15 research outputs found
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Effect of the gas blanket on the stability of the dense z-pinch
The stabilizing effect of the dense neutral gas blanket which surrounds the dense z-pinch as it is presently operated is studied. Because this gas can contribute to pressure balance, plasma pressure can fall to zero at some radius without introducing destabilizing surface currents which would be required for pressure balance without the gas. Thus pressure profiles are possible which are stable against interchange, and long wavelength kink modes are more easily stabilized by a wall. Internal kink modes are unaffected and are unstable
Magnetic Phase transitions in Plasmas and Transport Barriers
A model of magnetic phase transitions in plasmas is presented: plasma blobs
with pressure excess or defect are dia- or para-magnets and move radially under
the influence of the background plasma magnetisation. It is found that magnetic
phase separation could be the underlying mechanism of L to H transitions and
drive transport barrier formation. Magnetic phase separation and associated
pedestal build up, as described here, can be explained by the well known
interchange mechanism, now reinterpreted as a magnetisation interchange which
remains relevant even when stable or saturated. A testable necessary criterion
for the L to H transition is presented.Comment: 3 figures, 9 pages, equations created with MathType To be published
in Nuclear Fusion, accepted August 201
Resistive g-modes in a reversed field pinch plasma
First direct experimental evidence of high frequency, high toroidal mode
number (n>20), magnetic fluctuations due to unstable resistive interchange
modes (g-modes) resonant in the edge region of a reversed field pinch (RFP)
plasma is presented. Experimental characterization of time and space
periodicities of the modes is provided by means of highly resolved in-vessel
edge and insertable magnetic diagnostics. It is found that the spectral mode
properties are in good agreement with the predictions of the theoretical linear
resistive magnetohydrodynamic stability analysis. A simple model is proposed
for the observed saturation levels of the modes.Comment: Submitted to Physical Review Letter
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Can optical phase conjugators produce a very short conjugate pulse
In studying the transient behavior of phase conjugation via four wave mixing, it has been noted that very short probe pulses invariably produce considerably broadened conjugate pulses unless the conjugator is inordinately short. The present investigation was therefore undertaken to determine whether a short conjugate pulse might be produced by judicious programming of the probe. It was found that the probe can indeed be tailored to produce an arbitrarily short conjugate signal and, if the conjugating medium exhibits linear absorption, then the total energy in the probe signal is finite. The required probe programming can be determined by numerical evaluation of a Fourier transform
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Growth rates of interchange modes in the dense z-pinch and gas blanket effects
A class of models of the dense z-pinch in which the bulk of the plasma is neutrally stable against interchange is studied. At a radius a the plasma is abruptly terminated and surrounded by a gas blanket. Instability results from the surface current necessary to maintain pressure balance at the plasma-gas interface. A single dispersion relation applies to all models in this class and shows two effects of the gas blanket in reducing growth rate. (1) Partial support of plasma pressure by external gas pressure reduces the driving force for the instability, and (2) mass loading of the relatively dense neutral gas reduces growth rate
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Breakup of an accelerated shell owing to Rayleigh--Taylor instability
A simplified model for the Rayleigh-Taylor instability of an accelerated shell is examined, and it is found that the most dangerous wavelength to be about that of the shell thickness. The shell material is assumed to be an inviscid, incompressible fluid. Effects of finite compressibility and of surface tension are found to be negligible, but the effects of viscosity are shown to be very large. The need for better knowledge of viscosity at high pressure is pointed out