Chalain's Science Corner:
Technology Behind the BH-209 Plasgun

(Go read about it now, I'll still be here when you get back.)

I read about this in the November 2000 issue of Scientific American... I can't believe how cool this thing is. But why am I bothering you with this? Simple: this is 21st Century plasma technology; they just don't know that 900 years from now the device will be handheld.

(I'm diddling with a java program to demonstrate how the rocket/plasma cannon works, but I'm having trouble with the animation. Not really. I'm having trouble finding time to fix the animation. If/when I get it working, I'll post it here, and it it'll help you visualize how the technology works. --Ed.)

The VASIMR rocket uses three magnetic coils to contain the plasma.

Inside the first coil is the helicon antenna. You inject liquid hydrogen (LH2) into the helicon, which begins pumping HUGE amounts of RF energy into the LH2. The LH2 gets ionized by this, and flings electrons and protons everywhere (ions galore). BINGO--all of a sudden you have a swirling vortex of particles that will obey an electromagnet: heavy protons near the center, electrons orbiting on the outside. They get pushed past the first magnet and into the space between it and the second magnet.

The second magnet is coupled with the first one; their magnetic fields bulge outward in the space between them making a sort of sack. We have another radio array in this section: the ICRH array. ICRH stands for Ion Cyclotron Resonance Heating. It's a fancy word that means "microwave," only it operates at the cyclotron frequency of the ions, which is "a few Megahertz" for the protons and "in the gigahertz range" for the electrons.

This is important: after coming out of the helicon (root word: helix!), this whole mess is spinning. FAST. How fast? You guessed it--at the cyclotron frequency. Those protons poke around a few million times per second, while the electron cloud jets around them.

When ions are in the middle of the bulging "sack" between the magnets, the magnetic field lines are parallel and the ions are left to circle unhindered. But if they stray towards one of the magnets, the field lines start to constrict, forcing the ions to orbit faster and faster, just like a penny dropped into one of those charity toilet things at the mall. The difference is, in a really cool quantum exchange thingy, the ion gets reflected back towards the other magnet, all without any "work" having been done on it by the magnet. In other words, by letting it swap out its forward momentum into and back out of rotational energy, it's 100% efficient. There's no newtonian force exchange. That means the reactor can hold a little plasma or a lot, without having to increase the magnet strength. All you need is "enough", and above that you can keep adding plasma to the chamber until the ions reach saturation and start bumping each other out of the magnetic fields.

Now pay close attention. There's a third magnet, the aft array. It's purpose is to carefully modulate the plasma flow coming out the back end. There's a thing in plasma physics called the Alfvén speed. It works in about the same way that the speed of sound works in a jet engine: if you don't "fix" it, your engine can't go faster than that speed, because the exiting exhaust will cavitate and actually "suck" your engine backwards.

So here's what I'm thinking: the BH-209 DOESN'T have this decoupling array. It creates high-speed plasma and purposefully lets it cavitate... in order to eliminate thrust! You still have to deal with thrust from the sub-Alfvén plasma, but carefully modulated turbulence from the BH-209 can actually reduce some of this thrust.

Here's the result of all this wacky science:

• The BH-209 is "recoil-clamped". Not quite recoilless; it generates a few pounds of thrust when firing. The sensation is not unlike starting up an industrial-strength hair dryer.
• This recoil-absorbing cavitation involves turbulence. This means that at the muzzle, you get lots of side "lobes" and "muzzle flash." And you thought it was just a neat special effect. Hah. That's SCIENCE, baby.
• The specific impulse of the weapon is probably very high. This is a rocketry term; it means the plasma comes out at HOLY CRAP speeds and GOOD LORD temperatures. (About 3 million MPH and 10 megakelvins.) The reason has to do with energy-to-force coupling. At those speeds and temperatures, the plasma stream is very poorly coupled to the rocket--er, weapon. This is a good thing. Imagine you're pinned against a wall by the front bumper of my Jeep. I'm going to start the engine, put the car in gear, and dump the clutch. Do you want me to put it in first gear, or fifth? In fifth gear, the engine won't be able to couple its energy efficiently into squishing you. Same effect with the plasma in atmosphere.
• That leaves the Newtonian problem of "how to we make it GO that fast?" The answer is in the spinning of the plasma (those cyclotron frequencies again). If the plasma bottle has a cyclotron two inches in diameter, the electrons are cycling at 1 billion PI inches per second, or 2.9 million MPH. (A 1" field ejects at 750,000MPH, but a 2" bottle ejects at 12 MegaMPH.) Since we're dealing with helical acceleration on stuff that has negligible mass, we're good to go.
• The muzzle magnet (ICRH) can vary the amout of ejecta released, so the notion of having variable settings is totally feasible.
• The BH-209 must constantly be maintaining a plasma field (whenever the safety is off). It probably takes 10-60 seconds to reach firing temperature, so you'd want to turn your gun on before going into combat. (And you'd want to turn it back OFF in a bar, Schlock.)
• I hypothesize that the BH-209 has two safety states: ON and ARMED. In "ON" mode, the weapon makes its trademark OMMMMINOUS HUMMMM and collects hydrogen and helium for making plasma. Charge time here is whatever you want. In the ARMED state, the weapon actually begins turning the gas into ions and heating it up for playtime.
• Below 100 kiloKelvins, we are not dealing with fusion. (We could be, especially at 10 megakelvins.) The weapon probably operates at or near 100 kiloKelvins, too cold for fusion. If you're squeamish about neutron radiation, go with a "cold" plasma gun.

New word: Charge Exchange. Charge exchange is NOT your friend. What happens is this: an unionized bit of hydrogen makes it into the plasma chamber, and gets hit by a high-temperature proton. The cold atom says, "oh here, take this" and gives away its electron. Now you have a cold proton (ion) in the chamber, and--oh dear--a hot atom that is now neutral (no longer ionized) and thus cannot be held onto by the magnets. Nasa's Franklin Chang Diaz notes, "The resulting hot neutral is oblivious to the magnetic field and escapes, depositing its energy on nearby structures."

Roll that phrase around in your mind: "depositing its energy on nearby structures."

I would posit that the control logic circuits of the BH-209M suffer from a defect that, in the presence of alcohol, causes the helicon array to inject cold neutrals prematurely into the plasma array without ionizing them. The result, due to runaway amounts charge exchange, is instant, spectacular overheating of the weapon. Please note that this voids your warranty, and your next of kin will not be entitled to replacement or refund.

Man, I love science.