Plasma tubes becoming collimated as a result of magnetohydrodynamic pumping

Collimated magnetized plasma structures are commonly observed on galactic, stellar, and laboratory scales. The Caltech plasma gun produces magnetically driven plasma jets bearing a striking resemblance to astrophysical jets and solar coronal loops by imposing boundary conditions analogous to those plasmas. This paper presents experimental observations of gun-produced plasma jets that support a previously proposed magnetohydrodynamic MHD pumping model P. M. Bellan, Phys. Plasmas 10, 1999 2003 as a universal collimation mechanism. For any initially flared, magnetized plasma tube with a finite axial current, the model predicts i magnetic pumping of plasma particles from a constricted region into a bulged region and ii tube collimation if the flow slows down at the bulged region leading to accumulation of mass and thus concentrating the azimuthal magnetic flux frozen in the mass flow i.e., increasing the pinch force . Timeand space-resolved spectroscopic measurements of gun-produced plasmas have confirmed the highly dynamic nature of the process leading to a collimated state, namely, i suprathermal Alfvénic flow 30–50 km/s , ii large density amplification from 1017 to 1022 m−3 in an Alfvénic time scale 5–10 s , and iii flow slowing down and mass accumulation at the flow front, the place where the tube collimation occurs according to high-speed camera imaging. These observations are consistent with the predictions of the MHD pumping model, and offer valuable insight into the formation mechanism of laboratory, solar, and astrophysical plasma structures. © 2010 American Institute of Physics. doi:10.1063/1.3437075