Investigations of Spheromak plasma dynamics: High-speed imaging at the Sustained Spheromak Physics Experiment and magnetic diagnostics at the Caltech Spheromak experiment

helicity;image processing;magnetic fusion;magnetic self-organization;maximum entropy;multielement magnetic probe

Romero Talamás, Carlos Alejandro ; Bellan, Paul Murray

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: helicity;    image processing;    magnetic fusion;    magnetic self-organization;    maximum entropy;    multielement magnetic probe;   

Others  :  https://thesis.library.caltech.edu/491/1/RomeroTalamasPhDthesis11.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

This thesis consists of two parts. The first part describes a specially designed high-speed imaging system installed at the Sustained Spheromak Physics Experiment (SSPX). Thousands of images have been obtained at SSPX using a high-speed, 1280 x 1024 pixel, cooled and intensified CCD camera with double frame capability, and show unprecedented details of the SSPX plasma. From these images, three different stages were identified according to distinct plasma features. These stages are breakdown and ejection, sustainment, and decay.During the breakdown and ejection stage, JxB forces push the plasma and stretches the initial vacuum field into the flux conserver. As the plasma enters the field of view of the camera, undulations in the expansion front are visible. These undulations are caused by filaments formed in the gun region, and merge as they travel towards the flux conserver and rotate around the chamber axis. In less than 100 microseconds after breakdown, a transient plasma column is formed. Just microseconds after this, the column bends impulsively and seemingly merges in the toroidal direction (around the axis of the chamber). It is conjectured that the bending precedes a reconnection event that leads to magnetic flux amplification.Images taken during the sustainment stage show the presence of a central column which is very stable. Some images suggest nested current channels in this column. Comparisons of column diameter measurements versus numerical modeling (using the CORSICA code) are presented here. Bright and distinct patterns were observed on the surface of the source cathode, and appear to be related to the sustainment column and open flux surfaces. These patterns elongate toroidally in a constant direction which depends on the bias field polarity. It is conjectured that the pattern motion is caused by E x B drifts, or J x B effects near the cathode surface.Most of the hardware was specially designed for the high-speed imaging system, including a double-branch fiber bundle that was used to produce rough tomography (at midplane) of the transient central column. The algorithm used for tomographical reconstruction is based on a maximum entropy restoration method that was also used to improve noisy and blurry images.The second part of this thesis describes a 60-element magnetic probe array that was constructed using miniature commercial chip inductors. The coils are oriented in orthogonal directions to yield three-dimensional information. The probe has been used to investigate magnetic evolution at the Caltech Spheromak Experiment.