JUDGE HALVES FEDERAL MINIMUM TERM IN DRUG CASE FIRST-TIME OFFENDER GETS FIVE YEARS INSTEAD OF 10. By MIRANDA EWELL Mercury News San Francisco Bureau SAN FRANCISCO -- Friends and supporters of 23-year-old Christian Martensen burst into tears and applause Wednesday when the Grateful Dead fan was sentenced to five years in federal prison -- instead of 10 -- in a case that has helped reignite a national debate on federal drug sentencing laws. Martensen was arrested in 1991 after he agreed to introduce another Deadhead to someone who could sell LSD. The fellow Deadhead, however, turned out to be a federal undercover agent. And under stiff sentencing laws, which give federal judges no leeway to make distinctions based on the circumstances of a case or the background of the defendants, Martensen faced an almost certain 10 years in prison without parole for his offense. But San Francisco federal Judge Vaughn Walker, who has criticized the mandatory minimum sentencing law as irrational, has repeatedly rebuffed prosecution attempts to lock up Martensen for the duration of his young manhood. Critics -- including many judges -- say the laws, which were passed by Congress in 1986, are absurd. First time, non-violent offenders, like Martensen, typically serve more time for drug offenses prosecuted under federal law than violent criminals, like rapists. ''I think the drug laws are cruel and repressive,'' said Joanne Baughan, 58, a San Anselmo secretary who participated in a rally in support of Martensen just minutes before his sentencing. Jodi Libretti, a Palo Alto elementary school teacher also at the rally, said she was appalled at the resources the government was spending to jail non-violent drug offenders, typically at a cost of $20,000 per year for each. Her 30-year-old brother recently began serving a 20-year sentence in Phoenix for selling cocaine. ''I'm really afraid he's going to come out angry and hateful,'' Libretti said. ''This whole experience has completely shattered my faith in the criminal justice system.'' A Department of Justice report on mandatory minimum sentences released last week makes no recommendations on the controversial law, but its summary of facts support many of the critics' contentions. The report shows that roughly 21 percent of those in federal prison, or about 16,000 inmates, are first time non-violent offenders, said Virginia Resner, the Bay Area director of Families Against Mandatory Minimums (FAMM). Unlike some critics, who fear the current public concern over crime will doom efforts to repeal the sentencing laws, Resner believes people will make distinctions between violent criminals and non-violent offenders. ''Is a 23-year-old Deadhead the same as Richard Allen Davis?'' Resner asked, referring to the felon accused of kidnapping and killing Polly Klaas. ''Is he even a drug kingpin?'' Judge Walker made it clear he did not believe so. Walker had already refused once to sentence Martensen to 10 years, sentencing him instead to five. In doing so, the judge ignored laws -- and a Supreme Court ruling -- that require a judge to add in the weight of the paper or sugar cube containing the LSD when determining the sentence. In Martensen's case, the drug itself weighed 1.5 grams -- only enough to draw a five-year sentence. But if the blotter paper containing the LSD were added in, Martensen would draw a 10-year sentence. Walker was reversed on appeal, in a ruling widely expected to force the judge to impose a 10-year sentence. But Wednesday, with the case back before him, Walker accepted instead a complex and untested legal argument by defense attorneys that offered him a way to avoid that.