Linus Hägg: Analysis of neutron cameras and spectrometers with digital data acquisition at tokamaks

Abstract

The neutron emission from a fusion plasma can be measured with neutron diagnostics and analysed to estimate plasma quantities. This thesis covers the subject of neutron detection methods using scintillators. It follows the detection process from the scintillation phenomenon and the hardware solutions in the data acquisition, to the software data handling and pulse analysis in the data reduction. It also covers the relationship between the measured scintillation light pulse to the incident neutron energy, using light yield calibration methods and system response functions. In particular, this thesis has focused on the development of methods and codes that allows us to exploit the possibilities offered by fully digital data acquisition systems.

In the latter half of the thesis, these methods are put into practice for two neutron diagnostic systems at JET, the neutron camera and the MPRu neutron spectrometer. The neutron camera is used to estimate the estimate the volume integrated neutron yield. The method was developed in two iterations, and was applied to measurement data from recent JET DD and DT experiment campaigns. In its latest iteration, the method absolutely calibrates the neutron camera. Comparisons with neutron yield estimates from the JET fission chambers reveal inconsistencies between the two instruments, and between the two iterations of the method for the neutron camera. These discoveries prompt further investigation into the method.

For the MPRu, a framework was developed for estimating two fusion plasma quantities; the plasma rotation and the thermonuclear emission. The line of sight for the MPRu is advantageous for evaluating these quantities in the core of the plasma. The framework shows great promise, and has the potential of providing complementary measurements to diagnostics which have trouble penetrating into the core.

The techniques developed in this work can be refined for their current use, and may also be adapted for other similar neutron diagnostic systems.