Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation

Highly collimated, plasma-filled magnetic-flux tubes are frequently observed on galactic, stellar, and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic-flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic-flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.

Figure 1

Typical plasma discharge sequence (#6577, $2\times {10}^6\mathrm{fps}$, $40\mathrm{ns}/\mathrm{\text{frame}}$, log. color table). Eight collimated “spider legs” merge on axis to form a central column jet that collimates and propagates into the vacuum vessel.

Figure 2

Images from identical discharges. (a),(d) 4345; (b),(e) 4346; (c),(f) 4343. Nitrogen is puffed from the cathode nozzles and neon from the outer nozzles. Images (d)–(f) taken simultaneously with (a)–(c), respectively, using a second camera and 402 nm filter to image nitrogen-rich plasma. Arrow indicates specific spider leg analyzed in detail in Fig. 7.

Figure 3

Spider-leg collimation measurements. (a)–(c) Images from Figs. 2d, 2e, 2f. Identical white lines indicate the measurements of the spider-leg intensity. (d)–(f) Dotted line shows the quadratic term of the least-squares fit (solid line), dashed line shows the Gaussian term. FWHM (number) is assumed to be representative of the spider-leg width.

Figure 4

(a),(b) Measurement of flux tube flaring, on image 2f. FWHM (red solid vertical marks) constant to $\lesssim 20\%$. (c) Analytical flaring of flux tube, computed from vacuum poloidal magnetic field [20]. (d) Comparison of the analytical flux tube axis positions (white lines) with image 2f.

Figure 5

Planar section of neutral density distribution just before breakdown, as measured by the fast ion gauge. Dashed line indicates the central axis of the experiment.

Figure 6

Spectroscopic measurements: (a) center column “astrophysical jet” $n_e(t)$ inferred from Stark broadening. Solid line is a polynomial fit. Errors from 1–10 eV Doppler broadening and differing Lorentzian fit parameters [24]. (b) Doppler shift of single spider leg (N ii, 399.5 nm rest frame, dashed line). Solid lines are polynomial fits. Times correspond to end of $1\mu \mathrm{s}$ exposure, relative to plasma breakdown.

Figure 7

Plasma flow measurements: (a) Intensity along axis [of specific spider leg shown in Fig. 2f] at $2.5\mu \mathrm{s}$ [blue diamond, Fig. 2d], $2.75\mu \mathrm{s}$ [green triangle, Fig. 2e], $3\mu \mathrm{s}$ [red squares, Fig. 2f]. Solid lines are polynomial fits to the data between $4\le z\le 18\mathrm{cm}$. (b) Plasma velocities at $2.625\mu \mathrm{s}$ (blue diamond line), $2.75\mu \mathrm{s}$ (green triangle line), and $2.875\mu \mathrm{s}$ (red square line) obtained from Eq. (1).

Figure 8

Measured line spectra with Lorentzian fits: (a) single spider leg, H 486.133 nm (dashed line). Exposure $0–1\mu \mathrm{s}$ with respect to breakdown. Line of sight is perpendicular to the spider leg, parallel to the cathode plane. (b) Fully formed central column jet (N ii, dashed line is 424.178 nm rest-frame wavelength).