Self-organized magnetic equilibria in tokamak plasmas with very low edgesafety factor

Tokamak plasmas often exhibit self-organizing behavior in which internal modes shape thetoroidal current density profile, a common example being the sawtooth instability.However, such behavior has not been studied in detail for edge safety factor below 2 dueto disruptive kink instabilities that typically prevent operation in this regime. Now,steady tokamak plasmas with an edge safety factor down to 0.8 have been created in theMadison Symmetric Torus, where disruptions are prevented due to a thick, conductive walland a feedback power supply that sustains the plasma current. Internal measurements andnonlinear magnetohydrodynamic modeling reveal a family of safety factor profiles with acentral value clamped near unity as the edge safety factor decreases, indicating currentprofile broadening through a relaxation process. As the safety factor decreases, themagnetic fluctuations become irregular, and the electron energy confinement timedecreases.