This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. Free electron lasers (FELs) are coherent radiation sources based on radiation from “free” relativistic electrons rather than electrons bound in atomic and molecular systems. FELs can, in principle, operate at any arbitrary wavelength, limited only by the energy and quality of the electron beam that is produced by accelerators. Therefore, FELs can be used to fill gaps in regions of the electromagnetic spectrum where no other coherent sources exist and can provide radiation of very high power and extreme brightness. More than 50 FELs have been built around the world, serving a diverse array of scientific fields and applications. FELs are based on the resonant interaction of a high-quality electron beam with the radiation in a periodic magnetic device called an “undulator” and can have several operating modes. FEL oscillators use optical cavities to trap the radiation, so that the field is built up over many amplification passes through the undulator. FELs can also act as linear amplifiers that will magnify external radiation whose central frequency is close to the undulator resonance condition. Without any external signal, self-amplified spontaneous emission (SASE) can be used to generate intense coherent radiation starting from electron shot noise and is the most common approach for X-ray FELs. SASE will have limited temporal coherence and pulse stability due to its noisy start-up but is very flexible to generate ultrashort X-ray pulses down to attosecond durations. Various advanced schemes aiming at achieving fully coherent, stable X-ray pulses are proposed and are actively being investigated and developed.
Free Electron Lasers
Impedance-Induced Beam Instabilities
Modern particle accelerators require ever higher currents to meet user demands, both for high-energy physics experiments and for medical and industrial applications. These high currents, interacting with the accelerators’ environment, produce strong self-induced electromagnetic fields that perturb the external fields that guide and accelerate the charged particles. Under certain conditions, these perturbations can be so large as to limit the accelerators’ performance, giving rise to unwanted effects such as uncontrolled beam oscillations or instabilities. The self-induced fields are described in terms of the so-called wakefields and beam coupling impedances, two quantities that are used to evaluate their impact on beam dynamics and instabilities’ thresholds. The determination of wakefields and beam coupling impedances generated by the interaction of the beam with the different machine devices, and of the corresponding induced instabilities, is therefore very important, particularly for high currents. This is carried out with analytical approaches, through the use of simplified models, or, more rigorously and realistically, through simulation codes. The first step in this study is generally represented by a complete electromagnetic characterization of the different accelerator devices and the search for possible minimization of wakefields and beam coupling impedances. Once these quantities are known, their effect on beam dynamics can be evaluated, with both simulations and analytical methods, and a proper machine working point, far away from any impedance-induced beam instabilities, can be determined. As machine performance is pushed increasingly higher, new effects, produced by wakefields and beam coupling impedances, are found that are related, in many cases, to the coupling with other mechanisms (e.g., with beam–beam). All these effects can no longer be studied separately. Finally, mitigation solutions, such as beam coupling impedance optimization, feedback systems, the use of nonlinearities, and other techniques, must also be investigated so that different tools will be available to counteract unwanted beam-induced instabilities.
Self-Polarization in Storage Rings
This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. The conditions for the Sokolov-Ternov effect to occur are approximately satisfied by electrons (or positrons) circulating on the design orbit of a planar storage ring. Indeed, self-polarization was first observed in the electron/positron colliders Anneau de Collisions d’Orsay (ACO) and VEPP-2. Beam polarization offers an additional tool for understanding the physics events. The possibility of having polarized electron/positron beams for free is therefore appealing. However, the Sokolov-Ternov polarization time constant, proportional to 1/γ5 and to the third power of the bending radius, restricts the region of interest for self-polarization. For the about 100 km Future Circular Collider (FCC) under study at CERN, the polarization constant is about 10 days at 45 GeV beam energy. At high energy the randomization of the particle trajectory due to photon emission in a storage ring with finite alignment precision of the magnets introduces spin diffusion and limits the attainable polarization. In addition, in a collider the force exerted by the counter-rotating particles impact the beam polarization. This force increases with beam intensity and experiments are reluctant to pass up luminosity for polarization. To this day the electron(positron)/proton collider HERA has been the only high energy collider where electron (and positron) self-polarization was an integral part of the physics program.
Vladimir D. Shiltsev
Understanding the universe critically depends on the fundamental knowledge of particles and fields, which represents a central endeavor of modern high-energy physics. Energy frontier particle colliders—arguably, among the largest, most complex, and advanced scientific instruments of modern times—for many decades have been at the forefront of scientific discoveries in high-energy physics. Because of advances in technology and breakthroughs in beam physics, the colliding beam facilities have progressed immensely and now operate at energies and luminosities many orders of magnitude greater than the pioneering instruments of the early 1960s. While the Large Hadron Collider and the Super-KEKB factory represent the frontier hadron and lepton colliders of today, respectively, future colliders are an essential component of a strategic vision for particle physics. Conceptual studies and technical developments for several exciting near- and medium-term future collider options are underway internationally. Analysis of numerous proposals and studies for far-future colliders indicate the limits of the collider beam technology due to machine size, cost, and power consumption, and call for a paradigm shift of particle physics research at ultrahigh energy but low luminosity colliders approaching or exceeding 1 PeV center-of-mass energy scale.