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Plasma Diffusion
Plasma diffusion across a magnetic field is an important topic in magnetic confinement of fusion plasma. It especially concerns how plasma transport is related to strength of an external magnetic field, B. Classical diffusion predicts 1/B2 scaling, while Bohm diffusion, borne out of experimental observations from the early confinement machines, was conjectured to follow 1/B scaling. Hsu diffusion predicts 1/B3/2 scaling, which is presumably the best confinement scenario in magnetized plasma. See also * Bohm diffusion * Classical diffusion * Hsu diffusion How the plasma transport is reduced by the strength of the external magnetic field is of great concern in studying magnetic confinement of fusion plasma. The plasma diffusion may be classified by the classical diffusion of B−2 scaling, the Bohm d ... References Diffusion Plasma physics {{plasma-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Magnetic Field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron, and attracts or repels other magnets. In addition, a nonuniform magnetic field exerts minuscule forces on "nonmagnetic" materials by three other magnetic effects: paramagnetism, diamagnetism, and antiferromagnetism, although these forces are usually so small they can only be detected by laboratory equipment. Magnetic fields surround magnetized materials, and are created by electric currents such as those used in electromagnets, and by electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, cal ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Classical Diffusion
Classical diffusion is a key concept in fusion power and other fields where a plasma is confined by a magnetic field within a vessel. It considers collisions between ions in the plasma that causes the particles to move to different paths and eventually leave the confinement volume and strike the sides of the vessel. The rate of diffusion scales with 1/B2, where B is the magnetic field strength, implies that confinement times can be greatly improved with small increases in field strength. In practice, the rates suggested by classical diffusion have not been found in real-world machines, where a host of previously unknown plasma instabilities caused the particles to leave confinement at rates closer to B, not B2, as had been seen in Bohm diffusion. The failure of classical diffusion to predict real-world plasma behavior led to a period in the 1960s known as "the doldrums" where it appeared a practical fusion reactor would be impossible. Over time, the instabilities were found and addr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bohm Diffusion
The diffusion of plasma across a magnetic field was conjectured to follow the Bohm diffusion scaling as indicated from the early plasma experiments of very lossy machines. This predicted that the rate of diffusion was linear with temperature and inversely linear with the strength of the confining magnetic field. The rate predicted by Bohm diffusion is much higher than the rate predicted by classical diffusion, which develops from a random walk within the plasma. The classical model scaled inversely with the square of the magnetic field. If the classical model is correct, small increases in the field lead to much longer confinement times. If the Bohm model is correct, magnetically confined fusion would not be practical. Early fusion energy machines appeared to behave according to Bohm's model, and by the 1960s there was a significant stagnation within the field. The introduction of the tokamak in 1968 was the first evidence that the Bohm model did not hold for all machines. Bohm p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hsu Diffusion
How the plasma transport is reduced by the strength of the external magnetic field is of great concern in studying magnetic confinement of fusion plasma. The plasma diffusion may be classified by the classical diffusion of B−2 scaling, the Bohm diffusion conjectured to follow the B−1 scaling, and the Hsu diffusion of B−3/2 scaling. Here, B is the external magnetic field. The low-frequency fluctuating electric fields can cause particles to execute the ExB drift. Due to the long range nature of Coulomb interaction, the electric field coherence time is long enough to allow virtually free streaming of particles across the field lines. Thus, when no other decoherence mechanism exists, the transport would be the only mechanism to limit the run of its own course and to result in the Bohm diffusion of 1/B scaling in a 2D like plasma. In a 3D plasma, the parallel decoherence (the decoherence along the field line) is significant enough to reduce the transport of ExB drifts to only th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical potential. It is possible to diffuse "uphill" from a region of lower concentration to a region of higher concentration, like in spinodal decomposition. The concept of diffusion is widely used in many fields, including physics (particle diffusion), chemistry, biology, sociology, economics, and finance (diffusion of people, ideas, and price values). The central idea of diffusion, however, is common to all of these: a substance or collection undergoing diffusion spreads out from a point or location at which there is a higher concentration of that substance or collection. A gradient is the change in the value of a quantity, for example, concentration, pressure, or temperature with the change in another variable, usually distance. A change in c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
