Dual sightline measurements of MeV range deuterons with neutron and gamma-ray spectroscopy at JET

Abstract

Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radiofrequency (RF) heating coupled into a deuterium beam are reported. Measurements are based on a set of advanced neutron and gamma-ray spectrometers that, for the first time, observe the plasma simultaneously along vertical and oblique lines of sight. Parameters of the fast ion energy distribution, such as the high energy cut-off of the deuteron distribution function and the RF coupling constant, are determined from data within a uniform analysis framework for neutron and gamma-ray spectroscopy based on an one dimensional model and by a consistency check among the individual measurement techniques. A systematic difference is seen between the two lines of sight and is interpreted to originate from the sensitivity of the oblique detectors to the pitch-angle structure of the distribution around the resonance, which is not correctly portrayed within the adopted one dimensional model. A framework to calculate neutron and gamma-ray emission from a spatially resolved, two dimensional deuteron distribution specified by energy/pitch is thus developed and used for a first comparison with predictions from ab initio models of RF heating at multiple harmonics. The results presented in this paper are of relevance for the development of advanced diagnostic techniques for MeV range ions in high performance fusion plasmas, with applications to the experimental validation of RF heating codes and, more generally, to studies of the energy distribution of ions in the MeV range in high performance deuterium and deuterium-tritium plasmas.

Fast ion energy distribution from third harmonic radio frequency heating measured with a single crystal diamond detector at the Joint European Torus

“…This implies that, if either k ⊥ or C RF were known, there would be only a very narrow interval of values for the other parameter where an acceptable fit to the SDD data is obtained. It is also interesting to note that the minimum C-stat occurs for a k ⊥ value that is lower than that derived from TOFOR (vertical line of sight), in agreement with the NE213 analysis 34,35 (same line of sight; see Table II). This is further supported by the regions associated to 68% and 90% confidence levels around the k ⊥ and C RF values at the minimum C-stat (Min C) for the two pitch angle assumptions (1D model and 70…”

“…Quite surprisingly, a different cut-off energy was found for the energy distributions derived with the two instruments, being E * = 2.7 MeV for TOFOR and E * = 3 MeV for the NE213. As fully discussed in references, 34,35 this difference is well above the uncertainty on the cut-off energy (about 1%). In the same reference, the discrepancy is explained by the different phase space sensitivity of neutron measurements along a vertical and oblique line of sight.…”

“…For a quantitative analysis of the SDD data, we made use of the one dimensional model for 3rd harmonic ICRH with input values for k ⊥ and C RF as summarized in Table I. These were taken from results of an analysis based on data from the TOFOR time of flight spectrometer, 34,35 which was operational in the experiment and observes the plasma along a vertical line of sight. The corresponding deuteron distributions are shown in Figure 5.…”

Response function of single crystal synthetic diamond detectors to 1-4 MeV neutrons for spectroscopy of D plasmas

“…The measured pulse height spectra (see Figure 2) feature the characteristic broad response related to the scattering recoil on 12 C. The deposited energy scale of the pulse height spectra has been calibrated by using a triple alpha source ( 241 Am, 244 239 Pu). The energy released into the detector, E dep depends on the incoming neutron kinetic energy (E n ) and on the 12 C recoiling angle (θ R ) in the laboratory coordinate system with the elastic scattering law E dep = 0.284 × E n cos 2 θ R .…”

“…A SD prototype was installed at Joint European Torus (JET) in 2013 10,11 achieved results have successfully shown for the first time its spectroscopic capability of measuring 2.5 MeV neutrons from deuterium plasmas. Of particular interest is a recent experiment on radio frequency heating at the third ion cyclotron harmonic at JET, 12,13 where a SD was used to study the acceleration of supra-thermal deuterons in the plasma and provided complementary information to that of non-compact, advanced spectrometers, thanks to its different line of sight. 14,15 A key point for the development of a neutron spectrometer based on SDs is the measurement of the detector response function to quasi-monoenergetic neutrons.…”