Experimental investigations in spheromaks : injection into a tokamak and formation in an unbounded environment

Citation

Yee, Jimmy (2000) Experimental investigations in spheromaks : injection into a tokamak and formation in an unbounded environment. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Y45X-9C19. https://resolver.caltech.edu/CaltechETD:etd-08302005-134459

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Spheromaks are investigated in two separate experiments. In the first, the CT Injector experiment, spheromaks are accelerated by a coaxial rail gun for injection into a medium-sized tokamak ([...] = 10 kG). Speeds of 3 x 10[...] m/s and densities in the high 10[...] m[...] range are achieved upon implementation of impurity reduction techniques and high bias flux. Tokamak injection experiments indicate anomalous stopping of the spheromak within the injector at toroidal filed (TF) levels well below previous theoretical predictions. Modifications of the electrode geometry were found to substantially improve performance. An extension of the previously accepted theory is proposed which takes into account the effect of TF flux trapping within the electrodes. A 2-D magnetostatic analysis indicates that this phenomenon increases the spheromak kinetic energy density required for penetration by a factor dependent on spheromak size. A numerical analysis extends these results to finite length spheromaks. These calculations also show that the spheromak shape has an effect on penetration levels. The proposed theory indicates that a realistic assessment of spheromak injection requirements must take into account magnetic field interaction with the injector walls as well as the spheromak itself. The second part of this thesis describes an experiment examining the formation and decay of spheromaks in unbounded environments; i.e., without the influence of flux-conserving walls. A coaxial gun is discharged into a much larger vacuum chamber and the subsequent evolution of the plasma is observed, mainly using high speed cameras and a magnetic probe array. Photographic results suggest four distinct regimes of operation, labeled I-IV, each possessing qualitatively different dynamics, with the parameter [...] determining the operative regime. Plasmas produced in Regime II are identified as detached spheromak configurations. Images depict a roughly donut-like shape, while magnetic data, when interpreted using a propagation inference method, suggest that a closed toroidal flux-surface topology is present. Poloidal flux amplification shows that Taylor relaxation mechanisms are at work. Calculations of the spatial and temporal variation of [...] indicated that the spheromak is decaying and expanding in a manner consistent with the self-similar expansion model proposed for magnetic clouds. This implies that magnetic heating may be operative. Regime III is associated with stuffed coaxial gun operation. Images show several complex helical and twisted features, which are shown to be magnetic in nature through field line tracing of the magnetic data. Toroidal and poloidal flux and [...] calculations clearly demonstrate the stuffed nature of this regime.

Thesis Files