Nonlocal escape of nanoparticles from the negative glow in an acetylene DC discharge: Experiment and PIC modeling

Nanoparticle formation, confinement, and loss in a low-current DC glow discharge in acetylene with vertically oriented electrodes are investigated experimentally and numerically. Laser light scattering reveals the formation of a nanoparticle cloud in the negative glow region at acetylene pressures above 0.15 Torr, whereas no stable cloud is observed at lower pressures. The cloud is localized near the region of maximum plasma emission and evolves in time owing to particle growth and gravity-driven redistribution. A robust nonlocal loss mechanism is observed, whereby nanoparticles leave the confinement region and deposit on the discharge tube walls at positions significantly shifted toward the anode. Transmission electron microscopy demonstrates pronounced spatial size selection, with small nanoparticles confined near the cloud, whereas only large particles and aggregates are transported into the Faraday dark space. Two-dimensional particle-in-cell simulations combined with a force-balance analysis reveal a shallow axial potential pit in the negative glow and a spatially varying near-wall sheath that govern nanoparticle confinement and escape. The calculations show that the smallest nanoparticles carry, on average, less than one elementary charge because of the exceptionally low electron temperature in the negative glow, allowing stochastic charge fluctuations to produce neutral particles that can escape radially to the chamber walls. Larger nanoparticles remain electrostatically confined by the near-wall potential barrier. Because it is localized along the discharge axis, nanoparticles can drift along the confinement boundary toward the anode and bypass the barrier, escaping to the chamber wall. This mechanism provides a physical explanation for the observed nonlocal nanoparticle loss.