: I'd sure like to know about the internal structure of these features. Is
: material erroded along the paths or changed (melted) or fractured (making
: the paths permiable)?
: You also mention that the charge leaks from the exteroir surfaces, have
: you ever used a large round sphere to rapidly discharge a blocks surface
: with a lower potential difference between to two surfaces. Does surface
: etching result? Thanks in advance. dz
Eventually I hope to add some pictures of the microscopic structure of the discharge paths to the web site. The short answer to your first question is yes. The main discharge paths are roughly tubular where some of the Plexiglas has been evaporated and blasted out of the block. The channel walls are often discolored to a caramel color, or even blackened from carbonization if the specimen discharges while it is being irradiated by the electron beam. Because Plexiglas is fairly brittle, there is also considerable fracturing around the discharge paths. This causes the larger discharge paths to look almost crystalline when externally illuminated.
The sudden expansion (actually a shock wave) of the spark channel creates localized stresses and cracking within surrounding acrylic. This can be easily observed as color gradients around the discharge zone if the specimen is illuminated by polarized light and viewed through another polarizing sheet (cross polarizers). The combination of microscopic tubules and fractures does allow gas to enter the interior of the Plexiglas. The yellowish discoloration (solarization) from the irradiation process fades with time as oxygen diffuses into the Plexiglas. In older specimens, the solarized region fades both from the outside surface inward, and from the discharge layer outward. This causes the solarized region to become a thin sliver in the middle, eventually disappearing entirely.
When we charge the specimen, the interior acquires a huge negative charge. This attracts a correspondingly large number of positive ions from the air to collect on the outer surfaces of the specimen. The result is a very large voltage stress between the outside and interior of the Plexiglas. In addition, the radiation itself causes temporary increase in electrical conductivity (an effect called Radiation Induced Conductivity or RIC). The electrical conductivity also increases in regions that are subjected to extremely high electrical stress. The combination of these effects results in the discharge-free regions seen along the periphery of all specimens.
The outside of a charged specimen is actually relatively near ground potential since there is an approximate balance of charge between the interior space charge and exterior surface charges. Poking the specimen allows positive surface charges to penetrate more deeply into the specimen which hopefully induces electrical breakdown by increasing the local E-field beyond the material's dielectric strength. The actual shape of the rest of the discharge terminal or E-field configuration is not critical.
When the specimen discharges, there are also a set of external surface discharges that neutralize most of the positive surface charges residing on the exterior of the specimen. These surface discharges can sometimes be seen in photographs discharging specimens. These discharges alter the surface energy characteristics of the acrylic. Although these surface energy alterations are normally invisible, they can occasionally be made visible when water vapor lightly condenses onto the surface of a cold specimen.
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