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Atomic Nuclei: Nuclear Relaxation Processes01:23

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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A three-phase AC generator has a rotor with a rotating magnet placed within the stator mounted with the stationary three-phase winding to generate three-phase voltages via mutual induction. These windings are evenly distributed around the inner circumference of the stator and are arranged 120 electrical degrees apart. Three-phase stator windings consist of three separate coils or groups of coils, known as phases, each connected in Y (star) configuration or Delta configuration.
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All atomic nuclei are positively charged. When they have a nonzero spin, they behave like rotating charges. As a consequence of their charge and spin, these nuclei generate a magnetic field (B). This, in turn, gives rise to a magnetic moment (μ), which is randomly oriented in the absence of an external magnetic field. When an external magnetic field (B0) is applied, the magnetic moment vectors can align with the field or against it in 2 + 1 orientations. A hydrogen nucleus, which is just a...
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NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of...
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Understanding steady, laminar flow between parallel plates is essential for analyzing and designing flow in narrow rectangular channels, commonly found in various water conveyance and drainage systems. The Navier-Stokes equations govern fluid motion and are generally challenging to solve due to their nonlinearity. However, simplifications are possible in certain cases, like the steady laminar flow between parallel plates. For this scenario, we assume steady, incompressible, laminar flow.
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Optimization, Test and Diagnostics of Miniaturized Hall Thrusters
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Three-Dimensional Helical-Rotating Plasma Structures in Beam-Generated Partially Magnetized Plasmas.

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Beam-generated plasmas exhibit distinct azimuthal structures. Simulations reveal two instability regimes dependent on gas pressure, forming spiral or helical patterns with implications for plasma transport.

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Area of Science:

  • Plasma physics
  • Computational physics

Background:

  • Azimuthal structures in magnetized plasmas are crucial for understanding plasma behavior and transport.
  • Previous studies have explored plasma instabilities, but the transition between different structural regimes based on pressure requires further investigation.

Purpose of the Study:

  • To investigate azimuthal structures in beam-generated partially magnetized plasmas.
  • To identify and characterize distinct instability regimes and their dependence on gas pressure.
  • To develop analytical models for critical pressure thresholds and helical structure rotation frequencies.

Main Methods:

  • Three-dimensional particle-in-cell Monte Carlo collision simulations were employed.
  • Simulations analyzed plasma behavior under varying gas pressures.
  • Analytical formulas were derived for critical parameters.

Main Results:

  • Two distinct instability regimes were identified: a lower-hybrid instability at higher pressures leading to 2D spiral structures and enhanced cross-field transport, and a diocotron instability at lower pressures forming 3D helical-rotating plasma structures.
  • A critical threshold pressure separating these regimes was determined.
  • Analytical formulas for the critical pressure and helical structure rotation frequency were proposed.

Conclusions:

  • The study successfully characterized two distinct azimuthal structure formation regimes in beam-generated plasmas based on gas pressure.
  • The findings provide a theoretical framework and analytical tools for understanding and predicting plasma behavior in different pressure regimes.
  • Preliminary experimental verification supports the simulation results, paving the way for further experimental validation.