hg since the argon gas is circulating as described above the gas inflow rate and gas outflow rate can be modulated with valve assemblies to provide a net positive pressure in the interior chamber it also has been found that positive pressure in the interior chamber 616 can be useful in causing plasma filaments to be more uniform and more widely dispersed since the dielectric membrane is spaces away from the frame elements 620 a and 620 b in the central region of the interior chamber in another aspect of the invention the gas pressure in the interior chamber 616 of the dielectric membrane 605 can be modulated during the initiation and duration of a treatment cycle in one variation the pressure in the interior chamber prior to actuating rf delivery can be lowered to less than ambient pressure at the time of rf actuation the lower argon pressure will permit more instantaneous ignition of the plasma due to such lowered pressure since the argon gas is circulating the gas inflow and outflow rates can be modulated with the valve subsystems to provide a negative pressure in interior chamber 616 the pressure can be at least 5 below ambient pressure at least 10 below ambient pressure at least 15 below ambient pressure or at least 20 below ambient pressure after ignition of plasma in the interior chamber 616 the pressure can be regulated to a higher pressure as described above to expand the dielectric membrane away from the frame elements 620 a and 620 b fig2 illustrates another embodiment of an endometrial ablation system 800 with an elongated sleeve 810.