Reviews the neutron measurement techniques that are applied to the study of tokamak plasmas. The range of neutron energies of primary interest is limited to narrow bands around 2.5 and 14 MeV, and the variety of measurements that can be made for plasma diagnostic purposes is also restricted. To characterize the plasma as a neutron source, it is necessary only to measure the total neutron emission, the relative neutron emissivity as a function of position throughout the plasma, and the energy spectra of the emitted neutrons. In principle, such measurements might be expected to be relatively easy. That this is not the case is, in part, attributable to practical problems of accessibility to a harsh environment but is mostly a consequence of the time scale on which the measurements have to be made and of the wide range of neutron emission intensities that have to be covered; for tokamak studies, the time-scale is of the order of 1 to 100 ms and the neutron intensity ranges from 1012 to 1019 s-1.

https://iopscience.iop.org/article/10.1088/0741-3335/36/2/002/pdf

Neutron measurement techniques for tokamak plasmas

Recent progress in magnetically confined plasma research has brought the goal of controlled thermonuclear fusion within reach. Simultaneously, the increased size, temperature, and density of these devices has resulted in rapid changes in diagnostic techniques. In this article, recent developments in instrumentation are reviewed together with trends for the future. The topics discussed include far‐infrared laser interferometry and polarimetry, ruby laser television Thomson scattering, excimer and Nd‐glass laser scattering, ion temperature determination via large‐α scattering with FIR and CO2 lasers, collective scattering, Schottky diode mixer technology, synchrotron radiation diagnostics and imaging, ion beam probes, x‐ray diagnostics and imaging, neutron diagnostics, resonance fluorescence scattering, ultraviolet diagnostics, and internal magnetic field measurement.

Instrumentation for magnetically confined fusion plasma diagnostics

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

The d(d,p)t and d(d,n) 3He fusion reactions produce 1 MeV tritons and 08 MeV 3He ions which can subsequently undergo d(t, n) α and d(3He,p) α fusion reactions The magnitude of this triton and 3He ion 'burn-up' was measured on the PLT and PDX tokamaks by detection of the 14 MeV neutron and 15 MeV proton emission In discharges with B > 2 T, the measured 3He and triton burn-up is consistent (within a factor of three) with predictions based on classical theories of ion confinement and slowing down In discharges with weaker toroidal fields but constant plasma current, the burn-up of both ions fell by more than a factor of ten so that the observed burn-up was significantly less than expected classically

/pdf/burn-up-of-fusion-produced-tritons-and-3he-ions-in-plt-and-3qoymczfpg.pdf

Burn-up of fusion-produced tritons and 3He ions in PLT and PDX

Ultra-intense MeV photon and neutron beams are indispensable tools in many research fields such as nuclear, atomic and material science as well as in medical and biophysical applications. For astrophysical applications aimed for laboratory investigations, neutron fluxes in excess of 10$^{21}$ n/(cm$^2$ s) are required. Such ultra-high fluxes are unattainable with existing conventional reactor- and accelerator-based facilities. Currently discussed concepts for generating high-flux neutron beams are based on ultra-high-power multi-petawatt lasers operating at >10$^{23}$ W/cm$^2$ intensities. Here, we present a novel efficient concept for generating $\gamma$ and neutron beams based on enhanced generation of direct laser accelerated electrons in relativistic laser interactions with a long-scale near critical density plasma at 10$^{19}$ W/cm$^{2}$ intensity. New experimental insights in the laser-driven generation of ultra-intense well-directed multi-MeV beams of photons with >10$^{12}$ ph/sr and a ultra-high intense neutron source with >6$\times$10$^{10}$ neutrons per shot are presented. More than 1.4\% laser-to-gamma conversion efficiency above 10 MeV and 0.05\% laser-to-neutron conversion efficiency were recorded, already at moderate relativistic laser intensities and ps pulse duration. This approach promises a strong boost of the diagnostic potential of existing kJ PW laser systems used for ICF research. 

/pdf/forward-looking-insights-in-laser-generated-ultra-intense-g-f19ac9df.pdf

Forward-looking insights in laser-generated ultra-intense γ-ray and neutron sources for nuclear application and science

Neutron flux measurements around the Princeton large tokamak

Using Indium activation foils, the toroidal and poloidal neutron emission patterns were determined for PLT plasmas which include ICRF and neutral beam heating The activities produced the /sup 115/In (n,n') /sup 115m/In reaction were determined by counting the 336 keV gamma line of the /sup 115m/In decay This activation cross section falls just below 25 MeV so that the influence of scattered neutrons of degraded energies is reduced From the magnitude of the activity, the absolute calibration of the PLT fusion neutron emission is obtained with less than or equal to 40% accuracy

/pdf/neutron-flux-measurements-around-plt-11qhdq5p2i.pdf

Neutron flux measurements around PLT

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Neutron flux measurements around the Princeton large tokamak

Neutron flux measurements around PLT