Swept Langmuir Probe Research Articles

A field programmable gate array based Langmuir probe system for measurement of plasma parameters at 500kHz in a high-power impulse magnetron sputtering plasma.

By utilizing Field Programmable Gate Arrays in a configuration similar to that of the Mirror Langmuir Probe, it is possible to bias a single probe at three precise voltages in sequence. These voltages can be dynamically adjusted in real-time based on the measured plasma electron temperature to ensure the transition region is always sampled. The first results have been obtained by employing this method and have generated real-time outputs of electron temperature, ion saturation current, and floating potential on a low temperature pulsed-DC magnetron at 500kHz. These results are in good agreement with the analysis of a conventionally swept Langmuir probe. This probe is designed with the intention of being implemented on MAST-U to aid in the study of exhaust physics and enable further investigation into filamentary behavior.

Laboratory measurements of the performances of the Sweeping Langmuir Probe instrument aboard the PICASSO CubeSat

Abstract. The Sweeping Langmuir Probe (SLP) is one of the instruments on board the triple-unit CubeSat PICASSO, an ESA in-orbit demonstrator launched in September 2020, which is flying at about 540 km altitude. SLP comprises four small cylindrical probes mounted at the tip of the solar panels. It aims to perform in situ measurements of the plasma parameters (electron density and temperature together with ion density) and of the spacecraft potential in the ionosphere. Before the launch, the instrument, accommodated on an electrically representative PICASSO mock-up, was tested in a plasma chamber. It is shown that the traditional orbital-motion-limited collection theory used for cylindrical Langmuir probes cannot be applied directly for the interpretation of the measurements because of the limited dimensions of the probes with respect to the Debye length in the ionosphere. Nevertheless, this method can be adapted to take into account the short length of the probes. To reduce the data downlink while keeping the most important information in the current-voltage characteristics, SLP includes an on-board adaptive sweeping capability. This functionality has been validated in both the plasma chamber and in space, and it is demonstrated that with a reduced number of data points the electron retardation and electron saturation regions can be well resolved. Finally, the effect of the contamination of the probe surface, which can be a serious issue in Langmuir probe data analysis, has been investigated. If not accounted for properly, this effect could lead to substantial errors in the estimation of the electron temperature.

Application of the triple-probe technique to magnetized plasmas

The triple-probe technique (TPT) is a diagnostic widely used to determine the electron temperature in various devices and plasma conditions. It was developed for measurements in low-pressure gas-discharge plasmas in the absence of a magnetic field. This paper presents a comparison of the experimental results obtained by swept Langmuir probe (LP) measurements with those obtained by the TPT in magnetized plasmas in three experimental machines with different magnetic field magnitudes (0.01–1.15 T). The reliability of the triple-probe results for the electron temperature in tokamak plasmas at higher magnetic fields is discussed. It was found that the larger the magnetic field, the more the TPT overestimates the electron temperature compared with single swept LPs. The explanation proposed in this paper is based on a shift in the floating potential towards the plasma potential in the presence of a magnetic field, yielding a more positive voltage measured by the TPT and therefore higher electron temperatures. Using the extended formula for the electron probe current in the presence of a magnetic field a correction factor is derived such that the TPT yields a temperature similar to that of the swept LP techniques.

On the applicability of three and four parameter fits for analysis of swept embedded Langmuir probes in magnetised plasma

The problem of power exhaust is one of the grand challenges of nuclear fusion research today. In order to understand the physics phenomena occurring in the scrape-off layer and the divertor regions of tokamaks, it is essential to correctly determine the divertor plasma parameters, which are often measured by swept Langmuir probes (LPs). While the construction and operation of this diagnostic can be straightforward, the data analysis using three- or four-parameter fits presents a challenge and can potentially lead to erroneous values of electron temperature and ion saturation current. In this work, we present modelling and experiments aimed at determination of conditions for proper analysis of swept LPs using these two fitting models. Particle-in-cell modelling was employed to evaluate the sheath-expansion effects for particular probe geometry and plasma conditions, yielding a semi-empirical rule capable of predicting its magnitude. Experiments with unusually wide range of swept voltage in the divertor of the COMPASS tokamak explored the magnitude of voltage range required for successful analysis with either three or four-parameter fitting. With the use of our new semi-empirical rule, it is possible to improve the four-parameter fit reliability in situations where the available voltage range is limited. In addition, we introduce the tangent method—an independent and fast method of electron temperature estimation, which allows to reliably determine the available voltage range and as such assist more complex methods of probe analysis.

Climatology of Deep O+ Dropouts in the Night‐Time F‐Region in Solar Minimum Measured by a Langmuir Probe Onboard the International Space Station

AbstractThe Floating Potential Measurement Unit onboard the International Space Station includes a Wide sweeping Langmuir Probe that has been operating in the F‐region of the ionosphere at ∼400 km since 2006. While traditional Langmuir probe estimates include critical plasma parameters like electron density and temperature, we have also extracted the O+ percentage from the total ion constituents. This O+ composition data set from the recent minimum in the Solar Cycle 24 reveals orbits with dropouts in O+ to below 80% of the total background ion density at International Space Station orbital altitudes. The observed O+ percentages during these dropouts are much lower than the values predicted by the International Reference Ionosphere 2016 (IRI2016) empirical model. In this paper, we present the climatology of these O+ dropouts with their dependency on season, local time and geographical location. The results show that the lowered O+ percentages are more significant in the winter hemispheres and are routinely observed for orbits in the pre‐sunrise periods. The patterns in O+ dropouts can be explained in part from the lowering of the O+/H+ transition height during solar minimum along with patterns in neutral wind variation.

Development and application of limiter Langmuir probe array in EAST

A newly developed limiter Langmuir probe (LLP) array has been applied to the scrape-off layer (SOL) plasma measurement in EAST tokamak. The LLP system consists of two poloidal arrays located at opposite sides of the low field side guard limiter, and each poloidal array has 26 Langmuir probe channels with a spatial resolution of 35 mm in the vertical direction. The probe tip is polished to match the local graphite tile plane and avoid the leading edge and the corresponding high heat load. The probe can be assembled easily and has an excellent electric insulating property. Langmuir probes have three operation modes, i.e., floating potential mode, ion saturation current mode, and swept Langmuir probe mode. The LLP array could be operated in different modes according to the experimental purpose. The LLP array has been successfully applied to the EAST experiment in 2021, and the three-dimensional plasma structure and the plasma parameters (electron density and temperature) in the SOL have been measured.

Terrella for advanced undergraduate laboratory

A terrella developed for the undergraduate Advanced Laboratory course in the University of Wisconsin-Madison Physics Department is described. Our terrella consists of a permanent magnet, mounted on a pedestal in a vacuum chamber, surrounded by electrodes that may be biased in various ways. The system can confine a plasma, which may, in some ways, be considered as a toy model of the plasma confined in the Earth's magnetosphere. Our axisymmetric plasma forms in a region where the magnitude of the magnetic field B is 14 G ≤B≤ 550 G; for typical operation, the neutral gas pressure is p∼10−4 Torr. The plasma is created by thermionic emission from a hot filament. Available diagnostics are a swept Langmuir probe, a spectroscopic fiber and visible-wavelength spectrometer, and visible imaging. In two four-hour laboratory sessions, students are guided through vacuum pumpdown, connection of electrical circuits, establishment of plasma, acquisition of data, analysis of data, and critique of data. In this paper, we present student measurements of radial profiles of electron temperature Te and density ne as well as imaging of mirror trapping and ∇B drift and curvature drift. We conclude by outlining some opportunities for additional terrella-based student experiments.

Three-dimensional measurements of plasma parameters in an inductively coupled plasma processing chamber

Inductively coupled plasmas (ICPs) are extensively used for materials processing and microelectronics fabrication. However, their electromagnetic properties have not been fully characterized. In this regard, we have performed fully three dimensional (3D), time dependent measurements of the magnetic field, electron density, and electron temperature for an ICP sustained in argon in an industrial reactor designed for plasma etching in microelectronics fabrication. These measurements were compared to modeling results. The plasma was generated using pulsed power delivered at 2 MHz by a planar coil. The magnetic field was measured using a three axis magnetic probe at 15 366 locations throughout the plasma volume during the H-mode portion of the pulse at temporal intervals of 2 ns. A swept Langmuir probe was used to measure plasma parameters at the same locations. The plasma density measurement was calibrated with line-integrated densities obtained using a 96 GHz interferometer. During a single radio frequency (RF) cycle, the 3D current density [derived from B(r→,t)]is initially maximum just below the coil and moves downward toward the center of the chamber. Isosurfaces of current are nearly symmetric toroids. The total electric field, space charge field, and inductive electric field were derived and used to calculate the dissipated power, plasma current, and Poynting flux. Computer modeling of the experiment reproduces the phase dependent behavior. Animations showing the time dependent 3D measurements are presented in the supplementary material.

LEO Charging of the PICASSO Cubesat and Simulation of the Langmuir Probes Operation

The sweeping Langmuir probe (SLP) instrument is a payload of the European Space Agency's (ESA) in-orbit demonstrator PicoSatellite for Atmospheric and Space Science Observations (PICASSO) cubesat, measuring the plasma environment in a high-inclination low earth orbit (LEO) orbit. Because of the small size of the spacecraft (0.3 m × 0.1 m × 0.1 m), the spacecraft floating potential can significantly drift when sweeping the probe bias voltage, affecting the current-voltage characteristic from which plasma parameters are retrieved. In LEO, the spacecraft generally encounters a high-density, cold (less than 1 eV) plasma environment, but due to the high orbital inclination, PICASSO passes the auroral regions exposing the spacecraft to precipitating fluxes of hot (several kiloelectron volts) electrons with potentially harmful, high-level spacecraft charging as a result. To investigate the spacecraft charging and the in-orbit operation of the Langmuir probes in both of these regimes, simulations of PICASSO were carried out in Spacecraft Plasma Interaction System (SPIS). The conditions of the LEO plasma and the constraints imposed by the particle-in-cell (PIC) models on the mesh size make this type of plasma simulations a very challenging task.

Use of a Langmuir Probe Instrument on Board a Pico-Satellite

The sweeping Langmuir probe instrument is a part of the payload of PICo-satellite for atmospheric and space science observations (PICASSO), an ESA in-orbit demonstrator. It includes four thin cylindrical probes whose electrical potential is swept to measure both plasma density and electron temperature together with the spacecraft (S/C) potential. PICASSO is a triple unit CubeSat of dimensions $340.5~\text {mm} \times 100~\text {mm} \times 100$ mm. The orbit is expected to be a high inclination low-earth orbit. The main issue implied by the use of a pico-satellite platform for a Langmuir probe instrument is the limited conducting area of the S/C, which leads to S/C charging and drift of the instrument’s electrical ground during the measurement. A specific measurement technique that includes the simultaneous measurement of the potential and current of different probes has been developed to retrieve consistent current–voltage characteristics that can be used to estimate the plasma parameters mentioned above.

Elevated electron temperatures around twin sporadic E layers at low latitude: Observations and the case for a plausible link to currents parallel to the geomagnetic field

We present data from nighttime sounding rocket flights in the low latitude E region. The payloads carried a sweeping Langmuir probe, a plasma impedance probe, and electric field probes. A detailed examination of the plasma density, temperature, and electric field measurements show two strong sporadic E (Es) layers with very high electron temperatures (∼1000 K) on each side of the upper layer. The lower layer was consistent with the presence of a strong zonal neutral wind shear. The upper layer was strongly influenced by the presence of a strongly negative vertical electric field, with zonal winds and their shears also contributing. A strong downward motion of the plasma from the combined action of the downward electric field and negative zonal wind advected the upper layer far below the region of maximum growth. We have attributed the more puzzling high electron temperatures to frictional heating from parallel currents and shown that the F region nighttime dynamo could easily generate the necessary parallel current densities (1 μA m−2) near the electron density troughs. The electron temperature was also elevated in the Es layers themselves, implying parallel current densities of the order of 15 μA m−2 around the Es peaks. Those parallel currents were attributed to strong Hall current divergences driven by the zonal electric field around the Es peaks.

On the dynamic behavior of the anode–arc–root at the nozzle surface in a non-transferred plasma torch

The dynamic behavior of the anode–arc–root at the nozzle surface of a plasma torch was experimentally investigated in this work. A gas (N2) vortex–stabilized non-transferred arc torch with a thoriated tungsten rod (2wt %) cathode (3.2 mm diameter) and a coaxial anode (5 mm diameter, 30 mm length) was used in the experiment. By using a sweeping Langmuir probe in floating condition, the voltage of the plasma jet outside the nozzle was inferred. Arc voltage waveforms were also obtained. Data have been obtained for an arc current of 100 A and a gas flow rate of 30 Nl min−1. A typical sawtooth shape (i.e., restrike mode) (with a fluctuating level of ≈ ± 25 %) and a dominant frequency of ≈ 6.5 kHz was observed in the arc voltage waveforms, which is attributed to anode–arc–root movements along the anode surface followed by a restrike at a certain point close to the cathode. By performing a time correlation between the probe and arc voltage oscillograms together with simple estimations, the amplitude of the movement of the arc–root along the anode surface as well its velocity were inferred.

Understanding deposition rate loss in high power impulse magnetron sputtering: I. Ionization-driven electric fields

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a ‘sweet spot’ in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ΠTownsend is proposed. A parametric analysis of ΠTownsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength.

Towards Fast Measurement of the Electron Temperature in the SOL of ASDEX Upgrade Using Swept Langmuir Probes

AbstractOn ASDEX Upgrade first experiments were made using single probes with a voltage sweep frequency up to 100kHz. Possibilities and limitations using fast swept probes with a standard diagnostic and analysis tools are discussed. A good agreement between the data derived from fast swept single probe characteristics and floating as well as saturation current measurements was found. In a stationary (non ELMing) plasma the data of the fast swept probe are compared to standard slow swept probes (kHz range) showing an improvement of the measurement by faster sweeping. While ELM filaments already could be resolved the access of electron temperature fluctuations in small scale turbulence still has to be improved. Further comparisons are done in ELMy H‐mode with combined ball‐pen probe/floating potential measurements which can deliver electron temperatures with 25 μ s time resolution at reduced spatial resolution compared to pin probes. During ELMs the electron temperatures derived from the ball‐pen probe and fast swept single probes agreed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temporal limits of a rapidly swept Langmuir probe

The finite, electrostatically achievable, temporal resolution of plasma properties from a turbulent discharge is limited by an array of effects wherein the theory of Langmuir probes breaks down. Formulations for the particle transit time, sheath formation time, plasma-probe resonance, polarization current, sheath capacitance, stray capacitance, and mutual capacitance effects are all evaluated for time-resolved operation of a Langmuir probe. The resulting time scales serve to place a theoretical bound on the maximum rate of a rapidly swept Langmuir probe as analyzed with typical thin-sheath collisionless probe theory. For plasma typical to the plume of a Hall effect thruster [xenon plasma, ne=(1–1000)×10+15 m−3, and Te=1–20 eV], upper limits of 0.01–70 kHz are observed for a noncapacitive compensated Langmuir probe. With a high-speed dual Langmuir probe (HDLP) (a regular probe plus a null compensation probe), the upper probing frequency limits are increased to 0.04–11 MHz limited by sheath capacitance in the far and near field, and polarization effects for closer internal measurements. For a typical tokamak edge plasma (with HDLP), the thermally equilibrated hotter species (typically Te≈Ti≈10 to 20 eV) and lighter ions together lend higher limiting rates of ion transit, sheath formation, and sheath capacitance effects (in excess of 20 MHz), but the fully magnetized plasma complicates the collected probe current, limiting the allowable sweep rate to <1 MHz (for a magnetic field of 2 T). Thus we find that the upper rate of Langmuir probe sweeping is in the low megahertz range for both electric thruster and fusion plasma device diagnostics.

Publisher’s Note: “Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements” [Rev. Sci. Instrum. 75, 4334 (2004)

Publisher’s Note: “Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements” [Rev. Sci. Instrum. 75, 4334 (2004)

Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements

Swept Langmuir probes are widely used to measure electron temperature (Te) in laboratory plasmas by performing an exponential fit to the measured volt–ampere (I–V) characteristic. Often the probe voltage sweep frequency is much lower than the characteristic frequencies of the plasma fluctuations and a time-averaged I–V characteristic is used for the fit. We show by numerical modeling that in the presence of Te fluctuations with frequencies well above the voltage sweep frequency this standard technique applied to a swept single probe tends to read higher than the actual time-averaged Te provided no correlated plasma potential (Vp) fluctuations are present. In the presence of coupled Te and Vp fluctuations a slowly swept single probe may read either higher or lower than the average Te, depending on the relative amplitude and phase of the temperature and potential fluctuations. In contrast, swept double probe measurements of Te are virtually unaffected by either Te or Vp fluctuations.

Experimental evidence of significant temperature fluctuations in the plasma edge region of the TJ-I tokamak.

Density and temperature fluctuations have been measured in the plasma bulk side of the velocity-shear location of the TJ-I tokamak using a fast swept Langmuir probe technique. Evidence has been found that substantial temperature fluctuations have a phase difference of close to \ensuremath{\psi} with respect to the corresponding density fluctuations. This result suggests the possible role of radiation in determining edge fluctuation levels and calls into question the determination of the density and potential fluctuations from the Langmuir current probe and floating potential fluctuations.

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude ( E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn/ n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

The characterization of a laser-produced negative-hydrogen-ion plasma

The parameters of a plasma produced by irradiation of alkali hydride targets with a ruby-laser pulse are experimentally measured. Positive- and negative-ion abundances are determined with a quadrupole mass analyzer. Electron temperature was measured with rapidly swept Langmuir probes. Negative (H−) ion temperature was obtained using the mass analyzer and focusing grids. The electron density near the target was determined by optical holography. The quantity of negative ions produced per laser pulse make the source attractive for the future development of high-energy neutral-beam systems.