[FLASH-USERS] Question about magnetic field behavior in Flash's ZPinch simulation

[Hansen, Eddie]

The B-field drops at the liner/vacuum interface (not to zero, but low) because the liner has a much lower resistivity than the vacuum. The field doesn’t diffuse much through the liner, so when it gets to the liner/fill interface, there’s not much left to diffuse into the fill. Due to cylindrical geometry the field must go to zero at r=0.

There is not necessarily net current in the fill, or at least not a significant current. The current is determined by curl(B), which should be largest at the liner/vacuum interface for most, if not all, of the implosion. Some pinches will have a ‘secondary’ peak current at the liner/fill interface as well, but the specifics are problem-dependent.

In general, Z-pinches in FLASH are not literally simulating an applied current traveling through the domain. What we do is use a circuit model or given current value to set the B-field at the upper-r boundary with Ampere’s Law, and we let magnetic diffusion, with appropriate values for resistivity, determine the B-field profile throughout the domain. Then curl(B) x B will determine the pinch forces on the plasma. Real life doesn’t have a B-field diffusing inwards from some radius, but as long as the resulting B-field profile is correct, the approach will give correct results.

--
Eddie Hansen
Research Scientist
Flash Center for Computational Science


From: flash-users <flash-users-bounces at flash.rochester.edu> on behalf of Gines Garcia <gines.garcia at greentownsbyfusion.com>
Date: Tuesday, May 16, 2023 at 10:22 AM
To: flash-users at flash.rochester.edu <flash-users at flash.rochester.edu>
Subject: [FLASH-USERS] Question about magnetic field behavior in Flash's ZPinch simulation
Greetings everyone,

I have a question regarding the default ZPinch simulation in Flash. I'm curious about two aspects:

The magnetic field abruptly drops to zero when it encounters the fill material.  How can this happens if it is supposed to also have a net current inside the fill?

I appreciate any insights or explanations you can provide on these matters.

Thank you in advance for your help.

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