sawtooth theory for an axisymmetric plasma, the development of the two-color diagnostic used to characterize the sawtooth oscillation, and the properties of the sawtooth oscillation observed in the plasmas within CTH.