Finite plasma sheath corrections applied to electrostatic probes
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Langmuir or electrostatic probes are one of the oldest and yet the most widely used diagnostic in almost all fields of plasma physics research. Probe theories are, however, rather complex subject and invariably difficult to be experimentally checked with precision, even within the expected range of their validity. Testing probe theories against data from dedicated experiments using quiescent magnetized plasmas, is a very important task for assuring compatibility with the use of this diagnostic and validated models in more complex plasmas, notably those at the edge of the magnetic fusion plasmas. In these particular plasmas, attaining more precise plasma parameters directly impacts studies on a high variety of edge profile dependent phenomena, with impact in particle and heat transport and global confinement and local stability (e.g. H-mode pedestal and ELMs, respectively). It also helps to properly back-up more complex plasma edge diagnostics. The present work reports experiments to proper infer plasma parameters in low-magnetized glow discharge plasmas using electrostatic probes with different pin diameters, and operating in sweep mode. Particularly the plasma density parameter is detailed analyzed and inferred from different finite-plasma sheath theories, notably that from the non-magnetized collisioness plasma model proposed by Hutchinson.Keywords:
Langmuir Probe
Plasma parameter
Pedestal
Plasma stability
Plasma confinement
Plasma stability
BETA (programming language)
Field-reversed configuration
Fusion power
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Dusty plasma
Plasma parameter
Coupling parameter
Electron temperature
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This paper is devoted towards the study and comparison of different techniques for measurement of electron density in low temperature plasma. To study the plasma profile it is necessary that one should know about its parameter. Electron density is one of the important plasma parameters of plasma profile. Other plasma parameters like plasma frequency, Debye length and dielectric parameter are also depended on the density of electron. For the plasma density measurement, there are various techniques and each technique has its own unique advantages and some disadvantages as well. Basic techniques for plasma density measurement are Langmuir and Resonance probe based measurement techniques, microwave interferometer and optical emission spectroscopy. This paper also covers basics about plasma parameters. In the first section plasma parameters and their importance is described in brief. Then, principle, analysis and experimental setup for all basic four techniques are included in next section. The last section is devoted towards the comparison of these unique techniques.
Langmuir Probe
Electron temperature
Plasma parameter
Debye
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Impact of plasma core profiles on magnetohydrodynamics (MHD) stability at tokamak edge pedestal is investigated numerically to extend an operation regime for small amplitude grassy edge localized mode (ELM). With the hypotheses that pedestal pressure profile can be predicted with the EPED1 model and the trigger of grassy ELM is an ideal ballooning mode, the impacts of plasma poloidal beta and plasma internal inductance on edge MHD stability are investigated, the parameters of which are related to plasma core profiles and are important parameters for grassy ELMy H-modes in JET quasi-double null plasma. The numerical results indicate that a ballooning mode can be destabilized by decreasing poloidal beta and/or internal inductance. In contrast, it is confirmed that pedestal density, which is also an important parameter for realizing grassy ELMy H-mode, can stabilize a ballooning mode. In combination with these trends, it is possible to relax the necessary conditions for grassy ELMy H-mode by adjusting the parameters carefully, though this relaxation destabilizes type-I ELM more easily due to the increase in edge current density.
Pedestal
Ballooning
Edge-localized mode
BETA (programming language)
Plasma stability
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In this paper, the plasma uniformity was studied in a capacitively coupled plasma machine using Langmuir probe. The plasma uniformity is affected not only the process parameters, such as input power and pressure, but also the electrode setting. The increase of electrode gap and pairs of electrode significantly improve the plasma uniformity, but decreases the plasma intensity, which are represented by the value of electron temperature Te, electron density Ne, and electron energy density Te*Ne. The perforated electrode helps to improve the plasma uniformity significantly. The result indicates that the edge effect is the main reason caused un-uniform plasma distribution. Furthermore, the spatial plasma distribution along the Z axis is not uniform and plasma intensity in main plasma zone decreases with the distance to the powered electrode. The test result also indicates that the etch rate has the best correlation with electron energy density Te*Ne.
Capacitively coupled plasma
Langmuir Probe
Electron temperature
Plasma channel
Plasma parameter
Plasma cleaning
Plasma window
Intensity
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Analysis of the stability of the radial temperature and density profiles against radial collapse leads to predictive algorithms for plasma and neutral fueling density limits in tokamaks. The predicted plasma density limit agrees with the Greenwald limit in magnitude and parameter scaling for Ohmic plasmas. Auxiliary-heated plasmas are predicted to have a higher plasma density limit than comparable Ohmic-heated plasmas, but fusion alpha-heated plasmas are predicted to have a lower plasma density limit than comparable Ohmic-heated plasmas. Model problem calculations indicate a rather complex dependence of the predicted density limit on the plasma parameters; under certain conditions the weak sensitivity seen in experiments to edge temperature, impurity concentration, and heating power is found, but under other conditions a stronger sensitivity is predicted than is seen in experiments.
Ohmic contact
Plasma parameter
Plasma stability
BETA (programming language)
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The very large stellarator experiments LHD (operating) and W7X (under construction) move stellarator-confined plasmas into the near-reactor regime. Continuing experiments on smaller devices operating at heating powers from kilowatts to a few megawatts are exploring the effects of magnetic configuration stability and turbulence on plasma confinement to improve stellarator performance and our understanding of general toroidal confinement physics. Key issues being explored are the relation of rational magnetic surfaces and magnetic configuration characteristics such as helical ripple to plasma transport, confinement scaling and turbulence. The robust macroscopic stability of currentless stellarator plasma is a major contributing factor to these studies. Many of the phenomena most clearly evident in stellarators are increasingly implicated in tokamak experiments as well.
Plasma confinement
Plasma stability
Safety factor
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The Langmuir probe (LP) diagnostics is a powerful method for the evaluation of the plasma resistivity curve (I-V curve) and the characterization of the following plasma parameters: electron temperature, electron density, ion density, and plasma potential. In presence of a stable plasma it is possible to extrapolate the electron energy distribution function of the plasma electron population. Because of the long acquisition time (in the order of hundreds of msec or more), this method is suitable for cw plasmas in thermal equilibrium for which the physical properties vary in a time scale longer than the acquisition time. At INFN-LNS the LP diagnostics has been used in order to characterize the TRasco Intense Proton Source plasma, and the low temperature – high density plasmas of a plasma reactor designed for complex molecules dissociation. In the first case, it has been possible to evaluate the plasma properties for different magnetic field profiles and for several operating conditions. In the second case, the LP has permitted to characterize the plasma properties of the plasma reactor at different microwave powers and gas pressures, with the aim to find the optimal experimental conditions in terms of rate of molecules dissociation and of plasma stability and reliability. These series of measurements are here reported, together with measurements of the plasma reactor parameters. Finally, some considerations about the possibility to extend the LP diagnostics to the non-equilibrium plasmas in pulsed mode, as the plasmas obtained by means of laser ablation of solid targets, are given; the design of a possible experimental set-up is outlined.
Langmuir Probe
Electron temperature
Plasma parameter
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The high confinement mode (H-mode) plasmas in the pedestal region of tokamaks are characterized by steep gradient of the radial electric field, and sonic poloidal Up,m flow that consists of poloidal components of the E×B flow and the plasma flow velocity that is parallel to the magnetic field B. Here, E is the electric field. The bootstrap current that is important for the equilibrium, and stability of the pedestal of H-mode plasmas is shown to have an expression different from that in the conventional theory. In the limit where |Up,m| ≫ 1, the bootstrap current is driven by the electron temperature gradient and inductive electric field fundamentally different from that in the conventional theory. The bootstrap current in the pedestal region can be controlled through manipulating Up,m and the gradient of the radial electric. This, in turn, can control plasma stability such as edge-localized modes. Quantitative evaluations of various coefficients are shown to illustrate that the bootstrap current remains finite when |Up,m| approaches infinite and to provide indications how to control the bootstrap current. Approximate analytic expressions for viscous coefficients that join results in the banana and plateau-Pfirsch-Schluter regimes are presented to facilitate bootstrap and neoclassical transport simulations in the pedestal region.
Pedestal
Bootstrap current
Electric current
Pressure gradient
Plasma stability
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Citations (2)
Langmuir or electrostatic probes are one of the oldest and yet the most widely used diagnostic in almost all fields of plasma physics research. Probe theories are, however, rather complex subject and invariably difficult to be experimentally checked with precision, even within the expected range of their validity. Testing probe theories against data from dedicated experiments using quiescent magnetized plasmas, is a very important task for assuring compatibility with the use of this diagnostic and validated models in more complex plasmas, notably those at the edge of the magnetic fusion plasmas. In these particular plasmas, attaining more precise plasma parameters directly impacts studies on a high variety of edge profile dependent phenomena, with impact in particle and heat transport and global confinement and local stability (e.g. H-mode pedestal and ELMs, respectively). It also helps to properly back-up more complex plasma edge diagnostics. The present work reports experiments to proper infer plasma parameters in low-magnetized glow discharge plasmas using electrostatic probes with different pin diameters, and operating in sweep mode. Particularly the plasma density parameter is detailed analyzed and inferred from different finite-plasma sheath theories, notably that from the non-magnetized collisioness plasma model proposed by Hutchinson.
Langmuir Probe
Plasma parameter
Pedestal
Plasma stability
Cite
Citations (0)