Results 1 - 10 of 4312
Results 1 - 10 of 4312. Search took: 0.024 seconds
|Sort by: date | relevance|
[en] Experimental investigation of the Langmuir probe characteristic is a magnetized plasma with an electron current along the magnetic field direction shows that the standard procedure for determination of the electron temperature and plasma density, which is applicable in a current - free magnetized plasma, gives erroneous results for these plasma parameters. However, more precise values of the plasma parameters can be calculated from the ion saturation currents and electron temperatures obtained with that procedure for two opposite orientations of the one - sided planar probe collecting surface with respect to the direction of the electron drift. With the existing theoretical models only the order of magnitude of the electron drift velocity can be accurately determined from the measured electron saturation currents for the two probe orientations. (author)
[en] A finite length cylindrical Langmuir probe is modelled as an ellipsoid of revolution with spheroidal equipotential surfaces and confocal orthogonal hyperboloidal electric field lines. The theory is applicable in the transition regime of probe operation between the collisionless and fully collisional limits. The plasma is assumed to be weakly ionized, non-thermal and stationary, being characterized by frozen reactions and constant temperatures. It is further assumed that in an isotropic plasma the cold ions follow the field lines, as a result of ion-neutral collisions, in the presheath and sheath regions with collisionless Maxwellian electrons. The governing system of equations is derived and solved numerically with the results presented of the presheath and sheath solutions in collisionless and collisional regimes. These show convergence to the respective collisionless and collisional radial motion limits for spherical and cylindrical probes. Analytical approximations are also obtained for the sheath width (defined as the point where the ions reach the Bohm speed) and the Bohm potential over a wide range of collisionality. The collisional presheath drop according to the perturbation theory of Shih and Levi, as applied to cylindrical probes, is shown to significantly underestimate the numerical results. These are in better agreement with the collisional presheath drop for spheres even for long probes. Application of the theory to experimentally derived probe characteristics is also discussed.
[en] In the TORPEX [A. Fasoli et al., Phys. Plasmas 13, 055902 (2006)], a simple magnetized plasma device, low frequency electrostatic fluctuations associated with interchange waves, are routinely measured by means of extensive sets of Langmuir probes. To complement the electrostatic probe measurements of plasma turbulence and study of plasma structures smaller than the spatial resolution of probes array, a nonperturbative direct imaging system has been developed on TORPEX, including a fast framing Photron-APX-RS camera and an image intensifier unit. From the line-integrated camera images, we compute the poloidal emissivity profile of the plasma by applying a tomographic reconstruction technique using a pixel method and solving an overdetermined set of equations by singular value decomposition. This allows comparing statistical, spectral, and spatial properties of visible light radiation with electrostatic fluctuations. The shape and position of the time-averaged reconstructed plasma emissivity are observed to be similar to those of the ion saturation current profile. In the core plasma, excluding the electron cyclotron and upper hybrid resonant layers, the mean value of the plasma emissivity is observed to vary with (Te)α(ne)β, in which α=0.25-0.7 and β=0.8-1.4, in agreement with collisional radiative model. The tomographic reconstruction is applied to the fast camera movie acquired with 50 kframes/s rate and 2 μs of exposure time to obtain the temporal evolutions of the emissivity fluctuations. Conditional average sampling is also applied to visualize and measure sizes of structures associated with the interchange mode. The ω-time and the two-dimensional k-space Fourier analysis of the reconstructed emissivity fluctuations show the same interchange mode that is detected in the ω and k spectra of the ion saturation current fluctuations measured by probes. Small scale turbulent plasma structures can be detected and tracked in the reconstructed emissivity movies with the spatial resolution down to 2 cm, well beyond the spatial resolution of the probe array.
[en] A dense array of 99 Langmuir probes has been installed in the lower divertor region of the National Spherical Torus Experiments (NSTX). This array is instrumented with a system of elec- tronics that allows flexibility in the choice of probes to bias as well as the type of measurement (including standard swept, single probe, triple probe and operation as passive floating potential and scrape-off-layer (SOL) current monitors). The use of flush-mounted probes requires careful inter- pretation. The time dependent nature of the SOL makes swept-probe traces difficult to interpret. To overcome these challenges, the single- and triple-Langmuir probe signals are used in comple- mentary fashion to determine the temperature and density at the probe location. A comparison to mid-plane measurements is made.
[en] Following a review of the existing probe theory for a collisional plasma, a new theory is given for the collected ions by a spherical Langmuir probe in a collisional plasma, which goes over to the collisional theory of Allen, Boyd and Reynolds at low pressures and the continuum theory of Su and Lam at high pressures. The limitations of the theories are discussed. (author)
[en] A pulse technique has been presented for generating a complete Langmuir probe characteristic curve in a short time interval which can be varied from 1 sec to 1 millisec. The method has been applied successfully to different plasma systems of our laboratory. Several examples have been given for indicating the type of diagnostic studies being carried out with this pulsed probe. (author)
[en] A noninvasive method to measure the ion flux from the current flowing through the substrate (substrate current) in a capacitive discharge is proposed. The substrate current consists of a displacement current and a conduction current. In a high frequency capacitively coupled plasma, the displacement current is usually much higher than the conduction current. However, the displacement current becomes negligible at a moment that the second time derivative of the substrate current becomes zero (the first time derivative reaches its minimum), and from the measured substrate current at that time, the ion flux can be obtained. The measured ion flux from this method is compared with the flux measured by another diagnostic method
[en] An inexpensive, simple, and fast Langmuir probe sweeping circuit is presented. This sweeper completes a probe trace in 1.4 ms and has a maximum probe current capability of 5 A. It is suitable for pulsemode plasma operation with density greater than 1012 ions/cm3