KNOXVILLE, Tenn. — A retired University of Tennessee professor accused of passing secrets from his work on a U.S. Air Force contract to two foreign graduate students testified Friday that he didn't break the law because he hadn't even proven that his research worked.

J. Reece Roth, an expert in plasma physics, is charged with 18 counts of conspiracy, fraud and violating the Arms Export Control Act. Prosecutors claim Roth gave two graduate students — one from China and another from Iran — access to sensitive information while they researched a plasma-guidance system for unmanned aircraft.

Prosecutors presented several documents suggesting research by Roth's university laboratory or a spinoff company, Atmospheric Glow Technologies Inc., was restricted.

But Roth insisted he didn't violate the law.

"My understanding was that it only applied to things that worked, and we had not shown that. We had a lot of work to do," Roth testified.

Roth, 70, is also accused of taking reports and related studies in his laptop to China during a lecture tour in 2006, and having one report e-mailed to him there through a Chinese professor's Internet connection.

The government seized materials from Roth's office and took his computer from him at the airport when he returned from the trip. Prosecutors claim he violated the export control act simply by taking the laptop with sensitive materials outside the country, even if, as forensic evidence showed, he didn't open all of those files while he was in China.

"I have always obeyed the law," Roth said.

The charges involve work performed from 2004 to 2006 on two Air Force contracts by Roth, graduate students Xin Dai of China and Sirous Nourgostar of Iran, and university spinoff company Atmospheric Glow Technologies Inc. of Knoxville.

Roth said he never tried to hide that Dai worked on the project. He said the Air Force, the university and Atmospheric Glow Technologies all knew or should have known that he was from China.

It didn't become an issue until the university denied Roth's request to hire Nourgostar to replace Dai after he graduated.

Roth testified that if he had known from the outset that hiring foreign grad students as lab assistants would be a problem, "I simply wouldn't have submitted a proposal or participated in the proposal."

The jury will hear closing arguments Tuesday. Roth faces up to 160 years in prison and more than $1.5 million in fines if convicted.

Atmospheric Glow Technologies recently pleaded guilty to 10 counts of exporting defense-related materials and Roth protege Daniel Sherman has pleaded guilty to related charges. They are awaiting sentencing.

Over 30 years, Roth established a widely respected plasma lab in the university's electrical engineering department, attracting top students from around the world. Before arriving at the school in 1978, the MIT and Cornell graduate spent 15 years as a researcher with NASA.