Tag Archives: Accelerator Science

Frontiers in Physics: Particle Accelerators

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Joseph Lykken of Fermi National Accelerator Laboratory gave a great talk discussing the many ways we use accelerators today, and from there what we will expect the next generation of accelerators to do. For the non-physicists in the crowd it was a helpful overview and at such an interdisciplinary meeting it never hurts to explain the fundamentals.

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The big news in accelerator physics these days is the restart of the Large Hadron Collider at CERN in Geneva (shown above with the underground tunnel illustrated above ground). With a price tag of $10 billion and a time line of 20 years to build, Lykken emphasized the empirical trouble with operating such a machine. As the largest piece of machinery in theworld as well as one of the most complicated, the LHC holds twenty sevenkilometers of wiring, magnets, cooling systems and advanced technologies. You’rebound to run into a few bugs, like those that shut down the machine lastSeptember. But the LHC is on schedule torestart this fall when scientists will get to fullfill their ambitions offinding a particle that gives matter mass, uncovering the nature of darkmatter, and recreating conditions during the big bang. The science it hopes toproduce is truly tantalizing for scientists and science lovers everywhere.

The LHC is the largest and mostpowerful accelerator in the world, but the field also expands horizontally intosmaller, yet equally important applications. There are, of course, medicalapplications such as proton therapy which treats on the order of 8,000 patientsa year at 25 proton therapy centers in the US. There are even more biologicalapplications at synchrotron light sources: accelerators that give off X-rays asthey drive particles around a circle. These powerful X-rays are becoming a standard part of protein crystallography, and offer many other biological applications. Soon the first free electron laser will start up at SLAC NationalAccelerator Laboratory and offer scientists a chance to watch incrediblyfast processes such as protein folding and molecular motion in real time.

Accelerators may even give aid to the energy crisis and reduce global warming. Accelerator scientists may totally revamp nuclear power by developinga type of accelerator that drives a thorium reactor (rather than uranium orplutonium which are limited resources as well as major safety issues). Thewaste from these reactors would have a half life of 30 years, instead of thousands.The device would require a linear proton accelerator with particle energies of10 megawatts, which does not exist yet, but plans are stirring.  

Scientist all over the world arecontributing to plans for a project called the International Linear Collider,or ILC, a 31 kilometer electron-positron collider that would compliment theLHC. No country has granted space or funding yet, but there are already over2,000 collaborators from 300 countries working on preliminary plans.

Fermilab, Lykken’s home institution, continues to reach for the cutting edge of accelerator science in the US. Fermilab hosts a premier protontherapy center, and is home to the country’s most powerful proton-antiproton accelerator,the Tevatron, which will lead the hunt for the Higgs boson until the LHC comesback online. Fermilab has plans to continue leading the field by hosting atleast one of two proposed next generation projects. The first, Project X, wouldbe the first superconducting linear collider, the advantages of which areincreased stability and lowerwall-plug power consumption. There is also buzz for the first muon collider at Fermilab.The lifetime of a muon is less than a microsecond, so the challenge forbuilding such a machine is trifold: create, collect and make a beam out ofthese particles in less than a microsecond. The collisions would provide new physics to study and saveenergy and reduce size.

Particle-physicists certainly want to push the upper limit of accelerator power, but to go anybigger than the LHC would begin to cost in the hundreds of billions of dollars,which would require some major justification. So thenext generation, if they hope to continue to push the energy limits, must eventuallyreduce in size. This would not only mean big accelerators with even more power,but also making small accelerators smaller; so that, for example, protontherapy devices could it inside an office rather than a football field.

The leader in such technology comes from plasma wakefieldaccelerators, where particles sit in a plasma (rather than a vacuum) and in oneversion are accelerated with lasers. One bunch of electrons actually catches aride on the wake of a bunch in front of it – like a water skier behind a boat. These accelerators could move particles to the same speeds as current technologies,but at 1/1,000th the distance.

 “The traditionalR&D model for new accelerator technologies has been that the particlephysicists…do this R&D and everybody else benefits from [it],” said Lykken.”This model has worked very well in the past; it’s still working. But if wereally want…to turn around the time from the basic accelerator technologydevelopment to something you can use in a hospital, we need to get closerconnections between the people doing this kind of research and the people thatknow what they want to use it for.”

New Ideas for New Accelerators

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One of the largest disadvantages of therapies that require accelerators is the size and cost of the accelerators themselves. These factors limit who can receive care and where. So for medicine and the future of accelerators in general, the community is pushing for smaller and more affordable (but of course, that’s the case with all technology isn’t it?). At a very intriguing session I got to hear about an approach that a group of scientists at Stanford University and Fluence LLC are developing to make smaller particle accelerators for medical applications.  They are also collaborating with the Stanford medical school and SLAC National Accelerator Laboratory, who are contributing expertise in Monte Carlo simulations for beam transport and dose calculations.

Their technique is called electromagnetic plasma acceleration (not to be confused with other plasma accelerators that use lasers) and it goes something like this: get a neutral gas in-between two electrodes and let it break down into electrons and ions. The currents that flow through the plasma then generate  their own magnetic field (this is a big advantage because no magnets are needed); this exerts a force to the right, and the electrons start to move.

This alone is not a new approach.  But normally the ions upstream form a wall, sort of like a piston, and push the other particles forward.  This works for lower energies, but when one tries to accelerate the piston faster it starts to tilt and downstream gas can blow by it without being accelerated. This is like a piston with a hole in it, which doesn’t push anything. So there’s a definite limit to how fast you can get those particles to move.

But new research has shown that you can make tweaks to the system and cause the piston wall to disappear. Early work performed on this topic by Dah Yu Cheng in the 1970′s already showed that there are two ways to accelerate these particles. One (the piston) moves the particles like an explosion. The other moves them the way a jet engine does, in a more jet or bullet-like shape that is also more efficient. The Stanford team is now trying to understand this second mode of acceleration. While there was previously a limit on how fast you could accelerate the particles until the piston tipped, this limitation does not apply to the second mode.  They’ve got particles moving at around 100 keV, which is still far short of the 100 MeV’s needed for proton accelerators, but that’s what the Stanford team is dedicated to reaching. They’ll have other challenges as well, including reducing the wide energy distribution that the particle jets tend to have.

Flavio Poehlmann, who delivered the talk in place of professor Mark Cappelli who was unable to attend, said the team expects another ten years of work before they reach their energy goal. They’re currently in the process of submitting patents before they publish any papers on the subject.