UCLA

The arched plasma structures found throughout the Sun’s atmosphere can significantly impact the heliosphere and Earth through eruptive events like solar flares and coronal mass ejections. Coronal loops and solar prominences are examples of arched plasmas on the Sun. In this dissertation, we study arched plasma structures in the laboratory setting relevant to those found in the Sun. The current understanding of mechanisms leading to eruptions and general dynamics of arched solar plasmas remains limited. This is primarily due to a lack of extensive in situ diagnostics and, thus, a heavy reliance on remote imaging. Through proper scaling of relative plasma parameters, we can investigate the dynamics of arched plasma phenomena in a laboratory to better understand what is happening in the Sun. This research aims to contribute knowledge to the field and allow for the formulation of reliable predictive models.This experiment was designed with the primary goal of producing and studying arched mag- netized, current-carrying plasma. It is achieved with a hot lanthanum hexaboride (LaB6) cathode and cold copper anode plasma source operated in a vacuum vessel filled with a neutral Helium gas (up to 9 mtorr). Two magnetic fields are produced in this experiment, of which direction and magnitude can be tailored to experimental goals. The first is an arched guiding magnetic field con- necting the footpoints and guiding the plasma along the arch. The second is a horizontal overlying (ambient) magnetic field of uniform magnitude, directed perpendicular to the plane of the arch. A three-dimensional probe drive has been constructed for this work, allowing for automated in situ measurements of plasma parameters with high spatial resolution along the customizable grid. The probe drive control software was written in Python and integrated with the main data acquisition LabView software. The appropriate coordinate transformations between the probe tip position and the probe drive’s motors were determined and integrated into the control software. We have built four diagnostic in situ probes (magnetic field, 2- and 3-tip Langmuir, Mach), resistant up to 700◦C temperatures. This high temperature-resistant construction allows for measurements very close to the hot cathode source (up to 5 cm away). The probes were used along with two fast cam- eras to better understand phenomena in experiments presented here. We have carried out multiple arched plasma source maintenance and upgrade routines, improving its duty cycle and stability over the course of this work. A power supply control system was developed for both background and arched plasma source heaters. This system allowed us to gradually raise the heaters’ current in an automated and remote fashion. All data analyses were conducted in Python using the Jupyter Notebook environment. An extensive library of data analysis functions and procedures resulting from this work is tailored to this setup. One focus of this dissertation is the effects of a nearly horizontal overlying (strapping) magnetic field on the evolution and morphology of the arched magnetized plasma. The electric current in the arched plasma was kept low enough to keep it kink stable. The experimental results show that the sigmoid plasma structures are naturally produced in a sheared magnetic field configuration. The magnitude and handedness of the writhe of the arched plasma strongly depend on the structure of the guiding magnetic field. We found that orienting the guiding magnetic field nearly parallel to the electric current in the arched plasma results in a reverse-S shape. For an antiparallel orientation, the arched plasma assumed a forward-S shape. Moreover, the magnitude of the writhe and twist was correlated with the strength of the shear in the guiding field (strength of strapping magnetic field applied). These results are significant in light of the distribution of arched plasma structures on the Sun. Namely, the reverse-S shaped structures are more common in the Northern Solar Hemisphere, while forward-S shaped structures are characteristic of the Southern Hemisphere. The presence of strong shear in the magnetic field was observed to cause an eruption of a tran- sient jet structure out of the arched laboratory plasma. The detailed study of this phenomenon constitutes the second focus of this dissertation. Jet-like structures are commonly formed from arched plasma structures in the lower solar atmosphere (e.g., anemone jets and spicules). Due to our experiment’s relatively high density of neutrals, we can simulate the highly collisional con- ditions of the photosphere and lower chromosphere of the Sun. This capability is very unique to our laboratory setup. The ion-neutral collisions significantly impact the dynamics of lower atmo- sphere solar structures and our laboratory plasma structures. We employed the diagnostics of the magnetic field, density, temperature, and ion flow to characterize the jet structure formed in these experiments. We found that in its early stages, the laboratory jet has a supersonic (around Alfv�n speed) ion flow away from the arch, driven primarily by a large gradient in the magnetic field. On the Sun, structures like spicules are also found to carry an ion flow at velocities around the Alfv�n speed. The jet under study carries the electric current, which returns to the arch gradually with distance through an ion-neutral charge transfer collision mechanism. The electron current returns to the anode via a path crossing the weakest magnetic field lines, making a sharp turn near the magnetic null. The work presented here has contributed to our knowledge of the dynamics of arched magne- tized plasmas relevant to similar structures in the lower solar atmosphere. The arched plasma’s electric current in our experiments was naturally low enough to keep it kink-stable. This unique feature of our experimental setup allowed us to study the arched plasma’s sigmoid shape mor- phology and the plasma jet eruption dynamics purely in terms of the pre-existing magnetic field configuration. Our studies show that strong shear in the vacuum magnetic field not only impacts the morphology of the arched plasma but can also drive a formation of a jet structure. The signifi- cant presence of neutrals in our plasma is yet another unique aspect of this experiment. At around 2% ionization level, the plasma studied here is relevant to solar plasma found in prominences, chromosphere, and photosphere. With the new insights and all hardware and software developed during this dissertation, we have established a platform for further research on these topics. We hope this dissertation is but a building block of a future predictive model of eruptive arched plasma structures on the Sun.