Ion Current Collection by Cylindrical Electrostatic Probe in a Flowing Plasma
rectangular slot orifice, two other tube end conditions were used: an orifice plate perforated with a uniform pattern of i-in. diam. holes giving a total orifice area A+ = 5 in., and the condition of the tube end fully open with sonic flow at the exit (A^ = 1^ in. x 5 in.). The diaphragm configuration was identical for the three tube end conditions. The presence of an orifice plate appears to have limited effect on the waveform; the waveform for the perforated plate is essentially the same as for no orifice plate (Fig. 5). Figure 6 shows the effect of reduced p0 on e(ue) determined at the 7-ft station for the tube end fully open. At the lower pressure level two different diaphragm materials were used: 0.001 in. mylar, which tends to tear on rupture; and 0.0003 in. "red-zip" cellophane, which shatters into small fragments on rupture. No measurable difference in waveform was observed for the two materials. In conclusion, the present study shows diaphragm generated expansion waves to have wavehead first derivatives that are essentially discontinuous. A wavehead first derivative measured at any x then determines a unique x,t coordinate system. In that system, noncentered plane waves are conveniently described by the function e(ue) determinable from a measured pressure or velocity history at any x. It might be added that through e a local conical similarity exists for the inviscid noncentered wave' in terms of the similarity variable S = \+x/a0t—e/a0t.