Addressing issues in probing the magnetic cusp region

Summary form only given. There are several inherent difficulties in making spatially resolved electrostatic probe measurements of plasma properties within a current collecting magnetic cusp. It is difficult to insert a physical probe into the region without shadowing current flow to the anode as a result of the physical obstruction of field lines by the probe body. Current flow to the probe collecting surface can also be shadowed by the probe body blocking flux lines that curve away from the probe surface as a consequence of the physical magnetic field distribution in the region. Shadowing can therefore significantly reduce collected current at the probe, the anode, or both. The problem is complicated by the presence of the plasma sheath and the large magnetic field at the anode surface. Since the sheath potential varies spatially, it is not possible to simply set the voltage on a collecting probe and measure electron density when the potential is changing as a function of probe position. Unless the probe is biased at the local plasma potential at each point, the probe sheath will interfere with the local plasma conditions. A solution to these issues is proposed in the form of a joint emissive/collecting probe that will negate the shadowing issue as well. The probe will measure plasma potential and then interrogate the cusp region by actively biasing the probe in collection mode at the plasma potential. At the plasma potential, the maximum current flowing through that point is collected provided shadowing affects are accounted for.