Lev Landau | EPFL Graph Search

Lev Davidovitch Landau (en Лев Давидович Ландау, ), né le à Bakou (Empire russe) et mort le à Moscou (Union soviétique), est un physicien théoricien soviétique. Il est lauréat du prix Nobel de physique de 1962 mais ses contributions à la physique vont bien au-delà et couvrent de nombreuses branches où il apporta des formalisations théoriques des phénomènes de la mécanique des fluides à la théorie quantique des champs. Il élaborera ainsi un formalisme théorique des transitions de phase de deuxième ordre, de la supraconductivité (théorie de Ginzburg-Landau), du diamagnétisme, des liquides de Fermi, etc. Il est aussi crédité d'avoir anticipé l'existence d'étoiles à neutrons (ou, du moins, d'étoiles denses de particules subatomiques sans charge électrique) avant même la découverte des neutrons en 1932. Biographie Enfance et jeunesse Il naît à Bakou dans une famille juive dont le père, David, est ingénieur dans le pétrole et a été aussi administrateur de la compag

Transverse Noise, Decoherence, and Landau Damping in High-Energy Hadron Colliders

Sondre Vik Furuseth

High-energy hadron colliders are designed to generate particle collisions within specialized detectors. A higher number of collisions is achieved with high-quality beams of low transverse emittances, meaning a small transverse cross-section, and high intensity, meaning many particles per bunch. This thesis studies how noise negatively impacts the beam quality in high-energy hadron colliders, both in terms of beam instabilities and emittance growth. The impact is analyzed through the derivation of new theories, multi-particle tracking simulations, the numerical solving of partial differential equations, and dedicated experiments in CERN's Large Hadron Collider (LHC). The impact of noise on beam stability cannot be treated with the first-order, linear Vlasov equation, which is commonly used to study the thresholds of collective instabilities. Therefore, the Vlasov equation has in this thesis been expanded to second order in the perturbation of the beam distribution, finding a diffusion mechanism driven by the interplay between noise, decoherence, and wakefields. The diffusion leads to a local flattening of the distribution, which can cause a loss of Landau damping after a time delay referred to as the latency. An analytical formula for the latency and a specialized numerical diffusion solver were successfully benchmarked against the latency measurements in a dedicated experiment conducted in the LHC. Precaution in the machine operation has to be taken to account for this new mechanism. In particular, it is found that the machine must be operated with a margin to the linear stability threshold. For the case of the LHC, it has previously been found empirically that the octupole current during operation must be increased by about a factor 2, and this thesis provides the explanation as to why that is. Alternative operational settings are suggested to reduce the required octupole current in the LHC. In addition, the new theory allows for extrapolations to future machines, such as the High-Luminosity LHC, as well as the estimation of the impact of new devices, such as crab cavities. External noise and noise from the transverse beam feedback system cause an emittance growth rate due to decoherence of the noise kicks. Analytical theories for the suppression of the emittance growth rate with a bunch-by-bunch feedback have here been extended to a multi-bunch feedback. The numerical study of suppression during collision was conducted by means of a newly developed parallel multi-beam multi-bunch algorithm. For the typical case of low-frequency external noise and non-negligible feedback noise, a multi-bunch feedback has both analytically and numerically been found superior to a bunch-by-bunch feedback, as it can suppress the impact of the external noise equally well, while simultaneously reducing the noise generated by the feedback itself. Suggestions for a more optimal operation of the LHC are discussed, including a reduction of the upper cutoff frequency of the feedback system.

EPFL2021

Beam Transfer Function measurements and transverse beam stability studies for the Large Hadron Collider and its High Luminosity upgrade

Claudia Tambasco

The motion of high energetic particle beams in accelerators is influenced by their interactions with the accelerator environment through electromagnetic fields induced by the particle passages. Traveling with a speed close to the speed of light, the particles induce image charge and currents in the surroundings generating wake-fields that act back on the beams. Destabilizing effects may arise from the coupled motion between the circulating particles and the induced wake fields compromising the accelerator performances. The stability of the beams is ensured by the Landau damping of coherent motions generated by the diversity of oscillation frequencies of the particles in the beams. Under the effect of non linear forces produced by machine non linearities or beam-beam interactions the particles oscillate with slightly different frequencies depending on their amplitudes in the beams (tune spread). At the Large Hadron Collider (LHC) transverse instability thresholds are evaluated via the computation of the dispersion integral that depends on the tune spread in the beams as well as on the particle distribution. A large tune spread is beneficial for the Landau damping as long as no diffusive mechanism is present. In the presence of diffusive mechanisms, caused by resonance excitations or noise, the stability diagram can be deformed due to the modification of the particle distribution inside the beam leading to a possible lack of Landau damping of the impedance coherent modes previously damped by lying within the unperturbed stability area. This work aims to experimentally explore the transverse stability of the beams by means of Beam Transfer Function measurements at the LHC. They provide direct measurements of the stability diagrams and are sensitive to particle distribution changes. First measurements of the Landau stability diagrams at the LHC are presented and compared to model expectations. Experimental studies have been carried out in the presence of different sources of non linear effects such as octupole magnets and beam-beam interactions and compared to the model expectations. Limitations deriving from transverse coherent instabilities in the LHC are analysed and possible explanations for the observed LHC instabilities are discussed. In the perspective of the High-Luminosity LHC upgrade (HL-LHC), transverse stability studies for different beam-beam interactions and machine configurations are presented together with possible solutions to compensate reductions of the Landau damping during the operational phases of the HL-LHC.

EPFL2017

Long-term evolution of Landau damping in the presence of transverse noise, feedback, and detuning

Xavier Buffat, Sondre Vik Furuseth

The effect of Landau damping is often calculated assuming a Gaussian beam distribution in all transverse degrees of freedom, which agrees reasonably well with beam measurements. However, the stability of the beam is strongly dependent on the details of the distribution. The present study focuses on the slow evolution of the transverse bunch distribution, for a bunch excited by a coherent white noise source, damped by a transverse feedback system, and with detuning dependent on the transverse actions. The mechanism is modeled by the Fokker-Planck equation. It corresponds to a diffusion that is zero for particles with tune equal to the average tune of the bunch, and which is growing quadratically with the tune in the vicinity. The evolving distributions are then used to calculate the evolving stability diagrams, and thus the long-term evolution of the Landau damping. The relative effective octupole current is reduced faster for stronger noise, stronger linear detuning coefficients and weaker damper gain. The relevant parameters for this mechanism arc scanned. With relevant parameter values for the LHC, this mechanism can cause a reduction of the effective octupole current by at most 10%/h.