Measuring the plasma-wall charge by infrared spectroscopy

Abstract

We show that the charge accumulated by a dielectric plasma-facing solid can be measured by infrared spectroscopy. The approach utilizes a stack of materials supporting a surface plasmon resonance in the infrared. For frequencies near the Berreman resonance of the layer facing the plasma the reflectivity dip-measured from the back of the stack, not in contact with the plasmadepends strongly on the angle of incidence making it an ideal sensor for the changes of the layer's dielectric function due to the polarizability of the trapped surplus charges. The charge-induced shifts of the dip, both as a function of the angle and the frequency of the incident infrared light, are large enough to be measurable by attenuated total reflection setups.

Infrared spectroscopy of surface charges in plasma-facing dielectrics

“…Compared to our previous proposal for infrared spectroscopy of the wall charge [7], where avoided crossing of the Berreman mode and the resonance of a surface plasmon polariton was utilized to measure the surface charge through the shift of the reflectivity minimum, the configuration in this work is much simpler. In the stack approach of Ref.…”

“…In the stack approach of Ref. [7] a metallic layer on the prism is necessary for the surface plasmon resonance, and the plasmafacing dielectric layer needs to be thin, in order to host the Berreman mode. In order to prevent spill over of electrons from the dielectric to the metal layer, we suggested an additional, electro-negative layer between the metal and the plasma-facing dielectric.…”

“…Our previous proposal for this utilizes a stack of thin layers on top of a prism in order to utilize the Berreman resonance in the plasma-facing material, for which this layer may not be too thick [7]. The drawback of such a configuration is the need of at least two additional layers of materials, a metal and a dielectric, between the plasma and the prism used for the optics.…”

Electron energy loss spectroscopy of wall charges in plasma-facing dielectrics

“…The proposal relies on a layered structure supporting a Berreman mode in the infrared which turns out to be rather charge-sensitive. Combined with a self-consistent description of the electric double layer at the plasma-solid interface the method has the potential to provide not only the total charge deposited into the solid (which we demonstrated by an exploratory calculation [20]) but also its spatial distribution inside the solid.…”

“…To overcome the limitations of the existing methods we recently proposed infrared attenuated reflection (IR-ATR) spectroscopy as a tool for gaining access to the surplus charges in a dielectric exposed to a plasma [20]. The proposal relies on a layered structure supporting a Berreman mode in the infrared which turns out to be rather charge-sensitive.…”