Force-Free Current Flow in Z Pinches Imploded in an Axial Magnetic Field

Study of z-pinch implosions in an imposed axial magnetic field has a long history. Renewed interest in this field caused by the MagLIF approach to pulsed-power-driven ICF, has motivated new experimental studies with small-scale gas-puff
¹
and metallic-gas-puff
²
z-pinches. These studies have discovered that the presence of the axial magnetic field, B
z
, dramatically changes the distribution of current in the plasma compared to the predictions of the existing analytical models, 1-D, and 2-D simulations. A clear indication of this is the sensitivity of the implosion time to the magnitude of the initially imposed axial field, B
z0
. Theories and simulations which only account for the counter-pressure of the compressed axial field predict that the implosion time is almost independent of B
z0
in the parameter range characteristic of the experiments
¹
,
²
B
z0
(T) < 3 I
m
(MA)/R
0
(cm). Both experiments at IHCE and WIS indicate that the implosion slows down with increased B
z
whose estimated pressure is too small to account for the observed effect. Our explanation of these observations is that in the presence of the initial axial field a noticeable part of the pinch current does not participate in the implosion of the main plasma mass because it flows in a low-density plasma at the periphery of the plasma column, where the magnetic field is close to force-free. This conclusion is confirmed by the analytical models, 1-D RMHD simulations and the new experimental data from the Bi metallic-gas-puff pinch with a power-law density profile ensuring stabilization of implosion against the MRT growth. At I
m
= 0.45 MA, selfemission images and magnetic probes demonstrated that during the run-in phase the implosion velocity is equal to $( 11.5 ) \times 10 ^{7}~\text{cm/s}$ at B
z0
=0 and $0.6 \times 10 ^{7}~\text{cm/s}$ at B
z0
=0.6 T.