Slashdot Mirror
Laser Wakefield Particle Accelerator Realized
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."
* 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.
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:
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).
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.
The area in the plasma where the laser has passed, and conditions are right for acceleration. This is a cylindrical channel, usually.
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.
Units in which acceleration gradient is measured.
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.
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
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!
MacOS X, I've upped my standards, Up Yours...