Interpretation and implementation of an ion sensitive probe (ISP) as a plasma potential diagnostic.

An ISP is being developed as a robust diagnostic capable of measuring space potentials ($\Phi _{P})$ in the boundary plasma of tokamaks. The ISP relies on the large difference between the ion and electron gyroradii ($\rho _{i}$/$\rho _{e} \quad \sim $60) to reduce electron collection at a collector which is oriented parallel to the magnetic field, and recessed behind the ISP shield a distance of $\sim \rho _{i}$. The shield is independently biased. By sweeping the collector voltage, while maintaining a constant voltage difference of a few volts between the shield and the collector (with V$_{WALL} \quad <$ V$_{COLLECTOR})$, we obtain only ion current. The ISP was studied in a magnetized plasma chamber (DIONISOS, B = 0.04 T, n$_{e}$ = 10$^{16}$-10$^{18}$ m$^{-3}$, $\rho _{e}$ = 0.2 mm) at MIT. A model of the ISP potential shows that the collected ion current should reach zero for V$_{WALL}$ \underline {$>$} $\Phi _{P}$ and that space-charge effects are critical to the probe operation. The plasma potentials obtained from the ISP match well the plasma potentials measured with an emissive probe over a wide range of plasma conditions in DIONISOS, yet with a more robust physical design than the emissive probe.