Collapsing plasma makes for a controllable catastrophic event

In the fusion community, plasma collapse can be a nuisance, because it suddenly and unexpectedly changes the desired behavior. However, the collapse process, which connects two states by a very sudden event, makes for an interesting example of catastrophe theory.

Van Compernolle et al. studied plasma collapse in a magnetized plasma using the Large Plasma Device, a long cylindrical chamber.

By pulling electrons off a cathode with a potential difference, the team created a current to inject heat into the system. They used a ring-shaped cathode to emulate the off-axis heating in some fusion experiments. The ring induced rotation and flow shear in the system, driving the plasma flow in one direction on the outside and the opposition direction on the inside.

The heat traveled along magnetic field lines into the plasma, creating an annular temperature and density profile with regions of alternating cold and hot plasma. Initially, the innermost region of the plasma was cold and quiescent, a consequence of the flow shear.

“But then, after remaining stable for a long time compared to transport time scales, all of a sudden, the inside of the annular region gets filled in with hot plasma in a global collapse,” said author Bart Van Compernolle. “Initially, there was very little plasma there, and within 10 to 15 microseconds, you fill in the hole with a very sudden collapse of the system.”

After collapse, the electron temperature ring remained, but the density profile flattened. This instantaneously occurred throughout the entire 20-meter-long plasma cylinder.

The researchers are exploring how adjusting the ring-cathode potential can control these sudden processes.

Source: “Sudden collapse of a pressure profile generated by off-axis heating in a linear magnetized plasma,” by B. Van Compernolle, M. J. Poulos, and G. J. Morales, Physics of Plasmas (2022). The article can be accessed at https://doi.org/10.1063/5.0082247 .