The Theoretical Science behind the Sunsword

The Sunsword idea was most likely a knock off of the lightsaber from Star Wars. - We will never get an official response because the Thundarr show was canceled (the producers said it was too savage for the 80s) and Disney has taken Star Wars over and producing movies now.
So in a reimagining, we have to make our weapon distinct from Star Wars and keep faithful to the original series.
Here are my thoughts.

The Star Wars Lightsaber

The Star wars canon describes the power source of lightsabers called Kyber crystals. They are integrated with the "force" into the hilt of the lightsaber. Most fan articles delve into the color of the light and meanings, and we have very little on why they work. Most just say it's cool looking!

The Sunsword: A Portable Fusion Reactor

Our Sunsword concept is not based on the Force. It is technology pure and simple. The rudimentary theories can emerge from our very own 21st Century physicist Dr. Michio Kaku
For the "plausibility" scale for fans, the Sunsword can begin its design as a plasma-based weapon with a containment field in the shape of a sword. The energy source could be a mini fusion reactor.
Our Sunsword is based on technology from an advanced civilization capable of light speed interstellar travel. So we don't have to base our notions on our primitive scientists today, but we just want it to be believable so our audience can nod and think, "Yeah, I can see that." So does it have to look like the lightsaber of Star Wars?

No, it can appear as a pulsating rhythmic series of mini explosions if we could closely examine a fired up blade. Like the surface of the sun
Or to wrack up the coolness factor - a series of plasma arcs firing in random directions from the top of the blade to its hilt.
How would this blade be effective as a sword? Perhaps the containment sword is pliable. If it comes in contact with any substance denser than water, the containment field is displaced. The contents of the plasma then are exposed (its searing heat) and melting effect released. Upon removing the density of the micro ceramic like containment field, the molecules return to its position (like magnets) and the plasma blade is maintained.

GM ALERT

The Sunsword is very rare but for gaming purposes, it can easily be applied to known Starfinder items. If you wish to have low to a mid-tier game with Thundarr's Sunsword, use the Plasma sword, tactical. Higher tiers can have a higher level of plasma blades. Just avoid the restrictive rules of batteries of normal plasma blades. Thundarr's Sunsword lasts for months without the need to recharge.

Starfinder has a generous crafting system and the forging of weapons is essential to keep up as your characters advance. For those Thundarr wannabes, the plasma weapon series are reasonable. However, all restrictions should apply. We need to keep that Sunsword special!