Magnetized plasma experiment on layering and inhomogeneous mixing in a fluctuating vortex array: Melting staircases*

In this work we investigate layering and staircase dynamics using a fluctuating cellular array which intrinsically drives inhomogeneous mixing. To accomplish this, a vortex array is created in a large linear magnetized plasma device (LAPD at UCLA) that is designed to have a system of tangent eddies, thus forming a staircase structure which can exhibit “melting” through the interaction of neighboring cells in the lattice. To create the vortex lattice in the experiment, a smaller lanthanum hexaboride (LaB₆) cathode source is placed at the opposite end of the device relative to the main discharge large area LaB₆ cathode. A carbon mask containing a patterned array of holes is placed in front of this smaller cathode. When this smaller cathode is biased to a mesh anode in front of the main discharge cathode a dynamical vortex lattice with local and global vortical azimuthal and axial flows is established. In the near field region of the lattice, the local plasma potential forms a well, which induces ExB differential rotation of the lattice resulting in azimuthally symmetric inhomogeneous boundary layers that are on the scale of the holes in the initial lattice structure. A series of mixing layers is interspersed with the boundary layers and an analysis of the transport properties is discussed. A spectral analysis of the gradient-driven fluctuations and their relation to the diffusivity, as well as the coupling of this spinning lattice structure to the background plasma will be presented.