Measured Neutron Rates Research Articles

Initial testing of Alfvén eigenmode feedback control with machine-learning observers on DIII-D

A first of its kind fully data-driven system has been developed and implemented into the DIII-D plasma control system to detect and control Alfvén eigenmodes (AE) in real-time. Susceptibility to fast ion-induced AE is a challenge in fully non-inductive tokamak operation, which significantly reduces fast-particle confinement and results in degraded fusion gain. Controlling AEs in real-time to improve fast-ion confinement is, hence, important for future advanced tokamak fusion reactors. The models were implemented and tested in experiments which showed that neural networks (NN) are highly effective in detecting 5 types of AE (BAE, EAE, LFM, RSAE, TAE) using high resolution ECE. To estimate the neutron deficit, a NN has been trained that outputs the classical neutron rate using similar inputs to NUBEAM. Also a preliminary ML-based proportional control has been designed and gone through initial testing in experiment to use feedback-control on the neutral beam power to achieve desired amplitude of AE modes and neutron deficits. The effect of AEs on fast-ion confinement is measured by analysing the gap in classical neutron rate from the proposed NN-based NUBEAM and the measured neutron rate.

Dead time corrections for Bonner sphere measurements of secondary neutrons at a proton therapy facility

Radiation therapy with proton beams allows the deposition of high doses to the tumour while minimising dose to the surrounding tissue. During such treatment, the patient is also exposed to secondary radiation which produces an out-of-field dose that affects healthy tissue. The largest contribution to this out-of-field dose comes from neutron radiation; therefore, it is of interest to fully characterise the neutron field in the therapy room with measurements. This is usually done with Bonner sphere spectrometers using active detectors, typically 3He-filled proportional counters, as central thermal neutron sensors. Under the experimental conditions encountered in proton therapy facilities, a proper analysis of the measurements is impossible unless dead time corrections are implemented. In this paper, we present a method using a paralysable dead time model for carrying out such corrections for Bonner sphere measurements with 3He-filled proportional counters and apply it to data taken at the University Proton Therapy Dresden (UPTD) facility in double scattering mode. The neutron events were recorded with time stamps and, based on this time-resolved data, the measured neutron rate distribution was sampled. Since the neutron flux is proportional to the proton flux, the integral neutron flux is directly related to the proton dose. Hence, we were able to estimate the detector dead time from the measured rate distributions recorded for a set of measurements with different proton dose rates. Experimental measurements with different intensities of the proton field show that the corrections are in agreement within 0.5% for measured signal rates smaller than 15 × 103 counts per second and do not exceed 1% at 25 × 103 counts per second.

Neutron rate estimates in MAST based on gyro-orbit modelling of fast ions

A discrepancy between predicted and measured neutron rates on MAST using TRANSP/NUBEAM has previously been observed and a correction factor of about 0.6 was needed to match the two: this correction factor could not be accounted for by the experimental uncertainties in the plasma kinetic profiles nor in the NBI energy and power (Cecconello et al 2019 Nucl. Fusion 59 016006). Further causes of this discrepancy are here studied by means of TRANSP/NUBEAM and ASCOT/BBNBI simulations. Different equilibria, toroidal field ripples, uncertainties on the NBI divergence value and gyro-orbit effects were studied and simulations were performed with both transport codes. It was found that the first three effects accounted for only a 5% variation in the fast ion density. On the other hand, full gyro-orbit simulations of the fast ions dynamics carried out in ASCOT/BBNBI resulted in an approximately 20% reduction of the fast ion population compared to TRANSP/NUBEAM. A detailed analysis of the fast ion distributions showed how the drop occurred regardless of the energy at pitch values ⩽−0.4. The DRESS code was then used to calculate the neutron rate at the neutron camera detector’s location showing that the discrepancy is considerably reduced when the full gyro-orbit fast ion distribution is used, with now the correction factor, used to match experimental and predicted neutron rates, being around 0.9.

Quantitative modeling of neoclassical tearing mode driven fast ion transport in integrated TRANSP simulations

The TRANSP-‘Kick’ energetic particle transport model has been extended to study neoclassical tearing mode (NTM) driven fast ion (FI) transport with zero free parameters. FI transport induced by the NTM is obtained by calculating perturbed FI orbits via the guiding center particle following code orbit in the magnetic field of an experimentally characterized island chain. Transport probabilities are then used in TRANSP’s Monte Carlo module NUBEAM to modify the FI distribution in every time step of the full TRANSP analysis. This procedure retains all TRANSP functionality and self-consistently predicts the NTM impact on beam torque, current drive and heating. Comparisons to DIII-D steady state, hybrid and ITER baseline plasmas are encouraging with the model quantitatively recovering the measured neutron rates. FI effects depend on the location and width of FI phase space resonances which are not accounted for by the former ad hoc beam diffusivity model of TRANSP. Resonance overlaps result in a transport threshold, for the cases investigated herein, at . When the FI transport is dominated by energy and momentum redistribution. In particular, m/n = 3/2 NTMs lead to hollow profile at q = m/n and broadened profile in the core. When FI losses increase, the neutral beam torque () and driven current () decrease across the entire plasma. The effects on NB profiles can strongly depend on the NTM frequency and mode numbers with the 3/2 (2/1) broadening (peaking) near the magnetic axis.

Active real-time control of Alfvén eigenmodes by neutral beam and electron cyclotron heating in the DIII-D tokamak

A real-time Alfvén eigenmode (AE) controller is designed and implemented for the first time in DIII-D. The experimental test consists of two parts: (i) open-loop and feedback control using neutral beam power as an actuator and (ii) open-loop control using electron cyclotron heating (ECH) as an actuator. Part (i) demonstrates the feasibility of varying neutral beam power injected into plasma to control overall AE amplitude as well as the successful application of real-time electron cyclotron emission (ECE) as the process controller’s AE amplitude sensor. The temporal evolution of AE amplitude at a specific radial location has also been controlled by selecting a subset of the radial ECE array. Part (ii) finds a complicated dependence of AE evolution on ECH power and more dedicated investigation is needed to explore the dynamical ECH-control model. Additionally, the ratio between TRANSP calculated neutron rate and measured neutron rate is found to correspond closely with AE amplitudes in discharges with feedback control, verifying that this ratio can be another sensor for the control system.

Scenario development for the observation of alpha-driven instabilities in JET DT plasmas

In DT plasmas, toroidal Alfvén eigenmodes (TAEs) can be made unstable by the alpha particles resulting from fusion reactions, and may induce a significant redistribution of fast ions. Recent experiments have been conducted in JET deuterium plasmas in order to prepare scenarios aimed at observing alpha-driven TAEs in a future JET DT campaign. Discharges at low density, large core temperatures associated with the presence of internal transport barriers and characterised by good energetic ion confinement have been performed. ICRH has been used in the hydrogen minority heating regime to probe the TAE stability. The consequent presence of MeV ions has resulted in the observation of TAEs in many instances. The impact of several key parameters on TAE stability could therefore be studied experimentally. Modeling taking into account NBI and ICRH fast ions shows good agreement with the measured neutron rates, and has allowed predictions for DT plasmas to be performed.

Improving fast-ion confinement in high-performance discharges by suppressing Alfvén eigenmodes

We show that the degradation of fast-ion confinement in steady-state DIII-D discharges is quantitatively consistent with predictions based on the effects of multiple unstable Alfvén eigenmodes on beam-ion transport. Simulation and experiment show that increasing the radius where the magnetic safety factor has its minimum is effective in minimizing beam-ion transport. This is favorable for achieving high performance steady-state operation in DIII-D and future reactors. A comparison between the experiments and a critical gradient model, in which only equilibrium profiles were used to predict the most unstable modes, show that in a number of cases this model reproduces the measured neutron rate well.

Absolute calibration of neutron detectors on the C-2U advanced beam-driven FRC.

In the C-2U fusion energy experiment, high power neutral beam injection creates a large fast ion population that sustains a field-reversed configuration (FRC) plasma. The diagnosis of the fast ion pressure in these high-performance plasmas is therefore critical, and the measurement of the flux of neutrons from the deuterium-deuterium (D-D) fusion reaction is well suited to the task. Here we describe the absolute, in situ calibration of scintillation neutron detectors via two independent methods: firing deuterium beams into a high density gas target and calibration with a 2 × 107 n/s AmBe source. The practical issues of each method are discussed and the resulting calibration factors are shown to be in good agreement. Finally, the calibration factor is applied to C-2U experimental data where the measured neutron rate is found to exceed the classical expectation.

Integrated modeling applications for tokamak experiments with OMFIT

One modeling framework for integrated tasks (OMFIT) is a comprehensive integrated modeling framework which has been developed to enable physics codes to interact in complicated workflows, and support scientists at all stages of the modeling cycle. The OMFIT development follows a unique bottom-up approach, where the framework design and capabilities organically evolve to support progressive integration of the components that are required to accomplish physics goals of increasing complexity. OMFIT provides a workflow for easily generating full kinetic equilibrium reconstructions that are constrained by magnetic and motional Stark effect measurements, and kinetic profile information that includes fast-ion pressure modeled by a transport code. It was found that magnetic measurements can be used to quantify the amount of anomalous fast-ion diffusion that is present in DIII-D discharges, and provide an estimate that is consistent with what would be needed for transport simulations to match the measured neutron rates. OMFIT was used to streamline edge-stability analyses, and evaluate the effect of resonant magnetic perturbation (RMP) on the pedestal stability, which have been found to be consistent with the experimental observations. The development of a five-dimensional numerical fluid model for estimating the effects of the interaction between magnetohydrodynamic (MHD) and microturbulence, and its systematic verification against analytic models was also supported by the framework. OMFIT was used for optimizing an innovative high-harmonic fast wave system proposed for DIII-D. For a parallel refractive index , the conditions for strong electron-Landau damping were found to be independent of launched and poloidal angle. OMFIT has been the platform of choice for developing a neural-network based approach to efficiently perform a non-linear multivariate regression of local transport fluxes as a function of local dimensionless parameters. Transport predictions for thousands of DIII-D discharges showed excellent agreement with the power balance calculations across the whole plasma radius and over a broad range of operating regimes. Concerning predictive transport simulations, the framework made possible the design and automation of a workflow that enables self-consistent predictions of kinetic profiles and the plasma equilibrium. It is found that the feedback between the transport fluxes and plasma equilibrium can significantly affect the kinetic profiles predictions. Such a rich set of results provide tangible evidence of how bottom-up approaches can potentially provide a fast track to integrated modeling solutions that are functional, cost-effective, and in sync with the research effort of the community.

The effects of resonant magnetic perturbations on fast ion confinement in the Mega Amp Spherical Tokamak

The effects of resonant magnetic perturbations (RMPs) on the confinement of energetic (neutral beam) ions in the Mega Amp Spherical Tokamak (MAST) are assessed experimentally using measurements of neutrons, fusion protons and fast ion Dα (FIDA) light emission. In single null-diverted (SND) MAST pulses with relatively low plasma current (400 kA), the total neutron emission dropped by approximately a factor of two when RMPs with toroidal mode number n = 3 were applied. The measured neutron rate during RMPs was much lower than that calculated using the TRANSP plasma simulation code, even when non-classical (but axisymmetric) ad hoc fast ion transport was taken into account in the latter. Sharp drops in spatially-resolved neutron rates, fusion proton rates and FIDA emission were also observed. First principles-based simulations of RMP-induced fast ion transport in MAST, using the F3D-OFMC code, show similar losses for two alternative representations of the MAST first wall, with and without full orbit effects taken into account; for n = 6 RMPs in a 600 kA plasma, the additional loss of beam power due to the RMPs was found in the simulations to be approximately 11%.

Simulation of the neutron rate in ASDEX Upgrade H-mode discharges

The neutron rate is a direct indicator of the fusion performance in tokamaks. However, the accuracy of the measurement is limited by the wide range, covering several orders of magnitude, and by the delicate absolute calibration procedure. Moreover, a mere neutron counter does not distinguish between thermonuclear, beam–plasma and beam–beam fusion reactions. In this work we aim to use Monte Carlo simulations of the NBI deposition to improve our physics understanding of the correlation of the neutron yield with global plasma parameters. The modelling shows the beam–plasma reactions to be the main contribution to the neutron rate for PNBI > 5 MW. The comparison between experimental data and measurements allows the identification of systematic calibration factors for different calibration phases over several years of acquisition. The quality of the kinetic profile measurements is shown to play an important role in reducing the scatter of the simulated neutron rate around the measured value. The sensitivity to Zeff and to possible fast ion diffusion is investigated. Using an existing formula for the density dependence of Zeff the agreement between theoretical prediction and experimental measurements is considerably improved. Finally we derive a scaling law for the measured neutron rate in ASDEX Upgrade H-mode discharges depending on global parameters, in excellent agreement with a simple physics derivation for beam–plasma neutrons.

Toroidal ripple transport of beam ions in the mega-ampère spherical tokamak

The transport of injected beam ions due to toroidal magnetic field ripple in the mega-ampère spherical tokamak (MAST) is quantified using a full orbit particle tracking code, with collisional slowing-down and pitch-angle scattering by electrons and bulk ions taken into account. It is shown that the level of ripple losses is generally rather low, although it depends sensitively on the major radius of the outer midplane plasma edge; for typical values of this parameter in MAST plasmas, the reduction in beam heating power due specifically to ripple transport is less than 1%, and the ripple contribution to beam ion diffusivity is of the order of 0.1 m2 s–1 or less. It is concluded that ripple effects make only a small contribution to anomalous transport rates that have been invoked to account for measured neutron rates and plasma stored energies in some MAST discharges. Delayed (non-prompt) losses are shown to occur close to the outer midplane, suggesting that banana-drift diffusion is the most likely cause of the ripple-induced losses.

Fast-ion Dα measurements and simulations in quiet plasmas

The Dα light emitted by neutralized deuterium fast ions is measured in magnetohydrodynamics (MHD)-quiescent, magnetically confined plasmas during neutral beam injection. A weighted Monte Carlo simulation code models the fast-ion Dα spectra based on the fast-ion distribution function calculated classically by TRANSP [R. V. Budny, Nucl. Fusion 34, 1247 (1994)]. The spectral shape is in excellent agreement and the magnitude also has reasonable agreement. The fast-ion Dα signal has the expected dependencies on various parameters including injection energy, injection angle, viewing angle, beam power, electron temperature, and electron density. The neutral particle diagnostic and measured neutron rate corroborate the fast-ion Dα measurements. The relative spatial profile agrees with TRANSP and is corroborated by the fast-ion pressure profile inferred from the equilibrium.

System to monitor data analyses and results of physics data validation between pulses at DIII–D

A data analysis monitoring (DAM) system has been developed to monitor between pulse physics analysis at the DIII–D National Fusion Facility (http://nssrv1.gat.com:8000/dam). The system allows for rapid detection of discrepancies in diagnostic measurements or the results from physics analysis codes. This enables problems to be detected and possibly fixed between pulses as opposed to after the experimental run has concluded, thus increasing the efficiency of experimental time. An example of a consistency check is comparing the experimentally measured neutron rate and the expected neutron emission, RDD0D. A significant difference between these two values could indicate a problem with one or more diagnostics, or the presence of unanticipated phenomena in the plasma. This system also tracks the progress of MDSplus dispatched data analysis software and the loading of analyzed data into MDSplus. DAM uses a Java Servlet to receive messages, C Language Integrated Production System to implement expert system logic, and displays its results to multiple web clients via Hypertext Markup Language. If an error is detected by DAM, users can view more detailed information so that steps can be taken to eliminate the error for the next pulse.

Confinement studies of auxiliary heated NSTX plasmas

The confinement of auxiliary heated national spherical torus experiment discharges is discussed. From a database analysis, it is found that the energy confinement time in NBI heated plasmas with either L- or H-modes edge is up to 2.5 times the values predicted by the ITER97L scaling. A high power NBI heated H-mode discharge is discussed in detail. TRANSP calculations based on the kinetic profile measurements reproduce well the magnetically determined stored energy, but overestimate the measured neutron rate by 30%. Power balance calculations reveal that the ion thermal transport is above or near neoclassical levels, and significantly below the electron thermal transport, which constitutes the main power loss channel. Perturbative impurity injection techniques indicate the particle diffusivity is slightly above the neoclassical level in discharges with L-mode edge. High-harmonic fast-wave (HHFW) bulk electron heating is described and thermal transport is discussed. Thermal ion transport is found to be above the neoclassical level, and thermal electron transport remains the main loss mechanism. Evidence of an electron thermal internal transport barrier obtained with HHFW heating is presented. A description of H-mode discharges obtained during HHFW heating is made.

Diagnostic experience during deuterium–tritium experiments in JET, techniques and measurements

During 1997 JET was operated for an extensive period using a D–T mixture (DTE1). Changes in the design and operation of diagnostic systems made over the years in preparation for this phase are described. A number of diagnostic techniques have been deployed to measure the deuterium and tritium content of the plasma during DTE1 and their results are compared. All diagnostics with a direct vacuum interface with the main vessel have been fitted with tritium compatible pumps and their exhaust gases have been re-routed to the active gas handling plant. All items on the torus which could lead to a significant leak in the event of failure, were required to have double containment. Therefore, all windows, and a majority of bellows and feedthroughs, were designed and installed with a double barrier. Heated fibre hoses were installed to transmit plasma light beyond the biological shield for spectroscopic purposes. Blind fibres and fibre loops were also installed to study the effects of higher neutron fluxes on these fibres. A radiation-hardened video camera was installed to monitor the plasma during the DTE1 discharges. Extra shielding was installed on a number of diagnostics to deal with the higher neutron fluxes during DTE1. The effect of neutron radiation on electronics in the Torus Hall was studied. During DTE1 the tritium fraction was measured at the edge and in the core using several diagnostic methods. High resolution Balmer α line spectroscopy gave a measurement typical of the plasma edge region. In the JET sub-divertor volume the tritium concentration of the neutral gas was measured using Balmer α spectroscopy of a Penning gauge discharge. Using Neutral Particle Analysis, the tritium concentration was measured typically in a zone 20–40 cm from the plasma edge. Local core measurements of the tritium fraction have been made using active Balmer α charge exchange spectroscopy. The error on this measurement is, however, large,∼30%. After the discharge the tritium fraction of the exhaust was measured using the exhaust monitoring system. Using short deuterium neutral injection pulses allowed neutron rate measurements of the tritium concentration in the core region. A further technique used the measured neutron rate and calculated neutron rate from other plasma parameters to determine the tritium concentration.

Characterization of fast ion behaviour during tangential neutral beam injection in the Advanced Toroidal Facility

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (ne ⩽ 1.0 × 1019 m-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (ne ≈ 7.5 × 1019 m-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary