| Plasma Physics Seminar (Phys 769) |
Schedule - Fall 2006 September 26 -- Dan Barnes, Coronado Consulting -- "Lowering the Fusion Threshold" Abstract: Beam-target fusion is not of economic interest, as the beam drag power exceeds the fusion yield. This picture changes if one imagines using the heat produced by beam drag as a low entropy source (because of the high plasma temperature) rather than exhausting it. This elementary thermodynamics is fine, but the challenge is in the (conceptual) engineering. How to arrange a confined plasma to form a high efficiency heat engine and how to use the mechanical energy to form a beam? Known electrostatic fusion concepts are extended to "conventional" magnetically confined quasi-neutral plasmas. Rapidly (supersonic) rotating plasmas are particularly useful for this. The rotation forms a mechanical energy reservoir and the cross field electrical potentials are useful for particle acceleration. In this talk, two new physics ideas are developed and applied to this problem. First idea: electrostatic wells are replaced by centrifugal wells and non-neutral plasma replaced by quasi-neutral plasma. Previously known physics are applied, leading to many arrangements which form a high-efficiency (>90%) heat engine. Simplest of all is the Pastukov problem, in which a single well confines a low-collisionality nearly thermal plasma. It is shown that a proper arrangement of magnetic field (essentially the open field of a field-reversed configuration - FRC) can make this into a heat engine, so that plasma heat becomes rotation. The energy cycle is completed by converting rotation to beam energy. It is shown how to use the high electrical potentials induced by rotation to electrostatically accelerate a beam into the confined plasma. Second idea: plasma rotation can produce plasma waves from a static magnetic perturbation, using nothing more than the Doppler effect. These waves can also be used to produce a desired beam by resonant absorption. Another use for such waves is to drive currents. As already demonstrated experimentally, such currents can form a FRC. All of this leads to lowering the fusion threshold. In particular, required temperatures are greatly reduced, leading to very small, very high-power density systems. The non-thermal fusion also means that aneutronic fuel cycles can be used. Some examples are given. Finally, a small experiment to test these physics is being planned and some details of this design are given. October 18 -- Gennady Shvets, UT Austin -- "Controlling microwave-plasma interactions by helical fields: undulator-induced transparency" Abstract: Propagation of electromagnetic waves through magnetized plasmas can be strongly affected by adding an undulating magnetic field. The most dramatic effects occur near the cyclotron resonance frequency, where electron cyclotron heating can be suppressed by an undulator. For example, the phenomenon of "slow light" encountered in quantum optics can be realized in plasmas. The implication is that significant compression of microwave energy is possible, with applications to electron or ion acceleration. When the undulating field is helical, wave propagation in plasma becomes highly unusual because plasma behaves as both chiral and non-reciprocal medium. Implications of these propagation effects for high efficiency radiation generation by electron beams will be discussed. Finally, the possibility of using waveguide wall perturbations instead of the undulating magnetic field will be discussed. November 8 -- Ryusuke Numata, Australian National University, Canberra -- "Bifurcation structure in resistive drift wave turbulence" Abstract: Fusion plasmas and other turbulent flows in two dimensional (2D) geometry can undergo a spontaneous transition to a turbulence suppressed regime. In plasmas such transitions dramatically enhance the confinement and are known as L-H transitions. From theoretical and experimental work, it is now widely believed that generation of stable coherent structures such as shear flows suppresses cross-field turbulent transport and leads to the confinement improvement. In 2D plasmas and fluids, the net upscale energy flux from small scale turbulent modes to create large scale coherent structures can dominate the classical Kolmogorov cascade to dissipative scales. Recently, a low-dimensional dynamical model for L-H transition has been suggested and analyzed using bifurcation and singularity theories. The model consists of three macroscopic energy variables and, when validated against numerical and/or real experimental data, will provide an economical tool to predict transitions over a parameter space. In this study, we have analyzed the modified Hasegawa-Wakatani (MHW) model, which describes the electrostatic resistive drift wave turbulence in 2D slab geometry, by direct numerical simulation. We have shown that, at a certain parameter range, a coherent zonal flow structure is generated, and the zonal flow significantly suppresses cross-field turbulent transport. A parameter scan of the MHW model has been performed to construct a bifurcation diagram. The result shows that a sudden transition from a zonal flow dominated state to a zonal flow suppressed state occurs if we increase the turbulent drive (the length scale of the background density profile), or decrease the electron adiabaticity (inverse of the parallel resistivity). In the talk, we would also like to mention some interesting phenomena observed in the vicinity of the linear stability threshold where the bifurcation is expected to occur. 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