Development and calibration of a high-dispersive Thomson parabola for laser-driven ion beams

Modern high-power laser systems like the Vulcan Laser at Central Laser Facility at Rutherford Appleton Laboratory inOxfordshire, the OMEGA Laser at Laboratory for Laser Energetics (LLE) in Brighton in New York as well as the PHELIXLaser at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt can generate sub-ns laser pulses with an energyof several Joule. When focused on a few m2, power densites of up to 1021Wcm2 [1] can be achieved.If such a laser pulse interacts with matter, different dynamic processes can occur. These processes can lead to thegeneration of high-energy ion beams which were observed in experiments ([2], [3]) as well as in simulations ([4], [5]).This is called laser-driven ion acceleration, which itself can be divided in several underlying mechanisms, from which thetarget normal sheath acceleration (TNSA) maybe is the most prominent one.Because the acceleration happens inside a plasma, there is no limitation of the electric field strength due to breakdownas there is for the cavities of conventional accelerators. Instead, the electric fields can be in the TVm1 range, severalorders of magnitude above the field strength of conventional accelerators. Therefore ions can reach several MeV afteronly some m instead of several meters.Such a laser accelerated ion beam can contain up to 1013 particles [2], which is equivalent to a current of 1600 A forthe duration of 1 ns. An additional advantage of this kind of ion beams is their low emittance compared to conventionalaccelerator beams [6]. For laser accelerated beams it was observed to be smaller than 0.004mmmrad [7], while LINACion beams typically have an emittance of bigger than 0.8mmmrad [8].There are many different possible applications for laser-driven ion beams. One is the proton fast ignition, which could bea key on the way of realizing inertial confinement fusion ([9],[10]).Other possibilities are proton radiography [11] as well as the generation of exotic states of matter, namely the regime ofwarm dense matter [12].There are also attempts to develop a new generation of compact laser-driven ion accelerators [13], which could haveapplications for example in the cancer therapy [14].Despite the intensive experimental and theoretical investigation by many groups worldwide, the details underlyingthe different acceleration mechanisms, especially newer ones like RPA [15] and acceleration in the relativistic transparencyregime [16], are not fully understood yet.When doing experiments with laser-driven ion beams, it is essential to get information about the beam characteristic,especially the obtained ion charge states and the energy spectra. Therefor, a Thomson parabola spectrometer [17] isan excellent and commonly used diagnostic since it is easy to handle. In addition it can be relatively insensitive forelectromagnetic pulses (EMP) as they occur in high-power laser experiments and allows to receive the energy spectra fordifferent ion species separately. It is based on deflection of the ions in a magnetic plus an electric field, which separatesthe ions regarding to their q/m ratio and energy. The spatial detection of the projected traces allows to deduce a spectrum.The group of Prof. Dr. Markus Roth had one type of Thomson parabola available which was very limited in its applicationdue to its low energy resolution for the higher energies as well as due to perturbations from EMP.Therefore, the aim of this master’s thesis was to design and construct a new Thomson parabola which allows to investigatelaser accelerated ion beams of high energies. Since new acceleration mechanism let expect proton energies of85 MeV and beyond ([18], [19]), the spectrometer has to be able to detect and resolve such energies.Two important limiting factors of any Thomson parabola are its energy resolution and the merging of the different iontraces. Therefore the energy resolution and the separation of other ion traces, especially protons and C6+, have to matchthe requirements.From the energy spectra as well as the ratio of the different ion species, much can be learned about the underlyingacceleration mechanisms.This thesis is divided into six chapters:The first gives a short overview of the theoretical basics of laser ion acceleration mechanisms. Afterwards some importanttypes of diagnostics for investigation of such ion beams are introduced. The third chapter is a short summary of thesimulation results done during the proposal of this masters’s thesis. The next chapter describes the design of the newThomson parabola, and the fifth chapter is about the first experimental application and its results. The last chapter givesa summary and some suggestions for further improvements