Magnetothermodynamics: An experimental study of the equations of state applicable to a magnetized plasma

Abstract

Measuring the equations of state of a compressed magnetized plasma is important for both advancing fusion experiments and understanding natural systems such as stellar winds. In this paper, we present results from our experiments on the thermodynamics of compressed magnetized plasmas; we call these studies "magnetothermodynamics." In these experiments, we generate parcels of relaxed, magnetized plasma at one end of the linear Swarthmore Spheromak eXperimental device and observe their compression in a closed conducting boundary installed at the other end. Plasma parameters are measured during compression. Instances of ion heating during compression are identified by constructing a pressure-volume diagram using the measured density, temperature, and volume of the magnetized plasma. An axial scan of the ion temperature at upstream locations suggests that the increase in ion temperature arises due to the compression of the magnetized plasma in the conducting boundary. The theoretically predicted magnetohydrodynamic (MHD) and double adiabatic equations of state are compared with experimental measurements to estimate the adiabatic nature of the compressed plasma. The equilibrium of our magnetized plasmas is well-described by magnetohydrodynamics; however, we find that the MHD equation of state is not supported by our data. Our results are more consistent with the parallel Chew-Goldberger-Low equation of state, suggesting that there is significant anisotropy in the ion distribution function.