H. Henriksson

Neutron emission spectroscopy at JET—Results from the magnetic proton recoil spectrometer (invited)

The principles and operation at Joint European Torus (JET) of the first magnetic proton recoil (MPR) spectrometer for measurement of fusion neutron spectra are described. Some technical aspects of the instrument are discussed, including energy calibration and monitoring system. Data from the recent experimental campaign with tritium at JET are presented as examples of the MPR’s diagnostic abilities. It is shown that the spectrometer is a flexible and versatile instrument, capable of delivering high-quality data on a number of plasma parameters. In particular, the observation of the so-called α knock-on effect in neutron emission spectroscopy is discussed.

The TOFOR spectrometer for 2.5 MeV neutron measurements at JET

Neutron emission spectroscopy has been demonstrated to be a powerful plasma diagnostic at tokamaks. This was shown with the magnetic proton recoil spectrometer developed for measurement of the 14 MeV neutron emission from deuterium–tritium (DT) plasmas at the Joint European Torus (JET). For diagnosis of D plasmas, a 2.5 MeV spectrometer is needed with a factor of 100 higher efficiency to maintain the count rate because of the lower neutron emission rate. The time-of-flight (TOF) technique has the right attributes for these measurements. However, previous instruments have not achieved the full potential of the technique, especially, with respect to count rate capability. A TOF spectrometer for optimized rate (TOFOR) has been conceptually defined and is now under design and development for construction and use at JET. The TOFOR design goal is a count rate capability of about 300 kHz which should be attainable in high power D discharges at JET. The TOFOR project is now in a research and development stage whe...

Systematic spectral features in the neutron emission from NB heated JET DT plasmas

High power fusion plasmas produced in the world’s largest facility for magnetic confinement experiments (JET), have been studied using the neutron emission measured with the magnetic proton recoil (MPR) spectrometer. The MPR has been used to observe plasmas since 1996 including those of deuterium-tritium leading to several fusion records and corresponding observational achievements of neutron emission spectroscopy. Noteworthy are novel studies of the complex states of fuel ions arising through plasma heating by neutral beam (NB) injection and radio frequency (RF) power.This thesis concerns the analysis of MPR data on the neutron emission from NB heated discharges alone and in combination with RF. A main objective of these studies has been the effect of supra-thermal fuel ion reactions on the fusion power as compared to the basic thermal component. The analysis was based on dedicated models to describe the velocity distributions of the ion population under the influence of the NB and RF heating in a parametric form allowing trial neutron spectra to be calculated and fitted to the data to select the kinetic state of the fuel ions that best described the MPR observations.Spectral signatures of different underlying plasma states and phenomena were identified and results from up to five different ion reaction contributions to the fusion power were demonstrated besides the global plasma features of toroidal rotation. Moreover, the thesis presents examples of derived detailed plasma information from MPR data such as the kinetic energy densities for the thermal and supra-thermal parts of the fuel ion population as well as the synergetic coupling of RF power to the fast ions from NB injection. The results constitute a stepping-stone for neutron emission spectroscopy as a main diagnostic for ITER and other future fusion experiments on thermonuclear ignition.

Ion temperature and plasma rotation profile effects in the neutron emission spectrum

The instrumental factors and measuring conditions affecting neutron emission spectrometry measurements of tokamak plasmas are described and analyzed. The measured energy broadening and shift of the neutron emission is used to deduce ion temperature (Ti) and toroidal plasma rotation velocity (Vt) representing average (effective) values for the nonuniform plasma volume viewed. Analytical expressions are derived for the relationship between the line-volume integrated effective temperature (Teff) and the radial profile Ti(r) for the case of thermal plasmas with isotropic neutron emission; effects on Teff due to spectral broadening from the radial dependence Vt(r) were also considered. The analysis method presented here is applied to high quality data obtained with the magnetic proton recoil neutron spectrometer installed at Joint European Torus for measurements of deuterium–tritium plasmas. Similarly, cases of anisotropic neutron emission were quantitatively assessed.

Neutron emission spectroscopy of radio frequency heated (D)T plasmas

The energy spectrum of the d+t→α+n neutron emission has been measured in experiments carried out at JET for plasmas of deuterium–tritium subjected to minority ion cyclotron resonance heating (ICRH) tuned to deuterium. The data obtained with the magnetic proton recoil spectrometer were of sufficient quality to distinguish up to three spectral components of neutron emission some of which were time resolved. A new analysis model was used to derive information on the underlying deuteron velocity distributions and their corresponding energy densities in the plasma. This experiment represents the first use of neutron emission spectroscopy for detailed diagnosis of the response of fusion plasmas to the applied ICRH power for different plasma conditions, including the time evolution over the heating pulse duration for individual discharges. In particular, ICRH effects on the plasma, together with the power absorption mechanisms, were studied as a function of the minority ion concentration in the range 9–20%.

Neutron emission study of DT plasmas heated with tritium neutral beams

The neutron emission energy spectrum from deuterium–tritium fusion reactions has been measured in experiments carried out at the Joint European Torus for plasmas heated with high power neutral beam (NB) injection of tritium beams at Eb≈150 keV using a magnetic proton recoil neutron spectrometer. High quality data were obtained in which up to three spectral components due to different ion reactions could be distinguished with the help of dedicated model calculations of the fusion neutron emission based on Monte Carlo simulations. The analysis model involved neutrons from reactions of NB ions on passing and trapped orbits interacting with thermal bulk ions. Moreover, a narrow Gaussian component, due to neutrons from thermal d+t→α+n reactions, could be resolved in some cases. Results are presented and the plasma information obtained is discussed as an illustration of the capabilities of neutron emission spectroscopy diagnostics.

Time resolved neutron emission spectroscopy of DT plasmas with ion cyclotron resonance heating

The energy spectrum of the DT neutron emission was measured in experiments carried out at JET for plasmas of deuterium–tritium heated with different ion cyclotron resonance heating (ICRH) schemes. The spectrum was measured with the magnetic proton recoil spectrometer. The data were analyzed in up to three spectral components, which were ascribed to the underlying reactions involving thermal, epithermal, and high-energy ions. The results include time resolved information for individual discharges. High-energy components in the fuel ion population were found in most cases and analyzed in terms of a tail temperature. Changes in the (minority) deuterium concentration in the range of 12–20% were found to affect significant the ICRH power absorption and its perturbation of the minority velocity distribution.

Triton burn-up neutron emission in JET low current plasmas

The 14 MeV neutron emission from JET deuterium discharges is analysed on the basis of the information on all neutron diagnostics available on JET. This emission is due to the d + t → α + n reaction, mainly, the triton burn-up process, and is used to determine the fast triton confinement. A simplified model for triton burn-up neutron emission has been used and provides an adequate description of the 14 MeV emission. First orbit triton losses are found to amount to 50%, 20% and 10% at Ip = 1 MA, 2 MA and 3 MA, respectively. Neutron emission spectroscopy measurements with the magnetic proton recoil neutron spectrometer have detected a contribution to the 14 MeV emission due to residual tritium. For the selected (low impurity) discharges analysed in this paper 15% of the 14 MeV emission comes from the residual tritium reactions. It is also found that the residual tritium concentration tends to increase with increasing impurity content.

Control and monitoring system for fusion neutron spectroscopy on the Joint European Torus

A new control and monitoring (C&M) system is being developed for the TOFOR and MPRu fusion neutron spectrometers within the Joint European Torus enhancement program. The system, which is an evolution of the existing C&M system of the MPR spectrometer, consists of a controlled pulsed light source distributed by an optical fiber network to all photomultiplier tubes used in the plastic scintillator based spectrometers. The light source is a green Nd:LSB solid-state laser complemented by blue light emitting diode sources. Pulse height distributions for each detection channel are recorded to set the spectrometers to prescribed working points and monitor deviations. Absolute reference is obtained complementing the controlled light source with radioactive sources. In this article we report on the C&M prototype design and component tests for the MPRu spectrometer. The results show that the laser and the associated optics provide a controlled light pulse of intensity covering a dynamic range of more than four orders of magnitude. The choice of optical fiber diameters is critical for achieving the desired stability and uniformity of the light intensity collected by each MPRu detector.

Proposed Magnetic Proton Recoil Neutron Spectrometer Upgrade (MPRu)

The Magnetic Proton Recoil (MPR) neutron spectrometer was installed at the Joint European Torus (JET) in 1996. The present prototype instrument is designed for 14-MeV neutrons. Its capability as a neutron emission spectroscopy (NES) diagnostic was demonstrated during the Deuterium-Tritium Experimental (DTE1) campaign in 1997 and it has since been continuously used during subsequent D-plasma operation. In this paper we describe an upgrade of the spectrometer’s focal plane, recoil-proton detector and radiation shield. The MPRu is intended to make it possible fully to exploit the MPR capabilities in the forward JET experimental programme and to demonstrate the potential role of NES diagnostics in the next step fusion experiments such as planned with ITER.