Controlled Trapping in Laser Wakefield Accelerators

Sammanfattning: Experimental studies of laser-driven acceleration of charged particles, in particular electrons and protons, are described in this dissertation. Tightly focused femtosecond laser pulses with intensities exceeding 1018 W/cm2 were used to accelerate charged particles to high energies over distances of a few millimeters. Although the transverse fields of such laser pulses are sufficient to directly accelerate electrons to relativistic energies, the direction of the fields changes rapidly. Thus, direct acceleration using the electromagnetic fields of such laser pulses is not suitable for generating beams of charged particles. Instead, the directed electric fields that are generated in the interaction between the laser pulses and plasmas can be used to accelerate particles to high energies. The laser wakefield acceleration technique is based on the excitation of a plasma wave by a laser pulse that propagates through an underdense plasma. Electric fields on the order of 100 MV/mm, directed along the optical axis, are associated with the plasma wave with a wavelength on the order of 10 µm. A short pulse of electrons, with a length of only a fraction of the plasma wavelength, may be accelerated in the wave to several hundreds of megaelectron-volts. Simultaneously, strong electric fields act as a radially restoring force on the electrons undergoing acceleration, which therefore perform transverse oscillations. This leads to the generation of X-rays with energies on the order of a kiloelectron-volt in a beam directed along the optical axis, with an opening angle on the order of a hundred milliradians. The physical processes involved in laser wakefield acceleration are highly nonlinear, and the resulting beams of electrons are therefore sensitive to small fluctuations in the properties of the laser pulses, and to density variations in the plasma. Controlling the amount of charge in the electron bunches and the energy of the electrons is crucial to increase the possibility of using laser wakefield accelerators in practical applications. One step towards stable generation of beams of electrons using laser wakefield accelerators is to control the location at which the electrons are trapped in the accelerator and, at the same time, control the number of trapped electrons. This dissertation describes studies on different methods of controlled trapping of electrons in laser wakefields. The methods described include trapping triggered by plasma density modulations, trapping of tightly bound electrons released by photoionization, and trapping triggered by beating of electrons in the beat wave generated by two colliding laser pulses. The properties of the electron beams generated using different trapping techniques are then compared. The experimental studies showed that both the amount of charge and the electron energy distribution can be controlled using any one of these methods, and that shot-to-shot fluctuations in charge and peak electron energy below 10 % and 5 %, respectively, can be achieved. The target-normal sheath acceleration technique for proton and positive ion acceleration is based on the interaction between a focused femtosecond laser pulse and a micrometer-thick metallic foil. The surface of the foil is irradiated by a laser pulse at an intensity above 1019 W/cm2. Electrons heated in the interaction between the laser pulse and the plasma formed on the foil surface are driven through the foil and exit from the rear side. The resulting charge separation leads to an electric sheath field of the order of TV/m, which is quasi-static on the time scale of a few picoseconds. Contaminants, typically water molecules and hydrocarbon compounds, on the rear surface of foil, are ionized in the strong field and the positive ions, predominantly protons because of their high charge-to-mass ratio, are accelerated toward the electrons. Studies on spatial shaping of the proton beams generated in this process are described in the Appendix of this dissertation. The shape of the sheath field on the rear side of the foil was manipulated by modifying the profile of the laser pulse irradiating the foil. The divergence of the beam of protons generated using this technique could be decreased by splitting the laser beam, and irradiating the foil using two spatially separated pulses.