Locking the Plasma Potential with an Anodic Surface*

It is often assumed that a small positively biased electrode immersed into a bulk plasma has negligible impact on the bulk plasma properties, including the plasma potential. This is an assumption in many diagnostic devices, such as a Langmuir probe. In this poster we present a detailed study including simulations and experiments to determine the size scales when such an immersed positive interface has non-negligible impact on the plasma [1]. That is, we answer the question, “what is the largest size for an anodic surface before it influences the plasma?”. Letting $\mu = \sqrt{2.3m_e/m_i}$, we find that if the ratio of anode area ($A_A$) to grounded wall area ($A_W$) $A_A/A_W \lt 1\times\mu$, we can expect little impact on the bulk plasma, but as $A_A/A_W \to 1.7\times\mu$ we see significant influence, and at $A_A/A_W \gt 1.7\times\mu$, we expect the plasma potential to become locked to, and therefore controlled by, the anode potential. [1] Hopkins, Yee, Baalrud, Barnat, Phys. Plasmas 23, 063519 (2016).