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Experimental studies of the detailed dynamics underlying spheromak formation
| Author: | Bellan P. M. |
| Coauthor: | S. You, C. A. Romero-Talamas, G. Yun, E. Jorne, and S. E. Pracko |
| Institution : | Caltech |
| Abstract text: | The transient dynamics governing spheromak formation are being investigated using a planar magnetized coaxial plasma gun specially designed for this purpose. This gun has eight concentric gas puff ports on its 20 cm o.d. cathode and a corresponding set of eight gas ports on its concentric, co-planar annular 51 cm o.d. anode. Typical parameters are I~70-150 kA, poloidal flux ~0.5-2 mWb, and plasma duration ~10 microseconds.
High speed movies show that spheromak formation involves a definite sequence consisting of: (i) eight arched open magnetic flux tubes linking the gas ports fill up with plasma and become filamentary, (ii) the filled flux tubes merge to form a jet-like central column, (iii) the central column jet lengthens until it suddenly kinks, and, (iv) the kinked plasma detaches from the gun. In certain high-current situations, a localized sausage instability occurs after steps (i), (ii) or (iii).
Plasma symmetry depends strongly on the gas injection port symmetry. This is conjectured to be because the extent to which plasma fills up a flux tube in step (i) above is dictated by the availability of plasma for ingestion from the gas ports. This ingestion is due to J x B forces and so is a much stronger process than simple inward diffusion. Details of the ingestion process are being investigated by using different gas species at cathode and anode injection ports so that cathode/anode plasma inflows can be distinguished (see accompanying poster by You/Bellan). Also, preliminary spectroscopic measurements show blue shifts consistent with strong plasma jet flow in steps (ii) and (iii).
A related high speed photographic system installed on the Livermore SSPX spheromak provides photographs of spheromak formation in the presence of a flux conserver. This system provides imaging of concentric open flux surfaces near the geometric axis during sustainment (see accompanying poster by Romero/Bellan).
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