J.M. Adams

Compact NE213 neutron spectrometer with high energy resolution for fusion applications

Neutron spectrometry is a tool for obtaining important information on the fuel ion composition, velocity distribution and temperature of fusion plasmas. A compact NE213 liquid scintillator, fully characterized at Physikalisch-Technische Bundesanstalt, was installed and operated at the Joint European Torus (JET) during two experimental campaigns (C8-2002 and trace tritium experiment-TTE 2003). The results show that this system can operate in a real fusion experiment as a neutron (1.5 MeV<En<20 MeV) spectrometer with good energy resolution (ΔE/E<4% at En=2.5 MeV and ΔE/E<2% at En=14 MeV). First measurements performed under different plasma scenarios, including trace tritium experiments, are presented. The analysis of the pulse height data was carried out using a newly developed method based on maximum entropy unfolding. The results indicate that this efficient, inexpensive, and compact scintillator is suitable for use as a broadband spectrometer in large fusion devices (JET and the International Thermonucle...

Gamma ray emission profile measurements from JET ICRF-heated discharges

Helium 3 minority fuel ions have been observed to be accelerated to megaelectron-volt (MeV) energies by ion cyclotron radiofrequency (ICRF) heating. These energetic ions undergo nuclear reactions with impurity 9Be and 12C ions in the plasma, and characteristic gamma radiation is emitted. For special circumstances, this gamma ray emission has been detected with a 19 channel profile monitor constructed primarily for neutron measurements. Two-dimensional (2-D) profiles of the gamma radiation emission have been derived by tomographic reconstruction; these profiles correspond to a cross-section weighted density of 3He ions having energies of 2 or more MeV. As expected for ICRF heating, the observed spatial profile of the gamma radiation shows that access by the accelerated ions to the plasma volume on the high field side of the RF resonance layer is effectively inhibited. In addition, the profile appears to demand the presence of a group of passing particles that are localized in poloidal extent and are circulating some 15 cm to the low field side of the resonance layer. Apart from the 15 cm displacement, these features are well reproduced by a model that combines a 2-D bounce averaged Fokker-Planck calculation with an orbit-following code. A unique example of a gamma ray spatial profile captured at the time of a sawtooth crash shows that the profile is relatively unaffected apart from the sudden obliteration of the passing group, for which the ions are found to possess relatively high energy, presumably gained from the RF waves through the Doppler shift effect. On the basis of these observations, an explanation is proposed for the selective expulsion of RF heated fast ions from the central region of the plasma to the outer regions

JET neutron emission profiles and fast ion redistribution during sawtooth crashes

Measurements from the JET neutron profile monitor are analyzed tomographically to deduce the 2-D spatial distribution of neutron emissivity during NBI heating both before and after sawtooth crashes. In a discharge where the global neutron emission decreases slightly to 5/6 of the pre-crash rate of nearly 1016 n s-1, the axial emissivity drops much more to only 1/6 of the pre-crash value. The sources of neutron emissivity are analyzed with an analytic Fokker-Planck formulation. The main change in neutron emissivity is due to fast ion redistribution during a sawtooth crash and a decrease in beam-beam-produced emissivity. The deduced fast ion redistribution is consistent with other observations of sawtooth behaviour in JET.

Effects of sawtooth crashes on beam ions and fusion product tritons in JET

The JET neutron emission profile monitor is used to measure the 2.5 MeV and 14 MeV neutron emission line integrals before and after sawtooth crashes in high d-d neutron yield, hot ion H mode plasmas in the Joint European Torus (JET). Deuterium-deuterium (d-d) fusion produces 2.5 MeV neutrons and 1 MeV tritons (t) at nearly equal rates from its two reaction channels. A plasma current of 3 MA is sufficiently high to contain most of the fusion product tritons, which have birth orbit gyroradii and velocity space distributions similar to those of the 3.5 MeV or particles from d-t fusion. By examining neutron emission line integrals and tomographically deduced local emissivity profiles, an upper limit of 10% can be placed on the net fraction of fusion product tritons which are displaced from the plasma axis by those sawtooth crashes studied. This is a much smaller net fraction than that typically observed, 35-55%, for displaced injected neutral beam deuterium ions. A study of the response of beam injected deuterium ions to a sawtooth crash shows that the change in their axial density depends on the precrash spatial width of the neutron emissivity profile. The fusion product tritons, which have a large precrash spatial width, respond weakly to a crash. This weak response is consistent with the behaviour of the analogous d-d beam thermal neutrons when extrapolated to the corresponding emissivity spatial width. The implication of these observations is that beam ions and 3.5 MeV alpha particles in JET may be relatively resilient to sawtooth clashes, when the spatial width of their density is sufficiently large

Neutron measurements on Joint European Torus using an NE213 scintillator with digital pulse shape discrimination

A digital pulse shape discrimination system (DPSD) has been used in conjunction with collimated NE213 scintillators for neutron spectroscopic measurements at high count rates (MHz range) in Joint European Torus discharges (DD and DT fueled, neutral beam injection and rf heated). The system, developed at ENEA-Frascati, is based on a commercial 200 MHz 12-bit analog to digital transient recorder card, which digitizes the direct output signal from the anode of a photomultiplier. Among the unique features of this DPSD system are the possibility of postexperiment data reprocessing, high count rate operation, and simultaneous neutron and gamma (γ) spectroscopy. Separation between γ and neutron (n) events is performed by means of dedicated software exploiting the charge comparison method; separate n and γ pulse height distributions are obtained and an example of neutron spectrum unfolding is shown. Implications of the DPSD in future neutron diagnostic systems on large and next step tokamaks are discussed.

Neutron emission profile measurements during the first tritium experiments at JET

During a series of experiments with tritium (T) in deuterium (D) plasmas in the Joint European Torus (JET), the temporal evolution and the two dimensional (2-D) spatial profiles of the 2.5 and 14 MeV neutron emissivities from D-D and D-T fusion reactions were inferred from measurements with the JET neutron emission profile monitor. These experiments, involving triton production from D-D fusion, beam deposition and diffusion, D-T fusion, and tritium removed from wall tiles, were investigated in four plasma scenarios: (i) In high performance deuterium plasmas with deuterium neutral beam injection, the 14 MeV neutron emissivity due to triton burnup was observed. (ii) In discharges with 1% tritium beam injection, neutron emissivity ratios showed that approximately the same deposition profiles resulted from tritium as from deuterium beams. A thermalized tritium diffusion experiment was performed in which the T-D density ratio was found to be spatially constant across the plasma; in conjunction with similar particle source profiles, this indicates that deuterium and tritium have similar particle transport properties. (iii) In two high performance discharges for which two of the sixteen neutral beam sources operated with 100% tritium, the production rate of 14 MeV neutrons reached 6 × 1017 n.s-1 The alpha particle 2-D birth profile was directly inferred from the measured 14 MeV neutron emissivity profile. Both the axial 14 MeV neutron emissivity and the axial ion temperature saturated before the maximum global emission was reached. (iv) During the tritium cleanup phase, residual tritium entering the plasma produced a spatially constant ratio of tritium to deuterium, confirming the similarity of their particle transport properties

Tritium fast ion distribution in JET current hole plasmas

Current hole plasmas in JET are those in which the current density within r/a < 0.3 is close to zero. Tritium ions injected quasi-tangentially into such plasmas can fulfil a stagnation condition whereby their vertical drift is cancelled by the poloidal component of their parallel velocity. These ions remain trapped at approximately 0.2 m from the plasma axis and can be detected by a distortion in the neutron emission profile. Numerical modelling of the steady-state distribution reproduces the experimental results while the decay of neutron emission after the cessation of injection is found to be sensitive to small changes in the q-profile.

Determination of deuterium concentrations in JET plasmas

The concentration of deuterium in JET plasmas, expressed as a fraction of the electron concentration, has been determined using eight different methods, four of which involve neutron detection. The results from these various methods are found to be consistent within their experimental errors. The ratio nD/ne, measured at the moment of peak neutron emission strength, is found to lie in the range from nearly unity, for discharges into which deuterium pellets are injected, down to values of 0.4 or less, for some of the highest performance discharges. This finding is based on the analysis of discharges run in 1988, when the plasma facing components within the vacuum vessel were of carbon or were carbon coated.

Ohmic ion temperature and thermal diffusivity profiles from the JET neutron emission profile monitor

The JET neutron emission profile monitor was used to study ohmically heated deuterium discharges. The radial profile of the neutron emissivity is deduced from the line-integral data. The profiles of ion temperature, Ti, and ion thermal diffusivity, chi i, are derived under steady-state conditions. The inferred chi i, for 0.3 < p = r/a < 0.5, is in the range 0.5-2.5m2s-1. The ion thermal diffusivity is higher than, and its scaling with plasma current opposite to, that predicted by neoclassical theory.

Fast particle diagnostics at JET: status and plans

A comprehensive set of fast particle diagnostics is routinely used at JET. Some are in the process of being upgraded and others, completely new, are being prepared for use during the forthcoming tritium experiments. For fusion product studies, the strength and profile of the charged particle birth distribution is obtained by measuring the neutron emission with three pairs of absolutely calibrated fission chambers and a two-camera profile monitor. Information on the DD neutron energy spectrum is deduced from a time-of-flight neutron spectrometer. Neutrons of 14 MeV energy from triton burnup are measured using silicon diodes and a high energy branch of the profile monitor. Absolute calibration is obtained with an activation system. A prototype lost alpha particle detector (Faraday cup) has been tested in the laboratory and has been installed inside JET so that its noise immunity can be tested. Two 14 MeV neutron spectrometers are under commissioning and a third is under construction. A high energy neutral particle analyser is routinely used to diagnose fast RF driven particles and will also be available for alpha particle studies. The intensity of RF driven fast particles is also deduced by the spectroscopy of gamma rays emitted in reactions with impurity ions in the plasma. The gamma ray measuring branch of the neutron profile monitor adds spatial resolution to the measurement. A new antenna for measuring ion cyclotron emission (ICE) and a gyrotron for alpha particle scattering experiments are in the process of being commissioned. Active charge exchange spectroscopy to investigate the low energy range of the alpha particle population is being considered. Where appropriate, information gathered with these systems is presented to illustrate their performance