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GeV Acceleration In 3 Centimeters
ZonkerWilliam writes, "Here is a very interesting article, for the scientific community at least, on an advancement in laser wakefield particle accelerators. Being able to accelerate electrons to 1 Gev in the space of 3.3 cm calls up visions of portable devices that can be used anywhere: think of portable cancer therapies, if they can do the same for positrons, portable PET scans, possible use in compact fusion devices, capturing the dearly departed, etc. The uses are mind boggling." From the article: "By comparison, SLAC, the Stanford Linear Accelerator Center, boosts electrons to 50 GeV over a distance of two miles... The Berkeley Lab group and their Oxford collaborators... achieve a 50th of SLAC's beam energy in just one-100,000th of SLAC's length." I doubt that this tech will fit on a table top anytime soon. The article quotes the Berkeley researcher: "We believe we can [get to 10 GB] with an accelerator less than a meter long — although we'll probably need 30 meters' worth of laser path."
Seeing as how the LHC produces two beams in opposite directions with energy 7 Tev each (total collision energy is 14 Tev) this accelerator has 3 or 4 orders of magnitude to scale before it can even begin to compete with the LHC.
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RTFM: they're accelerating electrons, not photons, so mirrors won't work here.
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GeV = giga electron Volts
Also, TFA links to an illustrated version of the story.
We're talking about electron volts. You see, electricity is not the electrons themselves, but rather a wave of energy passing from one electron to the next as they collide with each other. (A bit simplified, but hey.)
You know those desk decorations that have about 5 metal ball suspended from wires? If you lift one and let go, gravity imparts energy on one of those balls. When it hits the next ball, it transfers energy to the other ball, which in turn hits the next ball, transfers its energy, so on and so forth. When the last ball has nothing more to hit, it swings out from the kinetic energy imparted on it. This is pretty much how electricity works.
An electron Volt is a method of measuring the kinetic energy for individual particles. It translates directly to the voltage/joules calculations we all know and love, except that it only involves one particle instead of a wire full of them. Most commonly, this term is used in particle physics where the energy of a single particle matters.
What has been built here is a micro particle-accelerator capable of imparting massive velocities on individual electrons. This is useful for things like advanced medical scanners which bombard a target with a small number of high energy particles in order to get 3D image of the object. With a small enough particle accelerator, we could begin building devices like the medical tricorders you see in Star Trek. That's never been possible before.
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You need the laser to be coherent across the entire path so that the particles are able to "surf" the wave fronts. Simply adding mirrors does not accomplish this and even if you had a coherents length of 30 m; you can't get the acceration to occur in higher energy density any faster. Basically you need to experience the full slope of the wave front to get maximal acceleration.
I actually do some research in this area (Plasma-wakefield particle acceleration, really really similar), and one of the biggest problems is getting the pulse-width incredibly small. They have to use something called Chirp Pulse Amplification and I think the beam length is somewhere on the order of 1-2 picoseconds. From the article, the power delivered by this beam is about 40 TeraWatts, which gives you some sort of idea. The acceleration gradient might be really high, but that doesnt mean youre going to get a desktop version any time soon. The equipment necessary to get the timing (pulse-length and power) right is incredibly difficult and expensive at the moment.
I'm perfect in every way, except for my humility.
For anyone who's interested, the actual velocity of the electrons is about 0.999999869 times the speed of light -- which is why talking about GeV is more instructive than talking about how fast the particle goes. The math follows, if you're interested.
... or you can type sqrt(1-1/(1GeV / electron mass / c^2)^2) into Google Calculator.
1GeV = energy = gamma * m * c^2 (gamma = 1/sqrt(1-v^2/c^2))
1 GeV / c^2 / m = gamma
1957 = gamma = 1/sqrt(1-v^2/c^2)
v/c = 0.999999869
Interesting fact: we usually hear about E = mc^2. That's the direct matter->energy conversion when the matter is at rest: if the matter is moving, we add on a factor of "gamma" -- which, at small velocities, is about 1 + 1/2 * v^2/c^2 (giving E = mc^2 + 1/2 mv^2, or rest mass + classical kinetic energy!)
Yeah the LHC produces proton collisions but protons are not fundamental particles, they are composite particles (3 quarks each + gluons)! They make "dirty" collisions with all sorts of particles flying everywhere. Electron positron collisions are much much "cleaner", the energy per individual fundamental particle is what matters. The international linear collider (a planned e- e+ collider) is hoped to achieve center of mass collisions of just 1 TeV and this will be sufficient to explore in depth the physics hinted at in the LHC at CERN. That means you only need two 500 GeV beams to do this. It looks to me like we're a mere ~2 orders of magnitude away from that point. All that has to be done now is a little succesive staging engineering and work to get the luminosity up.
- "Hear that?! The percolations are imminent! Cease your ingress!"
And just to elaborate on what you've hinted at for others, the reason this will never make the LHC redundant is to do with that the LHC does not have a fixed centre of mass energy in collisions as as you said, they are composite particles so while the protons collide at centre of mass energy of 14TeV, the individual quarks and gluons collide with a variable centre of mass energy (depends how much of the momentum is carried by that quark/gluon) upto a maximum of 14TeV. Anything produced by this development would have a fixed centre of mass energy. Hence to discover any new particle you have to scan through the centre-of-mass energies manually so to speak which means you could well miss something interesting. At the LHC, the scanning of the centre of mass energies is automatic so to speak making it very difficult to miss the new physics resonance. Hence you build messy hadron machines to find something and then precision lepton colliders to study it in detail as lepton colliders need to know the energy of the particle they're studing in advance.
You see, electricity is not the electrons themselves, but rather a wave of energy passing from one electron to the next as they collide with each other.
Well, this depends on what context you're talking about. In a metal conductor, you're absolutely right - an individual electron crosses a potential difference at a speed much much less (generally a fraction of a millimeter per second) than that of the effect of electricity (which is close to c). In a vacuum, when energy is imparted by a particle accelerator (such as the particle accelerator you are staring into right at this very moment), the electrons move much faster than they do in a conductor, and there are few particle collisions within the beam.
Of course, the energy imparted to the electrons that are flying at your face when you're looking at your monitor is in the keV range, many orders of magnitude less than the GeV we're talking about here. Still, they move fast enough when they strike the phosphor screen that relativistic effects are just beginning to creep up.
It is not for electron beams that this would be a boon. It is rather for other particles (protons, heavy ions). The footprint of such facilities is pretty large. In the US there are currently a number of proton treatment centers. Protons allow you to generate more conformal treatments (e.g. treating tumor not healthy tissue) with very low levels of doses elsewhere in the body. The latter is important for patients expected to have long survival times (these are becoming more prevalent as we are able to cure more and more tumors with less side effects). This is particularly important in treatment of childhood cancers.
Heavy Ions is another ball game. Now you need a synchrotron to get these up to the desired energies. This means building another building only to hold the accelerator (no talk about treatment rooms, rotatable gantries or anything). Heavy ions are very good at destroying cells as they generate a high density of ionizations along their path. They also have the same interesting conformal properties as protons. There are only about 4 or 5 heavy ion facilities in the world, most of them in Japan or Europe. There currently is no economic gain to be made building heavy ion facilities, protons are now reimbursed and facilities are starting to be generated although they are very costly ($100 10^6)
A "portable" accelerator would reduce footprint and building costs immensely making it economically feasible. Unfortunately, the accelerators presented here do not have a high enough flux yet to be used for clinically relevant doses.
In short: interesting, but don't hold your breath. Implementation, even in a research setting is at least 10-15 years down the line. Of course I could be completely wrong and have one on my doorstep tomorrow, with a note by a physician, "please calibrate, a patient starts tomorrow"PET scans don't use accelerated positrons. A radioisotope is injected into the patient, which emits a positron when it decays. The positron immediately annihilates with an electron and emits two gamma rays. The gamma rays are detected and used to build the scan. To make the radioisotope you need a proton accelerator, but these are already very compact at 2-3m diameter, and anyway don't need to be near the patient.
Fusion, of course, has nothing to do with accelerating electrons.
I thought geeks knew this stuff, or do they only need to pretend these days?