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Schemes have been suggested for transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high-power microwave (HPM) beams. All use power beaming. Microwave beams have been studied for propelling spacecraft for launch to orbit, orbit raising, launch from orbit into interplanetary and interstellar space, and deployment of large space structures. The microwave thermal rocket, called the 'microwave thermal thruster,' is a reusable single-stage vehicle that uses an HPM beam to provide power to a heat-exchanger propulsion system, with double the specific impulse of conventional rockets. Orbital missions include orbit raising and space solar power. Microwave-propelled sails are a new class of spacecraft that promises to revolutionize future space probes. Experiments and simulations have verified that sails riding beams can be stable on the beam for conical sail shapes. Beam-driven sail flights have now demonstrated the basic features of the beam-driven propulsion. Beams can also carry angular momentum and communicate it to a sail to help control it in flight. An early mission for microwave space propulsion is dramatically shortening the time needed for sails to escape Earth's orbit. A number of missions for beam-driven sails have been quantified for high-velocity mapping of the outer solar system, Kuiper Belt, the Heliopause, and the penultimate interstellar precursor mission. For large HPM systems at fixed effective isotropic radiated power, minimum capital cost is achieved when the cost is equally divided between antenna gain and radiated power. This is a driver when considering design of power-beaming systems such as interstellar Beacons, which the Search for Extraterrestrial Intelligence is searching for. Much of the technical means for these applications are already in hand. Microwave and millimeter-wave array antennas are already in use for astronomy; sources at high frequencies are being developed for fusion and the military. Development of high-power arrays is needed. A synergistic way to develop a space power-beaming infrastructure is incremental buildup, addressing lower power applications first, and then upgrading. Index Terms--Microwave antenna arrays, microwave oscillators, microwave power amplifiers, microwave power transmission, Search for Extraterrestrial Intelligence (SETI), space solar power (SSP), space-vehicle propulsion.

A model to account for the shortening of the relativistic magnetron microwave pulse length based on radial cathode plasma motion is put forth and corroborated with measurements. If the dense, conducting, cathode plasma (and not the cathode itself) sets the diode electrostatic and microwave boundary conditions, its motion changes the magnetron resonance condition. Time varying microwave power envelopes and frequencies are both expected and observed. based on the cathode plasma expansion rate, the main plasma ion constituent is inferred to be hydrogen. A magnetron experiment to defeat the pulse-shortening mechanism by heating the resonator successfully lengthened the pulse from 100 to 175 ns. Index Terms - Diode plasma motion, high power microwave, pulse shortening, low-Z contaminants, relativistic magnetron.

The Air Force Research Laboratory/Phillips Laboratory magnetically insulated transmission line oscillator (MILO) is a gigawatt-class, L-band, high-power microwave tube driven by a 500-kV, 60-kA electron beam. A previous version of this tube [1] generated 1.5 GW pulses, but with significant RF pulse shortening. This paper reports on improvements to the tube that have allowed us to increase the output power by 25% and to increase the RF pulse duration by a factor of two and a half. Index Terms - Crossed-field device, high-power microwave tube, magnetically insulated transmission line oscillator, MILO.

Microwave generation in devices that depend on synchronization between an electron beam and a resonant cavity or slow wave structure can be disrupted by changes in either. Explosive-emission-driven microwave sources use plasma as the electron source in the diode. This plasma is conductive enough to act as the boundary for both the applied diode voltage and the microwave electric field. The motion of this plasma can effectively change the dimensions of either the electron beam diode or the cavity and will thereby cause resonance destruction. This shortens the microwave pulse length [[Tau].sub.[Mu]]. A general model of the process predicts that, for a Child-Langmuir diode, microwave power will fall as [Mathematical Expression Omitted] and that pulse energy will fall as E [Mathematical Expression Omitted]. Therefore, energy efficiency declines as the pulse length is extended. We compare with data from magnetrons, MILO's and BWO's, and find that over some regions of operation the pulse length and energy from these explosive-emission-driven microwave sources agree with the plasma motion model scaling. At these applied drive voltages and output powers the microwave pulse length can be increased by finding cathode materials that generate slower plasmas. Index Terms - Child-Langmuir, diode plasma motion, high power microwave, pulse shortening.

Generation of microwaves at high power has progressed largely on single shot devices. Applications, however, require high average power, implying operation at substantial repetition rates. This is in a new domain for microwave devices. The technical challenge is to achieve very high electric fields in the source on a repetitive basis without breakdown. We describe experiments on CLIA, a Compact Linear Induction Accelerator, capable of generating 750 kV and 10 kA into a matched load using magnetic switching to produce 60 ns long pulses. As a first application we used CLIA to drive a water-cooled L-band magnetron at repetition rates as high as 250 Hz with no breakdown or pulse shortening. This gave 6.3 kW average power. A short burst at 1 kHz demonstrated operation that would translate to an average power of [is nearly equal] 25 kW. In this regime operation is not limited by gas build-up, electrode erosion or microwhisker depletion. We are now operating on CLIA with a high current relativistic klystron. Beams with modulated current powers of [is nearly equal] 1 GW have been generated at 100 Hz for bursts as long as 5000 pulses. There are no apparent obstacles to much higher average powers at higher peak powers.

CLIA (Compact Linear Induction Accelerator) is a 750 kV, 10 kA pulse-power generator using magnetic switching to produce 60-ns-long pulses at 200 Hz (90 kW average power) for longer than 1 s. As a first application, we used CLIA to drive an L-band magnetron. We observed that the magnetron was capable of consistent operation at repetition rates as high as 250 Hz with no breakdown or pulse-shortening induced by gas buildup or electrode erosion. As the repetition rate exceeded 100 Hz, the average power increased even as the peak power decreased, producing 1 GW peak power and 4.4 kW average power at 100 Hz and 600 MW peak and 6.3 kW average at 250 Hz. A short burst at a 1 kHz repetition rate indicated the possibility of 25 kW average microwave power operation.