Hot Fusion Plasmas Research Articles

A research program to measure the lifetime of spin polarized fuel

The use of spin polarized fuel could increase the deuterium-tritium (D-T) fusion cross section by a factor of 1.5 and, owing to alpha heating, increase the fusion power by an even larger factor. Issues associated with the use of polarized fuel in a reactor are identified. Theoretically, nuclei remain polarized in a hot fusion plasma. The similarity between the Lorentz force law and the Bloch equations suggests polarization can be preserved despite the rich electromagnetic spectrum present in a magnetic fusion device. The most important depolarization mechanisms can be tested in existing devices. The use of polarized deuterium and 3He in an experiment avoids the complexities of handling tritium, while encompassing the same nuclear reaction spin-physics, making it a useful proxy to study issues associated with full D-T implementation. 3He fuel with 65% polarization can be prepared by permeating optically-pumped 3He into a shell pellet. Dynamically polarized 7Li-D pellets can achieve 70% vector polarization for the deuterium. Cryogenically-frozen pellets can be injected into fusion facilities by special injectors that minimize depolarizing field gradients. Alternatively, polarized nuclei could be injected as a neutral beam. Once injected, the lifetime of the polarized fuel is monitored through measurements of escaping charged fusion products. Multiple experimental scenarios to measure the polarization lifetime in the DIII-D tokamak and other magnetic-confinement facilities are discussed, followed by outstanding issues that warrant further study.

Radiation Limit for the Energy Gain of the p–11B Reaction

The feasibility of positive energy yield in systems with the p–11B reaction is considered here by considering refined (optimistic) data on the reaction rate. The analysis was carried out within the traditional framework for magnetic confinement systems, but without taking into account a particular type of plasma configuration. The energy balance was considered both for the ions and electrons. The balance of particles includes all species as well as the products of fusion (alpha particles). Calculations have shown that accounting for the content of thermalized reaction products (alpha particles) leads to an increase in radiation losses and a decrease in gain to Q < 1. In the steady-state scenario, the energy gain Q~5–10 can be obtained in p–11B plasma, if only the fast (high-energy) population of fusion alpha particles is considered. For pulsed modes, the gain value is proportional to the content of alpha particles, and it is limited by the complete burn of one of the fuel components (boron), so it does not exceed unity. In the analysis we did not rely on any assumptions about the theoretically predicted mechanisms for increasing the cross section and the reaction rate, and only radiation losses (primarily bremsstrahlung) dramatically affect the gain Q. Thus, the regimes found can be considered as limiting in the framework of the classical concepts of processes in hot fusion plasma.

Compact Radiative Divertor Experiments at ASDEX Upgrade and Their Consequences for a Reactor.

We present a novel concept to tackle the power exhaust challenge of a magnetically confined fusion plasma. It relies on the prior establishment of an X-point radiator that dissipates a large fraction of the exhaust power before it reaches the divertor targets. Despite the spatial proximity of the magnetic X point to the confinement region, this singularity is far away from the hot fusion plasma in magnetic coordinates and therefore allows the coexistence of a cold and dense plasma with a high potential to radiate. In the compact radiative divertor (CRD) the target plates are placed close to this magnetic X point. We here report on high performance experiments in the ASDEX Upgrade tokamak that indicate the feasibility of this concept. Despite the shallow (projected) field line incidence angles of the order of θ_{⊥}=0.2°, no hot spots were observed on the target surface monitored by an IR camera, even at a maximum heating power of P_{heat}=15 MW. And even with the X point located exactly on the target surface and without density or impurity feedback control, the discharge remains stable, the confinement good (H_{98,y2}=1), hot spots absent, and the divertor in a detached state. In addition to its technical simplicity, the CRD scales beneficially to reactor-scale plasmas that would benefit from an increased volume of the confined plasma, more space for breeding blankets, smaller poloidal field coil currents, and-potentially-an increased vertical stability.

Helium plasma immersion ion implantation studies of tungsten and tungsten heavy alloys for fusion plasma facing components

Plasma fusion devices will require plasma-facing components (PFCs) which can withstand the extreme environment at the edge of a hot fusion plasma. Despite the excellent properties of tungsten(W) as a hard refractory metal, adverse effects such as embrittlement, melting, and morphological evolution have been observed in W when it is bombarded by a high-fluence of low-energy ions including helium. This study investigates the effect of helium ion bombardment on pure tungsten and a tungsten heavy alloy (W-HA)(NAECOMET 1000). Pure tungsten and NAECOMET 1000 samples were implanted with 3 keV helium ions with fluences ranging from to , using Plasma Immersion Ion Implantation (PIII). After PIII treatment, samples were analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM), which revealed differences in surface morphology and topography. Although the melting and cracking of the Ni/Cu binder phase in the NAECOMET 1000 samples was seen under all implantation conditions, the Ni/Cu presence did somewhat slow the formation of W fuzz in comparison to pure tungsten. X-ray diffraction (XRD) studies showed peak shifts increasing with helium ion fluence for both the pure W and NAECOMET 1000 samples, as well as increase in mean crystallite size, confirming the distortion of the lattice. XPS compositional analysis showed a strong oxidation (>97) near the metal surface, after helium PIII treatment, for both pure W and NAECOMET 1000 W-HA samples. Some conclusions about the potential suitability of W-HA materials for fusion plasma PFCs are drawn.

RELATIVISTIC KINETICS AND HYDRODYNAMICS OF HOT COLLISIONAL PLASMAS

In the paper, relativistic equations of local hydrodynamics for the laboratory fusion plasmas are obtained. Relativistic effects in the physics of electron transport appear primarily because of macroscopic features of relativistic thermodynamic equilibrium given by the Maxwell-Jüttner distribution function, and the characteristic velocity of plasma flow is significantly small: 𝑉 ≪ 𝑣𝑡𝑒 < 𝑐. We propose an approach in which the plasma electrons are treated as fully relativistic and the hydrodynamic flow is treated in the weakly relativistic approximation. For convenience, the obtained relativistic effects are divided between “quasi-relativistic” terms, which in the nonrelativistic limit coincide with well-known expressions, and fully relativistic terms, which disappear at 𝑐 → ∞. The considered mixed approach can be useful for construction of transport models for numerical studies of both astrophysical objects and hot fusion plasma.

The EU DEMO equatorial outboard limiter — Design and port integration concept

Abstract The equatorial outboard limiters (also called outboard midplane limiters (OMLs)) are an essential part of the DEMO wall protection concept. Limiters are foreseen in different areas of the DEMO first wall, namely in the equatorial ports, on the high-field side, in vertical ports and additional protection limiters between equatorial and lower ports. The limiters shall prevent the plasma to touch the first wall of the breeding blankets during all plasma transients. The port integration concept of the OMLs, used for plasma ramp-up/-down, is explained including (i) thermal, structural and electromagnetic loads, (ii) neutronic requirements and related material properties, (iii) remote handling considerations, (iv) space and mass constraints and (v) the required alignment precision to allow equal distribution of the heat exposure amongst the individual of the plasma facing (PFC) limiter components. While the hot fusion plasma during ramp-up is impinging directly on the limiter, its PFC components temperature is rising and can be measured by means of either thermocouples or by infrared (IR) thermography an estimation of the heat flux on the contact point can be made. This is the basis for the proposed alignment strategy.

General Relativistic Modification of the Lockheed Compact Fusion Reactor Concept

Taking into account Einstein’s general theory of relativity, a modification of Lockheed’s compact fusion reactor concept is proposed by replacing the two superconducting tori with rapidly rotating tori rotating in opposite directions. According to the general theory of relativity, two Coriolis force fields in opposite directions are set up, both of them having a negative mass density in their corotating reference systems, with a vanishing negative mass density in the center in between the rotating tori, where the hot fusion plasma is centered. Because of the Nernst effect going in the opposite direction, large toroidal currents are set up, repelling the hot plasma from the much cooler tori. This results in closed magnetic field lines for stable plasma confinement. The remaining problem, the removal of the heat released by neutron absorption in the metallic tori, can be resolved by a pulsed operation, axially injecting cool deuterium-tritium gas, from which the heat is externally removed by a radiator.

Plasma–metal junction

In this work, the concept of plasma–metal junction (pm-junction) is presented, in addition to well established physics of pn-junction, metal–semiconductor junction, semiconductor–insulator junction, etc. We demonstrate experimentally the dependence of I-V characteristics on work function of the metal for a pm-junction. The experiment is done using tungsten (W) and stainless steel (SS) having different work functions (Φm) of 4.55 eV and 4.30 eV, respectively. The W and SS planar metal probes of identical geometry were exposed to plasma simultaneously, making two distinct pm-junctions. The plasma exposed is identical for both the metals, and therefore, the difference observed in the I-V characteristics of these two pm-junctions is attributed only to the difference in an energy-band diagram of these junctions, owing to respective work function. The proposed concept of the pm-junction may simplify the complex plasma–material interactions in all laboratory plasmas, including cold glow discharges to hot fusion plasmas.

Charge exchange recombination spectra for 100 keV/u and 500 keV/u atomic hydrogen beam colliding with W64+

We present synthetic spectra for light emission following charge exchange (CX) recombination for Ne-like W64+ ions colliding with neutral atomic hydrogen at 100 and 500 keV/u, which is of relevance to the plasma diagnostics of the international experimental fusion device ITER now under construction. The spectra are calculated using a detailed collisional-radiative model for the W63+ ion that includes more than 6000 singly- and doubly-excited states and accounts for major physical processes in hot fusion plasmas. The CX cross sections into excited states are computed using the classical trajectory Monte Carlo method. A comprehensive analysis of the modifications to the spectra due to CX is presented.

Simulation of Spectra Code (SOS) for ITER Active Beam Spectroscopy

The concept and structure of the Simulation of Spectra (SOS) code is described starting with an introduction to the physics background of the project and the development of a simulation tool enabling the modeling of charge-exchange recombination spectroscopy (CXRS) and associated passive background spectra observed in hot fusion plasmas. The generic structure of the code implies its general applicability to any fusion device, the development is indeed based on over two decades of spectroscopic observations and validation of derived plasma data. Four main types of active spectra are addressed in SOS. The first type represents thermal low-Z impurity ions and the associated spectral background. The second type of spectra represent slowing-down high energy ions created from either thermo-nuclear fusion reactions or ions from injected high energy neutral beams. Two other modules are dedicated to CXRS spectra representing bulk plasma ions (H+, D+, or T+) and beam emission spectroscopy (BES) or Motional Stark Effect (MSE) spectrum appearing in the same spectral range. The main part of the paper describes the physics background for the underlying emission processes: active and passive CXRS emission, continuum radiation, edge line emission, halo and plume effect, or finally the charge exchange (CX) cross-section effects on line shapes. The description is summarized by modeling the fast ions emissions, e.g., either of the α particles of the fusion reaction or of the beam ions itself.

Polarization of K-shell Dielectronic Recombination Satellite Lines of Fe xix–xxv and Its Application for Diagnostics of Anisotropies of Hot Plasmas

We present a systematic measurement of the X-ray emission asymmetries in the K-shell dielectronic, trielectronic, and quadruelectronic recombination of free electrons into highly charged ions. Iron ions in He-like through O-like charge states were produced in an electron beam ion trap, and the electron-ion collision energy was scanned over the recombination resonances. Two identical X-ray detectors mounted head-on and side-on with respect to the electron beam propagation recorded X-rays emitted in the decay of resonantly populated states. The degrees of linear polarization of X-rays inferred from observed emission asymmetries benchmark distorted-wave predictions of the Flexible Atomic Code (FAC) for several dielectronic recombination satellite lines. The present method also demonstrates its applicability for diagnostics of energy and direction of electron beams inside hot anisotropic plasmas. Both experimental and theoretical data can be used for modeling of hot astrophysical and fusion plasmas.

Polarimetric Thomson scattering for high Te fusion plasmas

Polarimetric Thomson scattering (TS) is a technique for the analysis of TS spectra in which the electron temperature Te is determined from the depolarization of the scattered radiation, a relativistic effect noticeable only in very hot (Te ⩾ 10 keV) fusion plasmas. It has been proposed as a complementary technique to supplement the conventional spectral analysis in the ITER CPTS (Core Plasma Thomson Scattering) system for measurements in high Te, low ne plasma conditions. In this paper we review the characteristics of the depolarized TS radiation with special emphasis to the conditions of the ITER CPTS system and we describe a possible implementation of this diagnostic method suitable to significantly improve the performances of the conventional TS spectral analysis in the high Te range.

Impurity transport and bulk ion flow in a mixed collisionality stellarator plasma

The accumulation of impurities in the core of magnetically confined plasmas, resulting from standard collisional transport mechanisms, is a known threat to their performance as fusion energy sources. Whilst the axisymmetric tokamak systems have been shown to benefit from the effect of temperature screening, that is an outward flux of impurities driven by the temperature gradient, impurity accumulation in stellarators was thought to be inevitable, driven robustly by the inward pointing electric field characteristic of hot fusion plasmas. We have shown in Helander et al. (Phys. Rev. Lett., vol. 118, 2017a, 155002) that such screening can in principle also appear in stellarators, in the experimentally relevant mixed collisionality regime, where a highly collisional impurity species is present in a low collisionality bulk plasma. Details of the analytic calculation are presented here, along with the effect of the impurity on the bulk ion flow, which will ultimately affect the bulk contribution to the bootstrap current.

Pulsed fusion space propulsion: Computational Magneto-Hydro Dynamics of a multi-coil parabolic reaction chamber

Pulsed fusion propulsion might finally revolutionise manned space exploration by providing an affordable and relatively fast access to interplanetary destinations. However, such systems are still in an early development phase and one of the key areas requiring further investigations is the operation of the magnetic nozzle, the device meant to exploit the fusion energy and generate thrust. One of the last pulsed fusion magnetic nozzle design is the so called multi-coil parabolic reaction chamber: the reaction is thereby ignited at the focus of an open parabolic chamber, enclosed by a series of coaxial superconducting coils that apply a magnetic field. The field, beside confining the reaction and preventing any contact between hot fusion plasma and chamber structure, is also meant to reflect the explosion and push plasma out of the rocket. Reflection is attained thanks to electric currents induced in conductive skin layers that cover each of the coils, the change of plasma axial momentum generates thrust in reaction. This working principle has yet to be extensively verified and computational Magneto-Hydro Dynamics (MHD) is a viable option to achieve that. This work is one of the first detailed ideal-MHD analysis of a multi-coil parabolic reaction chamber of this kind and has been completed employing PLUTO, a freely distributed computational code developed at the Physics Department of the University of Turin. The results are thus a preliminary verification of the chamber's performance. Nonetheless, plasma leakage through the chamber structure has been highlighted. Therefore, further investigations are required to validate the chamber design. Implementing a more accurate physical model (e.g. Hall-MHD or relativistic-MHD) is thus mandatory, and PLUTO shows the capabilities to achieve that.

Conceptual design of a polarimetric Thomson scattering diagnostic in ITER

Polarimetric Thomson scattering (TS) is a novel diagnostic technique proposed as an alternative to conventional (spectral) TS, for the measurement of the electron temperature Te and density ne in very hot fusion plasmas. Contrary to spectral TS, which is based on the reconstruction of the Doppler broadened frequency spectrum, in polarimetric TS Te is determined from the depolarization of the scattered radiation. The technique is suitable for ITER, where it is expected to be competitive with conventional spectral TS for measurements in the highest Te range, specially in backward-like conditions with the scattering angle 90° ≪ θ ⩽ 180°. In this paper we consider a hypothetical polarimetric TS diagnostic for ITER and evaluate its performance for the θ = 145° scattering condition typical of the core TS system and also for a different scattering geometry in which, using a tangential laser beam, the central region of the ITER plasma can be observed under a scattering angle θ ∼ 75°. In both cases we calculate the expected errors on the measured Te and ne that can be obtained with a simple, two-channel polarimeter, and taking into account that only a fraction of the TS wavelength spectrum is detected. In both cases the expected performances are compared with those of the conventional spectral core TS diagnostic to determine the plasma conditions in which the polarimetric technique is more advantageous. A measurement of the depolarization effect of the TS radiation using the JET High Resolution TS system of JET is also discussed.

RETRACTED: New results on structure of low beta confinement Polywell cusps simulated by comsol multiphysics

The Inertial electrostatic confinement (IEC) is one of the ways for fusion approaches. It is one of the various methods which can be used to confine hot fusion plasma. The advantage of IEC is that the IEC experiments could be done in smaller size facilities than ITER or NIF, costing less money and moving forward faster. In IEC fusion, we need to trap adequate electrons to confine the desired ion density which is needed for a fusion reactor. Polywell is a device which uses the magnetic cusp system and traps the required amount of electrons for fusion reactions. The purpose of this device is to create a virtual cathode in order to achieve nuclear fusion using inertial electrostatic confinement (Miley and Krupakar Murali, 2014). In this paper, we have simulated the low beta Polywell. Then, we examined the effects of coil spacing, coils current, electron injection energy on confinement time.

Rotational Raman scattering as a source of polarized radiation for the calibration of polarization-based Thomson scattering

We discuss the use of rotational Raman scattering as a possible technique for the calibration of polarization based plasma diagnostics such as the polarimetric Thomson scattering (TS) systems proposed for the measurement of electron density ne and temperature Te in very hot fusion plasmas. First by using the Stokes vector formalism, we calculate the Mueller matrix of rotational Raman scattering from a diatomic gas (such as H2, D2 and N2) for a generic scattering angle and then we use it to describe in detail the polarization characteristics of the Raman radiation available in the typical conditions of a TS system in a fusion plasma. Then we consider the various experimental set-ups proposed for polarimetric TS and show that rotational Raman scattering can generate radiation with known intensity and polarization states that, although not equal to those of the TS radiation, are suitable for the calibration of polarimetric TS diagnostics. Finally we point out that in fusion experiments Raman polarimetry may be of interest also for the calibration of other polarization-based measurement systems, such as the motional Stark effect (MSE) diagnostics.

The Sparse Grid Combination Technique for Computing Eigenvalues in Linear Gyrokinetics

Using the five-dimensional gyrokinetic equations for simulations of hot fusion plasmas requires discretizations with a lot degrees of freedom due to the curse of dimensionality. The sparse grid combination technique could be one remedy to this problem, since it is dividing the original problem into smaller subproblems, which can be solved independently. For this study, the gyrokinetic code GENE has been used for solving the gyrokinetic eigenvalue problem using the combination technique. The performance of different combination schemes is used on a test problem and evaluated with respect to accuracy in retrieving an eigenvalue and the computational effort required. This evaluation suggests, that the sparse grid combination technique is a feasible method to compute eigenvalues of the gyrokinetic eigenvalue problem.

Influence of temperature fluctuations on plasma turbulence investigations with Langmuir probes

The reliability of Langmuir probe measurements for plasma-turbulence investigations is studied on GEMR gyro-fluid simulations and compared with the results from conditionally sampled I–V characteristics as well as electron-emitting probe measurements close to the last closed flux surface of the tokamak ASDEX Upgrade. In this region, simulation and experiment consistently show coherent in-phase fluctuations in density, plasma potential and also electron temperature. Ion-saturation current measurements turn out to reproduce density fluctuations quite well. Fluctuations in the floating potential, however, are strongly influenced by temperature fluctuations and, hence, are strongly distorted compared to the actual plasma potential. These results suggest that interpreting floating as plasma-potential fluctuations while disregarding temperature effects is not justified near the separatrix of hot fusion plasmas. Here, floating potential measurements led to corrupted results on the E × B dynamics of turbulent structures in the context of, e.g., turbulent particle and momentum transport or turbulence characterization on the basis of density–potential phase relations.

Plasma transport across magnetic field lines in low-temperature plasma sources

Plasma transport across magnetic field lines plays a key role not only in hot fusion plasmas but also in low-temperature plasma sources operating at low pressure, which often rely on external magnetic fields for their operation. Transport in these sources involves different physics than that in fusion plasmas: the ions are not (completely) magnetized, the plasma is sensitive to wall effects because the magnetic field lines intercept the chamber walls, and the neutral gas density is often much larger than the plasma density. This paper gives an overview of the main principles of magnetized low-temperature plasma transport as they are currently understood, including recent insights on the role of magnetic drift. Three important forms of magnetized low-temperature plasma transport are discussed: magnetized plasma diffusion, transport in E × B fields and magnetic drift. These phenomena are illustrated with recent numerical modeling results on a dipolar microwave source, an End-Hall ion source, and simplified version of the ITER negative ion source. For the latter source it is shown that obstructed magnetic drift can lead to plasma asymmetry and increased cross-field transport.