A laboratory investigation of Alfvén waves in an Alfvén speed gradient

Alfvén waves are important in many astrophysical and laboratory plasmas with complex inhomogeneous magnetic structures, such as the solar corona and the Earth’s magnetosphere [1]. Investigating Alfvén waves traveling through such inhomogeneous plasmas is essential for understanding turbulence, damping [2], and heating [3]. In theory, the Alfvén wave is predicted to reflect from the Alfvén speed gradient. In a recent laboratory experiment [4], it was observed that kinetic Alfvén waves (electron thermal speed >> Alfvén speed) lose energy while propagating through the Alfvén speed gradient. But, interestingly, no reflection was observed. We are studying the behavior of inertial Alfvén waves (electron thermal speed << Alfvén speed) as they propagate through the Alfvén speed gradients in the laboratory. We have performed a series of experiments at the Large Plasma Device [5] (LAPD) at the University of California, Los Angeles, to measure the reflection and transmission of inertial Alfvén waves through a gradient parallel to the background magnetic field. The inhomogeneity was created by varying the magnetic field in the direction of the propagation of the Alfvén wave. The Alfvén waves were excited using a loop antenna with multiple wave frequencies below the ion cyclotron frequency. We have observed the reflection of inertial Alfvén waves as they propagate through these Alfvén speed gradients. We will present the results of these experiments.