Bremsstrahlung | EPFL Graph Search

In particle physics, bremsstrahlung ˈbrɛmʃtrɑːləŋ (ˈbʁɛms.ʃtʁaːlʊŋ; ) is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into radiation (i.e., photons), thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the decelerated particles increases. Broadly speaking, bremsstrahlung or braking radiation is any radiation produced due to the acceleration (positive or negative) of a charged particle, which includes synchrotron radiation (i.e., photon emission by a relativistic particle), cyclotron radiation (i.e. photon emission by a non-relativistic particle), and the emission of electrons and positrons during beta decay. However, the term is frequently used in the more narrow sense of radiation from electrons (from whatever source) slowing in matter. Bremsstrahlung emitted from plasma is sometimes referred to as free–free radiation. This refers to the fact that the radiation in this case is created by electrons that are free (i.e., not in an atomic or molecular bound state) before, and remain free after, the emission of a photon. In the same parlance, bound–bound radiation refers to discrete spectral lines (an electron "jumps" between two bound states), while free–bound radiation refers to the radiative combination process, in which a free electron recombines with an ion. Larmor formula If quantum effects are negligible, an accelerating charged particle radiates power as described by the Larmor formula and its relativistic generalization. The total radiated power is where (the velocity of the particle divided by the speed of light), is the Lorentz factor, is the vacuum permittivity, signifies a time derivative of , and q is the charge of the particle.

Experimental analysis of suprathermal electrons generated by electron cyclotron waves in tokamak plasmas

Dahye Choi

This thesis focuses on the physics of suprathermal electrons generated by electron cyclotron (EC) waves in tokamak plasmas, which play an important role in the physics of current drive and energetic particle-driven instabilities. The suprathermal electron dynamics and their effect on the plasma stability have been experimentally studied utilizing high power EC waves, in the TCV tokamak of the Swiss Plasma Center at EPFL, Switzerland. A hard X-ray diagnostic, which measures the bremsstrahlung radiation of the suprathermal electrons in radial and energy spaces, has been mainly used for the analysis, and the measurement has been compared to an estimation made by Fokker-Planck modeling coupled with a hard X-ray synthetic diagnostic. In order to study the response of the suprathermal electrons to ECCD, ECCD modulation discharges have been developed. The time evolution of the hard X-ray profiles has been measured using coherent averaging techniques in order to observe the creation and relaxation of suprathermal electrons. Time-dependent Fokker-Planck modeling coupled with the hard X-ray synthetic diagnostic has been used to compare the experimental and simulation results, with various suprathermal electron transport models. A dependency of the radial transport of suprathermal electrons on the EC wave power has been demonstrated and a possibility of EC wave scattering has been addressed. The effect of the suprathermal electron population on the plasma stability has been studied, and in particular the destabilization and dynamics of the electron fishbone mode. The response of hard X-ray profiles to the internal kink mode has been observed directly by the hard X-ray diagnostic for the first time, at the frequency of the mode. The experimental evidence and a solution of a linear fishbone dispersion relation coupled to the Fokker-Planck modeling demonstrate the role of suprathermal electrons in destabilizing the fishbone mode and in particular the interaction of trapped electrons with the mode. This work provides the framework for a comprehensive understanding of the physics of suprathermal electrons related to ECCD, and explains how ECCD-generated suprathermal electrons behave in real and velocity spaces, and how they interact with and are redistributed by the MHD mode.

EPFL2020

SWISSCASE

Michael Bodendorfer

This PhD thesis consists of planning, simulating, building, testing and characterizing a new electron cyclotron resonance (ECR) ion source, SWISSCASE. the Solar Wind Ion Source Simulator for the CAlibration of Space Experiments located at the University of Bern, Switzerland. The ion source will be operated in the existing CASYMS ultra high vacuum (UHV) facility and will extend the application field of the existing filament electron collision ion source inside CASYMS and the existing electron cyclotron resonance ion source MEFISTO, operated in its own UHV facility. The chosen ECR ionization concept operating at 10.88 GHz delivers high currents of highly charged ions of up to 2 µA for Ar8+ and a maximum identified charge state of Ar12+. In addition to argon, the ECR plasma of SWISSCASE has been operated with krypton, xenon and carbon dioxide gas, revealing all of the expected charge states and featuring a charge states distribution in favor of highly charged ions. Design constraints were given by the final application of SWISSCASE being implemented in CASYMS with limited space and power. The new ion source was realized with limited funding. Numerical simulations gave insight in unprecedented quality and detail about the complex three dimensional extent of the magnetic field distribution of the full permanent magnet confinement of both, SWISSCASE and MEFISTO, the second ECR ion source, operated since 1997 at the University of Bern. For both confinement systems, the isocontour surface, defined by the electron cyclotron resonance condition of the incident microwave, is visualized in 3D. In addition to the realization of the ECR ion source, a plasma Bremsstrahlung measurement revealed an average ECR electron temperature of 10 keV for an argon plasma. This temperature was used to perform numerical simulations of electron trajectories inside the SWISSCASE and the MEFISTO confinement. The simulated magnetic field distribution of MEFISTO and the simulated electron distribution of SWISSCASE are closely related to the observed triangle shaped surface coating patterns found in hexapole confined ECR ion sources.

EPFL2008