Absence of Debye Sheaths Due to Secondary Electron Emission

A bounded plasma where the electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.Comment: 4 pages, 3 Figure

Self-amplification of electrons emitted from surfaces in plasmas with E x B fields

Emission from surfaces is known to cause enhanced wall heating and enhanced energy loss from plasma electrons. When E X B fields are present, emitted electrons are heated by the drift motion and cause enhanced transport along E. All emission effects are normally predicted to reach a maximum when the sheath becomes space-charge limited because any 'additional' emitted electrons return to the wall. But the returning electrons are also heated in the E X B drift, further enhancing transport, and return to the wall with extra energy, further enhancing the energy flux. Returning electrons can gain enough energy to induce secondaries, thereby self-amplifying to higher intensities. This newly analyzed mechanism could affect the wall heating, transport and global energy balance under certain conditions. Theory and simulations are presented

Cavitation energies can outperform dispersion interactions

The accurate dissection of binding energies into their microscopic components is challenging, especially in solution. Here we study the binding of noble gases (He-Xe) with the macrocyclic receptor cucurbit[5]uril in water by displacement of methane and ethane as 1H NMR probes. We dissect the hydration free energies of the noble gases into an attractive dispersive component and a repulsive one for formation of a cavity in water. This allows us to identify the contributions to host-guest binding and to conclude that the binding process is driven by differential cavitation energies rather than dispersion interactions. The free energy required to create a cavity to accept the noble gas inside the cucurbit[5]uril is much lower than that to create a similarly sized cavity in bulk water. The recovery of the latter cavitation energy drives the overall process, which has implications for the refinement of gas-storage materials and the understanding of biological receptors