Comparison of n=1 and n=2 stability limits in high qmin discharges

As the plasma pressure profile broadens, the lowest $\beta_N$ stability limit transitions from an n=1 to n=2 mode. Recent upgrades of DIII-D have increased the off-axis neutral beam (NB) power to give a total of 8 MW of on- and 7 MW of off-axis NB power. The increased off-axis NB power broadens the pressure profile. Discharges using similar total NB power with a different ratio of on- to off-axis NB power are analyzed to determine the effect of the pressure profile on the $\beta_N$ stability limits. This analysis focuses on discharges with q$_{min}$>2, a steady-state scenario being developed on DIII-D. The pressure peaking factor, peak pressure over volume averaged pressure, is used to quantify the breadth of the pressure profile. The pressure peaking factor varies between 2-3 for these discharges. The DCON stability code is used to calculate the ideal-wall stability limits of these plasmas. The first modes that often occur in these plasmas are tearing modes, which occur when the experimental $\beta_N$ approaches the ideal-wall $\beta_N$ stability limit. Tearing modes are triggered typically within 20% of the ideal-wall $\beta_N$ stability limit. Discharges with more off-axis NB power are limited by n=2 tearing modes compared to discharges with more on-axis NB power limited by n=1 tearing modes. Thus the width of the pressure profile correlates with most unstable mode in the plasma.