Negative plasma potential relative to electron-emitting surfaces

Abstract

Most works on plasma-wall interaction predict that with strong electron emission, a nonmonotonic "space-charge-limited" (SCL) sheath forms where the plasma potential is positive relative to the wall. We show that a fundamentally different sheath structure is possible where the potential monotonically increases toward a positively charged wall that is shielded by a single layer of negative charge. No ion-accelerating presheath exists in the plasma and the ion wall flux is zero. An analytical solution of the "inverse sheath" regime is demonstrated for a general plasma-wall system where the plasma electrons and emitted electrons are Maxwellian with different temperatures. Implications of the inverse sheath effect are that (a) the plasma potential is negative, (b) ion sputtering vanishes, (c) no charge is lost at the wall, and (d) the electron energy flux is thermal. To test empirically what type of sheath structure forms under strong emission, a full plasma bounded by strongly emitting walls is simulated. It is found that inverse sheaths form at the walls and ions are confined in the plasma. This result differs from past particle-in-cell simulation studies of emission which contain an artificial "source sheath" that accelerates ions to the wall, leading to a SCL sheath at high emission intensity.

Effect of total emitted electron velocity distribution function on the plasma sheath near a floating wall

“…In this study, only the classic sheath and critical SCL sheath, 6,[18][19][20][21][22][23][24][25][26] i.e. φ monotonically decreases along x direction, is considered.…”

“…6 They found that sheath potential drop decreases with increment of Γ, and thereby heat flux at wall increases; when Γ reaches a critical value Γ c,SCL = 1 8.3(m e /m i ) 0.5 , the sheath goes into space-charge limited (SCL) regime, with wall potential relative to sheath edge φ w = 1.02T e /e and wall electric field equals zero, where m e and m i are the masses of electron and ion respectively, T e is plasma electron temperature and e is the unit charge. Some other kinetic sheath models combined with PIC or Vlasov kinetic simulation also found that strong electron emission causes sheath structure transition from classic sheath to SCL sheath; [20][21][22][23][24] while the sheath was predicted to transit to an inverse Debye layer structure with a fixed ion flux through the sheath region, 25 and a more entirely inverse sheath structure with no ion through the sheath region; 26 and if ion collisions are included, these collisions cause ions get trapped in virtual cathode and force a transition to the inverse sheath. 27 The effect of emitted electron temperature (EET) on potential of critical SCL sheath was specially investigated in recent several years by Sheehan et.al with kinetic theory analysis, particle-in-cell simulation and experiment measurement.…”

“…from classic sheath to SCL sheath 6,[20][21][22][23] or inverse sheath. 25,26 Therefore, it naturally makes a question that how different total EEVDFs affect sheath potential and its structure transition.…”

Strongly Emitting Surfaces Unable to Float below Plasma Potential

“…Recent theory [10] and simulation [11,12,13] studies have demonstrated a second solution type. In the "inverse regime" the sheath potential is positive, opposite in sign from the SCL.…”

“…1 (see Ref. [10] for further discussion). For γ > 1, the SCL and inverse models both give valid solutions with the required emission barrier.…”