Evolution of Arched Magnetized Laboratory Plasma in a Strapping Field with Variable Decay-Index*

Arched magnetized plasma structures that carry electrical current ubiquitously exist in the solar atmosphere. The decay index of arched plasma measures the rate of the decrease of its overlying magnetic field with distance from its foot-point. Its role in driving the eruption through torus instability is an important topic [1]. Laboratory analogs of solar-arched plasma are produced on the Solar Plasma Device at UCLA [2]. The arched plasma (β » 10^-3, Lundquist number » 10²–10⁵,  plasma radius/ion-gyroradius » 20, B » 1000 G at footpoints, repetition rate = 0.5 Hz) evolves in the presence of a large-scale magnetic field with a variable decay index. The evolution of the arched plasma is recorded using fast-camera imaging and three-dimensional measurements of plasma density, electron temperature, and vector magnetic field. The role of decay index, magnetic shear, and relative strengths of the arched and strapping magnetic field in the formation of nearly stable, oscillatory, and eruptive will be discussed.