Author: W. E. HanPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
The Stability of the Plasma Sheath with Secondary Emission
Author: W. E. HanThe Effect of Secondary Electron Emission on a Plasma Sheath
Author: Peter J. HarbourStability of the Plasma Sheath
Author: R.N. FranklinPublisher:
ISBN: 9780853110859
Category : Plasma (Ionized gases)
Languages : en
Pages : 6
Book Description
Plasma-Sheath Instability in Hall Thrusters Due to Periodic Modulation of the Energy of Secondary Electrons in Cyclotron Motion
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 579
Book Description
Particle-in-cell simulation of Hall thruster plasmas reveals a plasma-sheath instability manifesting itself as a rearrangement of the plasma sheath near the thruster channel walls accompanied by a sudden change of many discharge parameters. The instability develops when the sheath current as a function of the sheath voltage is in the negative conductivity regime. The major part of the sheath current is produced by beams of secondary electrons counter-streaming between the walls. The negative conductivity is the result of nonlinear dependence of beam-induced secondary electron emission on the plasma potential. The intensity of such emission is defined by the beam energy. The energy of the beam in crossed axial electric and radial magnetic fields is a quasi-periodical function of the phase of cyclotron rotation, which depends on the radial profile of the potential and the thruster channel width. There is a discrete set of stability intervals determined by the final phase of the cyclotron rotation of secondary electrons. As a result, a small variation of the thruster channel width may result in abrupt changes of plasma parameters if the plasma state jumps from one stability interval to another.
The Effects of Space-charge Limited Electron Emission on the Plasma Sheath
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Theory and experiments are presented that explore the space-charge limited regime of plasma sheaths surrounding electron emitting surfaces. The separation point technique and the inflection point probe methods for measuring the plasma potential were compared to the floating potential of a highly emissive probe in a Hall thruster plasma. The inflection point techniques' measurement of the plasma potential was ~2Te/e above the floating potential measurement, supporting fluid theory predictions. The separation point technique was inconsistent with every other technique and did not accurately measure the plasma potential. A virtual cathode was observed in a multidipole chamber plasma sheath near a grounded, metal boundary coated with a dielectric. A combination of secondary electron emission and a relatively high density of primary electrons formed the space-charge limited sheath indicated by the virtual cathode. Low neutral pressure, primary electron energy, and discharge current allowed the virtual cathode to form. The discharge current greatly affected the sheath potential, reducing it as the current increased. A kinetic theory of space-charge limited electron emitting sheaths is presented which accurately treats the loss of plasma electrons to the boundary and the velocity distribution of emitted electrons. By considering those electrons lost to the wall, the predicted sheath potential was reduced by 10%. Using a kinetic description of the emitted electrons, assuming a half-Maxwellian distribution, greatly affects the sheath potential. It is shown that the kinetic theory predicts that the sheath potential goes to zero as the plasma potential. To test this kinetic theory of emissive sheaths, time-resolved measurements of the emissive sheath potential were made in the afterglow of a capacitively coupled plasma. The results showed that as the plasma electron temperature cooled and approached the emitted electron temperature, the normalized sheath potential was drastically reduced and went to zero when the two temperatures were equal, qualitative supporting the emissive sheath kinetic theory. The emissive sheath potential was unexpectedly large when the plasma electron temperature was larger than the emitted electron temperature by a factor of 4.
Instability, Collapse and Oscillation of Sheaths Caused by Secondary Electron Emission
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
The Debye sheath is shown to be unstable under general conditions. For surface materials with sufficient secondary electron emission (SEE) yields, the surface's current-voltage characteristic has an unstable branch when the bulk plasma temperature (Te) exceeds a critical value, or when there are fast electron populations present. The plasma-surface interaction becomes dynamic where the sheath may undergo spontaneous transitions or oscillations. Using particle-in-cell simulations, we analyze sheath instabilities occurring in a high Te plasma slab bounded by walls with SEE. As the plasma evolves, whenever the sheath enters an unstable state, its amplitude rapidly collapses, allowing a large flux of previously trapped electrons to hit the wall. These hot electrons induce more than one secondary on average, causing a net loss of electrons from the wall. The sheath collapse quenches when the surface charge becomes positive because the attractive field inhibits further electrons from escaping. Sheath instabilities influence the current balance, energy loss, cross-B-field transport and even the bulk plasma properties. Implications for discharges including Hall thrusters are discussed. More generally, the results show that common theories that treat emission as a fixed (time-independent) "coefficient" do not capture the full extent of SEE effects.
Fusion Energy Update
Author: General Cause of Sheath Instability Identified for Low Collisionality Plasma in Devices with Secondary Electron Emission
Author: Publisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
A condition for sheath instability due to secondary electron emission (SEE) is derived for low collisionality plasmas. When the SEE coefficient of the electrons bordering the depleted loss cone in energy space exceeds unity, the sheath potential is unstable to a negative perturbation. This result explains three different instability phenomena observed in Hall thruster simulations including a newly found state with spontaneous 2̃0MHz oscillations. When instabilities occur, the SEE propagating between the walls becomes the dominant contribution to the particle flux, energy loss and axial transport.
Plasma Physics and Engineering
Author: Alexander FridmanPublisher: CRC Press
ISBN: 1439884811
Category : Science
Languages : en
Pages : 1089
Book Description
Plasma plays an important role in a wide variety of industrial processes, including material processing, environmental control, electronic chip manufacturing, light sources, and green energy, not to mention fuel conversion and hydrogen production, biomedicine, flow control, catalysis, and space propulsion. Following the general outline of the bests
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