Plasma Physics and Fusion
Hot ionized gases, from fusion reactors to the solar wind, where charged particles and magnetic fields move together.
arXiv: physics.plasm-ph
16 topics
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Burning plasmas and alpha-particle physics
4 problemsIn a burning plasma, the helium nuclei (alpha particles) produced by fusion reactions supply most of the heating. These fast particles can excite waves that push them out, and self-heating can make the plasma regulate its own temperature in ways never tested.
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In space, shock waves form in plasma without particles colliding, held together by electric and magnetic fields. These shocks make cosmic rays, but how ordinary particles first get enough energy to enter the acceleration process is not understood.
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Tokamak disruptions and runaway electrons
5 problemsA tokamak plasma can lose its heat and electric current in a few thousandths of a second, an event called a disruption. The collapse induces a strong electric field that can accelerate electrons to nearly light speed, and a beam of these runaway electrons can melt the wall.
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Tokamak edge pedestal, ELMs and heat exhaust
5 problemsIn the best tokamak operating mode the plasma edge forms a thin insulating layer, the pedestal, whose pressure sets how well the whole plasma is confined. The pedestal keeps collapsing in bursts called edge-localized modes (ELMs), which hit the wall with heat pulses a reactor wall cannot survive, while the steady exhaust heat lands on a very narrow stripe.
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Hall thrusters (spacecraft engines that push ions with an electric field across a magnetic field) rely on the magnetic field to hold electrons back, but electrons cross it far faster than collisions allow. Without knowing why, these engines cannot be designed from first principles.
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Inertial confinement fusion at high gain
4 problemsInertial fusion crushes a small fuel capsule so quickly that it burns before it flies apart; in December 2022 a laser-driven capsule first released more fusion energy than the laser light delivered to it. A power plant needs about ten times higher gain, which is limited by fluid instabilities, mixing of shell material into the fuel and uneven drive.
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The solar wind is a hot, nearly collision-free plasma that stays much hotter than simple expansion predicts, so its turbulence must heat it, but without collisions it is unclear how the energy becomes heat. In similar plasmas, turbulent motion can amplify weak magnetic fields (a dynamo), and how that works without collisions is also open.
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QED pair cascades in lasers and pulsars
4 problemsA fast electron in a strong field emits a $\gamma$ ray, the $\gamma$ ray turns into an electron-positron pair, and the new particles repeat the process, creating an avalanche of matter and antimatter. This is predicted for the most intense lasers and is believed to fill pulsar magnetospheres with pair plasma.
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Classical and quantum radiation reaction
4 problemsAn accelerated charge radiates light, and the energy it loses must slow it down; this recoil is called radiation reaction. The classical textbook equation for it has unphysical solutions in which a free electron speeds up by itself, and strong-laser experiments now test which classical or quantum description is correct.
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Magnetic field lines in a plasma can break and rejoin, suddenly turning stored magnetic energy into heat, fast flows and very energetic particles. Simple theory says this should be far too slow to power solar flares, yet in nature it happens fast.
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Breakdown of perturbative strong-field QED
4 problemsIn extremely strong fields, the usual step-by-step approximation of quantum electrodynamics (QED, the quantum theory of light and charges) may stop working. This breakdown was predicted around 1980, but it is still not settled whether it happens in real fields or how to calculate beyond it.
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Limits of stellarator optimization
4 problemsA stellarator holds plasma with twisted external magnets instead of a plasma current, so it cannot disrupt, but its 3D shape lets particles drift out unless the field is very carefully designed. Computer optimization now finds excellent shapes, and the open question is how many goals can be met together with buildable magnets.
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Heat leaks out of a tokamak (a doughnut-shaped magnetic bottle for fusion fuel) mainly through small turbulent eddies. Above a threshold heating power the edge turbulence suddenly drops and an insulating layer forms (H-mode), but what triggers this, how dense the plasma may become and how the fuel mass changes the heat leakage are not understood.
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Quantum theory predicts that empty space, filled with short-lived virtual electron-positron pairs, should slow light differently depending on its polarization when a strong field is present, and should break down into real electrons and positrons in a strong enough field. Neither effect has been seen unambiguously in a laboratory with macroscopic fields.
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A laser pulse or particle bunch moving through plasma leaves a wake of electric field about a thousand times stronger than in ordinary accelerators, and particles moving with the wake gain energy quickly. Turning this into a collider needs beams that stay tiny and nearly monoenergetic over many stages, and needs positrons as well as electrons.
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Warm dense matter is material compressed to solid density or beyond and heated to tens of thousands of degrees, so it is neither an ordinary solid nor a thin plasma. It fills giant-planet interiors and every fusion capsule during implosion, yet its pressure, heat conduction and ability to slow fast particles are poorly known.