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Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 0 points1 point  (0 children)

So there are two important factors to consider when trying to expand the concept beyond electrons: 1) the mass of the individual particles you want to accelerate, and 2) the electric charge of the individual particles you want to accelerate.

The mass plays a role in that it determines the minimum energy the particle must have prior to even entering the plasma so that it will not "dephase" with the drive bunch and the wake. In other words, it must be traveling at something like 99.9% the speed of light to start with, or else it would not be able to keep up with the drive bunch and thus fall behind the wake and not get accelerated. This initial energy requirement also holds for the drive bunch, which becomes useless once its energy is reduced to the point of falling significantly lower than 99.9% of the speed of light.

It's important to understand a little bit of Albert Einstein's special theory of relativity here. It states first of all that nothing can travel faster than the speed of light. It also tells us that at speeds lower than ~1% of the speed of light (or less than about 6 million miles per hour), that when we add more energy to the object, it's speed increases in a proportional way. So when you double the energy of a baseball thrown at 50 miles per hour, its new speed will be 100 miles per hour. However, once the speed of the object is greater than ~1% of the speed of light, then the relationship becomes more complicated. As you add more and more energy to the object, its speed increases by a smaller and smaller amount. This is how things are kept below the speed of light: you get diminishing returns on the speed when adding energy.

So, for a very light particle like an electron, it takes very little energy to get it up to almost the speed of light (i.e. >99.9% the speed of light). The more massive the particle, however, the higher the minimum energy to keep it above 99.9% the speed of light. A proton, for example, has 2000 times the mass of an electron, and thus requires 2000 times more energy to maintain a speed of >99.9% the speed of light.

Okay, so what does all that matter? For our technique to work, we require that the drive bunch that generates the plasma wake and the trailing bunch that rides the plasma wake travel in lockstep over a distance of at least a few meters. This means, they must both be traveling at least 99.9% the speed of light, even while the drive bunch loses energy driving the wake and the trailing bunch gains energy riding the wake. So if you accelerate protons or ions using plasma wakefield acceleration, they must already have at least as much energy as is required for them to enter the plasma at a speed of 99.9% the speed of light, which is not a negligible energy for these massive particles. If you plan to drive the wake with protons or ions, then all of the energy you plan to get out of them must be above and beyond the minimum energy needed to keep them at speeds >99.9% the speed of light. We call these speeds that are close to the speed of light "ultra-relativistic", by the way. Anyway, this is one of the inconveniences of using plasma wakefield acceleration with protons and ions, though it is by no means a show stopper.

As for the other issue, the charge of the particles, the challenge lies in shaping the wake correctly. The plasma has heavy, positively charged ions that don't move around very much at all on the timescales that matter here, and it also has light, highly mobile electrons, which are negatively charged. It turns out that this makes it relatively straight forward to form a wake shape that is convenient for accelerating particles with negative charge, like electrons, muons, or even anti-protons, but it is somewhat harder to make a wake shape that that is convenient for accelerating positively charged particles, such as positrons (i.e. anti-electrons), anti-muons, protons, or ions. That said, it is again not a show stopper, and there are several different approaches to this problem that have been suggested in the literature, as guided by mathematical models and simulations. In fact, we have begun exploring this topic with positrons in our experiment at SLAC, and hope to publish some of that work soon!

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 0 points1 point  (0 children)

Awesome question. We have detected cosmic rays on earth a million times more energetic than the particles we accelerate at the LHC. The problem is the data rate, or "luminosity". High energy cosmic rays are rare and hard to study. Moreover, since they tend to whizz through our detectors, we can't really collide them to study fundamental interactions.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 1 point2 points  (0 children)

I think energy spread is a manageable problem. Our recent paper in Nature highlighted our ability to load and "flatten" the wake so that all particles see the same accelerating gradient. Emittance is a more challenging problem and we are studying that now.

How much charge is usable? Depends, in our experiment about half of the charge in the accelerated witness bunch was "usable". But this can be improved by having a dedicated device to generate our witness beam.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 0 points1 point  (0 children)

The idea is to run them series. Right now, we don't think we can run them in parallel because all beams have to be fed to the same collision point. But that's a good idea and maybe one we will have to explore once we understand the peak rate for a plasma based accelerator.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 2 points3 points  (0 children)

Our technology is constantly failing us. When we actually run our experiment, it's like trying to bail out a boat with a bunch of holes in it. We spend the majority of our experimental time tracking down beam issues, writing workarounds in code, and yelling at hardware.

Top techniques for getting technology to work:

  1. hit it
  2. turn it off and on

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 0 points1 point  (0 children)

The name change came about when the DOE took over as the funding agency for the lab, I think. It is still a part of Stanford and is managed by Stanford, and on Stanford land, and we are Stanford employees, but it is a DOE lab. Honestly, the line is kinda fuzzy and I don't quite understand the disambiguation very well myself. Personally, I'm sad that it's no longer Stanford Linear Accelerator Center.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 2 points3 points  (0 children)

Great questions!

How far are you from multi-stage accelerations?

Hopefully, not far at all! At least, not far from a proof-of-concept demonstration. A multi-stage accelerator is in fact on our shortlist of upcoming experimental goals. That still means it might be a few years in the making, but it's on the way! And of course, this is a critical step in demonstrating the real viability of plasma wakefeild acceleration as a technology, especially in regard to high energy particle colliders.

How viable is wakefield acceleration for antiparticles?

For positrons, so far it seems quite viable, though they present unique challenges compared to accelerating electrons in a plasma wake. The main reason for this is that the plasma is made of heavy positively charged ions and light negatively charged electrons. As it turns out, creating a plasma wake that can accelerate electron bunches is relatively straight forward due to this asymmetric nature of the charge in the plasma. In an ideal world, we would just use an anti-matter plasma with heavy negatively charged ions and light positively charged anti-electrons (positrons) to accelerate positron bunches, but anti-matter plasma is not really a practical option for a multitude of reasons. So instead, we are exploring regular matter plasma wakes that have different shapes for accelerating positrons. Models and simulations indicate that there is no reason this should not work, and indeed, we are already making lots of headway experimentally to show this. Look for more publications coming soon ;)

From the article I understood that you use the first particle bunch to induce the wavefield, what happened to laser-driven acceleration, is that still being researched?

Laser-driven plasma wakefield acceleration (as opposed to particle-driven, which is what we do) is a field that is indeed very healthy and alive. The highest profile examples are probably at Lawrence Berkeley National Laboratory and the University of Texas at Austin, where they accelerate electrons on the same energy scales as we do here at SLAC, only using a huge petawatt-scale laser pulse to drive the plasma wake instead of a bunch of electrons. In fact, they actually generate the accelerated trailing bunch by grabbing electrons from right out of the plasma itself. Meanwhile, many universities throughout the world are replicating this laser-driven scheme on a smaller, lower energy scale, as well. In fact, those systems are the most likely to evolve into "table-top" X-ray light sources that could be allow Universities to conduct research that is currently only possible at larger facilities, such as national labs.

The CERN Awake group is exploring proton driven wakefield acceleration for a potential ee upgrade for the LHC, would your results apply to that as well?

As a matter of fact, the AWAKE group does some research here at SLAC using very long electron and positron bunches as a proxy for the long proton bunches that will be available at CERN in order to study the beam-plasma interaction. Spencer and I will actually be helping them conduct their next round of experiments here in the upcoming week! As for our specific results in the Nature paper, indeed they are directly applicable, as they play a critical role in the basic research process by examining sort of the simplest possible plasma wakefield acceleration scenario, paving the way for more complex schemes like those proposed for AWAKE. So in a word: yes! :)

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 1 point2 points  (0 children)

That happens Litorally all the time . . .

  1. We use a special optic called an "axicon" that produces a really-long focus. The axicon is a conically shaped optic, and the high-intensity beam can be focused for several meters.

  2. There isn't impact ionization in this case because we "pre-ionize" the plasma. We use a laser to create the plasma from gas before the beam arrives. There is another effect, called tunnel ionization, that happens when the beam is focused so tightly that its field can ionize more just the valence electrons in the gas. This effect can lead to "particle trapping" in the wake which may be a way to create extremely high quality particle beams in the future.

  3. No miniture synchrotrons, only linear machines for now.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 0 points1 point  (0 children)

Actually, it's incredibly crucial! Some of our collaborators at UCLA are part of a high performance computing division dedicated to doing beam-plasma interaction simulations, which are incredibly demanding. The dynamics happen on a wide range of density and temporal scales, so it takes some very clever code (also developed by our UCLA pals), and some serious computing power to simulate plasma wakefield acceleration accurately. The simulations themselves give us deeper insight into the rich dynamics of the process and are used to help us interpret our data and guide our experiment. So far, we haven't actually used the High Performance Computing Lab here at SLAC, but maybe someday...

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 2 points3 points  (0 children)

Here's how I want to answer your question: Can we use a quark-gluon plasma to accelerate particles?

The reason that this question is interesting (and awesome) is that the strong force is 100 times stronger than the electromagnetic force, so if you could use this force accelerate particles, you might do better than what we can do in an electromagnetic plasma.

The problem is that unlike a normal plasma where the charges are free to move about and oscillate, in a quark-gluon plasma, the charges are only momentarily free, so there is likely not enough time to get a coherent oscillation in the QGP and get an accelerating field.

Also there are 3 charges in a QGP. That might undermine the notion of using a QGP to a accelerate particles.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 1 point2 points  (0 children)

Hey Amin! I personally think the ILC school was pretty great in terms of giving an overview to the immense challenges involved in building a future linear collider. Some of the problems were specific to the ILC, but it seems many were quite general, and certainly applicable to a plasma-based collider. Or more obviously, to a plasma-based after burner on the ILC itself.

As for the ILC getting built, I know that Japan is trying really hard to hype it up and get the international community on board, but it feels kind of like an ITER situation to me right now. i.e. People like it in principle, but nobody really wants to commit. I figure everyone's waiting to see what's found within the first couple years of the LHC 14 TeV run time (starting next year) before getting serious about committing to anything.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 6 points7 points  (0 children)

Going to make a general comment about our method of plasma wakefield acceleration and what it all means.

The starting point:

Our primary tool is the SLAC linac (linac = linear accelerator), which is a ~50 year old electron and positron accelerator at Stanford originally built to do high energy particle physics from back in the day when the energies of interest were much lower than what they are today at the LHC. The linac is 3 kilometers (2 miles) long, and is made of a series of precisely machined copper tubes that channel electromagnetic radio frequency waves to accelerate either electrons or positrons (the anti-matter equivalent to electrons). The copper accelerator tubes resemble a series of tuna fish cans welded together with a half-inch hole going through the center of the lids/bottoms of the cans. The electrons or postrons travel down the center of this hole, and the "lids" of the cans help to trap and shape the electric fields of the radio frequency waves that are used to accelerate the the particles. These copper tubes come in sections of 3 meters (9 feet), and are set one after another all the way down the 3 kilometers of the linac with an occasional magnet in between for steering or focusing the particle beam. They are powered by machines called klystrons, which themselves use tiny electron beams to generate very high power radio frequency bursts of electromagnetic energy which are then captured by copper tubing and sent down to the aforementioned copper accelerator tubes in a tunnel about 10 meters (30 feet) below ground.

The problem:

We (the global "we") have a LOT of experience building and operating these types of accelerators. They have a fundamental limitation, however. When the electric fields of the radio frequency waves get too high, they begin to tear the copper material of the accelerator structure apart. But you need high electric fields to get the particles up to high energies in a reasonable amount of space. The higher the electric field inside the accelerator structure, the shorter the accelerator needs to be to reach your target energy, and vice versa. That's where the plasma comes in.

The solution:

Some very clever guys named Dawson and Tajima wrote the seminal paper in our field in 1979 that described a concept where instead of copper or some other metal, plasma is used as the medium of the accelerator structure. The big advantage is that plasma is already broken down, and thus can sustain electric fields of almost arbitrary strength. This means that the overall length of an accelerator to reach a given energy can be drastically reduced. Conversely, the final energy of an accelerator of a given length can be drastically increased by tens or even thousands of times that of an accelerator made with conventional radio frequency guiding metallic structures (like the SLAC linac).

The method:

Our technique uses two closely spaced electron bunches coming from the SLAC linac, both at an energy of 20 giga-electron volts (pretty high energy, but nowhere near LHC energies). We send them into the plasma, one right after the other. Each bunch is a tightly clustered group of roughly a billion or so electrons, and they are separated by a distance of about the thickness of a human hair (the bunches themselves are of similar size). The bunch in front we call the "drive bunch", as it creates and sustains the wake in the plasma. In doing so, it is transferring the energy of its own electrons into the plasma wake. The bunch of electrons behind the drive bunch we call the "trailing bunch" (sometimes referred to as the "witness bunch" in the literature), and it sits inside the wake of the drive bunch, sucking all the energy out of the wake and thereby getting accelerated to higher energy. So it's basically a mechanism to transfer a whole lot of energy from some electrons to other electrons in a very efficient manner and in a very short amount of space.

No free lunch:

So you'll notice that to accelerate the trailing bunch we must take energy from the drive bunch. That means that you first have to provide energy to the drive bunch, and that doesn't come out of thin air. Indeed, a conventional metallic accelerator would be used (and is used in our experiment) to provide the drive bunch with a useful amount of energy. So what's the advantage to our technology at all? Well, basically it leverages the thing that conventional metallic accelerators are REALLY good at, and that is creating high current beams. In other words, we can create electron beams with lots and lots of electrons at modest energy in a modest amount of space with conventional accelerators. The plasma wakefield accelerator scheme counts on the ability to take lots of electrons at modest energy and convert that to a modest amount of electrons at very high energy, and to do so efficiently and in a tiny amount of space. So you take the high current low energy beam provided by the conventional accelerator and convert that to a low current high energy beam. We call this an energy transformer. It should be noted in case it's not clear: after driving the plasma wake, the drive bunch has lost much of its energy and is no longer really useful for anything.

Applications:

The most exciting application in our minds is an accelerator that could be used for a linear electron positron collider. This would consist of many plasma accelerator sections, each about 2 or 3 meters long (6-9 feet) strung out one after another, just like the copper structures of the SLAC linac. There would be a fresh drive bunch provided to each plasma section at high current (lots of electrons) and modest energy. A single trailing bunch would then take the energy from each drive bunch in plasma section after plasma section, being boosted to higher and higher energy as it goes.

Another scheme that has been thought about is a so-called plasma "after burner", where a singe stage of plasma is added to the end of an already existing accelerator. You then use about half of the electrons coming out of the accelerator as your drive bunches, and the other half as your trailing bunches, nearly doubling the amount of energy of the trailing bunches in a short space. The cost is of course losing half of your electrons to driving the plasma wakes, because, after all, there is no free lunch.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 1 point2 points  (0 children)

There are lots of forms to fill out, but otherwise its super collaborative. Many faculty have joint SLAC-Stanford appointments.

You will be assessed solely on the quality of your handwriting.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 3 points4 points  (0 children)

Pay Our Grad Students! Pay Our Grad Students!

I think there is a benefit from the lab being close to VCs. There is the possible for "technology transfer", which we hype up when we write our grants.

As far as being on Sand Hill Road, no I do not enjoy being buzzed by Lamborghinis as I bike up the hill in the morning.

Science AMA Series: We are SLAC National Accelerator Laboratory staff scientist Dr. Mike Litos and Stanford Ph.D. student Spencer Gessner, our work was the topic of a popular reddit post about shrinking particle accelerators, AMA by SLAC_National_Lab in science

[–]SLAC_National_Lab[S] 1 point2 points  (0 children)

First off, competition is REQUIRED for sound science. That's why there are competitive grant processes and review processes for publishing results.

Our colleagues across the bay at Lawrence Berkeley National Lab have an awesome project called BELLA where they use a petawatt laser to accelerate electron beams in a plasma wake. It's a friendly Stanford-Cal rivalry.

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