Measuring drift velocity and electric field in mirror machine by fast photography

The flute instability in mirror machines is driven by spatial charge accumulation and the resulting E$\times $B plasma drift. E$\times $B drift due to external electrodes can be used as a stabilizing feedback mechanism. In order to measure the plasma drift and the internal electric field distribution we used fast photography to visualize Hydrogen plasma in a small mirror machine. We use incompressible flow and monotonic decay assumptions to deduce the velocity field from the evolution of the plasma cross section. The electric field perpendicular to the density gradient is then deduced from E=-V$\times $B. Using this technique we measured the electric field of the flute instability and the field induced by electrodes immersed in the plasma.