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Register a new userCharacterization of Argon Plasma in a variable Multi-pole line Cusp Magnetic Field Configuration
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This paper demonstrates a detailed characterization of argon plasma in a variable multi-pole line cusp magnetic field (VMMF). The VMMF has been produced by placing six electromagnets (with embedded profiled vacoflux-50 core) over a large cylindrical volume (1 m axial length and 40 cm diameter). The magnetic field have been measured by hall probe method and compared with simulated magnetic field by performing simulation using FEMM tools. Results from magnetic field simulation indicate that the rate of change of pole magnetic field (maximum magnetic field) with respect to magnet current for vacoflux-50 core is high (7.53 G/A) as compared to the simple air core electromagnet (2.15 G/A). The area of the nearly field free region (null region) in the chamber volume can be controlled without changing a number of pole magnets. From the experimental results, it has been observed that in this field configuration the confinement of the primary electrons increases and leak width of plasma decreases with increasing the magnetic field. Thus the mean density, particle confinement time and the stability of the plasma increase with increasing magnetic field. In addition to this, it has been found that the radial uniformity of the plasma density explicitly depends on the VMMF. It is also shown that the VMMF controls the scavenging of confined primary electrons and confinement of primary electron increased with magnetic field which helps to boost up the plasma density.
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Two magnetic configurations of Multi-cusp Plasma Device (MPD) have been explored to obtain high quiescence level, large uniform plasma region with nearly flat mean density and temperature profiles. In particular, properties of plasma in a six-pole six magnet (SPSM) and twelve pole six magnets (TPSM) cusp configurations are rigorously compared and reported here. It is found that more uniform plasma with nearly flat profiles is found in TPSM along with increased quiescence level. Findings are verified across various magnetic field strengths for both configurations.
We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma ($< 20$ $mu$s), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2-D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of magnetic field (up to 70 G). We observe that the expansion velocity scales as B$^{-1/2}$, explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.
3D2V continuum gyrokinetic simulations of electrostatic plasma turbulence in a straight, open-field-line geometry have been performed using the full-$f$ discontinuous-Galerkin code Gkeyll. These simulations include the basic elements of a fusion-device scrape-off layer: localized sources to model plasma outflow from the core, cross-field turbulent transport, parallel flow along magnetic field lines, and parallel losses at the limiter or divertor with sheath model boundary conditions. The set of sheath boundary conditions used in the model allows currents to flow through the walls. In addition to details of the numerical approach, results from numerical simulations of turbulence in the Large Plasma Device (LAPD), a linear device featuring straight magnetic field lines, are presented.
The ionization efficiency of helicon plasma discharge is explored by changing the low axial magnetic field gradients near the helicon antenna. The highest plasma density is found for a most possible diverging field near the antenna by keeping the other operating condition constant. Measurement of axial wave number together with estimated radial wavenumber suggests the oblique mode propagation of helicon wave along the resonance cone boundary. Propagation of helicon wave near the resonance cone angle boundary can excite electrostatic fluctuations which subsequently can deposit energy in the plasma. This process has been shown to be responsible for peaking in density in low field helicon discharges, where the helicon wave propagates at an angle with respect to the applied uniform magnetic field. The increased efficiency can be explained on the basis of multiple resonances for multimode excitation by the helicon antenna due to the availability of a broad range of magnetic field values in the near field of the antenna when a diverging magnetic field is applied in the source.
Curvature-driven turbulence in a helical open-field-line plasma is investigated using electrostatic five-dimensional gyrokinetic continuum simulations in an all-bad-curvature helical-slab geometry. Parameters for a National Spherical Torus Experiment scrape-off-layer plasma are used in the model. The formation and convective radial transport of plasma blobs is observed, and it is shown that the radial particle-transport levels are several times higher than diffusive Bohm-transport estimates. By reducing the strength of the poloidal magnetic field, the profile of the heat flux to the divertor plate is observed to broaden.
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