Plasma Potential Measurements Research Articles

Plasma potential and ion energy characteristics in BP-HiPIMS discharge with double layer

Abstract As an emerging ion acceleration plasma source, the bipolar-pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge has been widely studied by academia and industry due to its ability to adjust the ion kinetic energy. Formation of the double layer (DL) potential structure during the BP-HiPIMS positive pulse is vital for accelerating ions, but its structural characteristics are still unclear. In this work, to understand the DL characteristics affected by various discharge parameters, the evolution of plasma potential V p and ion energy in BP-HiPIMS discharge with copper target has been investigated systematically using an emissive probe and mass spectrometer together. Spatial plasma potential measurements show that the DL is established in front of the target during the positive pulse, whose boundary potential drop U DL to accelerate ions can be increased to ∼60 V at a lower operating gas pressure (p= 0.6 Pa) and a higher applied positive pulse voltage (U + = 200 V). The ignition onset time of DL after applying the positive pulse can be shortened to ∼25 μs by decreasing the gas pressure and increasing the positive pulse voltage or negative pulse duration. After DL ignition, a group of high-energy copper ions with energy higher than the surrounding plasma potential can be recognized in the ion energy distribution function curves in the downstream plasma. This result illustrates that the copper ions can be ionized in the high-potential plasma region and be accelerated by the DL boundary potential drop. In addition, a global current balance model of the DL in BP-HiPIMS is developed, which suggests that the U DL can be well adjusted by increasing the positive pulse voltage U + especially for U + > 200 V as verified by the experimental potential measurements. All results suggest that the copper particles play an important role in the formation of DL and the DL plays an important role in accelerating copper ions.

Plasma potential measurements using an emissive probe made of oxide cathode

A novel emissive probe consisting of an oxide cathode coating is developed to achieve a low operating temperature and long service life. The properties of the novel emissive probe are investigated in detail, in comparison with a traditional tungsten emissive probe, including the operating temperature, the electron emission capability and the plasma potential measurement. Studies of the operating temperature and electron emission capability show that the tungsten emissive probe usually works at a temperature of 1800 K–2200 K while the oxide cathode emissive probe can function at about 1200 K–1400 K. In addition, plasma potential measurements using the oxide cathode emissive probe with different techniques have been accomplished in microwave electron cyclotron resonance plasmas with different discharge powers. It is found that a reliable plasma potential can be obtained using the improved inflection point method and the hot probe with zero emission limit method, while the floating point method is invalid for the oxide cathode emissive probe.

The First Application of the HIBP Diagnostics for co-NBI Plasma Potential Measurement in the TUMAN-3M Tokamak

The First Application of the HIBP Diagnostics for co-NBI Plasma Potential Measurement in the TUMAN-3M Tokamak

Plasma sputtering of biased electrodes in an oblique magnetic field

The surface erosion of biased electrodes immersed in a radio frequency (RF) plasma was investigated in oblique magnetic fields ranging from 0.1 T to 3.5 T. The plasma potential and density in the vicinity of the biased electrode have been measured using an RF-compensated cylindrical Langmuir probe moved by a three-dimensional motorised arm in the ALINE facility at 0.1 T. The erosion profiles were determined for different gases (argon and helium), magnetic field strength and plasma volume for the same electrode surface area at an angle of 5∘ with the magnetic field in three different experimental geometries. The radial electric field (perpendicular to the magnetic field) in the plasma column has been deduced from plasma potential measurements. This radial field makes the ions converge to the core of the plasma column at grazing angles (30∘) and is reversed at larger angles. The ion flux on the sample increases with the magnetic field magnitude leading to a strong erosion inhomogeneity. At lower plasma volume, the erosion rate is sensitively enhanced. Local redeposition was identified using x-ray photoelectron spectroscopy on the surfaces exhibiting preferential deposition directions. Finally, the asymmetric erosion patterns obtained from a molybdenum layer etching are explained by the similar asymmetric density maps obtained with the probe and by the drift and the local sheath acceleration.

The longitudinal energy spread of ion beams extracted from an electron cyclotron resonance ion source

We present a study of factors affecting the energy spread of ion beams extracted from a Charge Breeder Electron Cyclotron Resonance Ion Source (CB-ECRIS). The comprehensive simulations, supported by experiments with a Retarding Field Analyser (RFA), reveal that the longitudinal and transverse energy spread of the extracted beams are strongly affected by the electrostatic focusing effects, namely the extraction geometry and plasma beam boundary, to the extent that the electrostatic effects dominate over the magnetic field induced rotation of the beam or the effect of plasma potential and ion temperature. The dominance of the electrostatic focusing effect over the magnetic field induced rotation complicates parametric studies of the transverse emittance as a function of the magnetic field strength, and comparison of emittance values obtained with different ion sources having different extraction designs. Our results demonstrate that the full ion beam energy spread, relevant for the downstream accelerator, can be measured with the RFA only when all ions are collected. On the contrary, studying the effect of plasma properties (plasma potential and ion temperature) on the longitudinal energy spread requires heavy collimation of the beam accepting only ions near the symmetry axis of the beam for which the electrostatic and magnetic effects are suppressed. As the extraction system of the CB-ECRIS is similar to a conventional ECRIS, the conclusions of the study can be generalised to apply for all high charge state ECR ion sources. Finally, we present the results of systematic plasma potential measurements of the Phoenix-type CB-ECRIS at LPSC, varying the source potential, the microwave power and the axial magnetic field srength. It was observed that the plasma potential increases with the extraction magnetic field and the microwave power.

Determination of Plasma Potential Using an Emissive Probe with Floating Potential Method

Despite over 90 years of study on the emissive probe, a plasma diagnostic tool used to measure plasma potential, its underlying physics has yet to be fully understood. In this study, we investigated the voltages along the hot filament wire and emitting thermal electrons and proved which voltage reflects the plasma potential. Using a circuit model incorporating the floating condition, we found that the lowest potential on the plasma-exposed filament provides a close approximation of the plasma potential. This theoretical result was verified with a comparison of emissive probe measurements and Langmuir probe measurements in inductively coupled plasma. This work provides a significant contribution to the accurate measurement of plasma potential using the emissive probe with the floating potential method.

Affect of Secondary Beam Non-Uniformity on Plasma Potential Measurements by HIBD with Split-Plate Detector.

In a Heavy Ion Beam Diagnostic (HIBD), the plasma potential is obtained by measuring the energy of the secondary ions resulting from beam-plasma collisions by an electrostatic energy analyzer with split-plate detector (SPD), which relates the secondary ion beam energy variation to its position determined by the difference in currents between the split plates. Conventionally, the data from SPD are analyzed with the assumption that the secondary beam current is uniform. However, the secondary beam presents an effective projection of the primary beam, the current of which, as a rule, has a bell-like non-uniform profile. This paper presents: (i) the general features of the secondary beam profile formation, considered in the simplistic approximation of the circular primary beam and the secondary ions that emerge orthogonal to the primary beam axis, (ii) details of spit-plate detection and the influence of the secondary beam non-uniformity on plasma potential measurements, (iii) supported experimental data from the tokamak ISTTOK HIBD for primary and secondary beam profiles and the SPD transfer characteristic, obtained for the 90° cylindrical energy analyzer (90° CEA) and (iv) the implementation of a multiple cell array detector (MCAD) with dedicated resolution for the measurements of secondary beam profile and MCAD operation in multi-split-plate detection mode for direct measurements of the SPD transfer characteristic.

Comparative studies of cold/hot probe techniques for accurate plasma measurements

The emissive probe technique and the cold Langmuir probe technique for the plasma potential measurement are compared in microwave electron cyclotron resonance plasmas. With different results of plasma potential, discrepant results of electron temperature and electron density are obtained from a hot emissive probe I–V curve and a cold Langmuir probe I–V curve, respectively. A comparison of the experimental data shows that the plasma parameters obtained from the cold Langmuir probe I–V curve are always grossly underestimated, while the results determined from the hot emissive probe I–V curve are much more reliable. Additionally, based on the experimental results, a novel emissive probe technique named the hot probe with zero emission limit method is proposed to easily obtain the accurate plasma potential and other reliable plasma parameters.

The first application of the HIBP diagnostics for co-NBI plasma potential measurement in the TUMAN-3M tokamak

The heavy ion beam probe (HIBP) diagnostics at the TUMAN-3M tokamak was updated to provide measurements in the regime with neutral beam injection co-directed with plasma current (co-NBI). By means of HIBP, plasma potential measurements in the center of plasma were carried out. Plasma potential evolution in the discharge with the LH transition (transition to the improved confinement mode) is in good agreement with the concept of negative radial electric field generation during formation of the transport barrier. Keywords: plasma physics, tokamak, heavy ion beam probe, H-mode, neutral beam injection.

Первое применение диагностики плазмы пучком тяжелых ионов для измерения потенциала плазмы при ко-инжекции высокоэнергичных нейтральных атомов в токамак ТУМАН-3М

Heavy ion beam probe (HIBP) diagnostics on TUMAN-3M tokamak was updated to provide measurements in the regime with neutral beam injection co-directed with plasma current (co-NBI). By means of HIBP plasma potential measurements in the center of plasma were carried out. Plasma potential evolution in the discharge with L-H transition (transition to improved confinement mode) is in good agreement with the concept of negative radial electric field generation during the formation of transport barrier.

Spatial evolution characteristics of ion and electron flow for 300 W class low-power Hall thruster

The Faraday probe and cylindrical Langmuir probe were used to characterize the downstream ion and electron spatial evolution of a 300 W class low-power Hall thruster. The time-averaged ion current density, electron energy probability function, plasma potential, electron temperature, and electron density measurements were conducted at discharge voltages of 200–400 V and anode mass flow rates of 0.65 and 0.95 mg s−1 in the range of 100–500 mm axially and −100 to 100 mm radially downstream of the thruster. The results show that the ion and electron flows exhibit a bipolar diffusion characteristic along the radial direction. Meanwhile, the radial diffusion rate of ions in the plume is greater than the axial diffusion rate. The plasma potential decreases from 18 V at 100 mm axially from the thruster exit to 4 V at 500 mm axially and 100 mm radially. Correspondingly, the electron temperature decreases from 4.2 to 1.0 eV. The electron number density decreases from 2.6 × 1016 to 4 × 1014 m−3. A variable exponential relationship between electron temperature and electron density was observed from the measurements of electron energy probability distribution functions, with an adiabatic factor γ ranging between 1.3 and 1.4 (below the adiabatic value of 5/3). The adiabatic factor γ is considered to correlate with the anode mass flux and the spatial location of plasma, which suggests a possible dependence on the collision rate. These data are of great importance for plume model validation, improvement, plume effect evaluation, and protection mechanisms.

Measurement of Plasma Potential, Ion Saturation Current and Mach number using Retarding Field Energy Analyzer

This article deals about the experimental measurement of plasma potential, ion saturation current and Mach number obtained with the variation of power, operating gas pressure and radial position using retarding field energy analyzer. We employed a retarding field energy analyzer by rotating with different angles such as 0° (facing toward source), 90° (facing side walls) and 180° (facing opposite the source). The coil current is varied from 0 to 15 A to produce the magnetic field which is used to confine the plasma. The flow of plasma has been characterized which was found to be subsonic. The low-temperature plasma is produced by means of a 13.56 MHz helicon plasma source at 300-1000 kW radio frequency power. The plasma is expanding from 13.8 cm diameter source into a 150 cm long diffusion chamber of 60 cm diameter.

Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas.

Langmuir probes have long been used in experimental plasma physics research as the primary diagnostic for particle fluxes (i.e., electron and ion fluxes) and their local spatial concentrations, for electron temperatures, and for electrostatic plasma potential measurements, since its invention by Langmuir in the early 1920s. Emissive probes are used for measuring plasma potentials. The protocols exhibited in this work serve to demonstrate how these probes may be built for use in a vacuum chamber in which a plasma discharge may be confined and sustained. This involves vacuum techniques for building what is essentially an electrical feedthrough, one that is rotatable and translatable. Certainly, complete Langmuir probe systems may be purchased, but they can also be built by the user at considerable cost savings, and at the same time be more directly adapted to their use in a particular experiment. We describe the use of Langmuir probes and emissive probes in mapping the electrostatic plasma potential from the body of the plasma up to the sheath region of a plasma boundary, which in these experimentsis created by a negatively biased electrode immersed within the plasma, in order to compare the two diagnostic techniques and assess their relative advantages and weaknesses. Although Langmuir probes have the advantage of measuring the plasma density and electron temperature most accurately, emissive probes can measure electrostatic plasma potentials more accurately throughout the plasma, up to and including the sheath region.

Direct experimental comparison of krypton and xenon discharge properties in the magnetic nozzle of a helicon plasma source

Helicon plasma sources currently represent an active field of research in the domain of low-temperature plasmas due to several interesting characteristics for in-space propulsion applications. This work reports direct comparison of krypton and xenon plasma properties through spatially resolved measurements performed in the near-field plume of a sub-kilowatt-class 13.56 MHz helicon source. The set of developed and employed intrusive diagnostics is comprehensively described. The magnetic components of the plasma rf field are inferred using a 3D B-dot probe along the reactor axial direction with and without the externally applied DC magnetic field. Plasma floating potential fluctuations are measured using a capacitive probe to draw design requirements for a rf-compensated Langmuir probe. The latter is used to perform axial and radial measurements of plasma density, electron temperature, and plasma potential. A four-grids retarding potential analyzer is used to infer the ion kinetic energy downstream the source exit. The effect of magnetic field magnitude on ions and electrons properties is especially investigated.

50 YEARS OF HOT PLASMA DIAGNOSTIC WITH HEAVY ION BEAM PROBING (HIBP) AT THE KHARKOV INSTITUTE OF PHYSICS AND TECHNOLOGY

An overview describes the evolution of HIBP diagnostics from the origins till today. The progress in the beam technology is presented by examples of HIBPs in tokamaks and stellarators. At the beginning, HIBP provided timeaveraged measurements of plasma potential in single space location, then it evolves to time-resolved measurements of radial distributions and finally it becomes a multi-purpose diagnostics to study the temporal evolution of 2D distributions of potential and turbulence, including the long-range potential correlations with dual HIBP. Highlights in plasma potential profile evolution, a link between potential, density and confinement, geodesic acoustic modes, steady and chirping Alfvén eigenmodes, turbulent particle flux are presented.

Measurement of Plasma Potential in the Gas Dynamic Trap by Doppler Spectroscopy

Measurement of Plasma Potential in the Gas Dynamic Trap by Doppler Spectroscopy

Plasma Potential Measurement in Detached Plasmas by Emissive Probe Considering Space-Charge-Limited Effect

Plasma Potential Measurement in Detached Plasmas by Emissive Probe Considering Space-Charge-Limited Effect

Plasma potential and fluctuations measurements by HIBD with 90° cylindrical energy analyzer on the ISTTOK tokamak

The Heavy Ion Beam Diagnostic (HIBD) is known as unique tool for direct measurements of plasma potential in fusion devices such as tokamaks and stellarators. On small ISTTOK tokamak (R = 0.45 m, a = 0.085 m, B = 0.5 T, Ip = 4 kA, <ne> = 4 × 1018 m−3) the HIBD is operated with primary Xe+ ion beam with E = 22−24 keV of energy and a Multiple Cell Array Detector (MCAD) to measure the product of plasma neσ and its fluctuations simultaneously at a number of sample volumes along primary beam trajectory across the plasma. To upgrade the ISTTOK HIBD capabilities to measure the plasma potential and its fluctuations a multichannel 90° cylindrical electrostatic energy analyzer (CEA) operated in a beam deceleration mode has been proposed.This paper presents the first results of the plasma potential and its fluctuations measurements by the one-channel CEA prototype operated in two-times deceleration mode and in real conditions of the HIBD experiments on ISTTOK. Using an indirect CEA calibration approaches, the value of the potential Φ ∼ −350 V has been measured in the plasma core. Also, the plasma potential fluctuations Φ∼ ∼ 40 V have been observed. The obtained results demonstrate and confirm the applicability of the 90° cylindrical electrostatic energy analyzer and its operation in deceleration mode for the plasma potential and its fluctuations measurements by HIBD.

Evaluating the uncertainty for a complex experiment: the case of the plasma potential measurement

A general method for the determination of the uncertainty of a physical quantity measured during an experiment is presented. This procedure is described step-by-step so it can be applied to any measurement and experiment available for undergraduate and graduate students, from the simplest to the most complex. The method is then applied to a classic measurement of plasma physics: the plasma potential determination using an emissive probe. This kind of experiment is routinely performed in plasma physics laboratories and may be realized by graduate students. The emissive probe diagnostic relies on a data analysis method called the inflection point. This method follows an indirect procedure on which the step-by-step uncertainty calculation strategy presented in the first part of the article is presented.

Single probe diagnostics for the study of plasma parameters in the expander of an open magnetic trap

Method of plasma potential and electrons mean energy measurement by single Langmuir probe in highly fluctuated plasma of magnetic open trap is described. Diagnostics based on this method allows for two types of measurements: plasma potential in the emission probe mode, as well as current-voltage characteristics of the probe, followed by calculating the plasma potential and the average electron energy. All diagnostic elements, including pulse shape recorders, are fully galvanically isolated. Power supply is provided by battery placed on board of the measuring system. Control and synchronization are realized by means of optical communication lines. This diagnostic design allows measurements relate to plasma electrodes, the potential of which fluctuates with a frequency of up to hundreds of kilohertz.