Measuring fast ions in fusion plasmas with neutron diagnostics at JET

Abstract

Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfvén eigenmodes and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.

Generation and observation of fast deuterium ions and fusion-born alpha particles in JET $\mathrm{D-^3He}$ plasmas with the 3-ion radio-frequency heating scenario

“…First, we note several peaks in the interval between 2 MeV and 5 MeV. Those in the energy range E γ =2 MeV to 4 MeV originate from nuclear reactions between fast D ions and 9 Be, namely d + 9 Be → n + 10 B * and d + 9 Be → p + 10 Be * , and that result in the production of the excited heavy nuclei 10 B * and 10 Be * , which decay through the emission of a gamma-ray [22]. 9 Be is the main low-Z impurity in JET-ILW plasmas.…”

“…A direct confirmation of the co-passing nature of the fast deuterons comes from a detailed analysis of the shape of the 2.87 MeV gamma-ray peak born from D + 9 Be nuclear reactions and measured with a high purity Germanium (HpGe) detector. In Fig.…”

Plasma heating and generation of energetic D ions with the 3-ion ICRF + NBI scenario in mixed H-D plasmas at JET-ILW

“…(Fig. 3(b) in [29]). (b) TOFOR neutron energy spectrum for JET pulse #91256, t = 9-11.5s (PICRF/PNBI ≈ 0.7, PICRF ≈ 2.5MW, PNBI ≈ 3.5MW).…”

“…Thus, shorter times-of-flight correspond to higher neutron energies, and higher neutron energies are a sign of more energetic D ions in the plasma. Based on the knowledge of the response function of TOFOR and from the kinematics of the D-D fusion reactions, a relation between a given time-of-flight and a minimum D energy corresponding to a given tTOF has been derived [29], see Fig. 6(a).…”

Fast-ion orbit sensitivity of neutron emission spectroscopy diagnostics

“…At the JET tokamak, neutron spectroscopy is well-established 38 , velocity-space weight functions have already been developed 33,34 and it is used as a main diagnostic for fast-ion studies. 38,39 . The development of NES orbit weight functions will enable more detailed studies of fast ions in burning plasmas at JET in the upcoming DT campaign.…”