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 ldquomicrowave thermal thruster,rdquo 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.

Space Applications of High-Power Microwaves

Experiment on a new type of high-power, millimeter wave amplifier, (a harmonic-multiplying, inverted gyrotwystron), are reported. Superior stability resulted from two factors: (1) interaction between a relatively low order waveguide mode (TE/sub 22/) and the beam wave at the fundamental cyclotron frequency in the traveling wave input section; (2) an internal mode filter in the highly overmoded (TE/sub 42p/), second harmonic output cavity. Bandwidth was 1.3% with saturated gain of 33 dB around 31.8 GHz. The gain-band width performance represents a significant advance for gyrotron amplifiers operating in such high order modes, and is thought to be due in part to operation of the long output cavity in several modes with different axial eigennumber (p=3, 4, 5) but with overlapping spectral regions.

Operation of a high performance, harmonic-multiplying, inverted gyrotwystron

Summary form only given. A variety of schemes have been suggested for transferring energy from Earth-to-space, space-to-Earth, and space-to-space using high power microwave beams. All use power beaming, scaling of which will be summarized. This talk summarizes a recent review. Microwave beams have been studied for propelling spacecraft for launch to orbit, launch from orbit into interplanetary and interstellar space and deployment of large space structures. The microwave thermal rocket is a reusable single stage vehicle that uses a high power microwave beam to provide power to a heat-exchanger type propulsion system, called the microwave thermal thruster with double the specific impulse of conventional rockets. It could transform the economics of launch to space. The principle is analogous to nuclear thermal thrusters, which have experimentally demonstrated specific impulses exceeding 850 seconds. The cost of such large HPM systems is driven by two elements-the capital cost, including microwave source cost and radiating aperture cost, and the operating cost, the cost of the electricity to drive the system. Cost must also account for the learning curve, the decrease in unit cost of hardware with increasing production. For fixed effective isotropic radiated power, minimum capital cost is achieved when the cost is equally divided between antenna gain and radiated power. Microwave propelled sails are a new class of spacecraft that promises to revolutionize future space probes. Beam-driven sail flights have now demonstrated the basic features of the beam-driven propulsion. An early mission for microwave space propulsion is dramatically shortening the time needed for sails to escape earth orbit. Simulations of trajectories and escape time for sails driven by a microwave beam from the surface show that resonance methods can reduce escape times from earth orbit by over two orders of magnitude. A number of missions for beam-driven sails have been quantified: high velocity mapping of the outer solar system, Kuiper Belt, Plutinos, Pluto and the Heliopause, and the interstellar medium. The penultimate is the interstellar precursor mission. For this mission class, operating at high acceleration the sail size can be reduced to less than 100 m and accelerating power ~100 MW focused on the sail. At 1GW, sail size extends to 200 m and super-light probes reach velocities of 250 km/s for very fast missions. Will such sails riding beams be stable? Experiments have verified that beam-riding does occur. Beams can also carry angular momentum and communicate it to a sail to help control it in flight. Although technical feasibility is the focus, advocates must also deal with societal issues: interference with unintended targets in the sidelobes, spectrum allocation and potential weaponization.

Space Applications of High Power Microwaves

The inverted gyrotwystron (phigtron) is a millimeter wave frequency-doubling amplifier that has been demonstrated to produce over 300 kW peak power at twice the input frequency (centered at f/sub in/=16.85 GHz and f/sub out/=33.7 GHz) over a 0.5% bandwidth with a saturated gain of 30 dB and efficiency greater than 35%. The device has also been studied both theoretically and experimentally in a different operating regime where frequency-doubled, phase-locked oscillation is possible. A signal, injected via a fundamental gyro-traveling wave tube input section, modulated a 55 kV, 10 A electron beam. After transit through a drift section, the prebunched electron beam produced phase-locked, second harmonic oscillations in a TE/sub 03/ mode output cavity. RF output centered at either of two frequencies, 34.42 and 34.62 GHz, with a maximum output power of 180 kW, an efficiency of 32% and a locked signal gain of 35 dB was measured. A theoretical prediction of locking bandwidth, a design overview, and the experimental results are presented followed by a summary and discussion of the results.

High efficiency, phase-locked operation of the harmonic-multiplying inverted gyrotwystron oscillator

The dependence of output signal phase on the electron beam accelerating voltage and guiding magnetic field has been studied in a frequency-doubling inverted gyrotwystron (phigtron). The phigtron is capable of producing 720 kW of peak power, centered at 33.75 GHz with a bandwidth of 0.7%, saturated gain of 30 dB and an electronic efficiency of 35%. This paper is motivated by the need for compact millimeter wave final amplifiers for systems such as high-resolution coherent radar and high-gradient linear accelerators, in other words, applications requiring very stable radio frequency (RF) phase. Previous studies of the phigtron concentrated on the operating parameters gain efficiency and bandwidth. Here, we investigate the phase characteristics of the frequency-doubling gain mechanism and the sensitivity of the phase to fluctuations in the external operating parameters. RF drive power is injected into a TE/sub 02/ gyro-TWT section to modulate a 51.2 keV, 20 A electron beam. In the output cavity, the prebunched beam excites the TE/sub 03/ mode at twice the input frequency near the second harmonic of the cyclotron frequency. Fluctuations in the accelerating voltage, guiding magnetic field, and RF phase were measured and analyzed for correlation. The results show that phase deviation is well correlated with fluctuations in pulse voltage and that straightforward improvements in the stability of power supplies would significantly reduce the measured phase deviation. Furthermore, the frequency-doubling mechanism employed in the phigtron is demonstrated to produce good gain near the second harmonic of the cyclotron frequency with low phase deviation.

Experimental study of phase deviation and pushing in a frequency doubling, second harmonic gyro-amplifier

Phase-locking in a 34.5-GHz special complex cavity gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency was studied. Injection of the locking power was made via a quasi-optical circulator connected to the gyrotron output. Locking bandwidth was measured by comparing the phase of the injection signal and output signal using a balanced mixer. Locking was observed with input power level as low as 40 dB below the gyrotron output power. The locking bandwidth is, however, narrower than in gyrotrons operating at the fundamental cyclotron frequency which may be attributed to the longer resonant cavity in the second harmonic gyrotron and the corresponding larger value of external quality factor. The measurements are roughly in agreement with predictions of Adler's phase-locking equation which is given for our system in terms of powers propagating in the output waveguide toward and away from the gyrotron cavity. >

Phase-locking of a second-harmonic gyrotron oscillator using a quasi-optical circulator to separate injection and output signals

A beam waveguide (BWG) design suitable for high-power applications is described. The design features a transmit-only, four-port high-gain horn as input to a BWG system with a single parabolic mirror and three flat plates. The use of a single parabolic mirror is such that the highest field concentration is no greater than that caused by the horn itself. The horn is linearly polarized and a grid reflector is used to reflect the orthogonal polarization into the receive feed. A rotatable dual polarizer provides for arbitrary transmit polarization. The dual-reflector system is shaped to provide uniform illumination over the main reflector and therefore maximum gain for the given size aperture. Measured data from a scale model BWG system are presented.

B. L. Conroy

Papers

Phase-locking of a second-harmonic gyrotron oscillator using a quasi-optical circulator to separate injection and output signals

A beamwaveguide design for high-power applications