Electronically steerable plasma mirror for radar applications

An alternative to using a phased array to steer a high frequency microwave beam is to electronically control the orientation of an inertialess broadband microwave reflector. Such a system could steer one or more high power beams for search or tracking radars, and the system could possess wide instantaneous bandwidth. Experiments have demonstrated that a planar plasma mirror can be formed with electron densities high enough to reflect X-band microwaves. Estimates of microwave beam power handling capabilities of such a plasma Sheet exceed 3 kW/cm/sup 2/ for microsecond duration pulses at kilohertz repetition rates. The plasma minor is formed by driving a glow discharge between a hollow cathode and a flat plate anode which are immersed in an axial magnetic field of 150-300 Gauss. The plasma sheet could in principle be steered in one plane using the magnetic field, and in another by designation of the cathode. A 50 cm wide, 1 cm thick, 60 cm long plasma sheet is formed between the cathode and the anode 10 /spl mu/s after the voltage is applied. The discharge also extinguishes within 10 /spl mu/s of the voltage being turned off. For a 4 kV voltage pulse, plasma densities /spl les/10/sup 13/ cm/sup -3/ are achieved within the plasma sheet. The discharge gas is air at 130 mTorr. Single mirrors have been formed with pulse widths of 20-1000 /spl mu/s, and burst mode operation at 10 kHz has been achieved with 30-70 /spl mu/s long pulses. Measurements of the phase using a heterodyned microwave interferometer show that the critical surface is stationary during most of the pulse. A 30 cm diameter Cutler-feed antenna is being used to illuminate the mirror at 10 GHz. The H-plane radiation pattern (for a 90/spl deg/ reflection) measured 4.3 m from the plasma mirror, is very similar to that obtained from a metal mirror of comparable dimensions.