Pressure-driven plasma morphology transition in helium grid-electrode DC glow discharge: Imaging and fluid simulation

This work investigates the plasma morphology of helium DC glow discharge with grid electrodes using optical imaging experiments and three-dimensional fluid simulations. The experimental images show that, with increasing pressure, the plasma image changes from a center-dominated bright region inside the aperture to a cross-shaped bright region near the grid corner. The simulations show that this transition is associated with the evolution of the in-aperture plasma. The potential and electron density distributions change from center-dominated circular morphology governed by volume reactions and diffusion at low pressure to star-like morphology governed by electrode geometry at high pressure. Meanwhile, the aperture center develops into a low-field region, and the excited-species distribution develops a weak-excitation region at the aperture center. At high pressure, sheath contraction inside the aperture causes the electron density distribution to shift toward the post-cathode region, leading to a more distinct post-cathode electron layer. The three-dimensional electron-density isosurface reveals local electron accumulation above the aperture intersections, showing the shaping effect of the grid structure on the post-cathode electron layer. These results indicate that the ratio of characteristic Debye length to aperture size, together with the grid geometry, plays an important role in determining the plasma morphology.