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The Hard-Core Pinch. Part I

Description: It is well known that a pinch column with internal H/sub Z/ and external conducting shell can be made grossly stable, but that small-scale instabilities persist; especiaily in the tubular region of maximum current density. To investigate further these small-scale instabilities of the stabilized pinch,'' we are using 12-in.-i.d. linear pinch tube with a 3-in.-o.d. insulated center rod. By controlling a current along this rod, as well as a current along external conducting straps, and a third current in an external H/sub z/ coil, it is possible to create many grossly stable pinch configurations. The small-scale stability of the tubular region of maximum current density can thus be studied for a wide range of internal and external magnetic field vectors. The magnetic field distribution in each discharge is obtained by a string of 10 magnetic pick- up loops. The distribution of plasma density is determined by modulating the inner or outer wall current and measuring the radial velocity of the resultant compressional Alfven waves. In one experiment, an initial H/sub z/ is entrapped in plasma by preionization, and then pushed radially outward from the rod by a rising H/sub theta /. The resultant field distribution, in which H/sub theta / everywhere falls more rapidly than 1/r, should have absolute hydromagnetic stability. The persistence of small-scale instabilities, as observed by the magnetic probes, in this inverse stabilized pinch'' suggests that the basic trouble is nonhydromagnetic. It is also found that, when the magnetic field approximates a vacuum field distribution, perfectly smooth and reproducible probe signals can be obtained. The absolute plasma current densities at which nearvacuum field distributions have been found stable are larger than current densities at which the 'stabilized pinch'' and inverse stabilized pinch'' distributions have been found unstable. (auth)
Date: July 31, 1959
Creator: Birdsall, D.H.; Colgate, S.A. & Furth, H.P.
Partner: UNT Libraries Government Documents Department
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THE HARD-CORE PINCH. PART II

Description: The toroidal version of the hard-core pinch tube is created by levitating a ring conductor inside a toroidal shell. The magnitude of induced H/ sub theta / necessary for levi-tation is small in terms of field strengths normally desired for energetic pinches. In a 3-in. glass-and-copper toroid of square cross section a 3/4-in. hollow copper ring has been levitated with a 60- cycle current of 3 kiloamperes. A 12-in stainless steel tube of round cross section is being built. The stability of nearvacuum field hard-core configurations is best investigated in toroidal geometry. At high power levels and low plasma densities, the conventional toroidal stabilized pinch'' is subject to an anomalous plasma energy leakage to the wall, which cannot be explained by the observed ultraviolet radiation alone. A critical question is, therefore, whether the relative stability of some hard-core pinches, as reflected by the smoothness and reproducibility of magnetic probe traces, is reflected by an improved containment of the plasma en-ergy leading to high temperature. A toroidal hard-core tube is also useful in studying the nature of the nonhydromagnetic instabilities observed in the linear inverse stabilized pinch.'' The presence and condition of electrodes appear to have a substantial effect on the magnitude of these instabilities, as would be expected if they were, for instance, of electrostatic origin. In order to complement the plasma study of the hard-core pinch, we have developed an analogue method using sodium tubes to simulate the current-carrying layer. In this way the purely hydromagnetic aspect of the plasma behavior can be isolated. (auth)
Date: July 31, 1959
Creator: Birdsall, D.H.; Colgate, S.A. & Furth, H.P.
Partner: UNT Libraries Government Documents Department
open access

The Hard-Core Pinch. I

Description: This report analyzes a linear, hard-core pinch tube built to examine tubes afflicted by small-scale instabilities evident from many observations.
Date: July 31, 1959
Creator: Birdsall, D. H.; Colgate, Stirling A. & Furth, Harold P.
Partner: UNT Libraries Government Documents Department
open access

The Hard-Core Pinch. II

Description: This report describes a toroidal version of the hard-core pinch and the additional information it can obtain to what the analogous linear pinch can get.
Date: July 31, 1959
Creator: Birdsall, D. H.; Colgate, Stirling A. & Furth, Harold P.
Partner: UNT Libraries Government Documents Department
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