Dottorato in FISICA DEGLI ACCELERATORI

Titolo della tesi: Radiation effects for the next generation of synchrotron radiation facilities

High energy radiation is an important tool for many fields of research as it allows for the measurement of smaller structures and atomic interactions. The current best method of generating coherent and narrow bandwidth synchrotron radiation is with a free electron laser. It requires very high charge density, to start the amplification process and concurrently leads to its high level of coherency, and high energies (GeV to obtain keV photons). The stringent parameters on the electron bunch are met by linear accelerators. These are typically kilometre long straight structures that operate from tens to 100 Hz repetition rate. A novel design was proposed by the INFN Milan research group called MariX [1]. Here a LINAC is used in combination with a com- pression arc. This reduces the size of the facility, because the electron bunch can be accelerated twice by the same LINAC. As the electrons pass through dipoles in the compression arc the fields emanating from the particles in the bunch can cause deterioration to it. These fields, consisting out of the relativistic Coulomb- and radiation field, travel with the speeds of light, and thus originate from a point in the past. For this reason the behaviour of these retarded fields is investigated from first principles and developed into a 3D algorithm for calculating the forces within a bunch. An in depth overview is given on how the constituent fields behave over a large range of electron energies. Proportionality relations are given that determine which one is dominant. To reach unprecedented high energy photons is through the scattering of intense lasers with electron bunches; (inverse) Thomson or Compton scattering. Photon energies of keV can be reached with tens of MeV electrons, and MeV photons with GeV electrons. High repetition rate collisions are possible with cavity based laser systems. Currently the power in- side the cavity is several hundreds of kW with an intensity at the focus up to 1014−15 [W/cm2 ]. With these high powers the cavities can become degenerate, i.e. higher order transverse modes are excited, either by imperfections of the mirrors or deformations caused by heat dissipation. A short study provides insights to the observability of these modes in the Thomson spectrum. The general method for Thomson scattering is to have a (quasi) monochromatic laser pulse collide with an electron bunch with a very small energy spread. The latter usually leads to a reduction of the number of charges, and therefore the flux of scattered photons. The frequency of the scattered radiation is linearly dependent on that of the laser’s, and therefore the energy spread of the electrons could be compensated by including a frequency modulation. The highest intensity lasers obtained are by chirped pulse amplification and thus readily available. Two schemes have been investigated: longitudinal and transverse chirp. Both can reach the limit in bandwidth and number of photons scattered of the mono- energetic and mono-chromatic case. For ultra shorted pulses the carrier envelope phase becomes an important variable. Thomson scat- tering can be used to measure For intensities where non-linear effects dominate, because the scattered radiation contains the information of the laser pulse.. A model of its signature in the Thomson spectrum has been developed: it shifts the peaks of higher harmonics that overlap. This shift is also correlated to the emission direction of harmonics. A detailed analysis is given how to measure it experimentally.