The chemical laser weapon being developed with top priority on President Reagan's secret order would be the first "death ray" tested in space, according to highly sensitive documents.

The president, eager to speed research on a laser space weapon, ordered accelerated development last December. Code-named Zenith Star, the program aims to test a cylindrical, hydrogen-fluoride-fueled laser in space as early as 1990.

Under the traditional interpretation of the 1972 Antiballistic Missile Treaty, the United States would have to give the Soviets six months' notice of intent to scrap the treaty by testing a missile-killing weapon in space. If the next president does this, he will provoke a row with Congress.

In fact, the battle would have been joined months ago if Reagan had heeded the advice of Lt. Gen. James Abrahamson, director of the Strategic Defense Initiative Organization. The general urged Reagan to go public with the program last December, but the president, taking a battering over the Iran-contra scandal, elected to keep Zenith Star a closely held secret.

Since then, Abrahamson has been driving full speed ahead on the weapon's development -- and the space test that would shoot down the ABM Treaty.

As often happens, Congress helped to set itself up. Abrahamson has total discretion to shift tens of millions of dollars from one project to another within five general SDI programs Congress has voted funds for. The major program that covers Zenith Star is directed-energy weapons. It had $843 million last year, and the president is asking for $1.1 billion next year.

Congress was notified earlier this year, in a secret report, that a chemical laser test in space was contemplated for the "early 1990s." Such a test, the administration admitted, would be acceptable only under a new, "broader" interpretation of the ABM Treaty.

An unclassified version of that secret Pentagon report was released on Sept. 21. But it was ignored by the general press, which failed to spot the enormous implications of even the limited, carefully worded admissions in the 24-page public version.

The overlooked key was "LISE," an acronym for "Laser Integrated Space Experiment." That will be the product of the Zenith Star project.

"LISE is a fully integrated, completely autonomous experiment that would test the ability of a space-based chemical laser to negate a booster in ballistic flight and also to demonstrate interactive discrimination," the report states. In plain English, the test would try to destroy a missile in its liftoff phase and to pick out warheads from decoys.

LISE would use the Alpha chemical laser being developed by TRW Inc. in San Juan Capistrano, Calif. Sources said there is confidence the laser can perform at its designed five-megawatt power level.

The Pentagon's unclassified report concluded that "planning for this experiment would have to begin now to conduct the experiment in the early 1990s." In fact, Abrahamson has been secretly planning the test since Reagan's order last December.

Intelligence sources believe that 1990 is earlier than the Soviets could test a similar device, which would make the U.S. laser the first space "death ray" in history -- if the next president so chooses.