Are Electron Sheaths Common for Positive Electrodes in Higher Pressure Discharges?

In low pressure discharges (P<100mTorr), the balance of global electron and ion current loss dictates that an electron-rich sheath can only occur at a positively biased electrode when the electrode area (A_E) is small compared to the area of the other walls (A_W) bounding the plasma. At these low pressures it is well known that the area ratio must satisfy A_E/A_W<(2.3m_e/m_i)^1/2, a relation that has been verified by experiments and simulations[1]. In this work simulations are used to show that electron sheaths can occur when A_E/A_W>(2.3m_e/m_i)^1/2 if the neutral pressure exceeds roughly 300 mTorr. The increased neutral pressure accompanies larger density gradients in the plasma approaching the positive electrode without the presence of a correspondingly large bulk electric field. These gradients reduce the electron current collection at the electrode relative to the ion current collection at the walls. This feature lessens the area ratio requirement for global balance of current loss and makes the presence of electron sheaths at positive electrodes more likely at higher pressures. Further studies will be needed to determine the limits on the range of conditions under which electron sheaths may occur at positive electrodes.   

 

[1] Interaction of Biased Electrodes with Plasmas: Sheaths, Double Layers, and Fireballs, Plasma Sources Science and Technology 29, 053001, S. D. Baalrud, B. Scheiner, B. T. Yee, M. M. Hopkins, E. V. Barnat