Riassunto analitico
A great deal of information on the fuel ions in a hot plasma can be extracted from characteristic features in the neutron emission spectrum. In particular, neutron emission spectroscopy (NES) can provide information on the ion temperature, density, fuel ion ratio and can be used to measure the fuel ions distribution function. By the same token, this already well-established and prolific plasma diagnostic technique might become in the future the primary fast-ion diagnostic for magnetically confined plasmas, following the increase in neutron yield that will accompany the next high power fusion devices. At present, the highest performance in the NES diagnosis of D plasmas is attributed to the time-of-flight spectrometer optimized for rate (TOFOR) installed at JET and based on the time-of-flight technique. However, the TOFOR spectrometer requires a large experimental area, and cannot be implemented where space constraints are present, as for instance into a neutron camera configuration.
The work presented in this thesis concerns the development and characterization of a 2.5 MeV neutron spectrometer based on a C7LYC crystal, hereby called CLYC detector for simplicity, which has been recently installed at the ASDEX Upgrade mid-sized tokamak. This relatively new inorganic scintillator features a good resolution and neutron/gamma discrimination via pulse shape discrimination (PSD), and a Gaussian response function to 2.5 MeV neutrons that favors spectroscopic measurements. Most importantly, when compared to the bulky design of TOFOR and its operational complexity, the CLYC detector strikes as low cost, simple, and compact alternative that allows for installation in a multi-line-of-sight camera. These characteristics make a CLYC neutron spectrometer rather easy to handle and implement in large-sized tokamaks.
This dissertation begins with a general smattering on the principles of neutron and gamma-ray detection and spectroscopy, to frame the purpose within the current context of plasma diagnostics, followed by a description of the CLYC detector, the characterization measurements, and the experimental setup, concluding with the results.
The study starts with the development of a Python algorithm for waveform post-processing and the identification of the best integration gate lengths for optimization of the PSD technique. Then, two CLYC detectors of different sizes are taken into consideration and tested for their linearity and energy resolution using several gamma-ray sources of different energies. Afterwards, the neutron response is investigated in dedicated experiments carried out at the Frascati Neutron Generator (FNG-ENEA) facility, where the two detectors have been exposed to fast neutrons in the 2 to 3 MeV energy range. The neutron-to-gamma-ray light yield ratio of the CLYC crystal and the energy resolution to neutrons is estimated from these measurements. A comparison with a pristine CLYC crystal is subsequently used to assess any long-term activation due to neutron bombardment. Lastly, a first measurement is conducted on a real tokamak plasma at ASDEX Upgrade, showing the CLYC capability to acquire valuable neutron spectra in short discharges. A preliminary data analysis allowed the assessment of the presence of fast ions due to external heating.
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