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

Abstract

The flute instability in mirror machines is driven by spatial charge accumulation and the resulting E×B plasma drift. On the other hand, E×B drift due to external electrodes or coils can be used as a stabilizing feedback mechanism. Fast photography is used to visualize Hydrogen plasma in a small mirror machine and infer the plasma drift and the internal electric field distribution. Using incompressible flow and monotonic decay assumptions we obtain components of the velocity field from the temporal evolution of the plasma cross section. The electric field perpendicular to the density gradient is then deduced from E=-V×B. With this technique we analyzed the electric field of flute perturbations and the field induced by electrodes immersed in the plasma.

Flute growth rate of plasma jet in mirror machine

“…Instead the rotation is probably due to the faster escape of electrons, leaving behind positively charged plasma that rotates by E × B drift. In experiments with pronounced perturbation, some rotational shear is frequently observed [19], but it is much lower than the level required for flute stabilization [7,8]. According to equation ( 4), when wall stabilization is small, the rotation rate of all the modes is expected to be similar.…”

“…The fast camera records a projection of the H α radiation from the recombination of the plasma. As discussed in [19], this radiation is a good indicator for the study of the plasma shape and dynamics, although it is not strictly proportional to the plasma density.…”

“…As a result, the overall plasma luminosity decays, so if the shape of the plasma is tracked by examining the contour lines with the same luminosity in consecutive images, the plasma appears to be shrinking (figure 2, left). To correct for the luminosity decay, it is assumed that the decay is local and the plasma flow is incompressible [19]. Using these assumptions it was shown in [19] that the effect of the decay can be negated by equating the cumulative histograms of consecutive frames.…”

“…To correct for the luminosity decay, it is assumed that the decay is local and the plasma flow is incompressible [19]. Using these assumptions it was shown in [19] that the effect of the decay can be negated by equating the cumulative histograms of consecutive frames. In practice, it is identical to following the evolution of luminosity contour lines that encircle equal areas in successive frames (figure 2, right).…”

Stabilization of magnetic mirror machine using rotating magnetic field

“…A Phantom camera with an exposure time of 2 µs was used. Differences between consecutive frames were calculated in order to resolve velocity profiles (Be'ery et al 2013). This technique has better spatial resolution and can resolve flow also for the high magnetic field experiment.…”