typodupeerror
• The example is stupid... (Score:3, Insightful)

on Wednesday February 14, 2007 @06:54PM (#18017626)
'Imagine a car that accelerates from zero to sixty in 250 feet, and then rockets to 120 miles per hour in just one more inch. First of all at non relativisitc speeds, doubling the speed results in a four fold increase in kinetic energy and not a doubling. Why give a bad classical mechanics analogy and then tell us that the speed didn't actually double because of relativistic effects.
• So let's try this: (Score:2)

Imagine a bunch of cars that accelerate from zero to sixty in 250 feet, then slam into a barrier, causing a multi-car pileup from which, starting just one inch further down the road, one of them rockets out at 85 miles per hour.

(Obviously they were inspired by the traffic on Interstate 280 on their way to SLAC. B-) )
• I actually work on this at USC!!! (Score:5, Informative)

on Wednesday February 14, 2007 @07:01PM (#18017692)
I actually do some work on this with the PWFA group at USC (i'm an undergrad research assistant). It really is amazing! We can reach acceleration gradients of around 60 GeV/m, compared to something like 40 MeV/m for a normal accelerator. It works like this:
1. The electrons travel down the main linac in carefully spaced "bunches", and get accelerated to around 43 GeV over a course of ~3KM (this is at the main beam at SLAC).
2. A (in the last experiment) 1.2m long Lithium plasma "oven" is at the end of the beam, which the electrons are directed into.
3. The first, or "driving," bunch goes through the plasma, and repels all of the electrons it gets near, leaving an "empty" wake behind it, where only the positively charged ions are.
4. The positive charge behind the driving beam pulls it backwards, causing it to lose energy. At the same time, a "witness" bunch placed strategically within the wakefield gets pulled forward by the positively charged ions. The witness gains energy while the driver loses energy.
5. Voila! One bunch now has twice the energy, and one bunch now has none . . .or at least something close to that!

The main caveat is that you're upward-limited by your entering energy, so you still need a huge Linac to accelerate the bunches to begin with. This will likely get tacked on in the form of a "plasma afterburner" to a normal linac, such as in the setup at SLAC.
• Re: (Score:2)

I've not read any of the technical papers on this yet but off hand I wonder if a) you can ever get the same luminosity as more traditional methods and b) how well can this be used for heavier particles?   I suppose the first real world use
would be as an injector?
• Re:I actually work on this at USC!!! (Score:4, Informative)

on Wednesday February 14, 2007 @07:27PM (#18017976)
As far as I understand it, it doesn't work nearly as well for heavier particles (I assume you are thinking protons?). Especially ones with a positive charge. The heavy mass of the protons compared to the electrons in the plasma cloud are what allows the "wakefield" to be created in the first place. When we model this stuff, the ions move so slowly compared to the electrons that we generally just assume that they are static for the duration of the beam passing through the oven (pico-femto second range). As I mentioned earlier, this will most likely always show up as an "afterburner" that goes at the end of a traditional linac.
• Re: (Score:2)

Can you maybe use this thing for accelerating other exotic particles, like muons? I'd read that muon colliders are being researched because they as point particles might arrange for cleaner point-collisions with enough kinetic energy to serve as "Higgs factories". A "Higgs factory" device which could produce lots of Higgs particles might then make it much easier to find/study the "God particle". (Even exploit it?)

Because laser wakefield / plasma wakefield accelerates things so quickly and so suddenly, perha
• Re: (Score:1)

You still have one issue with this form of wake field acceleration, Luminosity! The luminosity goes way down, ~1% of what the original electron "bunch" was.
• Re: (Score:2)

4. The positive charge behind the driving beam pulls it backwards, causing it to lose energy. At the same time, a "witness" bunch placed strategically within the wakefield gets pulled forward by the positively charged ions. The witness gains energy while the driver loses energy.
sounds like a slingshot pass in NASCAR. Shake-n-Bake, baby
• Re: (Score:2)

Understandably, one wants the bunches to be small. Wouldn't this mechanism tear the bunches apart, and give them much higher temperature (i.e. spread out energy distribution)?
• Re:I actually work on this at USC!!! (Score:5, Informative)

on Wednesday February 14, 2007 @10:23PM (#18019518) Homepage
I attended a talk from one of the primary investigators on this project a few months back. The system does indeed spread out the distribution, which can be bad for some circumstances. When all you care about is the peak energy, however, it's great. They call it a plasma afterburner.

One thing that isn't obvious is that you can't use two of these devices to double the energy twice. One doubling is all you got. Apparently there's some theorem in plasma physics that a Gaussian distributed pulse (as SLAC is) can only be energy-doubled by any method or methods once. I don't know the details of this, and I might be misrepresenting it, but there you go.

By the way, I think you have a misconception about temperature. It's true that a higher temperature gas has a wider energy spectrum, but the primary piece of information you're interested in is the average velocity. The statistical distribution is a function of only one variable -- you can't "spread out" the distribution to increase the temperature without simply dumping energy into the system. If you somehow separated the particles into low average energy and high average energy, you'd just have two classes of particles with two temperatures, not one cumulatively higher one.
• Re: (Score:1)

Is it likely that these accelerators can be cascaded to get higher energies?
• Re: (Score:2)

Excuse my ignorance, but I don't have the slightest clue as to why this is important.

Based on my limited understanding of what you said, its application could be used for some "Energy Weapon". Yes? If so, just imagine how much the military would pay USC for it. At that point, I guaranty you wouldn't need another grant for a long long time ;)
• Re: (Score:2)

my laymans understanding is: particle accelerators are used to study the basic building blocks of our universe, and as they get more powerful (ie they can give the accelerated particles higher energies ) they see new levels of building blocks EG:

imagine the world is made of Lego shaped into the form of duplo. Before we had particle accelerators, we might have thought we were made of duplo. By smashing the "faux duplo" together, we realize there's Lego in there(*). if we throw it even harder at each other,

• Err, 120mph? (Score:1)

"Imagine a car that accelerates from zero to sixty in 250 feet, and then rockets to 120 miles per hour in just one more inch.... Because electrons already travel at near light's speed in an accelerator, the physicists actually doubled the energy of the electrons, not their speed."

Hmm. Well which is it, are we doubling velocity or engery? I'm no rocket surgeon, but I'm pretty sure these are different things. If it's energy then I think the analogy should have been, "...and then rockets to 85 miles per h

• Re: (Score:2)

Imagine a car...

You KNOW when someone use a car analogy to "explain" a highly technical or abstract concept that it will make no sense. And worse, will start up a whole bunch of threads about cars, driving, etc, etc.

• Improved accelerators? (Score:1)

Is this going to improve my 3d graphics accelerators?
• Re: (Score:1)

Is this going to improve my 3d graphics accelerators?

No, but watching you being bombarded by relativistic subatomic particles would be much funnier than reading your comments.
• Slashdotism (Score:1)

All your electron-Volts are belong to us.

Only Old North Koreans need souped up particle accelerators.

In Soviet Russia, particles accelerate YOU!

What did I forget?
• Re: (Score:2)

Step 1: Double Particle energy.
Step 2: ???
Step 3: PROFIT!!!

Not really a Slashdotism, but it is WAY over used here...

• Loss of beam focus? (Score:2)

If the collimated beam going into the plasma chamber gained a bunch of energy (42GeV) in the space of 84cm, that means total time of flight in the chamber was in the nanosecond range. That means a jillion collisions with the plasma. Wouldn't this screw up the focus of the beam? Sure, you've got a more powerful pulse of electrons coming out, but they will be sprayed all over the place, so the amount of energy that you can put into your incoming target beam of positrons (or into a stationary target) is dilute

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