Resolving the controversy surrounding the value of the on-axis magnetic safety factor during the sawtooth cycle
2024 Research Campaign, Transient Control
Purpose of Experiment
Create a plasma that shows a sawtooth cycle with q(0)~0.7 instead of q(0)~1.0 to verify theoretical predictions for sawtooth activity near q(0)=0.7 and confirm that long−debated measurements of q(0)~0.7 are valid. For reaching q(0)~0.7 the 1/1 mode, which is responsible for the commonly observed (Kadomtsev) sawtooth cycle, has to be stabilized. This can be achieved by using a circular plasma shape with an on−axis peaked current profile.
This experiment has been designed based on predictions for a m/n=2/3 instability that can explain historical sawtooth instability observations in which q(0)~0.7 was found instead of q(0)~1 [Smiet
et al. Nucl. Fusion 60 (2020) 084005]. The ideas for stabilizing the m/n=1/1 sawtooth cycle were presented in [Bussac et al. PRL 35 (1975) 1638] where a toroidal plasma with a circular plasma cross section were considered.
This experiment should resolve a 30 year controversy for the value of q(0) and hopefully shows that tokamak plasmas can exhibit sawtooth cycles with q(0)~0.7 and with q(0)~1.0.
Experimental Approach
Repeat the circular shot but change the plasma ellipticity slowly (in 2−3 sec) from 1.0 to ~1.4 during the steady state phase after the 2/3 sawtooth instability is well established. During the ellipticity ramp the 1/1 mode should become unstable and the 2/3 sawtooth activity should change into the 1/1 sawtooth. Signs for this change is a sudden increase in sawtooth period and amplitude. If we don’t observe a sawtooth period change and if MSE does not indicate that q(0)=0.7 we can try to lower q(0) down to 0.7 by driving drive extra current at the magnetic axis with ECCD.