Development of a 2.5 MeV neutron spectrometer for JT-60SA – plasma strand project

Supervisors: Marco Cecconello (Durham University), Anders Hjalmarsson (Uppsala).

JT-60SA is the largest superconducting tokamak in the world and it has just recently achieved its first plasmas indicating that it is ready to continue to the physics exploitation phase. The scientific and technical goals of JT-60SA is to address key physics and engineering issues for the realization of commercial fusion energy and to support the operation of ITER and the design of DEMO. The aim of JT-60SA is to complement ITER in key physics areas necessary for the development of DEMO. For this reasons, JT-60SA has been designed to be able to achieve break-even in deuterium plasmas with long plasma operations (about 100 s) by non-inductive current drive using superconducting magnetic field coils. Key areas of research are the development of operating regimes with feedback control at high performance using ITER approach in steady state with high plasma pressure and with real-time control of MHD instabilities, the study of fast particle physics, plasma transport and turbulence for the confirmation ITER scaling laws with a focus on the transition from low to high confinement regimes and associated edge localized modes, the demonstration of the handling capability of the power exhaust in the divertor in high performance plasmas at a high heating power for long pulses. JT-60SA will be equipped with external auxiliary heating systems in the form of positive and negative neutral beam injection (NBIs) of different energies (85 and 500 keV) resulting in fast ions populations with different energy and pitch-angle distribution, driving different fast particle instabilities.

In addition, the orbits of the MeV D ions (from the negative NBI) are comparable to α-particles orbits expected in ITER and DEMO. The emission of neutrons from these plasmas can be harnessed for teal-time plasma position control based on collimated neutron flux monitors, to study the off-axis NBCD efficacy and optimization since the behaviour of fast-ions during their slowing-down processes is also a key issue to understand the current drive dominated by these fast-ions as well as for the development of operating scenario development via fuel ion density and temperature measurements. These can be inferred by the measurement of the neutron emissivity energy spectrum as they are highly sensitive to the underlying fast ion population in the plasma. A key instrument for these studies is a neutron spectrometer optimized for 2.5 MeV neutrons. Such an instrument (TOFOR) is installed at JET and is based on the so-called time-of-flight method where the neutron energy is determined by measuring the time the neutrons take to travel between two detectors (start and stop detector) with a well characterized geometrical relationship. These detectors however detect also a large component of neutrons that have not been detected in both detectors (false coincidences) and in recent years optimized detection techniques have been developed to reduce the false coincidences thus enhancing the signal to noise ratio enabling more accurate physics studies on JET.

The aim of this project is to improve the design of the present TOFOR neutron spectrometer that is installed at JET, the development and refurbishment of some scintillators and their characterization, and eventually the installation and initial commissioning of the system on JT-60SA. The initial phase of the project will focus on the replacement of the so-called S1 detectors (those that are used for providing the start time of the neutron flight time). This work will require the detailed characterization of the detectors both experimentally and via Monte Carlo particle transport codes such as MCNP and GEANT 4. Another important part of the project is the development of TOFOR synthetic diagnostics that will be used to model the expected spectra to be observed as well as the scattered neutron contribution and in optimizing methods for the time-of-flight and energy selection of the coincidence events for the reconstruction of the neutron spectrum. JT-60SA neutron emissivity and spectra for the machine operating scenarios will have to be calculated as part of the project. Development of the electronic data acquisition system and of the data analysis software is also part of this first phase of the project. In particular, an important part of the project is the development of data handling using FPGAs for long pulses operations. The next step would be the installation and commissioning of TOFOR at JT-60SA. This would involve travels to JT-60SA to manage the installation of the system, its integration in the JT-60SA data acquisition system and the initial operation to characterize among others the level of background noise (electronic and neutronic). Eventually, it is expected that the project will include the analysis of the first neutron energy spectra observed on JT-60SA.

The project will develop skills in a range of computational and experimental areas, as well as provide expertise in neutron diagnostics, in data acquisition systems, Monte Carlo particle transport codes and a broad knowledge in fusion plasma physics.

The project will be mainly based at Durham University but with extended visits to Uppsala (Sweden) and to JT-60SA (Japan).

This project is offered by Durham University. For further information please contact: Marco Cecconello,