The Death Star prototype was a test-bed for the first Death Star. It was constructed in the Empire's secret Maw Installation.
Death Star prototype schematics.
The Death Star prototype was a simple durasteel frame with a reactor core, superlaser, engines, and a control sector. First conceived by Grand Moff Wilhuff Tarkin, it was constructed by Bevel Lemelisk and his engineers at the Maw Installation.
The Death Star prototype measured 120 kilometers in diameter. Its superlaser was only powerful enough to destroy a planet's core, rendering it uninhabitable, although it was powerful enough to totally destroy larger moons. The superlaser also took a large amount of power and drained the vehicle's batteries. It took 15 minutes to recharge the batteries and superlaser.
The targeting system on the Death Star prototype was also never calibrated, greatly hurting its accuracy. The prototype's interior consisted of multiple levels, one of which housed a control room where all the computers including the superlaser were slave linked, and at least one or two other decks that were able to hold a majority of the Maw Installation's personnel. This meant the station could be run by only a small crew, although only one person was needed to pilot the structure. An intercom system was installed that could be heard throughout the entire station. Multiple catwalks and access hatches were present throughout the prototype, notably the core. The interiors of the later Death Stars were taken up by the hyperdrive engines and other components.
The Death Star prototype was constructed to see if the planet-destroying superlaser could be built. As soon as the laser worked, the prototype was left anchored above the Maw Installation since it had fulfilled its mission. It was never completed.
Tol Sivron and his colleagues used the Death Star Prototype to flee an attack by the New Republic on the Maw Installation in 11 ABY. After escaping out of the Maw and into the Kessel system, the crew began to repair the damage they sustained. Once it was fixed, Sivron decided to test the superlaser. Using Kessel as a target, the prototype fired but missed, due to the poor calibration, and hit the moon instead, completely destroying it. The station was driven off by the Millennium Falcon and assorted pirate forces of the Smugglers' Alliance, eventually retreating back into the Maw Cluster after firing the superlaser at the oncoming ships a few times. After a quick meeting, Sivron and the other crew members decided to return to the Maw Installation and use the prototype to destroy the facility, in order to not let their research fall into enemy hands. The Millennium Falcon, piloted by Han Solo, Lando Calrissian, and Mara Jade; had flown into and attached itself to the core while the station moved.
As Solo repaired the Falcon, Jade and Calrissian used enviro-suits to exit the ship and place thermal detonators on the core in an attempt to destroy it and cripple the prototype. The intruders were detected by the scanners in the control room and two spacetroopers were sent to stop them. The two troopers emerged through a trap door and began to scuffle with Jade, who managed to kill one of them but retreated with Calrissian as the timed detonators were close to exploding. The Falcon escaped as the surviving spacetrooper managed to disable all but one of the detonators, which exploded when the timer ran out, killing the trooper. Soon, the Death Star Prototype reemerged at the Maw Installation and entered the battle that was taking place between General Wedge Antilles' New Republic forces and the recently arrived Admiral Nataasi Daala's Imperial II-class Star Destroyer, Gorgon. However, Sivron only wished to destroy the Maw Installation and refused to fire on Antillies' forces and help Daala, as she and her fleet had previously abandoned the Maw to attack the New Republic, leaving the scientists unprotected. As Sivron moved to fire on the Installation, the explosion in the core crippled the station, requiring immediate repairs and temporarily preventing the superlaser from shooting, giving Wedge and his men enough time to evacuate the facility before it was destroyed by Daala's Star Destroyer after the facility's shields failed.
Kyp Durron, who had come to the Installation to destroy the Sun Crusher once and for all, attempted to destroy the prototype by firing resonance torpedoes at the superstructure. Despite large portions of the station sustaining damage, the prototype was still operational and began to fire its superlaser at Durron, who quickly ran out of torpedoes. Sivron himself took the controls and chased Durron around the Maw, while the superlaser fired, trying to destroy the Sun Crusher. The Death Star prototype was finally destroyed when Durron lured it into one of the Maw's black holes, but not before finally being struck by the superlaser, although taking very little damage due to the Sun Crusher's invulnerable Quantum-crystalline armor.
Though it is never directly stated, there are indications that other prototypes might have existed.
Originally, the Death Star prototype was considered to be just that—a project begun prior to work beginning on the Death Star I. However, following the appearance of the Death Star plans in Attack of the Clones and the first Death Star in Revenge of the Sith (confirmed by George Lucas himself), The New Essential Chronology retconned the prototype into being built alongside the main Death Star because concerns were raised.
Champions of the Force describes the Death Star prototype to have an interior superstructure consisting of a maze of beams and catwalks, similar to the second Death Star. However, drawings of the station show it to have an entirely open interior other than the reactor core and superlaser.