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Historical and pop-culture perspective
The "Death Ray" was a comic book staple in the 1920s, spreading the notion that those crafty Germans (for example) would soon be able to point a "ray gun" at people and dust them to oblivion with a beam of energy. Yikes!
In the 1930s, England offered a £1000 prize to whoever could kill a sheep at 100 yards with electro-magnetic energy. No sheep came to any harm. Robert Watson-Watt himself calculated that with available RF power/frequency and practical antenna gain, the idea was impossible. The experiment paved the way for the Chain Home radar system, which protected England during the Battle of Britain. The Brits lost a staggering 1250 aircraft, but the Germans lost even more (1700). Never was so much owed by so many to so few, as Churchill pointed out. Learn more about Chain Home from Professor Thomas Withington, in his "Bandits at Three O'Clock!" webinar presented to the Association of Old Crows. Cough up a few bucks and join this group, and tell your employer to reimburse you if you are a cheapskate. Good luck.
By 1936, ray guns were all that was needed to keep Emperor Ming and the Sea Beasts in their place. Dale Arden, you go, girl!
Flash Gordon 1936
By the 1960s, every kid was gonna want one of these...
Raygun toy from the 1960s
And of course, who is not familiar with the Star Trek phrase "Phasors on Stun?"
What phasors do
Modern active denial
Active denial is a means for inflicting pain without permanent tissue damage, to stop bad guys from doing bad things. The active denial system (ADS) idea has been around for a hundred years, but a workable concept has only been around during the 21st century. If Watson-Watt could have even imagined the ability to generate 100kW power at 95 GHz, he never would have said directed energy was impossible. Collimate a beam of 95 GHz power, and shine it one someone you want to feel pain. Simple? Anything but...
Why 95 GHz? First, it is a frequency that has relatively low atmospheric absorption. And second, 95 GHz penetrates your skin to right around where pain nerves are, a depth of about 1/64 of an inch (0.5mm). Cook a potato? use 2.45 GHz. Toast a cheese bagel? 95 GHz could do that.
Two early proponents fielding this idea were Sandia National Labs, and The Lazy R Ranch. Later, Air Force Research Labs took the lead. Who could have predicted that the biggest private proponent of active denial has the word "ray" in the company name? That's something you'd expect in a novel, not real life.
Here's info on some of the first ADS work, which included a gyrotron supplied by Communications Power Industries (Palo Alto, previously Varian), described in a 2004 paper. You can't plug this into a normal home wall outlet, you'll probably need 480 VAC and a hundred amperes of current. And your power supply never gets a rest, as you have to cool the tube 24/7. By 2010, 1MW was reported on an AFRL contract. By 2011, 2MW had been achieved. $6M was spent to deliver just two of these tubes. Ray-o-the-Gods added a truck, a large fuel tank, a turret and a reflector and the modern ADS system was born.
100 kW Gyrotron
A version of ADS was fielded and sent to Afghanistan, where presumably the citizens did not have adequate legal representation if they were put in harm's way. It was sent back a few months and never deployed. Maybe it broke during shipment. Remind yourself of all the other war materiel that was left behind. What would the Taliban do with an active denial system anyway?
The "safety" of ADS depends on turning it off after a quick jolt. If you hot-wired the kill switch, you would have a means of frying people alive. Even without modification, you could tie someone in front of it and blast them repeatedly until skin starts to peel off. Proponents would say, there are way cheaper ways to burn people, like a Bic lighter, so this would never happen. We can all take comfort in that assessment. Did you know the founder of Bic was Marcel Bich? His full name did not translate well into English.
In spite of presumably intense lobbying, the military never procured any ADS systems. If this is such a great weapon, why was it not fielded? It's not man-portable, and if you are in the army, you have many other options for neutralizing a threat a little more permanently. Plus, there's a lot of stigma associated with the US using an industrial-sized pain ray in the third world. Also, it's kind of large... and large generally does not translate to affordable.
Original ADS
Next, the system was shrunk to 30kW so it could be mounted on a smaller truck (or ship or ...). It was called Silent Guardian, and it weighed 10,000 lbs so it will not fit on your Ford Ranger. That name is kind of like the intercontinental ballistic missiles with multiple nuclear warheads which were named "Peacekeepers". Not a single Silent Guardian system was sold. With lower power, it takes longer to cook skin... 250 seconds was what the marketting people came up with.
Silent Guardian
Later, Uncle Ray marketed a joystick version of ADS which almost resulted in a sale to a Los Angeles prison, for use on prisoners in their cafeteria. Here's an article entitled "Pain Ray, Rejected by the Military, Ready to Blast L.A. Prisoners" which should put the subject in some perspective. The deal was never closed. Possibly the root cause of prison fights is overcrowding, which is unavoidable when a country locks down 2M people and allows for-profit companies to run the jails. Brown jello for dinner, again?
Most recently, there were discussions about using the technology for crowd control in Washington DC, during the Black Lives Matter Protest Summer of 2020. But the National Guard does not have such a system so it was all a moot point. Lawyers were probably salivating at the prospect of representing all that pain and suffering that would have ensued. Can you imagine your neighborhood hippies with their petitions against puny 5G towers, when they hear about 100kW being actually aimed at people?
In a total coincidence, the US Secretary of Defense at the time of demonstrations was Mark Esper, who previously was a lobbyist representing Big Red. Among other career highlights, he failed to get the DOD to buy the Silent Guardian. Why do we keep using euphemisms for a major defense contractor? A long time ago one of their lawyers told us not to post their good name on this unprofessional website. There is a lot more to the story, but it ends like this: Pfft! (definitely click that link!)
Solid State ADS
Gyrotrons can put out a lot of power efficiently, but you have to keep them on all the time (not "warmed up", more like "cooled down"). What if you only need a few seconds of pain in a given day? The efficiency of such a system is ridiculously low.
GaN power amplifiers were emerging right around the time that ADS was. No one knew how high in frequency the technology could go. But it turns out that 95 GHz was possible by the usual methods of shrinking or minimizing critical device dimensions, access resistance, and parasitic capacitance. By 2010, a publication showed 2W power amplifier and a 1.2W amplifier with 20% power added efficiency. Consider that all this was done on an experimental process at a fab in Andover Massachusetts without verified large signal models (or perhaps even small signal models!) The designs were done at a facility in Rancho Cucamonga, which was closed a few years back. Would it still be open if ADS was a thriving business? Who knows.
2W GaN power amp
2W GaN power amp results
For now, 2W seems to be the end of the line for a single chip at 95 GHz, additional power comes from power combining. If you need kilowatts from a 2W device, wired power combiners will eat your lunch. A path forward was found in spatial combining. In a hold-my-beer moment Uncle Ray's Rest Camp combined ~8000, "1W+" MMIC chips together to net 7000W at 95 GHz, which is by far a record for a solid-state power amplifier. This was reported in 2016. The aperture measures 25.6 x 25.6 inches, the weight of it was not reported. However, there is a picture of it being moved with a chain hoist. This is not gonna fit in your grandfather's death ray.
7kW 95 GHz amplifier
Conclusion
The pain ray works, but it is a long, long way be the hand-held version that comic-book readers and Trekkies have long imagined. Twenty years of active denial work has not resulted in a single system sale.
The technologies that have been developed for ADS certainly have value in many other systems. No careers were killed working on ADS, as you can always say "I told you someone was going to need GaN power amplifiers for E-band radio!
Epilog
The principal investigator for much of ADS development was one of the most talented microwave engineers on the planet, Dr. Kennith (Ken) W. Brown. He worked on the tube version and also developed the solid-state 7kW amplifier and its GaN PA MMICs. We have nothing but respect for his incredible contributions to active denial and many other defense programs. Ken died in an unfortunate car accident in 2019. His LinkIn profile lives on... a lesson to all of us, to leave behind passwords for cleaning up our digital DNA when eternity calls.
References
K. Felch et al., "Demonstration of a 95 GHz, 100 kW, CW gyrotron oscillator," Fifth IEEE International Vacuum Electronics Conference (IEEE Cat. No.04EX786), 2004, pp. 63-64, doi: 10.1109/IVELEC.2004.1316199.
M. Blank, P. Borchard, P. Cahalan, S. Cauffman and K. Felch, "Experimental demonstration of multi-megawatt 95 GHz gyrotron," 2010 Abstracts IEEE International Conference on Plasma Science, 2010, pp. 1-1, doi: 10.1109/PLASMA.2010.5534059.
S. Cauffman, M. Blank, P. Borchard, P. Cahalan and K. Felch, "Recent tests on a multi-megawatt 95 GHz GYROTRON," 2011 Abstracts IEEE International Conference on Plasma Science, 2011, pp. 1-1, doi: 10.1109/PLASMA.2011.5992993.
A. Brown, K. Brown, J. Chen, K. C. Hwang, N. Kolias and R. Scott, "W-band GaN power amplifier MMICs," 2011 IEEE MTT-S International Microwave Symposium, 2011, pp. 1-4, doi: 10.1109/MWSYM.2011.5972571.
K. W. Brown, D. M. Gritters and H. Kazemi, "W- and G-Band Solid State Power Combining," 2015 IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS), 2015, pp. 1-4, doi: 10.1109/CSICS.2015.7314529.
K. Brown et al., "7kW GaN W-band transmitter," 2016 IEEE MTT-S International Microwave Symposium (IMS), 2016, pp. 1-3, doi: 10.1109/MWSYM.2016.7540034.