History and Future of High Power Microwaves

High power microwaves ( $>$ 100-MW peak power, frequencies between 1 and 300 GHz) began emerging some 50 years ago as a new technology enabling new applications and offering innovative approaches to the existing applications. It is the result of the confluence of several historical trends: efforts to control thermonuclear fusion, development of intense electron beams, and plasma physics. The initial "golden age" of HPM, which lasted till the 1990s, had the character of a power derby, achieving $\sim$ 10 GW. This ended with the sobering realization that device development was stalling around 10-GW peak power and 1-kJ pulse energy. This was in part because of pulse shortening, a decrease in pulselength as peak power increases. The ultimate limits on HPM single-source peak power are still not well known; combining sources via phase locking is a way forward. Now, there is increasing interest in higher frequencies for directed energy weapons in the $X$ -band to Ka-band. During the Cold War era, the most comprehensive HPM research programs were in the United States and Russia, spreading in Europe to the U.K., France, Sweden, and Germany. China now has the world's largest HPM program, investigating most source types, including some innovative configurations, which combine source types. Particle-in-cell (PIC) codes have altered the basic paradigm of HPM source design. Future trends will be HPM weapons deployment (several new types of HPM weapons have emerged, including space-to space weapons, a new theater for HPM), multispectral sources, dispersion engineering, phase coherence used to combine two or more sources, increasing interest in higher frequencies, particularly the Ka-band, accelerating research on HPM amplifiers, and use of metamaterials and 3-D printing ("additive manufacturing"). Supporting technologies that are advancing are solid-state pulsed power drivers, linear transformer drivers, and compact pulsed power.