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Experimental investigations of the shockwave propagation in the direction parallel to the electric field in low-pressure longitudinal glow discharge argon plasmas are performed by the simultaneous multipoint laser deflection technique. In the newly developed shock tube at Troy University, Mach 1.5-2.2 shockwaves are produced by a fast capacitor discharge (quarter period tau1/4 = 1.4 mus ). In this paper, the shock propagation measurements are extended to the low pressure limit down to 3.6 torr while confirming the earlier measurements performed at gas pressures 15 torr and above. The shockwaves are launched through a plasma medium inside the shock tube, where the deflections of the laser beams are recorded on a fast oscilloscope. An average shockwave velocity in the plasma is determined from the time history of the laser deflection signals. The shockwave speed and the broadening of the laser deflection signals in the plasma are found to be dependent on the plasma discharge current. Shockwave speeds increase by 18% for the plasma at 3.6 torr over a range of plasma discharge current I = 0-150 mA and by 46% for the plasma at 15 torr over I = 7-150 mA. In addition, shockwave amplitudes are attenuated in the plasma and show linear dependence on the shockwave speed or Mach number.