atomic_fungus (atomic_fungus) wrote,

#1506: How a laser works.

No, this isn't a basic physics primer. If you want that, go do a search; I'm talking about something a little more esoteric.

Here is why:
According to Thomas Friedman, the author of a March 14, 2009 New York Times piece titled The Next Really Cool Thing, the NIF has already taken a major step on their planned path - they fired all 192 lasers at the same time at an empty chamber. That experiment occurred at 3:00 on Monday, March 9. I am not positive, but I am pretty sure that the time was carefully chosen based on electrical power availability - a story in the Sunday Times indicates that the combined power of the 192 beams is 500 trillion watts, 1,000 times the power of the US national grid. Obviously, there are some large capacitors involved that can be charged up to allow that kind of instantaneous power blast.
He is not entirely correct.

There are, in fact, large capacitors which are charged up, and which are then discharged very quickly, to provide the power needed by the lasers.

But there is a catch: the laser pulse is very, very short, on the order of 20 nanoseconds. (Two "shakes", in nuclear weapons parlance.)

Take the power that the average light bulb--say, a 100-watt bulb--emits in an hour: 100 watt-hours. Now dump it all in a second. 100 watt-hours becomes 360 watt-seconds.

The same thing happens with high-energy lasers. You take a moderately large amount of power--industrial levels--and dump it ina very short span of time. 10,000 kilowatt-hours turn into 36,000,000 kilowatt-seconds, and the shorter the time you spend using that power, the greater it goes: in half a second, 72,000,000 kilowatts. In a tenth of a second, 360,000,000 kilowatts.

In 10 nanoseconds, 3,600,000,000,000,000 kilowatts.

It's something like pressure: one pound per square inch won't cause you any trouble if it's actually spread over one square inch of skin...but if you concentrate that much pressure in, say, the point of a hypodermic needle, it has no trouble puncturing your skin at all. And it will calculate out to many thousands of PSI.

It's not a case of inventing power from some magical place; it's just that the power is expended much faster. Power consumption is a function of draw versus time, hence the hyphenated "kilowatt-hour".

The articles referenced by that blog post use watts as the unit of energy because it's a word everyone is familair with. But physicists will use joules in that context, not watts. (One kilowatt-hour is 3,600,000 joules of energy.)

In fact, in this context, saying that the lasers will use "500 trillion watts" is essentially meaningless. It's like saying your car is fast enough to go 500 miles; it says nothing about how much time is required. Again, most people are used to hearing "watts" used as a measure of power: this hair dryer is a 1500-watt hair dryer; this microwave oven is an 800-watt oven; this light bulb is a 100-watt light bulb. But that's a measure of the device's output, not its power consumption--for that, you've got to factor in time. So a 100-watt light bulb uses 0.1 kilowatt-hour in an hour's time. It uses 0.025 kilowatt-hour in 15 minutes. It uses 1 kilowatt-hour in 10 hours. But the entire time it's on, the light emits 100 watts of energy. (Mostly heat if it's an incandescent bulb--99% of its output, in fact, is heat.)

I can't blame the layman for not understanding this. There is a difference between power and energy; one is measured in watts and the other is measured in watt-hours. (Or -seconds or -minutes or -years or -centuries, whatever unit of time is most convenient.)

Anyway, in the laser field, there are at least two things used to bump the power output of a laser. One is called the mode lock; the other is the Q switch. Both of these techniques are used to increase the output amplitude of a given laser.

Please note that I am not an expert in high-energy physics. I know enough about lasers to build one from scratch; that's basic physics. Given the right materials, parts, tools, and time, I could build a bench-mounted gas laser. But that's where my knowledge trails off.

I know enough to realize that a "500 trillion watt" laser doesn't actually consume 500 trillion watt-hours or -seconds of power, though.

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