The Swarthmore Spheromak Experiment is a laboratory focused on mainly basic plasma physics research. Over the last twenty years, subject areas studied have included magnetic reconnection; spheromak formation, evolution and relatation; and energy and particle confinement for fusion applications. Most recently, SSX research has centered on exploration of MHD turbulence and laboratory astrophysics.
Primary Research Questions
Is the statistical character of magnetohydrodynamic (MHD) turbulence universal?
Though turbulence in fluids has been observed for centuries, a complete mathematical description and precise physical explanations allude physcists to this day. While many mathematical models of turbulence have been proposed, no one model has been proven to fully describe physically observed turbulence, even in basic neutral fluids such as water or air. Plasma can be modeled as a non-neutral fluid--a fluid composed of separate positive and negative charges--and as such can exhibit typical fluid turbulence. Many more complications can arise in this already complicated, unsolved physical state including the interaction between particles electric/magnetic fields as well as the presence of many types of fluctuations and instabilities not present in neutral fluids. Though a complete model of neutral turbulence has not been made, many characterics of turbulences have been observed and mathematically described. One of these is the so-called "universality" of fluid turbuence. In a turbulent system, energy is transported from large scales to small scales. For a certain range of scales, called the inertial range, the rate at which energy is tranfered to smaller scales is constant and can be shown to follow a power-law scaling as a function of inverse scale size. Kolmogorov in 1941 showed how the spectral index for this scaling should be -5/3 for all fluids. Observations of many different turbulent fluids (from water, to air, to even liquid helium) have exhibited this "universal" behavior. In plasmas, such universality is not necessarily expected given the greater degree of complexity in the system. Research on the SSX seeks to explore this concept by studying the nature of turbulence in SSX and compare the findings to other turbulent plasmas such as the solar wind or the magnetosphere.
How can plasma turbulence be measured and quantified?
Turbulence research on SSX has spanned a broad spectrum of analysis techniques. By accumulating a large number of metrics on SSX plasma and on other turbulent plasmas, the first question can be better answered. The primary analysis tools used to study turbulence on SSX have been using fluctuation spectra and intermittency. The examines the energy content of the plasma by Fourier transforming measurements made in the time domain. The fluctuation energy content as a function of frequency can be related to the energy transfer rate between scales and thus compared to predictions made for fluids and plasmas akin to Kolomogov's original theory for neutral fluids. The second technique seeks to durming the size and nature of structure that is found in the turbluence by examining differences in measurements separated by time or distance. Other techniques are being explored include spatial and temporal correlation of signals and most recently the entropy and complexity of the turbulent plasma.
What are the physical mechanisms that inject energy into the turbulent system and disspate energy from it into heat?
In the simplest terms, turbulence can be thought to have three elements: energy injection, energy transfer, and energy dissipation. In plasmas, the process through which energy is injected into a system and disspiated (that is, turned to heat) can be quite varied. On SSX, the energy can be injected both magnetically during the spheromak formation process, and kinetically by translational forces just as the JxB (current cross magnetic field) force. Dissipation can occur in a number of ways as well including through magnetic reconnection, through Landau damping, and collisional resisitivity. The nature of injection and dissipation is explored on SSX as well as how these processes affect turbulent metrics and universaility.