Accelerator and Beam Physics
How to accelerate, focus and control beams of particles and light, the machines behind colliders, light sources and cancer therapy.
arXiv: physics.acc-ph
19 topics
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Instead of microwaves, small dielectric tubes can be driven by an intense electron bunch, or tiny glass structures by a laser, to reach much higher accelerating fields. The open questions are how efficient, stable and scalable such accelerators can be.
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Physical origin of the beam-beam limit
2 problemsWhen two colliding beams cross, each one's electric field kicks the particles of the other, and above a certain beam density the beams blow up. Why the limit has the value it does is not fully explained.
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A bright beam packs many electrons into a small, nearly parallel, short package, which decides how sharp an x-ray source or electron camera can be. Limits come from the cathode, from the electrons pushing each other apart, and from collisions inside the beam.
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Cooling hadron, ion and muon beams
5 problemsCooling shrinks the spread of particle motions in a beam, making it denser and brighter. Existing methods are too slow for high-energy hadrons, and muons decay in about two millionths of a second, so faster methods are needed.
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A beam interacts with the fields it leaves in the vacuum pipe, with clouds of stray electrons and with the opposing beam, and these can make the whole beam oscillate and blow up. Several effects seen in present colliders are not yet predicted by theory.
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When a short electron bunch bends in a magnet, radiation from its tail catches up with its head and pushes on it, which can blur the beam or break it into tiny density ripples. Predicting these self-forces in three dimensions, and using them to make light on purpose, are open.
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A free-electron laser makes x-rays by sending a bright electron beam through a row of alternating magnets, where the electrons gather into thin slices and radiate in step. How short a wavelength, how narrow a spectrum and how quiet a beam can be reached are open.
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In X-ray lasers the electron bunch is compressed to boost its current, but tiny density ripples grow during compression and spoil the beam. The X-ray pulses also start from random noise, so their colors are messy and making them clean is still hard.
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Six-dimensional ionization cooling of muons
3 problemsMuons live only two microseconds, so a muon beam must be shrunk very fast by passing it through matter that slows it down and then re-accelerating it. Doing this in all six position and momentum directions down to collider size has never been done.
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Performance gap of Nb3Sn cavities
2 problemsNb3Sn is a superconductor that could work at higher temperature and reach twice the field of niobium. Real Nb3Sn cavities reach well under half of that predicted field, for reasons not fully known.
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Particles in a collider circle billions of times through slightly imperfect magnets, and tiny nonlinear kicks can slowly push some out. Predicting which orbits stay stable for hours from simulations of seconds is an unsolved dynamical-systems problem.
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Minimum intrinsic emittance of photocathodes
3 problemsElectron beams for X-ray lasers and electron microscopes start when light knocks electrons out of a surface, and the sideways jitter of those electrons sets how sharp the beam can ever be. How small this jitter can be made is unknown.
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Particles in a beam carry spin, like tiny magnets, and keeping the spins aligned during acceleration and storage is hard. Some precision experiments need them to stay aligned for minutes to hours.
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Copper accelerating structures can hold only so strong an electric field before sparks (breakdowns) jump across the vacuum and spoil the beam. Nobody can yet predict from basic physics where that limit lies.
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The magnets that steer high-energy beams are superconducting coils that often lose superconductivity (quench) below their design current and reach it only after many repeated quenches, called training. Why they train, and what limits their field, is not understood well enough to design around.
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In very intense proton beams the particles' mutual repulsion drives a few of them onto large orbits, forming a faint halo that hits the walls. Losing even a part per million per turn makes the machine radioactive.
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Superconducting niobium cavities accelerate particles while wasting almost no energy, but they quench or lose energy at fields below what theory allows. How the losses grow or shrink with field is only partly understood.
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Superconducting niobium cavities lose almost no energy, but at strong fields the losses suddenly grow (the Q-slope), while cavities treated with nitrogen lose less as the field grows (the anti-Q-slope). Why either happens is still debated.
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Approach to the superheating field in SRF
3 problemsA superconductor can keep magnetic field out somewhat beyond the field at which entering becomes energetically favorable, up to the superheating field, before vortices (tiny whirlpools of current carrying magnetic flux) enter. Real cavities fail below this limit, and the true maximum is uncertain.