SANTA ANA, Calif. — Taking the witness stand in his own defense, an accused Chinese spy said yesterday that he was engaged in legitimate scientific exchange and not espionage when he sent technical papers on electrical systems for American submarines and Navy warships to China.

“I like to look at some technical exchange,” the defendant, Chi Mak, said. “I’m curious to know what’s their development stage and I’d like to have their comment on the work.”

Mr. Mak, an electrical engineer who worked for a California defense contractor, said the papers were intended not for the Chinese government, but for two former academic colleagues in Hong Kong and a family friend in the mainland. Mr. Mak said he saw nothing wrong with sending the papers to China because they were handouts he got at engineering conferences where foreigners were present.

“All three CDs contained presentation materials that were presented in conferences with international attendees already,” Mr. Mak said. Encrypted files from the disks were found in the hand luggage of Mr. Mak’s brother and sister-in-law when they were arrested at Los Angeles International Airport in October 2005 as they prepared to fly to China.

With those key questions out of the way, a defense attorney, Ronald Kaye, led his client through a lengthy discussion of his education and the years he spent working in a Hong Kong tailor shop before immigrating to America. The prosecution contends that his history has many of the hallmarks of an intelligence agent, but Mr. Mak testified that his journey to Hong Kong from the Chinese mainland in the 1960s was due to a severe bout of rheumatism and that his interest in American military ships was a natural byproduct of the tailoring business.

Mr. Mak said he came to America to “seek higher power,” meaning work on more sophisticated electrical systems. He acknowledged omitting two of his siblings from an immigration form but said he had no nefarious intent.

The defense painted their client as an introverted pack-rat who obsessively gathered and analyzed data, even plotting his projected Social Security income on a graph.

Mr. Mak said he never sought out defense-related work and did none of it for about a decade after arriving in America. He acknowledged that he stored thousands of power-engineering documents at home, but said such practices are common. “This is a tradition almost with every engineer,” he said.

None of the documents investigators found at Mr. Mak’s home were marked as classified, but some were designated as “NOFORN,” meaning they could not be given to foreigners. Mr. Mak said he was unaware of a policy against taking those home. “I don’t think it’s practical,” he said. “Marketing and even my boss encourage us to work at home.”

Mr. Mak is expected back on the witness stand today. He has not yet explained documents found at his home that the government says are “tasking lists” specifying the kinds of technology China wanted to acquire. Four members of Mr. Mak’s family are also charged and face a separate trial, but a retired FBI agent, Ivian Smith, testified for the defense that such family-based spying is unheard of with the Chinese. “There’s not a single instance anywhere,” he said.

Mr. Smith, who once headed the FBI’s counterintelligence program focusing on China, said Chinese who study in America or migrate here are expected to help the Chinese government, but that doesn’t mean everyone who gets such requests is a spy. The ex-agent also said a spy probably would not give up his job at a military contractor and move to a purely commercial firm, as Mr. Mak once did. “I can’t imagine anybody in Beijing would be very happy if they went back there and said, ‘My source had an opportunity to have access to classified information and declined,'” he said.

A prosecutor, Greg Staples, said Mr. Smith’s experience with Chinese espionage was fleeting and came about two decades ago. “Your career working Chinese intelligence was really more like a 50-yard dash,” the prosecutor said.

“I wouldn’t agree with that,” Mr. Smith replied. The New York Sun played an unexpected walk-on role in yesterday’s proceedings when Mr. Staples challenged Mr. Smith about an interview he gave in March, after the defense said it planned to call him. “I’m not naïve. I expect the government to try to beat me up a little bit,” the former agent told the Sun. “To disagree with the government is parallel to calling you treasonous.”

“That’s a little melodramatic,” Mr. Staples said yesterday. “Who in the government called you treasonous?”

“Some of my former colleagues,” Mr. Smith replied, though he acknowledged that no current officials had threatened him.

The prosecutor also suggested that it was improper for Mr. Smith, as a witness, to have spoken with the press. The former agent said he was aware of no such restriction on retired officials.

Mr. Smith did not dispute the quote but said he had not seen the article. “We don’t get The New York Sun in Essex County, Va.” he said with a chuckle.