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I wanna make a microwave gun for futuristic infantry that's both smaller than a rifle and able to output a megawatt's worth of power for a sustained period of a few seconds. Is this possible in a gun the size of a large revolver or a sawed-off shotgun, or would I need to make the laser's components out of adamantium in order to handle the waste heat? Would I be better off making a laser with a tighter beam for killing? And if so, could a less-powerful but more focused laser still make people's heads and limbs boil and explode?

For the sake of clarity, the gun is powered by a high-energy density nanobattery charged by magnetic induction through the tiny piezoelectric motors and solar cells woven into the user's clothing or artificial skin. The battery has a density of about x100 current lithium-ion batteries, so if the battery itself weighed a kilogram, it could fire off nearly 90-100 full-charged shots before being depleted.

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    $\begingroup$ Let's suppose that the maser is 95% efficient at converting electric power into collimated microwaves; this means that the gun dissipates as heat 5% of 1 MW, or 50 kW. I would want to be the soldier ordered to hold in his hand a 50 kW heat source for "a sustained period of a few seconds". $\endgroup$ – AlexP Sep 21 '17 at 22:06
  • $\begingroup$ Okay so dialing it down a bit is probably the smarter thing to do then lol. $\endgroup$ – Z.Schroeder Sep 21 '17 at 22:17
  • $\begingroup$ You probably should skip the charging of the battery. If I fired that laser for 5 seconds, and we assumed its efficiency was near 100%, that's 5MJ of energy. Recharging that means I need to pull 5MJ of energy from the user. A skilled cyclist can put out about 250W, so it'll take about 5.5 hours of endurance-cycling level effort to charge a single shot, much less 100. The solar cells might be able to do better, if they were perfectly efficient you might get 2 or 3kW. At 3kW, it only takes about 30 minutes to recharge a single shot from solar power. $\endgroup$ – Cort Ammon Sep 21 '17 at 22:33
  • $\begingroup$ For some sense of scale, we have developed chemical lasers which operate on the megawatt scale for a few seconds. Here's one of them. Oh, and your real enemy is retroreflectors. Them be bad mojo! $\endgroup$ – Cort Ammon Sep 21 '17 at 22:37
  • $\begingroup$ So could I make it a 50-100kW handheld laser and still explode people's heads? Or is exploding body-parts a little too much for handheld lasers? $\endgroup$ – Z.Schroeder Sep 21 '17 at 23:00
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Let's Get Real

Many users on this site get a bit wound up in the physics. You're writing a story. You do not need a scientifically accurate solution. You might not even need a scientifically plausible solution. What you need is a solution with enough science supporting it to allow readers to suspend their disbelief. So, really, what do you need?

Most people don't know about energy density

But they do know about batteries, and increasingly, they know about solar panels. They'll get that you can't run the gun even with the suit being all solar panel, but they'll wonder about what happens when the suit gets all torn up, or dirty, or the skies get cloudy (especially when things that go "boom!" are pushing smoke and debris into the air). It's a tough sell. On the other hand, let's make the gun battery a backback battery and to recharge it I have one word for you, just one word... piezoelectricity.

Piezoelectricity is electricity derived from materials that are under stress. Simplistically, a material is squeezed, and electricity is the result. Put the plates in the soles of the soldier's shoes! Make the flak jacket out of it! And the helmet! Why? Because no war would be fought with only beam weapons any more than it would be fought with only infantry. A variety of weapons are needed, including projectile weapons. Besides, soldiers are suseptible to flying debris ... and every object that hits them helps recharge the gun.

Combine this with a battery of Clarkian nature and we have it made.

The Problem with Heat

You're going to need to deal with the heat issue. I have a phrase I've coined: "technology dichotomy." It describes the tendency for writers to have time machines without their society having first invented the wheel. In your case, the problem is an advanced energy society that will obviously be able to pinpoint a heat source from orbit.

A heat source that makes grandpa's Yuletide fire look like a snuffed-out match.

The most obvious issue here is the heat due to inefficiency, which has already been pointed out by others in answers and comments. The RPG Traveller had the concept of a "cold cannister." (At least it did a million years ago when I was playing it....) It attached to sealed combat armor (like Battledress) and absorbed heat (body & equipment). This seriously reduced the thermal signature of the soldier. When the can was consumed, it was replaced. You're going to need something along those lines... a way to absorb heat or convert it to something more benign.

Enter the thermoelectric generator: a solid-state chunk of awsome that converts thermal gradients into electricty. Add a solid dose of completely believable efficiency and you have dumped a ton of heat. And it will help charge the gun!

But this isn't the only problem with heat: you're pushing a bazzillion-watt whoopin' toward the other dude. That's going to draw a wonderful line of heat with a big ole' arrow at your end that says "kill this guy right here."

And frankly, I've got nothing for this one. If you'll excuse a bit of hyperbole: the air will turn to plasma. It will glow like the Northern Lights. It's actually a really good reason why directed energy weapons may never be popularly used. You could throw a bunch of heat generators out onto the battlefield to try and raise the ambient temperature, which would hide your beams... but you'd lose the temperature differential to get that extra electricity to keep the gun working.

This issue might be insurmountable. Which means you need a distraction. Like mortars sending super-heated BBs into the field. Won't do a lot of damage themselves, but will create a criss-cross of image-confusing thermal lines that might, just might, confuse enemy computers and keep you alive to roast the next guy in line. Yeah.... distractions.

Let those limbs explode!

In reality, a focused energy weapon that had enough energy behind it to kill a person wouldn't boil or explode a limb or a head. Like industrial lasers, they'll cut, not boil. Boiling occurs when heat has the time to propagate through the material, raising the temperature of (e.g.) the pot of water. Apply a high-energy beam and you cut through the water... and the pot... and the stove... because all that energy is in one spot at one moment rather than a lot of spots for a long time.

However, if you're talking about emitting microwaves... your microwave oven heats food by heating water (I'm being a bit simplistic, but it's basically what happens). Pass a high-energy microwave through a human and the water vaporizes. "Boil" isn't the correct word. Nothing would boil... but the jets of steam bursting from his arm and the gooey explosion of his eye when the aqueous humor instantly vaporizes would be satisfying to any connoisseur of B-grade drive-in horror movies!

And by the end of the story, people might not even question the science behind your madness artistry.

Because the goal isn't scientific accuracy... it's suspension of disbelief. All that needs is a good premise.

Bonne Chance!

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Well, there are 3600 seconds in an hour. Let's assume that a few seconds is 3 to make the math easy. So 1MW for 3 seconds is the same as 833 watts for an hour. I found a 1200 watt-hour battery that weight 42 pounds, but it had some additional stuff. Assume we get a really good one down to 30 pounds.

So you are going to have a very heavy weapon to cart around and per the comment the shooter is going to be dealing with 50kw of heat energy. I don't think you are going to get very many volunteers.

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