I´m building a hard-sifi setting and want to use Stellasers (abbreviation for stellar laser) to provide thrust and power to interstellar vessels. This part one is only concerned with the construction and physics of the laser and its abilities. Matters concerning the ship-side issues will be addressed in Part II.
I possess a basic understanding of the way lasers work. I encountered the concept of Stellasers only two times before. On the website http://www.laserstars.org/ which seems to be no longer available and in videos made by the futurist Youtuber Isaac Arthur , who mentions the concept regularly. Episodes, where the concept is described, are "Colonising the Sun", "Interstellar Highways", "Exodus Fleet" and many more.
Everything should happen with known physics. Should "optimistic estimates" be required to make the Stellaser work, so be it. I also want this to be a project for a sub-Kardashev-Type-2 civilization. That said, the simpler the proposal the better.
What should be delivered
- a continuous beam delivering 5 Petawatt should be sufficient for my purposes (the value is somewhat arbitrary, if you want to change it by a few orders of magnitude feel free to do it)
- the beam needn´t be strongly focused, as the target is several thousand $km^2$ big and should survive
- none of the designs ask for one giant beam source, large arrays containing thousands of the chosen design them may be built
The first idea is to place two mirrors as low as possible in the star's atmosphere to use it as my gain medium.
- energy is delivered to the system via the light of the star in whose corona it orbits
- tethers would connect the mirrors to prevent them from drifting apart
- an outside laser would be used to target, stabilize and force population inversion
- the laser would be placed in a polar orbit tangential to the star
- protection from the element of fire would be delivered by maintenance, clever shading, supermaterials, magnetic fields (also useful for keeping height and stabilization), and active cooling (radiators are kept in the mirrors shadows)
- a Fresnel Lens  could be used to behind the partially reflective mirror
Dance of Mirrors
This isn´t a laser but rather a partial Nicolle-Dyson-Beam. But it might get the job done. So it´s on the list.
- several huge mirrors are placed in specific orbits around the star, allowing them to focus their light on the same point
- they are stabilized and maneuvered using other mirrors, lasers or rockets
- a satellite gathers energy with solar panels and uses it to power a laser
- also placed in a polar orbit
- stabilized and targeted with rockets
- Which is the best one concerning energy output, practicality, and efficiency? (all designs are based on a spherical disc with a radius of 50 km for reference)