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If you want the lowdown from an expert, the issue with these acceleration schemes with lasers is the accelerated beams aren't that great in terms of characteris
by noobermin 4y ago
If you want the lowdown from an expert, the issue with these acceleration schemes with lasers is the accelerated beams aren't that great in terms of characteristics, specifically they tend to not have the best divergence and energy spread compared to traditional linacs (divergence is better for electron beams from LWFA but for the ion accelerators which are another major application of intense laser interaction with plasma, the beam characteristics are much worse). With a cyclotron or a linac you control everything a lot better because the involved fields are fields you put in yourself with equipment you can tinker with, but here the plasma bubble is microscopic and the relevant physics happens on a femtosecond-to-picosecond time scale, and any small issues with the input laser (specifically the wings of the gaussian pulse) mess things up.
One of the issues that I sort of sniffed out in the field is as a whole they are pushing for higher and higher peak energy without really looking to improve beam quality, that is, everything below the peak. Like this paper (which an old colleague is an author on!! Congrats to him) is actually pretty good on the divergence as 1mrad (mili radians) but the energy spread is 15% around the peak, which is good for some things, but there are experiments with linacs that require percent spreads less than a percent. Now, you can probably "filter" out the energies you don't want out of the beam to make it even more monoenergetic, but for example this beam has tens of picocoloumbs. Assuming (very generously, haven't read beyond the abstract yet) that most of the tens of pico they report is in that 15% spread, 1% means you might be dipping into less than a picocoloumb territory on target for your experiment, which might not give enough dose for what you need to probe or otherwise do.
Anyway, in terms of the field, this work is fantastic, and generally Howard and Jorge are fantastic scientists (never met Jorge but Howard's a real decent man), but this is the cream of the crop of what we as a field can currently do. I feel like getting to the point where we give the traditional accelerator types a run on their money is still a few years out.
- fnands 4y agoThanks for the input. I've been hearing about wakefield accelerators for a long time, but never got into the nitty gritty of it. Is the beam quality a fundamental limiting factor, or is it solvable (at least theoretically). It sees they're going for the get the energy up and solve the beam characteristics later approach
- wolfram74 4y agocaution: Haven't studied particle accelerator design My understanding of most particle accelerators is that they accelerate their subjects directly with fields, either oscillating or steady. Wakefield accelerators use a combination of fields (the laser pulse) and other particles (the "stationary" ions). My suspicion is that the ion background introduces enough noise/chaos into the process that it would be a great effort to make the output more monoenergetic and retain total brightness. But I dunno, I always found particle physics a little tedious and didn't pay as much attention as I could have.
- noobermin 4y agoThat is also true, basically target imperfections plus noise in the laser cause imperfection in the acceleration process. Essentially the shape of the bubble can change the accelerating fields uniformity, leading to the divergence and spread issues. They usually cryo their target to minimize just plain chaotic behavior from it being a gas, so generally most of the jitter is due to wings of the laser itself causing perturbations in the gas/plasma (it's a plasma once the peak reaches it). That said, you're totally right about the target itself being a source of noise too.
- marcosdumay 4y ago> Is the beam quality a fundamental limiting factor, or is it solvable (at least theoretically). At least frequency noise is inherent for any accelerator where the particles stay for a very short time.
- noobermin 4y agoEverything is solvable theoretically :) I don't think it's necessary fundamental limitation to the process, but it seems a bit far away just at this moment. I will be a little more charitable to myself and the field generally, some people do care about characteristics, it's just what gets the phys revs+ seem to be higher peak energy. It's just a shame because for applications the general beam quality is a potential show stopper. I think the problem is that as anyone who works with numbers knows, including hackers, the peak value of something is what you put on the slide because it's easy and sounds sexy and is more exciting than a histogram.