Self-Generated Plasma Rotation in a Z-Pinch Implosion with Preembedded Axial Magnetic Field

Using detailed spectroscopic measurements, highly resolved in both time and space, a self-generated plasma rotation is demonstrated using a cylindrical implosion with a preembedded axial magnetic field ($B_{z0}$). The rotation direction is found to depend on the direction of $B_{z0}$ and its velocity is found comparable to the peak implosion velocity, considerably affecting the force and energy balance throughout the implosion. Moreover, the evolution of the rotation is consistent with magnetic flux surface isorotation, a novel observation in a Z pinch, which is a prototypical time dependent system.

Figure 1

(a) Schematic description of the experimental setup and spectroscopic systems showing the geometry of the electrodes in mm in the vicinity of the A-K gap. The Helmholtz coils for the $B_{z0}$ generation (not shown) are outside the vacuum chamber. Spectroscopic system 1: the spectral resolution is 0.7 Å and spatial resolution is $<0.2\mathrm{mm}$. Spectroscopic system 2: the spectral resolution is 0.3 Å and spatial resolution is $\approx 0.7\mathrm{mm}$. (b) The 5-ns gated 2D plasma images for $B_{z0}=0$. (c) The 2D plasma images for $B_{z0}=0.26\mathrm{T}$. (d) Plasma radius, hollow circles and black squares, and current evolution, dashed and solid line, for $B_{z0}=0$ and for $B_{z0}=0.26\mathrm{T}$, respectively.

Figure 2

The left column shows 2D images of the plasma self-emission (in visible). The middle column shows the spectral images of the chordally integrated O VI 3811.35 Å transition (marked by the vertical dashed line) recorded at $z=7\mathrm{mm}$ (the position that is marked by the white rectangle on the 2D images). The right column shows the line shapes of the O VI transition together with the best fits. They are obtained from the lineouts of the spectral images taken at the $y$ positions marked by the dashed white rectangles. The Doppler shifts due to rotation are symmetric around the unshifted wavelength. (a) For $B_{z0}=0$, $t=-2\mathrm{ns}$; (b) for $B_{z0}=+0.26\mathrm{T}$, $t=2\mathrm{ns}$; (c) for $B_{z0}=-0.26\mathrm{T}$, $t=-1\mathrm{ns}$.

Figure 3

Radial distribution of $v_{\theta }$ for different times and $z$ positions ($z=0$ is the anode surface). The corresponding open symbols are the $v_r$ values measured at the radial position of O V. The red, blue, and green lines represent the radial distribution of the square root of the continuum intensity (which is proportional to $n_e$) at different times and $z$ positions, scaled to unity.

Figure 4

The evolution of $v_{\theta }$ (with dashed lines as a guide to the eye) as measured at the radial position of O V (see Fig. 3) as a function of time for different axial positions $z$. Near the cathode ($z=9$) the rotation slows, while near the anode ($z=1$) it speeds up.

Figure 5

Schematic of the plasma, electrodes, velocities, and poloidal B field configuration at (a) $t\approx -300\mathrm{ns}$ and (b) $t\approx -80\mathrm{ns}$. $\omega =v_{\theta }/r$ is the rotation angular velocity and $\mathbf{B}=B_z\widehat{z}+B_r\widehat{r}$ is the magnetic field.