Measurement of Electron Parameters using Thomson Scattering

Laser-plasma acceleration (LPA) [1] oers acceleration gradients several orders of magnitudehigher than in conventional radio-frequency accelerators, enabling the accelerationof electrons to hundreds of MeV in just a few millimetres. These small-scale acceleratorsand in particular X-rays generated from LPA electron beams could potentially be used inmaterial science or medicine [2]. However, LPAs are not commercially used yet, partlybecause repetition rate and shot-to-shot beam stability are not sucient for industrial ormedical applications and the investigation of the long-term stability on the order of severalhours required for the industrial use of LPA has only just started [3, 4].In this thesis, an electron source was developed to match parameters required for theuse of LPA in X-ray uorescence imaging experiments [5-7]. To generate stable electronbeams, self-truncated ionisation injection using a weakly relativistic laser was experimentallyimplemented for the rst time [8]. The long-term stability of these beams wasinvestigated in an 8 hour run at a repetition rate of 2:5 Hz, resulting in a total of 72000acceleration events. During this time, the average charge stayed constant while the meanenergy decreased by 7 percent, showing the necessity to further investigate long-term stabilityLPA, as such a drift would likely not have been detectable from the only tens tohundreds of shots that are usually considered.Furthermore, a new diagnostic technique based on Thomson scattering [9] was developed,which oers the rst in-situ measurement of electron parameters during the accelerationinside the wakeeld. Using this technique, the electron-energy evolution wasmeasured inside the plasma, showing an energy increase from 35MeV to 61MeV over adistance of 400 µm. The results agree well with particle-in-cell simulations and were usedto study the dephasing of the electron bunch inside the accelerator. By combining measurementsand simulations, the inuence of the laser strength parameter on the dephasinglength could be isolated, showing the power of this method.This newly developed diagnostic technique could be transferred to other electron parameterssuch as divergence or emittance [10-12] to obtain a full picture of the electronbunch during acceleration. This could help to study experimentally issues of LPA such asemittance growth and increase the stability of electron beams from plasma accelerators.In combination with further long-term stability studies, these measurements could helpto transfer LPA from proof-of-principle experiments to X-ray sources for a broad range ofapplications