Density Measurements in Low Pressure, Weakly Magnetized, RF Plasmas: Experimental Verification of the Sheath Expansion Effect

Zhang, Yunchao; Charles, Christine; Boswell, Roderick

doi:10.3389/fphy.2017.00027

Cited by 8 publications

(7 citation statements)

References 21 publications

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“…where c is the speed of light in cm s −1 and n is the plasma density in cm −3 . Figure 5 shows the calculated RF skin depth for the standard and z ext = 0 cm cases found using equation (2) and the radial plasma density obtained using an uncompensated Langmuir probe and Sheridan's method [20,21]. The figure shows that the RF skin depth is very similar between the two cases and has a minimum closer to the centre of the source, increasing radially with decreasing plasma density.…”

Section: Discussionmentioning

confidence: 92%

Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

Bennet¹,

Charles²,

Boswell³

2018

J. Phys. D: Appl. Phys.

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A thorough understanding of particle transport in expanding plasma devices under different geometric and magnetic configurations is necessary for future thruster and reactor design. Recently, we have shown that the development of high density conics via hot electron transport along the most radial field lines escaping the source region of an expanding plasma device is crucial for the development of a high velocity ion beam on axis. In this study, a gap between the source region and a movable extension tube inserted in the downstream of an expanding plasma device is used to create a geometric window of variable size. Only those electron populations situated on field lines that pass through this window can be transported to the region where high density conics normally form. Critical field lines for the plasma behaviour are identified by increasing the size of the geometric window while concurrently measuring the conics density and ion beam characteristics. A step change in the beam velocity is observed when high density conics are able to fully reform. The field lines responsible for full conics formation are found to be those that pass within ∼2 skin depths of the source chamber wall. As such, the downstream plasma behaviour is dominated by electrons situated on field lines that are most effectively heated by the radio-frequency antenna and escape the source.

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Section: Discussionmentioning

confidence: 92%

Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

Bennet¹,

Charles²,

Boswell³

2018

J. Phys. D: Appl. Phys.

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“…This method treats the energy of an incident ion population separately to its density, and the calibration of the density measurement can therefore be treated independently of the velocity. Calibration with a Langmuir probe using Sheridan's analysis 31,32 at a stationary location in the plasma (z ¼ À25 cm) must still be undertaken because the current measurement has not been adjusted for effects like the transmission factor of the biased grids or the acceptance angle of the RFEA, for example; however, these effects are assumed to be consistent across measurements with the same RFEA and can be accounted for through calibration. For a radialfacing RFEA, only one population of ions is expected and…”

Section: Drifting Maxwellians Methodsmentioning

confidence: 99%

In situ electrostatic characterisation of ion beams in the region of ion acceleration

2018

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In situ and ex situ techniques have been used to measure directional ion beams created by a sharp axial potential drop in low pressure expanding plasmas. Although Retarding Field Energy Analysers (RFEAs) are the most convenient technique to measure the ion velocities and plasma potentials along with the plasma density, they are bulky and are contained in a grounded shield that may perturb the electric potential profile of the expanding plasma. In principle, ex situ techniques produce a more reliable measurement and Laser Induced Fluorescence spectroscopy (LIF) has previously been used to characterise the spatial velocity profile of ion beams in the same region of acceleration for a range of pressures. Here, satisfactory agreement between the ion velocity profiles measured by LIF and RFEA techniques has allowed the RFEA method to be confidently used to probe the ion beam characteristics in the regions of high gradients in plasma density and DC electric fields which have previously proven difficult.

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“…Figure 3(d) shows the theoretical prediction of sheath potential according to Equation(1) and Equation(10). Particularly, the black curve in Figure3(d)is exactly the emissive sheath potential predicted by Hobbs, serving as a benchmark for the code validity.…”

mentioning

confidence: 79%

“…A sheath becomes emissive due to surface emission processes, including secondary electron emission (SEE), backscattering, field emission, thermionic emission, photoemission, etc. Emissive sheath widely appears in confined laboratory plasmas and plays a vital role in numerous industrial plasma applications such as plasma processing, electric proportion, plasma diagnostics, plasma source and many others (1)(2)(3)(4). The present research focuses on the algorithms to implement the interactions between plasma and dielectric surface in the kinetic simulation and the underlying sheath physics.…”

Section: Introductionmentioning

confidence: 99%

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

et al. 2022

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A one-dimensional Vlasov–Poisson simulation code is employed to investigate the plasma sheath considering electron-induced secondary electron emission (SEE) and backscattering. The SEE coefficient is commonly treated as constant in a range of plasma simulations; here, an improved SEE model of a charged dielectric wall is constructed, which includes the wall charging effect on the SEE coefficient and the energy dependency of the SEE coefficient. Pertinent algorithms to implement the previously mentioned SEE model in plasma simulation are studied in detail. It is found that the SEE coefficient increases with the amount of negative wall charges, which in turn reduces the emissive sheath potential. With an energy-dependent SEE coefficient, the sheath potential is a nonlinear function of the plasma electron temperature, as opposed to the linear relation predicted by the classic emissive sheath theory. Simulation combining both wall-charging effect and SEE coefficient’ energy dependency suggests that the space-charged limited sheath is formed at high plasma electron temperature levels, where both sheath potential and surface charging saturate. Additionally, different algorithms to implement the backscattering in the kinetic simulation are tested and compared. Converting backscattered electrons to secondary electrons via an effective SEE coefficient barely affects the sheath properties. The simulation results are shown to be commensurate with the upgraded sheath theory predictions.

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Density Measurements in Low Pressure, Weakly Magnetized, RF Plasmas: Experimental Verification of the Sheath Expansion Effect

Cited by 8 publications

References 21 publications

Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

Selective radial release of hot, magnetised electrons downstream of a low-pressure expanding plasma

In situ electrostatic characterisation of ion beams in the region of ion acceleration

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

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