Ions In Fusion Plasmas Research Articles

Optimization of the collective Thomson scattering diagnostic for the GDT open magnetic trap

The collective Thomson scattering of high-power millimeter-wave radiation on plasma density fluctuations (CTS) is widely used for diagnosing the velocity distribution function of energetic ions in fusion plasma. In this paper, we discuss a non-standard scheme of CTS measurements, which exploits strong refraction of probing and scattered microwave beams when reflecting from the cut-off layer in dense plasma. The scheme may be realized with the existing CTS diagnostic system at the Gas-Dynamic Trap (GDT) facility, a large open magnetic trap operating at Budker Institute (Novosibirsk, Russia). This requires a minor upgrade of the available hardware and essentially increases the range of plasma densities allowed for CTS measurements, as well as its sensitivity and spatial resolution. A detailed study of CTS efficiency for different geometries and plasma conditions at GDT is performed by means of an advanced numerical model that allows for an accurate description of non-Gaussian beam scattering in inhomogeneous plasma. To perform this task, we develop a quasi-optical theory of scattering, which itself may be of general interest.

Observation of reduced-turbulence regime with tungsten injection in HL-2A edge plasmas

A reduced-turbulence regime has been observed in HL-2A NBI-heated deuterium plasmas. The transition to this regime is achieved by injecting a certain amount of tungsten into the plasma based on the laser blow-off technique. It has been found that the amplitude of geodesic acoustic mode (GAM) zonal flow and turbulent vortex size together with eddy tilting angle are all significantly increased in the edge region after tungsten injection. However, the frequency of GAM zonal flow remains nearly unchanged. Measurement shows the nonlinear coupling degree of turbulence dramatically increases while the collisional damping of GAM zonal flow drops slightly. We conclude that the increased nonlinear coupling is the main cause of the excitation of GAM zonal flow, which consequently results in the reduction in turbulent transport as observed in this experiment. These results indicate that tungsten ions play an active role in turbulence-GAM dynamics through a symmetry-breaking mechanism, which could help us to better understand the inherent physical mechanisms governing turbulent transport in the presence of high-Z impurity ions in fusion plasmas.

Development of a high-temporal resolution neutron flux measurement system for the HL-2M tokamak

To enhance the understanding of the physics of energetic ions in fusion plasma, a high-temporal resolution neutron flux measurement (HTRNFM) system, which is equipped with a fast-neutron scintillation detector embedded with ZnS:Ag phosphor, has been developed for the HL-2M tokamak. It has a temporal resolution of 10 μs during conventional operations. Its dynamic range is sufficiently wide for neutron flux measurements by adopting the combination use of the scalar mode and the Campbell mode. Based on the Monte Carlo calculations, the applicable count rate ranges of both the scalar mode and the Campbell mode are respectively 0.1–10 Mcps and 10–200 Mcps. The performance validation of the HTRNFM system has been performed by neutron flux measurements in magnetohydrodynamic (MHD) quiet plasmas in the HL-2A tokamak. In another plasma with abundant MHD instabilities, both the continuous neutron flux decreases and the rapid neutron flux decreases caused by different MHD instabilities are observed in a more detailed manner for the first time with the HTRNFM system than with other neutron flux measurement (NFM) systems that have a lower temporal resolution of 1 ms. The HTRNFM system will serve as a powerful diagnostic tool for research on energetic ion confinement quality in the HL-2M tokamak.

Simulating the impact of charge exchange on beam ions in MAST-U

A model for simulating charge exchange (CX) of fast ions with background atoms in magnetically confined fusion plasmas has been implemented in the ASCOT orbit-following code. The model was verified by comparing simulated reaction mean free paths to analytical values across a range of fusion-relevant parameters. ASCOT was used to simulate beam ions slowing down in the presence of CX reactions in a MAST-U target scenario. ASCOT predicts the CX-induced loss of beam power to be , which agrees to within with the TRANSP prediction. Due to CX, plasma heating and current drive by beam ions are strongly reduced towards the edge. However, an overall lower but noticeable increase of up to in current drive is predicted closer to the core. The simulated deposition of fast CX atoms on the wall is concentrated around the outer midplane, with estimated peak power loads of 70–80 kW m−2 on the central poloidal field coils (P5) and the vacuum vessel wall between them. This analysis demonstrates that ASCOT can be used to simulate fast ions in fusion plasmas where CX reactions play a significant role, e.g. in spherical tokamaks and stellarators.

Fast-ion orbit sensitivity of neutron emission spectroscopy diagnostics.

Fast ions in fusion plasmas often leave characteristic signatures in the plasma neutron emission. Measurements of this emission are subject to the phase-space sensitivity of the diagnostic, which can be mapped using weight functions. In this paper, we present orbit weight functions for the TOFOR and NE213 neutron diagnostics at the Joint European Torus, mapping their phase-space sensitivity in 3D orbit space. Both diagnostics are highly sensitive to fast ions that spend a relatively large fraction of their orbit transit times inside the viewing cone of the diagnostic. For most neutron energies, TOFOR is found to be relatively sensitive to potato orbits and heavily localized counter-passing orbits, as well as trapped orbits whose "banana tips" are inside the viewing cone of TOFOR. For the NE213-scintillator, the sensitivity is found to be relatively high for stagnation orbits.

60 Years of neutral particle analysis: from early tokamaks to ITER

Academician A. D. Sakharov’s idea concerning the emission of atomic flux from hot plasma (1951) inspired scientists of A. F. Ioffe Physico-Technical Institute to create the first in the world instrument called Neutral Atom Analyzer in 1960 and then in 1961 to use it successfully on the Alpha device (USSR, 1958–1963). Now the analysis of fluxes of fast atoms referred to as Neutral Particle Analysis (NPA) is one of the main diagnostic methods for the ion component of plasma in tokamaks, stellarators, and other devices. NPA provides a unique opportunity for studying the ion distribution functions, ion temperatures and hydrogen isotope ratio in hot plasma. Neutral particle analyzers developed at the Ioffe Institute were widely used in the USSR until the late 1970s, and afterwards began to be employed worldwide. Since then, most of the information on the ion distribution functions and the behavior of fast ions in fusion plasma is obtained from NPA measurements on all leading magnetic confinement fusion systems worldwide. The specialized complex of atom analyzers currently being created at the Ioffe Institute is included in the primary list of ITER diagnostics. The integration of this complex on ITER is expected to begin in 2025.

Collective Thomson scattering with 77, 154, and 300 GHz sources in LHD

Collective Thomson scattering (CTS) is one of attractive diagnostics for measuring locally and directly the fuel temperature and the velocity distribution of fast ions in fusion plasmas. A mega-watt class source of millimeter or sub-millimeter waves is required to detect a weak scattered radiation superimposed on background radiation owing to electron cyclotron emissions (ECEs) from plasmas. Based on electron cyclotron resonance heating (ECRH) system with the frequencies of 77 GHz and 154 GHz in the Large Helical Device (LHD), the CTS diagnostic system has been developed to measure bulk ion temperatures from a few keV to ∼10 keV and fast ions originated from 180 keV-neutral beam injection in the LHD. The measured CTS spectra and their time evolutions are analyzed with the electrostatic scattering theory. The bulk ion temperatures obtained from CTS spectra increase with the neutral beam injections and decrease with the heating terminated. The velocity map of simulated fast ions explains that the bumps on tail of measured CTS spectra are caused by the co- and counter- fast ions. A new prescription for anisotropic velocity distribution function is proposed. As for 154 GHz bands, the CTS spectrum broadenings for D and H plasmas are distinguished reasonably at the same temperature, and its ion temperatures are comparable to those of the charge exchange recombination spectroscopy. As reactor-relevant diagnostics, a 300 GHz gyrotron and a corresponding receiver system have been implemented in LHD to access high density plasmas with low background ECEs. The recent progress for CTS diagnostics and their spectrum analysis with the probe frequencies of 77 GHz, 154 GHz, and 300 GHz in the LHD experiments is described.

Measuring fast ions in fusion plasmas with neutron diagnostics at JET

Fast ions in fusion plasmas often leave characteristic signatures in the neutron emission from the plasma. In this paper, we show how neutron measurements can be used to study fast ions and give examples of physics results obtained on present day tokamaks. The focus is on measurements with dedicated neutron spectrometers and with compact neutron detectors used in each channel of neutron profile monitors. A measured neutron spectrum can be analyzed in several different ways, depending on the physics scenario under consideration. Gross features of a fast ion energy distribution can be studied by applying suitably chosen thresholds to the measured spectrum, thus probing ions with different energies. With this technique it is possible to study the interaction between fast ions and MHD activity, such as toroidal Alfvén eigenmodes (TAEs) and sawtooth instabilities. Quantitative comparisons with modeling can be performed by a direct computation of the neutron emission expected from a given fast ion distribution. Within this framework it is also possible to determine physics parameters, such as the supra-thermal fraction of the neutron emission, by fitting model parameters to the data. A detailed, model-independent estimate of the fast ion distribution can be obtained by analyzing the data in terms of velocity space weight functions. Using this method, fast ion distributions can be resolved in both energy and pitch by combining neutron and gamma-ray measurements obtained along several different sightlines. Fast ion measurements of the type described in this paper will also be possible at ITER, provided that the spectrometers have the dynamic range required to resolve the fast ion spectral features in the presence of the dominating thermonuclear neutron emission. A dedicated high-resolution neutron spectrometer has been designed for this purpose.

Radiative losses of alpha particles on heavy ions in fusion plasma

A new channel of radiative losses in fusion plasma with impurities of multielectron ions associated with their excitation by fast alpha particles has been studied. It has been shown that this loss channel for tungsten impurity is comparable with standard radiative losses of plasma electrons.

Tritium Aspects of Fueling and Exhaust Pumping in Magnetic Fusion Energy

Magnetically confined fusion plasmas generate energy from deuterium-tritium (DT) fusion reactions that produce energetic 3.5 MeV alpha particles and 14 MeV neutrons. Since the DT fusion reaction rate is a strong function of plasma density, an efficient fueling source is needed to maintain high plasma density in such systems. Energetic ions in fusion plasmas are able to escape the confining magnetic fields at a much higher rate than the fusion reactions occur, thus dictating the fueling rate needed. These lost ions become neutralized and need to be pumped away as exhaust gas to be reinjected into the plasma as fuel atoms.The technology to fuel and pump fusion plasmas has to be inherently compatible with the tritium fuel. An ideal holistic solution would couple the pumping and fueling such that the pump exhaust is directly fed back into pellet formation without including impurity gases. This would greatly reduce the processing needs for the exhaust. Concepts to accomplish this are discussed along with the fueling and pumping needs for a DT fusion reactor.

Radiative transition of hydrogen-like ions in quantum plasma

ABSTRACTAt fusion plasma electron temperature and number density regimes of 1 × 103–1 × 107 K and 1 × 1028–1 × 1031/m3, respectively, the excited states and radiative transition of hydrogen-like ions in fusion plasmas are studied. The results show that quantum plasma model is more suitable to describe the fusion plasma than the Debye screening model. Relativistic correction to bound-state energies of the low-Z hydrogen-like ions is so small that it can be ignored. The transition probability decreases with plasma density, but the transition probabilities have the same order of magnitude in the same number density regime.

Performance of the prototype LaBr3 spectrometer developed for the JET gamma-ray camera upgrade.

In this work, we describe the solution developed by the gamma ray camera upgrade enhancement project to improve the spectroscopic properties of the existing JET γ-ray camera. Aim of the project is to enable gamma-ray spectroscopy in JET deuterium-tritium plasmas. A dedicated pilot spectrometer based on a LaBr3 crystal coupled to a silicon photo-multiplier has been developed. A proper pole zero cancellation network able to shorten the output signal to a length of 120 ns has been implemented allowing for spectroscopy at MHz count rates. The system has been characterized in the laboratory and shows an energy resolution of 5.5% at Eγ = 0.662 MeV, which extrapolates favorably in the energy range of interest for gamma-ray emission from fast ions in fusion plasmas.

Validating modeling assumptions of alpha particles in electrostatic turbulence

To rigorously model fast ions in fusion plasmas, a non-Maxwellian equilibrium distribution must be used. In this work, the response of high-energy alpha particles to electrostatic turbulence has been analyzed for several different tokamak parameters. Our results are consistent with known scalings and experimental evidence that alpha particles are generally well confined: on the order of several seconds. It is also confirmed that the effect of alphas on the turbulence is negligible at realistically low concentrations, consistent with linear theory. It is demonstrated that the usual practice of using a high-temperature Maxwellian, while previously shown to give an adequate order-of-magnitude estimate of the diffusion coefficient, gives incorrect estimates for the radial alpha particle flux, and a method of correcting it in general is provided. Furthermore, we see that the timescales associated with collisions and transport compete at moderate energies, calling into question the assumption that alpha particles remain confined to a flux surface that is used in the derivation of the slowing-down distribution.

The Effect of Nuclear Elastic Scattering on Temperature Equilibration Rate of Ions in Fusion Plasma

A plasma with two different particle types and at different temperatures has been considered, so that each type of ion with Maxwell-Boltzmann distribution function is in temperature equilibrium with itself. Using the extracted nuclear elastic scattering differential cross-section from experimental data, solving the Boltzmann equation, and also taking into account the mobility of the background particles, temperature equilibration rate between two different ions in a fusion plasma is calculated. The results show that, at higher temperature differences, effect of nuclear elastic scattering is more important in calculating the temperature equilibration rate. The obtained expressions have general form so that they are applicable to each type of particle for background (b) and each type for projectile (p). In this paper, for example, an equimolar Deuterium-Hydrogen plasma with densityn=5×1025 cm−3is chosen in which the deuteron is the background particle with temperature (also electron temperature)Tb=1 keV (usual conditions for a fusion plasma at the ignition instant) and the proton is the projectile with temperatureTp>Tb. These calculations, particularly, are very important for ion fast ignition in inertial confinement fusion concept.

Radiative power losses and opacity calculations in fusion plasmas1This article is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

We present recent results on radiative power losses of carbon and gold ions in fusion plasmas. Our approach is based on a detailed calculation, where the atomic database is provided by the MCDF code. A lineshape code, allowing for NLTE ionic populations, was adapted to the calculation of radiative power loss profiles. Because the calculation time is sometimes prohibitive, an alternative approach based on statistical distributions is proposed. This approach involves the first four moments of the radiative power losses. We have focused our attention on carbon and gold. The first element could be present in the ablator in some inertial confinement schemes. The second element is present in holraums and its X-ray emission contributes to the heating in inertial confinement fusion. Finally, opacity calculations are compared to measured transmission in aluminum ions.

Resonance between passing fast ions and MHD instabilities both in the tokamak and the RFP configurations

Suprathermal ions in fusion plasmas have different confinement properties in tokamak and reversed field pinch (RFP) configurations. These differences are explained in terms of guiding centre drifts by an analytical approach. Magnetic field perturbations and passing fast ions can satisfy resonance conditions which do not hold for thermal or low energy particles. Such a resonance is controlled by a generalization for passing fast ions of the safety factor parameter. Applications to experimental situations show good agreement in predicting the radial position of the resonances in the fast ion phase space. Numerical simulations in RFP devices are qualitatively consistent with the longer confinement times of fast ions, especially in helical regimes.

Magnetohydrodynamic activity and energetic ions in fusion plasmas

Selected issues of mutual influence of the energetic ions and collective phenomena in toroidal plasmas are considered. This includes peculiarities of energetic-ion-driven Alfvén instabilities in stellarators, fishbone instability and magnetohydrodynamic activity during tangential neutral beam injection in tokamaks and spherical tori with the shearless core.

Temperature of ion cyclotron heated impurity ions in fusion plasmas

The temperature of impurity ions in fusion plasmas is normally assumed to be the same as that of background particles. This assumption can be, however, violated for selective impurity species intensively heated by radio frequency waves or in reactor plasmas of low collisionality. An approach to compute the impurity temperature under such conditions is elaborated. It is based on the construction of a hierarchy of approximate solutions to the impurity heat balance equation and takes into account that the density and, thus, the heat conductivity of heated ion species can change by many orders of magnitude with the position in the plasma. Coupled with the wave propagation code, the particle and heat balances for impurity and main plasma species provide a self-consistent approach to model the ion cyclotron resonance heating scenario.

Reflection coefficients of D, T ions with Maxwell energy distribution

The reflection coefficients of D, T ions in fusion plasma incident to the first wall of a fusion reactor are necessary for us to understand and evaluate the energy balance, fuel recycling in the fusion reactor operation. Due to different energy and different direction of D, T ions in fusion plasma, the calculation of reflection coefficients of D and T ions in fusion plasma is generally a heavy task. In this paper we will present a new method, which is based on the scaling property of ion transport, for calculating the reflection coefficients of Maxwell D, T ions of fusion plasma. A systematic and comprehensive calculation for D, T reflection coefficients has been completed for getting the scaling formulas, and some reflection coefficients of Maxwell D, T ions with different temperature are given.

Effective charge and collision logarithm for slow ions in fusion plasmas

We find a general expression for the effective charge ${Z}_{\mathrm{eff}}$ of slow ions in dilute plasmas at temperature $T$, which applies for atomic numbers $5<~{Z}_{1}<~92$. The ratio $\frac{{Z}_{\mathrm{eff}}}{{Z}_{1}}$ becomes a function of $\frac{T}{{Z}_{1}^{2}}$. The effective-charge dependence of the collision logarithm for slow ions in the plasma is analyzed. A transition from classical to quantum regimes occurs with increasing $T$, for $\frac{T}{{Z}_{1}^{2}}\ensuremath{\approx}{10}^{6}$ K. The effective ion charge is relevant in the classical domain. For temperatures across and over the transition the ions are fully stripped of electrons.