Results of fiber-optic-interferometer density measurements performed in the plasma-operating-switch region of DMI

Fiber optic interferometer (FOI) measurements were performed in the plasma opening switch (POS) region of DMI. These are the first such measurements of the behavior of the plasma when driven by the DMI current. It is found that when the cable guns are fired they produce a plasma, measured at z/spl ap/2.8 cm, r/spl ap/8.0 cm, of density n-1/spl times/10/sup 15/ cm/sup -3/ which falls off rapidly in z. The plasma has a transverse velocity of v/sub z//spl sim/1 cm//spl mu/s and is estimated to have a density at z=0 cm of about n/spl ap/5/spl times/10/sup 15/ cm/sup -3/. The shot to shot variation in the cable gun plasma is measured to be about /spl Delta/n/n/spl sim/10 to 20%. When the Marx bank is fired, a burst of plasma is observed to travel axially toward the diode with velocity v/spl ap/25 cm//spl mu/s. The velocity of the burst increases with z, reaching v/spl ap/42 cm//spl mu/s by z=8.8 cm. These velocities are close to the calculated Alfven velocity, which is v/sub A/=31 cm//spl mu/s for DMI parameters. The density of the burst is estimated to be about n/spl sim/10/sup 16/ cm/sup -3/ and appears to increase somewhat with z. The opening of the POS takes place when the leading edge of the burst is at about z/spl ap/4.2 cm. The burst of plasma is estimated to have a width of about w/spl sim/3.8 cm at z=2.8 cm, prior to opening, and a width of w/spl sim/10 cm at z=8.8 cm, after opening. Prior to opening, no plasma is observed downstream of the POS (z=8.8 cm) greater than about n/sub e/=1/spl times/10/sup 13/ cm/sup -3/.