Entwicklung und Optimierung sphärischer Ionisationskammern zur Bestimmung der Ionentemperatur in Fusionsplasmen

The development of spherical Pulse Ionisation Chambers for neutron spectroscopy on DD-fusion plasmas is presented. The chambers are filled with Methane plus a small amount of $^{3}$He. Two chambers with different aspect ratios, $\alpha$, (i. e. the ratio of outer to inner electrode radii) have been investigated. The energy resolution for the 765 keV peak from the $^{3}$He(n$_{th}$,p)T reaction without correction of electronic noise is 3.27 % and 3.07 % for aspect ratios of 30 and 40, respectively. These results are confirmed using quasi monoenergetic neutrons generated by a proton beam impinging on a thin solid Ti-T target. Hence, from a practical point of view, ion temperatures $\geq$ 1 keV can be determined if the statistics of the data are not too poor. Instead of unfolding the proton recoil spectrum, which requires good statistics, the response function is deduced from a normal neutron distribution together with the physical dimensions of the counter and fitted to the high energy flank of the spectrum. A detailed investigation of the time behaviour of this chamber type is performed because the charge collection time and the resultant pulse shaping time for optimum counter resolution are strongly correlated with each other. The experiments performed with the Van de Graaff accelerator clearly indicate that massive shielding and collimation of the detector will be essential for the performance of high resolution measurements. Preliminary results from the experiment JET are presented and compared with results of other methods of ion temperature measurements