Observation Of A Relaxed Plasma State In A Quasi-Infinite Cylinder

A helical relaxed plasma state is observed in a long cylindrical volume. The cylinder is long enough so that the predicted minimum energy state is a close approximation to the infinite cylinder solution. The plasma is injected at v \u3e= 50 km/s by a coaxial magnetized plasma gun located at one end of the cylindrical volume. The relaxed state is rapidly attained in 1-2 axial Alfven times after initiation of the plasma. Magnetic data are favorably compared with an analytical model. Magnetic data exhibit broadband fluctuations of the measured axial modes during the formation period. The broadband activity rapidly decays as the energy condenses into the lowest energy mode, which is in agreement with the minimum energy eigenstate of del x B = lambda B. DOI: 10.1103/PhysRevLett.110.08500

Calibrated Cylindrical Mach Probe In A Plasma Wind Tunnel

A simple cylindrical Mach probe is described along with an independent calibration procedure in a magnetized plasma wind tunnel. A particle orbit calculation corroborates our model. The probe operates in the weakly magnetized regime in which probe dimension and ion orbit are of the same scale. Analytical and simulation models are favorably compared with experimental calibration. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3559550

Stable Spheromak Formation By Merging In An Oblate Flux Conserver

An axisymmetric spheromak formed by the dynamic merging of two smaller spheromaks of the same magnetic helicity in the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] has been observed and characterized. The spheromak is formed in an oblate (tilt stable), trapezoidal, 6 mm wall copper flux conserver in SSX, which is 0.5 m in diameter and L=0.4 m in length at its largest dimensions. This configuration is formed by cohelicity merging of two spheromaks (either both right-handed or both left-handed) in which the merging poloidal fluxes are parallel (i.e., no field reversal for reconnection to occur initially). After a period of dynamic and nonaxisymmetric activity, the configuration ultimately relaxes to an axisymmetric state. A nonaxisymmetric tilted state, very close in total energy to the axisymmetric state, is also sometimes observed. This configuration is characterized by a suite of magnetic probe arrays for magnetic structure B(r,t), ion Doppler spectroscopy for T(i) and flow, and interferometry for ne. The magnetic structures of both states match well to computed eigenstates. (C) 2010 American Institute of Physics. [doi:10.1063/1.3334324

Three-Dimensional Reconnection And Relaxation Of Merging Spheromak Plasmas

Plasma relaxation inside a highly conducting cylindrical boundary is studied both experimentally and computationally. Dynamics are initiated by the introduction of two equal helicity spheromaks at either end of the cylinder. In the experiment, dense, high-magnetic-flux spheromaks are injected into the flux conserving volume with magnetized plasma guns. In the simulation, identical spheromaks initially occupy both halves of the cylinder and a perturbation is introduced. Merging commences with a single three-dimensional null-point that moves radially out of the flux conserving volume at velocities up to 0.2 of the reconnection outflow velocity. Relaxation to the minimum energy state occurs in about ten Alfven times. An important conclusion is that even though the dynamical activity is limited to a few modes, this activity is sufficient to promote relaxation to the final, minimum energy state. The dynamical activity appears to conserve magnetic helicity while magnetic energy is converted to flow and heat. The final state arrived at dynamically is identical to that described by C. D. Cothran et al. [Phys. Rev. Lett. 103, 215002 (2009)] using static, eigenvalue analysis. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3492726

Three-Dimensional Magnetohydrodynamics Simulations Of Counter-Helicity Spheromak Merging In The Swarthmore Spheromak Experiment

Recent counter-helicity spheromak merging experiments in the Swarthmore Spheromak Experiment (SSX) have produced a novel compact torus (CT) with unusual features. These include a persistent antisymmetric toroidal magnetic field profile and a slow, nonlinear emergence of the n = 1 tilt mode. Experimental measurements are inconclusive as to whether this unique CT is a fully merged field-reversed configuration (FRC) with strong toroidal field or a partially merged doublet CT configuration with both spheromak- and FRC-like characteristics. In this paper, the SSX merging process is studied in detail using three-dimensional resistive MHD simulations from the Hybrid Magnetohydrodynamics (HYM) code. These simulations show that merging plasmas in the SSX parameter regime only partially reconnect, leaving behind a doublet CT rather than an FRC. Through direct comparisons, we show that the magnetic structure in the simulations is highly consistent with the SSX experimental observations. We also find that the n = 1 tilt mode begins as a fast growing linear mode that evolves into a slower-growing nonlinear mode before being detected experimentally. A simulation parameter scan over resistivity, viscosity, and line-tying shows that these parameters can strongly affect the behavior of both the merging process and the tilt mode. In fact, merging in certain parameter regimes is found to produce a toroidal-field-free FRC rather than a doublet CT. (C) 2011 American Institute of Physics. [doi:10.1063/1.3660533

Observation Of A Helical Self-Organized State In A Compact Toroidal Plasma

A nonaxisymmetric stable magnetohydrodynamic (MHD) equilibrium within a prolate cylindrical conducting boundary has been produced experimentally. It has m=1 azimuthal symmetry, helical distortion, and flat lambda profile, all in agreement with the computed magnetically relaxed minimum magnetic energy Taylor state. Despite varied initial conditions determined by two helicity injectors on the device, this same equilibrium consistently emerges as the final state. These results therefore describe a new example of self-organization in an MHD plasma

Spectroscopic Observation Of Simultaneous Bi-Directional Reconnection Outflows In A Laboratory Plasma

We report a precise, direct spectroscopic measurement of simultaneous bi-directional outflows from a reconnection event in a laboratory plasma. Outflow speeds are as Alfvenic and Abel analysis shows that the outflows are generated in the plasma core. A Sweet-Parker like analysis of outflow speed coupled with external measurements of reconnection electric field and assumption of Spitzer resistivity predict an aspect ratio of the reconnection layer and reconnection rate that are close to that measured in the experiment and in simulations. However, this analysis underestimates the absolute scale of the layer, indicating other than 2D resistive physics is at play. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747345