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/cm² 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 μs after the voltage is applied. The discharge also extinguishes within 10 μs of the voltage being turned off. For a 4 kV voltage pulse, plasma densities ⩽10¹³ cm^-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 μs, and burst mode operation at 10 kHz has been achieved with 30-70 μs 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° reflection) measured 4.3 m from the plasma mirror, is very similar to that obtained from a metal mirror of comparable dimensions