Analysis of magnetic field plasma interactions using microparticles as probes

The interaction between a magnetic field and plasma close to a nonconductive surface is of interest for both science and technology. In space, crustal magnetic fields on celestial bodies without atmosphere can interact with the solar wind. In advanced technologies such as those used in fusion or spaceflight, magnetic fields can be used to either control a plasma or protect surfaces exposed to the high heat loads produced by plasma. In this paper, a method will be discussed for investigating magnetic field plasma interactions close to a nonconductive surface inside a Gaseous Electronics Conference reference cell employing dust particles as probes. To accomplish this, a magnet covered by a glass plate was exposed to a low power argon plasma. The magnetic field was strong enough to magnetize the electrons, while not directly impacting the dynamics of the ions or the dust particles used for diagnostics. In order to investigate the interaction of the plasma with the magnetic field and the nonconductive surface, micron-sized dust particles were introduced into the plasma and their trajectories were recorded with a high-speed camera. Based on the resulting particle trajectories, the accelerations of the dust particles were determined and acceleration maps over the field of view were generated which are representative of the forces acting on the particles. The results show that the magnetic field is responsible for the development of strong electric fields in the plasma, in both horizontal and vertical directions, leading to complex motion of the dust particles.

Figure 1

Schematic of the experimental setup. The magnet is placed on the lower electrode and covered by a glass plate. Dust is dropped from above the upper electrode into the plasma and illuminated by a fanned diode laser. A high-speed camera perpendicular to the laser plane records the dust particle trajectories.

Figure 2

Magnetic field lines for the experimental setup discussed. The left side of the plot represents the symmetry axis. The magnetic flux density is plotted with dashed lines representing lines of constant field in increments of 0.02 T. Solid lines represent the magnetic field lines themselves.

Figure 3

Representative experimentally measured particle trajectories. Only a small number of data points (1619 out of 84 727) are shown for clarity. The left side of the plot is the symmetry axis of the experiment. The light-gray box indicates the glass plate above the magnet, represented by the dark-gray box. The black lines in the background are the magnetic field lines and the dashed circle indicates the region used for the acceleration determination at this spot.

Figure 4

Radial velocity versus radial acceleration for all data points within the circle shown in Fig. 3. The dashed line shows the regression curve as described in the text.

Figure 5

Contour plot of radial accelerations of 12 micron MF particles with (a) no magnetic field and (b) with magnetic field. The numbers on the contours represent the acceleration in units of g. The gray lines in (b) represent the magnetic field lines.

Figure 6

Contour plot of the vertical accelerations of 12 micron MF particles with (a) no magnetic field and (b) with magnetic field. The numbers on the contours represent the acceleration in units of g. The light-gray lines in the case with a magnetic field represent the field lines. Vertical lines mark the position of the sheath profiles in Fig. 7.

Figure 7

Vertical acceleration profiles at different radial positions with (wm) and without (nm) magnet. The positions of the sheath profiles are indicated in Fig. 6.