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

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.

Plasma diagnostic systems and methods used on the stellarator TJ-II

The TJ-II is a heliac-type stellarator device with major radius of 1.5 m and averaged minor radius ⩽0.22 m that has been operated at CIEMAT, Madrid since 1998. Its full magnetic field is created by a system of poloidal, central, toroidal and vertical field coils, thus it possesses a fully 3-dimensional plasma structure and a bean-shaped plasma cross-section. Although this results in a complicated vacuum-vessel layout, it has excellent port access for diagnostics (96 portholes). During its initial operational phase, it was equipped with a limited set of essential diagnostics. Since then, a broad variety and large number of both passive and active diagnostics have been installed. The former include Hα monitors, light spectrometers, an electron cyclotron emission radiometer, X-ray filter monitors, neutral particle analysers, magnetic diagnostics, as well as cameras, among others, while the latter include various laser, atomic and ion beam based diagnostics, microwave probe beams, Langmuir probes plus impurity injection techniques. In this paper, after describing the TJ-II stellarator, its heating and fuelling systems, the diagnostic systems employed are outlined and discussed briefly here. Finally, results obtained with selected systems are highlighted.

Tungsten Langmuir probes from JET-with the ITER-Like Wall: Assessment of mechanical properties by nano-indentation

Tungsten Langmuir probes retrieved from the JET tokamak with the ITER-Like Wall (JET-ILW) after the second ILW campaign were examined by nano-indentation, microscopy and X-ray diffraction in order to determine changes in mechanical properties and phase composition. Not-exposed probe served as a reference material. Two regions were studied: (i) recrystallized region below the tip and, (ii) the lower probe structure, called “support structure”. A large difference between the hardness in the tip and the other region has been found: 5 GPa versus 15 GPa, respectively. The measured values of the Young’s modulus in both zones of exposed probe are at the same level of 260 GPa. From the force-displacement curves, it can be concluded that the material in the tip has a smaller range of elastic deformations compared to that characteristic for the support structure. The values obtained for the material in its initial state are consistent with the available literature data for tungsten. With X-ray diffraction and microscopy only tungsten has been detected in the probe tip. It remained clean and free from impurities and undesirable compounds, which could have a negative impact on the probes electrical properties.

Four-electrode probe for plasma studies in the GOL-NB multiple-mirror trap

A system of four-electrode Langmuir probes developed for the GOL-NB multiple-mirror trap is discussed. The system is used for studies of a low-temperature start plasma (1019–1020 m-3, 5 eV) that fills the device during the initial phase of the experiment. The probe allows simultaneous measurements of plasma density, electron temperature and radial electric field. The accuracy of the probe measurements is also discussed.

A New Ionospheric Index to Investigate Electron Temperature Small-Scale Variations in the Topside Ionosphere

The electron temperature (Te) behavior at small scales (both spatial and temporal) in the topside ionosphere is investigated through in situ observations collected by Langmuir Probes on-board the European Space Agency Swarm satellites from the beginning of 2014 to the end of 2020. Te observations are employed to calculate the Rate Of change of electron TEmperature Index (ROTEI), which represents the standard deviation of the Te time derivative calculated over a window of fixed width. As a consequence, ROTEI provides a description of the small-scale variations of Te along the Swarm satellites orbit. The extension of the dataset and the orbital configuration of the Swarm satellites allowed us to perform a statistical analysis of ROTEI to unveil its mean spatial, diurnal, seasonal, and solar activity variations. The main ROTEI statistical trends are presented and discussed in the light of the current knowledge of the phenomena affecting the distribution and dynamics of the ionospheric plasma, which play a key role in triggering Te small-scale variations. The appearance of unexpected high values of ROTEI at mid and low latitudes for specific magnetic local time sectors is revealed and discussed in association with the presence of Te spikes recorded by Swarm satellites under very specific conditions.

Swarm Langmuir Probes' data quality and future improvements

Swarm is ESA's (European Space Agency) first Earth observation constellation mission, which was launched in 2013 to study the geomagnetic field and its temporal evolution. Two Langmuir Probes (LPs) on board of each of the three Swarm satellites provide very accurate measurements of plasma parameters, which contribute to the the study of the ionospheric plasma dynamics. To maintain a high data quality for scientific and operational applications, the Swarm products are continuously monitored and validated via science-oriented diagnostics. This paper presents an overview of the data quality of the Swarm Langmuir Probes' measurements. The data quality is assessed by analysing short and long data segments, where the latter are selected sufficiently long to consider the impact of the solar activity. Langmuir Probes data have been validated through comparison with numerical models, other satellite missions, and ground observations. Based on the outcomes from quality control and validation activities conduced by ESA, as well as scientific analysis and feedback provided by the user community, the Swarm products are regularly upgraded. In this paper we discuss the data quality improvements introduced with the latest baseline, and how the data quality is influenced by the solar cycle. The main anomaly affecting the LP measurements is described, as well as possible improvements to be implemented in future baselines.