The gas avalanche switch is a recently conceived, laser-activated, high-voltage, picosecond-speed switch. It basically consists of a set of pulse-charged electrodes immersed in a gas at high-pressure (2-800 atm). A picosecond-scale laser pulse initiates an avalanche discharge in the gas between the electrodes. With the proper configuration, the avalanche, which is fueled by the immense number of electrons available in the gas, causes the applied voltage to collapse in picoseconds and generates electromagnetic waves. A two-dimensional (2D) electron fluid computer code solves Maxwell's curl equations and a set of electron fluid equations for transverse magnetic (TM) modes in air between parallel plane conductors. Collision frequencies for ionization and momentum and energy exchange to neutral molecules are taken to scale directly with neutral pressure. Electrode charging and initial electron deposition are considered to be instarnaneous. Calculations are performed for a Blumlein pulse generator geometry, featuring a charged rectangular center electrode between grounded parallel plates spaced 2 mm apart. In one mode of operation, initial electrons are induced in the lower air gap, the gap between the center electrode and the lower plate. At 292 kV on the center electrode, 27 atm pressure, and uniform ionization under the full width of the center electrode, induced voltage pulses of 300 kV, 2.4 ps rise time, 9.1 p5 full width at half maximum (FWHM), 38 ps duration, and 22 GHz bandwidth at 3 dB occur at the ends ofthe parallel plates. Reduction ofthe initial electronnumber over eleven orders ofmagnitude has very little gross effect on these pulses. Concentration ofthe initial electrons into a narrow, centered column generally results in reductions in rise time, FWHM, and pulse duration, as well as increases in peak voltage. Movement of the narrow column to the extreme left of the center electrode causes the voltage pulses at the left and right sides of the parallel plates to differ somewhat in their characteristics. In a second mode of operation, slightly delayed laser pulses initiate avalanches in both the upper and lower air gaps. Voltage pulses of reduced widths (1 .1—1.8 Ps FWHM) and amplitudes (7—51 kV) result from the interference of the opposite polarity electromagnetic waves generated in the two gaps.