The permanent siren hasn't arrived and the county police helicopter isn't ready for duty, but Fairfax County officials proceeded anyway yesterday with a mock escape at a medium-security prison camp west of Fairfax City.

Supervisor Marie B. Travesky of the Springfield District called the new alert system at the state-run Camp 30 a "concession" from prison officials to neighbors upset by the addition of 35 work-release prisoners from a Woodbridge prison camp to the 150-inmate facility in her district.

Precisely at 2 p.m. yesterday, a make-shift siren wailed across the rolling hills of the prison camp, on West Ox Road near the county's I-66 landfill. (Prison officials said the real siren is on order and won't arrive for about 10 weeks.)

Six minutes later, county police cruisers sped up the driveway to the camp's red-brick prison barracks. Eleven minutes after the alarm, the police department's new helicopter, which won't be available for regular service for several more weeks, whirred over the cluster of neighbors, public officials and media representatives below.

It was all part of an event designed to alleviate some anxieties of neighbors who were troubled by the State Corrections Department's transfer of the work-release prisoners.

Most of the dozen neighbors who turned out for the demonstration appeared mildly impressed. Larry Eichel, who lives just over the tree line in the Dixie Hills subdivision, said he'll probably "go to sleep a little more comfortably now."

Travesky said area residents won't be satisfied until the prison is closed, a request she said she will urge the General Assembly to consider at its next session. She said, however, any such effort would admittedly "involve a long process." Neighbors said the once-rural community has become too populous to co-exist with a prison.

Superintendent James E. Collins said one prisoner has escaped in the past two years.