Experimental investigation of sheath effects on <i>I–V</i> traces of strongly electron emitting probes

Yip, Chi-Shung; Jin, Chenyao; Zhang, Wei; Xu, Guo Sheng; Hershkowitz, N.

doi:10.1088/1361-6595/ab60dd

Cited by 14 publications

(19 citation statements)

References 30 publications

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“…It should also be noted that the zero-capacitance point 'predicted' in figure 1 never appeared in figures 5 and 6, and the overshot of the electron saturation current is also quite different between figures 5 and 6 and the predictions shown in figure 1. This suggests that the transition from ion sheath to electron sheath and electron sheath formation has been more complicated than that predicted by conventional theories, as recent studies suggested [25,26].…”

Section: Resultsmentioning

confidence: 98%

Fast-sweeping Langmuir probes: what happens to the I –V trace when sweeping frequency is higher than the ion plasma frequency?

Jin¹,

Yip²,

Zhang³

et al. 2022

Plasma Sci. Technol.

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Limited particle transit time is one of several limiting factors which determines the maximum temporal resolution of a Langmuir probe. In this work, we have revisited known fast sweep Langmuir probe techniques in a uniform, quiescent multi-dipole confined hot cathode discharge with two operation scenarios: one in which the probe sweeping frequency fsweep is much lower than the ion plasma frequency fpi, another one where fsweep is much greater than fpi, respectively. This allows the investigation of the effect of limited ion-motion on I-V traces. Serious distortions of I-V traces at high frequencies, previously claimed to be ion-motion limitation effect, was not found in the degree previously claimed unless shunt resistance is sufficiently high, despite achieving a ratio of ~ 3 between the probe sweeping frequency and the ion plasma frequency. On the other hand, evidences of sheath capacitance on the I-V trace have been observed. Distortions of I-V traces qualitatively agrees with predictions of sheath capacitance response to the sweeping voltage. Additionally, techniques in fast sweep Langmuir probe are briefly discussed. The comparison between a High-speed dual Langmuir probe (HDLP) and the single probe setup shows that the capacitive response can be removed via subtracting a leakage current for the single probe setup almost as effective as using an HDLP setup, but the HDLP setup does remain advantageous in its facilitation of better recovery of weak current signal common in low plasma density situations.

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Section: Resultsmentioning

confidence: 98%

Fast-sweeping Langmuir probes: what happens to the I –V trace when sweeping frequency is higher than the ion plasma frequency?

Jin¹,

Yip²,

Zhang³

et al. 2022

Plasma Sci. Technol.

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“…Particularly with the LaB 6 cathode being 2 mm thick, increase of heating power is generally realized by a higher heating current, which eventually forms a strong enough magnetic field near the rod to prevent electrons from escaping the virtual cathode, extinguishing the discharge, as illustrated in figure 1 [15]. Note that any hot cathode can be TL or SCL depends on plasma parameters, just as emissive probes always experience a TL regime and a SCL one depending on the probe bias and the parameters of the plasma in which the probe is immersed in [23][24][25][26]. Physically speaking, an emissive probes shares the same underlying sheath formation mechanism as a hot cathodes immersed in an existing plasma and theoretical formulations solving sheath formation problems for hot cathodes are often tested with emissive probes [23,[27][28][29].…”

Section: Space-charge Limited Hot Cathodesmentioning

confidence: 99%

“…Note that any hot cathode can be TL or SCL depends on plasma parameters, just as emissive probes always experience a TL regime and a SCL one depending on the probe bias and the parameters of the plasma in which the probe is immersed in [23][24][25][26]. Physically speaking, an emissive probes shares the same underlying sheath formation mechanism as a hot cathodes immersed in an existing plasma and theoretical formulations solving sheath formation problems for hot cathodes are often tested with emissive probes [23,[27][28][29].…”

Section: Space-charge Limited Hot Cathodesmentioning

confidence: 99%

Improving discharge performance of space-charge limited hot cathodes via local neutral collisionality

Yip¹,

Jin²,

Zhang³

et al. 2023

Phys. Scr.

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The effects of neutral gas inlet location to the performance of SCL hot cathodes is investigated. It is found that placing the neutral gas inlet very close (~5mm) to the hot cathode can significantly improve the emission characteristics via immersing the hot cathode in a volume of elevated neutral density. Such placement of the neutral gas inlet was found to make no observable effect on global plasma parameters when the discharge conditions are otherwise identical, facilitating the implementation of this measure onto existing plasma devices. Even with only a small part of the hot cathode is immersed in a volume of an elevated neutral density, the improvement of electron emission is found to be very significant. Thus, one can envision a design of a hot cathode assembly with an integrated neutral gas inlet to significantly improve its emission performance. The underlying mechanism of this improvement of hot cathode performance is the reduction of SCL effects, thus applicable to SCL hot cathodes regardless of heating configurations. This will be particularly useful in low pressure applications where a widened range of operable plasma parameters are often desirable.

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“…The thickness of electron sheath near the probe increase as probe bias voltage increasing respect to plasma potential. In addition, the electron sheath thickness 𝜆 𝑒,𝑠ℎ𝑒𝑎𝑡 ℎ ∼ (𝑉 bias − 𝑉 𝑝 ) 3/4 𝜆 Debye ∼ (𝑉 bias − 𝑉 𝑝 ) 3/4 (𝑇 𝑒 /𝑛 𝑒 ) 1/2 is coupled with the perturbations of 𝑇 𝑒 , 𝑉 𝑝 , and 𝜆 Debye by the effects described in (c) [29] at high probe bias, with which the effective area of the probe expands considerably. Thus, the perturbations to the plasma parameters via the probe as an electron loss area and sheath formation near the probe becomes a self-consistent problem at sufficiently high probe bias.…”

Section: Expanding Fitting Range Of Power Law Parametrizationmentioning

confidence: 99%

Parametrization of electron sheath expansion for Langmuir probe I-V traces

et al. 2023

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The Langmuir probe is a common plasma diagnostic widely utilized in various fields of plasma physics, and its associated methods of data analysis remains in active development. In this work, we present an attempt using a power law parametrization model similar to Sheridan's empirical formulation is to describe the sheath expansion effect on electron current in an argon plasma. The fitting results using the power law parametrization method show observably better consistency compared to those using conventional interception method and inflection method. It is also observed that when used to fit the electron saturation current at probe bias voltages well beyond the plasma potential plus the argon ionization energy, resultant curve fitted by power law parametrization, as with that by linear fitting, deviates significantly from the curve fitted at lower bias voltages, albeit the saturation obtained fitted using the power law method remained consistent with that fitted at lower probe bias voltages. Probable causes of this deviation include anode glow or spots near the probe and the changes of the global plasma parameters via electron absorption at very high probe bias. The fact that the resultant electron saturation current remained consistent hints that these non-trival effects can also be approximated by power law parametrization.

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Experimental investigation of sheath effects on I–V traces of strongly electron emitting probes

Cited by 14 publications

References 30 publications

Fast-sweeping Langmuir probes: what happens to the I –V trace when sweeping frequency is higher than the ion plasma frequency?

Fast-sweeping Langmuir probes: what happens to the I –V trace when sweeping frequency is higher than the ion plasma frequency?

Improving discharge performance of space-charge limited hot cathodes via local neutral collisionality

Parametrization of electron sheath expansion for Langmuir probe I-V traces

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