Speaker
Description
We investigate the effects of an axial magnetic field ($B_z$) on the current distribution in imploding plasma and the efficiency of the $B_z$-field compression by the imploding plasma. In the experiment, a cylindrical argon gas puff, in which is initially embedded quasi-static magnetic flux (up to 0.4 T), prefills the volume between two electrodes. Subsequently, a pulsed-current (rising to 300 kA, in 1.6 µs) driven through the gas, ionizes it, and generates an azimuthal magnetic field that compresses the plasma and the embedded $B_z$-field. Here, for the first time, we directly and simultaneously measure the evolution of the axial and azimuthal magnetic fields during the implosion and stagnation. This measurement was achieved by employing a spectroscopic technique based on the polarization properties of Zeeman split emission, combined with laser-doping technique that provided mm-scale spatial resolution. The measurements show that for implosions with $B_z$(t=0) = 0.4 T, the azimuthal magnetic field ($B_θ$) in the imploding argon plasma shell is much smaller than expected from the measured current and plasma radius, demonstrating that $B_z$ dramatically affects the current distribution. It is found that in the presence of a low $B_z$, a significant part of the current flows at large radii through a non-imploding dilute plasma ($n_e$ ≤ $10^{17}$ cm$^{-3}$). In addition, simultaneous $B_z$ and $B_θ$ measurements at stagnation for $B_z$(t=0) = 0.4 T show that $B_z$ is compressed about 12 × relative to its initial value, giving at stagnation a $B_z$-magnitude ~ 4 × larger than $B_θ$. The pressure in the stagnated plasma (including the thermal pressure) becomes 16 × higher than the pressure of $B_θ$. This demonstrates the large role of the ram pressure of the imploding plasma on the compression of Bz in this experiment.
This work is supported by the US-Israel Binational Foundation under Grant 2012096.
