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Science

Laser Wakefield Particle Accelerator Realized 40

Posted by michael from the keep-remaining-eye-out-of-the-beam dept.
deglr6328 writes "Researchers at Lawrence Berkeley National Lab's "l'OASIS" group have, for the first time, discovered a way to create high quality monochromatic beams of relativistic electrons using a 10 terawatt laser pulse focused on a specially formed plasma channel. The work is considered a landmark in new accelerator physics due to the fact that they are theoretically capable of creating extraordinarily high field accelerating gradients in the 100's of GeV per meter range; much higher than what's possible with the current gradients created by microwave frequency accelerators. The discovery could therefore open the door to far more efficient and compact staged particle accelerators utilizing next generation petawatt power lasers to achieve TeV scale particle energies and at lower energies, allow things like proton beam cancer therapy to be made affordable and widely available."
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Laser Wakefield Particle Accelerator Realized More

Laser Wakefield Particle Accelerator Realized

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  • What? (Score:3)

    by Scyber ( 539694 ) on Friday October 01, 2004 @11:10AM (#10405247)
    You lost me at hello
    • You lost me

      No kidding. I somewhat understood the article, but the buzzword count goes off the technobabble meter. It's like a Star Trek writer made the submission or something...

      Let's see...

      • monochromatic beam
      • relativistic electron
      • plasma channel
      • accelerating gradient
      • GeV per meter
      • compact staged particle accelerator
      • next generation petawatt power
      • TeV scale particle energy
      • proton beam cancer therapy

      Half that stuff sounds like lingo pulled from an audiophile magazine; all it needs is "vacuum tube am

      • Re:What? (Score:5, Informative)

        by Eric Sharkey ( 1717 ) <sharkey@lisaneric.org> on Friday October 01, 2004 @12:27PM (#10406178)
        * monochromatic beam

        All the electrons have the same energy and are moving at the same speed (more or less).

        * relativistic electron

        The speed of the electron is almost the speed of light.

        * plasma channel

        An evacuated pipe with electromagnetic fields to hold high speed particles.

        * accelerating gradient

        The ratio of the electron energy to how long the accelerator has to be to get the particles up to that speed.

        * GeV per meter

        The units used to measure accelerating gradient. One GeV is the energy of one electron accelerated by a 1 billion volt electrical potential difference.

        * compact staged particle accelerator

        The accelerators don't have to be big, and you can build several and stick 'em together.

        * next generation petawatt power

        Bright light.

        * TeV scale particle energy

        1,000 times more than GeV.

        * proton beam cancer therapy

        Like traditional radiation cancer therapy, but with protons rather than gamma rays or other types of radiation. Protons are better since they are most effective when relatively slow. They can penetrate the skin and other healthy tissue (slowing down in the process, but having little effect) and then have a large impact on a deeply embedded tumor. Current proton accelerators are too expensive to use on a large scale.
        • I like your explanation it's concise, but I'm going to make one point. You said that a relativistic particle is one that is nearly travelling at the speed of light. This isn't incorrect, but it isn't the whole picture. Relativistic effects can become really noticeable at as little as 30% of the speed of light. At these speeds gamma values of 1/sqrt(1.3) which is approximately 0.9 are calculated. This means basic special relativistic effects such as length contraction and time dilation could be observed.

      • Explanation. (Score:5, Informative)

        by Christopher Thomas ( 11717 ) on Friday October 01, 2004 @12:32PM (#10406249)
        I'll take a stab at explaining this.

        Particle accelerators use electric fields to accelerate charged particles to high speed. Normally you literally have a set of electrodes producing the field. This makes for a bulky device, because you're limited in how close together electrodes can be that have a given voltage difference (due to hardware constraints).

        Laser wakefield acceleration is one of a family of acceleration schemes that work by making a disturbance in a plasma, and using the ripples in that to accelerate charged particles. These ripples can be thought of as being similar to sound waves, but because plasma consists of charged particles, you get voltage differences between the peaks and troughs. Laser wakefield acceleration (and beat-wave acceleration and particle wakefield acceleration and so forth) use these voltage differences to accelerate particles (the usual analogy is to say that the accelerated particles are "surfing" on the slope of a moving wave, picking up speed the whole time).

        The advantage of plasma accelerator schemes is that the voltage gradients are much steeper than in conventional accelerators (large voltage change in a very short distance). This means that as long as you can keep them behaving nicely, you can use a plasma accelerator that fits on a tabletop to produce particle energies you'd otherwise need a huge linear accelerator to generate.

        Unlike conventional accelerators, there's no easy way to chain plasma accelerators together to get arbitrary energies. This is being worked on. They're also working on using better lasers to create larger plasma disturbances and get single-stage accelerators to work better; that's the focus of this article.

        Now, the Star Trek terms:
        • monochromatic beam

          This means that all of the accelerated particles wind up at more or less the same energy, instead of being at different energies. The analogy is with monochromatic light (all photons at the same energy, and hence colour).
        • relativistic electron

          Electron accelerated to energies much higher than its rest mass. For an electron, this means they're above the 1 MeV range. For a proton, it would be above the GeV range. For relativistic heavy ions (e.g. the ones in the RHIC device), it's the TeV range.

          At ultrarelativistic speeds, particles are travelling almost exactly at the speed of light, which makes accelerator design a bit easier.
        • plasma channel

          The area in the plasma where the laser has passed, and conditions are right for acceleration. This is a cylindrical channel, usually.
        • accelerating gradient

          How much voltage changes with distance. This determines the acceleration felt by the particles you're driving, which tells you how big a device you need to reach a given energy or what particle energy you can expect to get out of a given device.
        • GeV per meter

          Units in which acceleration gradient is measured.
        • compact staged particle accelerator

          A particle accelerator that's small (this is the advantage of plasma accelerators), and that use multiple stages to reach higher energies than any single acceleration stage could. This is tricky to do with plasma accelerators, but not impossible. Very handy if you can get it working reliably for hundreds of stages.
        • next generation petawatt power

          These are the ultra-short-pulse, high-energy lasers you may have been hearing about. Right now, you can get off the shelf systems that dump a few joules of energy into a pulse less than a picosecond long. Power during the pulse is in the terawatt range (which is why these are called "T3 / Table-Top Terawatt" lasers). Having a short, sharp pulse instead of a long, drawn-out one makes laser wakefield acceleration work better. The next generation of ultra-short-pulse laser delivers higher power in an even shorter time. The goal is to get petawatt power du

        • Re:Explanation. (Score:3, Insightful)

          by DLWormwood ( 154934 )
          I'll take a stab at explaining this

          As I wrote, I had an inkling of what the article talked about; I still remember some of my College Physics (including some relativity, oddly enough). It's just that even by /. standards, the vocabulary content of the "article summary" was over the top.

          I wasn't kidding about the audiophile comment... With high end entertainment centers advertising "fibre channels" and "DTS encoders" and other such junk only an electrician or a acoustic engineer can understand fully, the

          • "DTS encoders" and other such junk

            DTS encoders aren't junk. DTS is "Digital Theater System". It's for surround sound. You've probably heard of it before. There's a DTS decoder in most DVD players and surround sound systems, almost all DVDs contain DTS soundtracks. It's a digital format, and therefore non-lossy, which I would think would appeal to an audiophile, but then these are the same people who slobber over vacuum tube amps, so who really knows...

            A DTS encoder, like the one on my nVIDIA nForce2 moth
            • DTS encoders aren't junk.

              I wasn't trying to disparage digital audio, but I was trying to make the point that most people don't care about exactly what something means when they use terms in communication. That's what "technobabble" is, generally. I suspect the original article submitter copy/pasted parts of the text to write the summary, rather than trying to write it in layman's terms. For example, I'm a Mac programmer, so I can understand some of why a G5, as a 64-bit processor, can be better than 32-bi

              • Re:Explanation. (Score:3, Informative)

                by deglr6328 ( 150198 )
                Hi, I would like to personally assure you that I most certianly did NOT cut and paste any part of the article summary above. I'm sorry if you found it too full of "technobabble" but I was merely using proper terms to describe why this is an important discovery in the most accurate and concise way I could. I know not everyone knows what TeV or petawatt means and that's why I provided links to explanations and pictures of terms I thought might be too obscure. Simply reading the story should've provided a rea
            • Gah! I hope your kidding! Their website's too much of a mess for me to figure out, but do they really sell gold-tipped fibre!?!

              Egad, you weren't [monstercable.com] kidding... That exposed tip looks suspiciously golden to me, probably part of their "springloaded" attachment head.

              Not only that, they "polish" the cable to reduce "signal noise." Not worth the $80... (I paid $20 for the fibre I installed in my parent's surround system.)

  • OK, God! Lemme have it! (Score:5, Funny)

    by Linux_ho ( 205887 ) on Friday October 01, 2004 @11:16AM (#10405311) Homepage
    10 Terawatt laser? This is really a diversion to fund laser weapons research. I mean, what do you think a phase conjugate tracking system is *FOR*?
  • ..a way to make popcorn from orbit!

    http://www.imdb.com/title/tt0089886/
  • What I want to see is some of these babies aimed at giant solar sails which provide accelration to a spaceship ...

    • Re:Petawatt power lasers ... (Score:5, Informative)

      by Christopher Thomas ( 11717 ) on Friday October 01, 2004 @12:40PM (#10406355)
      What I want to see is some of these babies aimed at giant solar sails which provide accelration to a spaceship ...

      These lasers produce very high power, but for an extremely short time. Typical pulse energy is on the order of tens or hundreds of joules, so your space ship won't be moving very fast.

      Ultra-short-pulse lasers are used to investigate chemical reactions, and the exchange of energy between lasers and plasma (useful to understand how to get inertial confinement fusion working properly). They're also handy for creating the kind of plasma disturbance needed for laser wakefield acceleration.

      For driving a solar sail, you'd want a very large array of continuous-wave lasers, phase-locked to provide an effective aperture size of hundreds of kilometres (so that you can stay focused on the sail at a distance of light-days to light-years, depending on whether you're going for a flyby or a Forward-style decelleration scheme). If the aperture's any smaller, divergence causes most of your beam to be wasted on empty space.

      Individual lasers in this system have to be powerful, but not "petawatt" powerful. You're limited by the amount of light your sail reflector can safely handle, and by the heat sinking your laser requires.
      • We have one next to our physis lab that pulses for femtoseconds, that is 10^-15 seconds. At an energy of around 3milliwatts, which using the formula Power = Energy * Time gives a power of about 3TW. This kind of power laser gives focussed energies greater than 10^20 W/cm^2. This actually means that you can use a single laser of this power to initiate fission, because the energy of bound nuclei is less than this!
        • At an energy of around 3milliwatts, which using the formula Power = Energy * Time gives a power of about 3TW. This kind of power laser gives focussed energies greater than 10^20 W/cm^2. This actually means that you can use a single laser of this power to initiate fission, because the energy of bound nuclei is less than this!

          Can you give me more information on exactly how an ultrashort pulse is better for initiating fusion than a slower pulse? My understanding was that it was plasma temperature and density
          • Can you give me more information on exactly how an ultrashort pulse is better for initiating fusion than a slower pulse?

            Must - read - comment - carefully. One character apart, but it's a very important character.
            He was talking about using the terawatt laser to initiate (spontaneous) fission not fusion. -- I.E. the laser generates enough energy density to rip apart the nucleus of a atom (not smash two of them together)

            • Must - read - comment - carefully. One character apart, but it's a very important character. He was talking about using the terawatt laser to initiate (spontaneous) fission not fusion. -- I.E. the laser generates enough energy density to rip apart the nucleus of a atom (not smash two of them together)

              I'd understood this, but brain faulted and written about fusion as well.

              My comment re. photon energy holds, though. I don't really see how photons with energy that low would even _interact_ in a meaningful
              • I think that the value (in terms of nuclear fusion) of pumping that sort energy in such a short time is rather like the difference between 2 ounces of plastique vs a 3 ounce jelly donut. They both contain roughly the same ammount of chemical energy, but the plastique releases that energy much faster -- and thus more destructively (unless you feed the donut to a hyperactive 3 year old).

                BTW: I'm presuming that he meant to say 3 mili-joules because: [samuel@me tmp]$ units 3milli-joule/femtoseconds tera

                • In any case, there are two things to focus on. One is the energy density and the other is the speed at which it is
                  imparted. If you pump the energy in slow enough, then the atoms can dissapate it faster than it comes in. At some point, you toss in the energy fast enough that a portion of it accumulates -- hopefully enough to blow apart the atom (if that's what you're intending to do). I dunno the actual accumulated energy necessary to rip apart a nucleus. I never got that far in my physics
              • I'm not sure about fission or if indeed laser induced fission is even possible...but I can speak to the fusion issue though. Here at the LLE we're building a >2 Petawatt laser (to be done in ~2 years) to work with the current 60 beam 30Kj ~40-50 terawatt laser that's already here. The idea is that the 60 beam (spherically symmetric) laser will fire and compress a fuel pellet to a fraction of its original size. At the point of maximum temperature and pressure just before stagnation and then expansion, the
          • "My understanding was that it was plasma temperature and density that mattered, which depends only on the energy deposited and how symmetrical you can get the pellet implosion...."

            You are correct as far as I know, but as Stephen correctly pointed out I was talking about fission. In the US a project which might be called something like the national combustion lab, and uses 192 similar (but less powerful) high powered lasers, to initiate fusion.

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