Knobloch-Maas, Renate (2009)
Measurement of the Energy Loss of Heavy Ions in Laser-produced Plasmas.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The interaction of ions with plasma is not yet fully understood today, although it is important for inertial fusion technology. During recent years, the energy loss of heavy ions in plasma has therefore been a subject of research in the Laser and Plasma Physics group of Darmstadt University of Technology. Several experiments were carried out at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt using laser-created plasma, thereby taking advantage of the unique combination of GSI's accelerator facility and the laser system nhelix, which is also described in this work. The experiments focus on the measurement of the energy loss of medium heavy ions in a plasma created by directly heating a thin carbon foil with the nhelix laser, at an energy of about 50 J. In order to measure the energy loss using a time-of-flight method, a stop detector is used to register the arrival of the ion pulses after passing the plasma and a 12 m drift space. At the beginning of the work on this thesis, the ion detector types formerly used were found to be inadequately suited to the difficult task; this was changed during this thesis. The ion detector has to be able to temporarily resolve ion pulses with a frequency of 108 MHz and a width (FWHM) of 3 ns at a very low current. It also has to withstand the X-ray burst from the plasma with a dead time shorter than the difference between the X-ray and the ion time of flight between the plasma and the detector. In order to satisfy these and other demands, a new diamond detector was designed and has now been used for several measurements. In addition to the new detector, other improvements were made concerning the diagnostics and the laser. The laser-created plasma now reaches a maximum temperature exceeding 200 eV and a free electron density of up to 10^22 cm^-3. With this greatly improved setup, energy loss data could be obtained with a temporal resolution several times better than before, using an ion beam with a diameter of only 500 µm. Additionally, thin foils of 0.5 µm thickness could be used, compared to 1.5 µm used before. These foils are very quickly turned into a fully ionized plasma by the laser pulse, so the resulting plasma is more homogeneous than with thicker foils. With the previous detectors, thicker foils had to be used due to the lower detector resolution. The newly acquired energy loss data exhibits some unusual characteristics. At the beginning of the interaction of laser and plasma, the energy loss first decreases, then increases again, up to an energy loss 30% higher than the energy loss in the cold matter, then continues to decrease slowly. Several changes were made to study possible explanations for this behavior, including a change in the geometry of the setup to investigate whether fields could be responsible for the decrease.

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