Experimental Identification of the Kink Instability as a Poloidal Flux Amplification Mechanism for Coaxial Gun Spheromak Formation

The magnetohydrodynamic kink instability is observed and identified experimentally as a poloidal flux amplification mechanism for coaxial gun spheromak formation. Plasmas in this experiment fall into three distinct regimes which depend on the peak gun current to magnetic flux ratio, with (I) low values resulting in a straight plasma column with helical magnetic field, (II) intermediate values leading to kinking of the column axis, and (III) high values leading immediately to a detached plasma. Onset of column kinking agrees quantitatively with the Kruskal-Shafranov limit, and the kink acts as a dynamo which converts toroidal to poloidal flux. Regime II clearly leads to both poloidal flux amplification and the development of a spheromak configuration.

Figure 1

Schematic of coaxial gun, showing bias field coil, $\psi$ contours, gas feeds, and magnetic probe.

Figure 2

Image sequence of shot 2472 (DRS Hadland Imacon 200 CCD camera). The circular gap between outer and inner electrodes is visible toward right side of each frame. $B$ probe is also visible. Kink is fully developed by $13\mu \mathrm{s}$.

Figure 3

Images of three plasma regimes which depend on increasing value of ${\lambda }_{\mathrm{g}\mathrm{u}\mathrm{n}}$ (Cooke Co. HSFC-PRO CCD camera): (I) stable column (shot 1210), (II) kinked column (shot 1247), and (III) detached plasma (shot 1181). $B$ probe was not installed for these shots.

Figure 4

Flattening of $q(R)$ profile toward unity (shot 2472) right before kink onset. Error bars are due to uncertainty in $L$. Also shown are $B_{\varphi }$ and $B_{\mathrm{Z}}$ profiles.

Figure 5

Plot of ${\lambda }_{\mathrm{g}\mathrm{u}\mathrm{n}}$ vs column length $L$, showing agreement with Kruskal-Shafranov limit.

Figure 6

Plots (shot 2472) vs $R$ and time of (top) $B_{\mathrm{p}\mathrm{o}\mathrm{l}}$ vectors and $\psi$ contours (mWb), and (bottom) $\psi$ along horizontal dashed line of top panel.

Figure 7

Plots showing $B_{\mathrm{Z}}$, $B_{\varphi }$ profiles (shot 2472) compared to Taylor solutions ($\lambda =15{\mathrm{m}}^{-1}$ and radial offset of 4 cm). Profiles are plotted for vertical dashed line in Fig. 6 (top).