A compact, high-power 167-174.8 GHz travelling-wave tube amplifier for planetary boundary layer differential absorption radar

Our objective is to develop a 100 W traveling-wave tube (TWT) amplifier that is tunable from 167 to 174.8 GHz to enable spaceborne profiling of planetary boundary layer (PBL) water vapor in cloudy and precipitating volumes using differential absorption radar (DAR). Such measurements are inaccessible by any current remote sensing method, and directly address the PBL targeted observable outlined in the 2017 Decadal Survey, which called for technology incubation to enable profiling of PBL water vapor and temperature from space with 200 m vertical resolution. Global-scale, high-resolution measurements of PBL thermodynamics are essential in order to improve the representation of unresolved PBL processes in climate and weather models, with impacts to several high-value science questions including low-cloud climate feedbacks and severe weather predictability. The TWT amplifier is a key component in the envisioned architecture for a spaceborne DAR that is capable of sensitively detecting clouds in a variety of cloudy PBL scenes while satisfying the 200 m vertical resolution requirement for humidity profiles. Therefore, we propose to advance the Technology Readiness Level (TRL) from 2 to 4 of a TWT-based high-power amplifier module that meets the requirements for spaceborne DAR operation. This work builds on technology heritage at Northrop Grumman Mission Systems in compact, high-power vacuum source development at millimeter and submillimeter-wave frequencies, and at the Jet Propulsion Laboratory (JPL) in DAR instrument development and measurement validation through the Vapor In-cloud Profiling Radar (VIPR, IIP-16 and AITT-19) project. The TWT-based high-power amplifier proposed here represents a state-of-the-art, compact solution to the problem of efficient high-power generation at unconventionally high radar frequencies. The TWT design will be based on a proven narrowband component at 233 GHz developed for a DARPA-initiated airborne radar program, but under this effort significant innovations will be incorporated into the source design to make it specifically relevant to deployment in an orbital DAR system. Relative to previous G-band high-power amplifier modules, this subsystem will feature a 10x reduction in both volume and mass, achieved by using a periodic permanent magnet instead of a solenoid to confine the vacuum source electron beam; the TWT circuit design will have higher gain to ensure 100 W operation despite lower available input power levels from solid-state 167-174.8 GHz drive sources; and the cathode high-voltage supply will be able to switch at the microsecond time scale between two states in order to tune the TWT center frequency between the online (174.8 GHz) and offline (167 GHz) DAR frequencies at a rate much faster than the radar pulse repetition frequency. During the first two years of this three-year effort, multiple TWT amplifier prototypes will be designed, fabricated, and evaluated to ensure that the final integrated module meets the performance requirements of 100 W output power, 25% duty cycle, 5% tuning bandwidth, and 10 kHz frequency-switching speed. In the final year, the VIPR system will serve as a testbed for verifying the TWT performance in realistic atmospheric measurement scenarios and for demonstrating its in-cloud profiling capability.

Accelerate mission development and reduce risk by developing critical components and subsystems for advanced instruments and observing systems

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