Magnetic Confinement Devices Research Articles

Helimak control system

The excellent wall condition is essential for fusion devices to generate reproducible high-parameter plasma. Wall conditioning techniques should be validated before applying on large fusion devices to reduce potential risk. Helimak, a toroidal steady state magnetic confinement device, was reassembled and upgraded to a technical validation platform for wall conditioning. A new control system has been developed to achieve the upgrade of Helimak. The control system is based on a distributed control architecture, which provides not only efficient development but also flexible control mechanisms. For each auxiliary subsystem, a corresponding local control module is developed to drive and control a variety of equipment. The control network ensures real-time transmitting of the parameters and data. A dedicated trigger network and timing control module can improve synchronization and real-time performance. Data acquisition system based on mature commercial busses provides sufficient data acquisition capabilities for diagnostic tools. The data management system uses MDSplus to provide convenient data access. An independent interlock and safety system is designed for comprehensive and reliable personal protection. After engineering commissioning, the Helimak has realized wall conditioning by electron cyclotron resonance heating (ECRH) plasma at different resonance positions. The flexibility of control systems allows the installation of more wall conditioning tools in the future that will provide the ability to validate combined wall conditioning techniques. Helimak will become a low-cost, reliable technology verification platform, which will help to achieve advanced scenarios in tokamak and stellarator.

On the Formation of a Plasma Cloud at the Ablation of a Pellet in a High-Temperature Magnetized Toroidal Plasma

The investigation of cold secondary plasma clouds near pellets ablating in the hot plasma of magnetic confinement devices (tokamaks and stellarators) provides valuable information on the physical characteristics of a pellet cloud. In this work, the characteristic sizes of emitting clouds around fusible polystyrene pellets and refractory carbon pellets have been analyzed. The calculation of the ionization length of C+ ions in both carbon and hydrocarbon clouds has shown that the contribution of only hot electrons is insufficient to ensure the experimentally observed decay lengths of the CII line intensity. Taking into account the strong shielding of the electron flux of the background plasma in the hydrocarbon pellet cloud, the ionization of C+ ions in this cloud is determined predominantly by electrons of the cold plasma of the cloud. Shielding near a refractory carbon pellet is weak because its ablation rate is lower. The contributions from hot electrons of the surrounding plasma and cold electrons of the pellet cloud to the ionization of C+ ions are comparable in the case of carbon pellets.

The Princeton Field-Reversed Configuration for Compact Nuclear Fusion Power Plants

The Princeton Field-Reversed Configuration (PFRC) nuclear fusion reactor concept is an innovative approach to fusion power generation prioritizing low neutron production and small size. A combination of analytical modeling and numerical simulation shows that the novel heating approach generates an FRC with closed field lines. Simulation data from a single-particle Hamiltonian code predicts ms-scale plasma heating in reactor-scale conditions while PIC codes predict formation of warm FRC plasmas from initial mirror fields. The PFRC-1 and PFRC-2 experiments have heated electrons to energies well in excess of 100 eV and plasma durations to 300 ms, more than 10\(^4\) times longer than the predicted tilt instability growth time. From these data, we have created a development plan and anticipated performance metrics for a fusion reactor based on the PFRC concept. The resulting 1–10 MW PFRC reactors would be suitable for diverse applications, from submarines to urban environments to space propulsion. PFRC is a steady-state, driven magnetic confinement device. Plasma, inside a cylindrical array of coils, is confined and heated by external RF antennae. PFRC would be ultra-low radiation due to both its fuel and small size. The choice of advanced fuels, deuterium and helium-3 (D–3He), may be enabled by the high-\(\beta\) FRC configuration. The small size of the reactor would enable rapid exhaust of the dangerous tritium ash. Low radiation would make the reactor safer to operate and, in combination with simple geometry and small size, dramatically lowers development and maintenance costs. This review paper gives an introduction to the physics of the PFRC and a summary of the PFRC-2 experiment results to date. It then discusses the future program plan and how PFRC reactors would be commercialized.

Electromechanical behaviour of REBCO coated conductor toroidal field coils for ultra-high-field magnetic-confinement plasma devices

The development of rare-earth barium copper oxide (REBCO) coated conductors with an extremely high critical current density under ultra-high fields opens up a high-field path towards large-scale fusion. The latest technology has inspired cable-in-conduit conductors such as conductor on round core wires, twisted stacked tape conductor cables and Rutherford cables with outstanding current-carrying capacities. In order to realise an inductance balance and decrease magnetic diffusion, these cables have been twisted or folded to a certain extent, thus breaking the mechanical behaviour of the ceramic superconductor and limiting their potential for ultra-high-field applications. One possible solution is to employ a non-twisted cable, which offers maximum protection of its mechanical properties and enables a parallel orientation of the toroidal field vector to the surface of REBCO coated conductors, and at the same time decreases the influence of perpendicular fields on the critical current of REBCO cables. However, the applied physics community’s attitude towards using non-twisted, parallel REBCO stacked-tape cables is one of scepticism, the main argument being that the nonlinear E–J behaviour associated with screening current in the parallel stack might lead to a field distortion and reduce the performance of superconductivity. Recent analyses have demonstrated that the effect of screening current decreases significantly owing to a wavelike magnetic field distribution along the cable. The authors obtained similar results using H-formulation and T–A formulation based finite element methods and demonstrated that the non-twisted cable may be feasible for DC current transmission toroidal field coils in magnetic-confinement devices. Furthermore, the electromechanical behaviour of toroidal field coils has been evaluated via the Maxwell stress, solved by using an A–V formulation. It was discovered that the stress generated by the toroidal field coils is within the stress tolerance of the REBCO coated conductor, something which is of great significance in promoting the application of REBCO coated conductors for ultra-high-field magnetic-confinement plasma devices.

Heavy Ion Beam Probing Diagnostics on the TUMAN-3M Tokamak for Study Plasma Potential and Electric Fields in New Operational Regimes

Heavy Ion Beam Probing (HIBP) diagnostic is a powerful tool for electric field studies in the hot dense plasma of modern-day toroidal magnetic confinement devices. On the TUMAN-3M tokamak, the HIBP have been used in regimes with improved plasma confinement to clear up the role of the radial electric field in the transition to good confinement regimes. Recently, a modernization of the TUMAN-3M HIBP diagnostics was performed, aiming to reconfigure it for a work with a reversed plasma current direction and improvement of the overall stability of the diagnostic. The results of the first measurements of the plasma potential in the co-NBI scenario are reported and discussed.

Tracking blobs in the turbulent edge plasma of a tokamak fusion device

The analysis of turbulence in plasmas is fundamental in fusion research. Despite extensive progress in theoretical modeling in the past 15 years, we still lack a complete and consistent understanding of turbulence in magnetic confinement devices, such as tokamaks. Experimental studies are challenging due to the diverse processes that drive the high-speed dynamics of turbulent phenomena. This work presents a novel application of motion tracking to identify and track turbulent filaments in fusion plasmas, called blobs, in a high-frequency video obtained from Gas Puff Imaging diagnostics. We compare four baseline methods (RAFT, Mask R-CNN, GMA, and Flow Walk) trained on synthetic data and then test on synthetic and real-world data obtained from plasmas in the Tokamak à Configuration Variable (TCV). The blob regime identified from an analysis of blob trajectories agrees with state-of-the-art conditional averaging methods for each of the baseline methods employed, giving confidence in the accuracy of these techniques. By making a dataset and benchmark publicly available, we aim to lower the entry barrier to tokamak plasma research, thereby greatly broadening the community of scientists and engineers who might apply their talents to this endeavor.

The Influence of Fast Particles on Plasma Rotation in the TUMAN-3M Tokamak

In most present-day tokamaks, the majority of the heating power comes from sources such as neutral-beam injection (NBI) and other types of auxiliary heating which allow for the transfer of energy to the plasma by a small population of externally introduced fast particles. The behavior of the fast ions is important for the overall plasma dynamics, and understanding their influence is vital for the success of any future magnetic confinement devices. In the TUMAN-3M tokamak, it has been noted that the loss of fast particles during NBI can lead to dramatic changes in the rotation velocity profiles, as they are responsible for the negative radial electric field on the periphery.

Minimization of magnetic forces on stellarator coils

Magnetic confinement devices for nuclear fusion can be large and expensive. Compact stellarators are promising candidates for cost-reduction, but introduce new difficulties: confinement in smaller volumes requires higher magnetic field, which calls for higher coil-currents and ultimately causes higher Laplace forces on the coils—if everything else remains the same. This motivates the inclusion of force reduction in stellarator coil optimization. In the present paper we consider a coil winding surface, we prove that there is a natural and rigorous way to define the Laplace force (despite the magnetic field discontinuity across the current-sheet), we provide examples of cost associated (peak force, surface-integral of the force squared) and discuss easy generalizations to parallel and normal force-components, as these will be subject to different engineering constraints. Such costs can then be easily added to the figure of merit in any multi-objective stellarator coil optimization code. We demonstrate this for a generalization of the REGCOIL code (Landreman 2017 Nucl. Fusion 57 046003), which we rewrote in python, and provide numerical examples for the NCSX (Zarnstorff et al 2001 Plasma Phys. Control. Fusion 43 A237–49) (now QUASAR) design. We present results for various definitions of the cost function, including peak force reductions by up to 40%, and outline future work for further reduction.

New Approach to Cross-Correlation Reflectometry Diagnostics of Nonlocality of Plasma Turbulence

One of the most important properties of stochastic nonlinear processes, including the turbulence of the hydrodynamic motion of continuous media, is distant spatial correlations. To describe them, an approach was proposed by Shlesinger and colleagues based on a linear integro-differential equation with a slowly decaying kernel, which corresponds to superdiffusion (nonlocal) transfer in the regime of Lévy walks (Lévy flights when the finite velocity of the carriers is taken into account). In this paper, we formulate a similar approach that makes it possible to formulate the problem of determining these properties from the scattering spectra of electromagnetic (EM) waves and cross-correlation reflectometry. A universal description of the relationship between the observed symmetric quasi-coherent component in the spectrum of scattered EM waves in plasmas and a process of the Mandelstam–Brillouin scattering type is obtained. It is shown that the nonlocality of spatial correlations of density fluctuations in a turbulent medium is due to long-free-path carriers of the medium’s perturbations, for which the free path distribution function is described by the Lévy distribution. The effectiveness of the proposed method is shown by the example of the interpretation of the data of cross-correlation reflectometry of EM waves in the radio-frequency range for the diagnosis of turbulent plasma in magnetic confinement devices for axisymmetric toroidal thermonuclear plasma.

Kinetic investigations of nonlinear electrostatic excitations in quantum plasmas.

For plasmas in an extremely high-density state, like stellar cores or compressed fuel in inertial fusion facilities, their behavior turns out to be quite different when compared with those plasmas in interstellar space or magnetic confinement devices. To figureout those differences and uncover the kinetic physics in electrostatic excitations, a quantum kinetic code solving Wigner-Poisson equationshas been developed. Basic plasmon decay, Landau damping, and two-stream instability of extremely high-density plasmas are investigated by using our newly developed code. Numerical simulations show that in the linear region, the dispersion relations of intrinsic modes can be significantly affected by quantum effects, and such simulation results can be well described by the existing analytical theory. Especially in the nonlinear region, since the space-time scale of collective modes of plasmas is comparable to the electron de Broglie wavelength, their couplings produce some new physics: the energy exchange between the electron and the collective mode results in an abnormal oscillation that does not exist in classical plasmas.

2D Fourier finite element formulation for magnetostatics in curvilinear coordinates with a symmetry direction

We present a numerical method for the solution of three-dimensional linear magnetostatic problems by embedding their geometry into a symmetric domain and Fourier expansion in the symmetry coordinate, leading to a set of decoupled two-dimensional problems. Special cases include axial and translational symmetry. To keep the treatment universal, a framework for the Fourier decomposition of the vector potential formulation is developed in the covariant formalism with general curvilinear coordinates. Under the restriction of homogeneous material parameters along the symmetry direction, but not on fields and current densities, this leads to a decoupled set of two-dimensional equations in both, zeroth (non-oscillatory) and non-zero (oscillatory) harmonics. For the latter it is possible to eliminate one component of the vector potential resulting in a fully transverse vector potential orthogonal to the transverse magnetic field. In addition to the Poisson-like equation for the longitudinal component of the non-oscillatory problem, a general curl-curl Helmholtz equation describes the transverse problem covering both, non-oscillatory and oscillatory case. The resulting variational forms are treated by the usual nodal finite element method for the longitudinal problem and by a two-dimensional edge element method for the transverse problem. The numerical solution can be computed independently for each harmonic using fewer degrees of freedom than a three-dimensional simulation at comparable accuracy. These properties make the developed approach useful for the analysis of perturbation harmonics in magnetic plasma confinement devices in cylindrical coordinates or magnetic flux coordinates. The method is validated on analytical and numerical test cases and applied to a racetrack-shaped coil setup.

Sudden collapse of a pressure profile generated by off-axis heating in a linear magnetized plasma

The features of an unexpected, large event that arises spontaneously during a basic heat transport experiment are presented. It consists of the sudden collapse of the radial plasma pressure profile, akin to disruption events observed in toroidal magnetic confinement devices. The experiment is performed on the Large Plasma Device at the University of California, Los Angeles (UCLA). It uses a LaB6 thermionic emitter of annular shape to induce off-axis heating of a cold, afterglow plasma, in a linear magnetic geometry. The temporal evolution consists of three regimes. During an early, quiescent period, classical heat transport along and across the magnetic field arises from Coulomb collisions. After significant pressure gradients develop, drift-Alfvén waves become unstable. Upon reaching large amplitude, they trigger avalanche events that flatten the outer part of the heated region, which, in turn, quenches the instability. Due to the sustained heating, the pressure profile rebuilds and the process repeats, leading to a relatively long, second regime that displays multiple avalanches, but suddenly, the annular pressure profile is observed to collapse. After this collapse, the system enters a third regime with large fluctuations. Before the collapse, a rapid, runaway heating environment arises whose time evolution exhibits a self-similar dependence on the applied voltage. The time evolution, morphology, and scaling of the collapse event are presented, and an examination is made of the underlying mechanisms.

Classical Thermodynamic Analysis of Deuterium-Based Fusion Reactions

The fusion reactions involving deuterium are of great interest for the exploitation of the fusion energy via magnetic-confinement devices. In classical thermodynamics, the spontaneity of a process is established through the assessment of the change in Gibbs free energy. So far, the feasibility of nuclear reactions has been characterized in terms of cross section and Q-value while the entropic term (T ΔS) has been neglected. Such an assumption is always justified for fission reactions where the term ΔS is positive. In the case of fusion reactions that operate at very high temperatures (106–107 K) and where ΔS is negative, the change in Gibbs free energy may be positive, making the reaction non-spontaneous. This paper proposes a classical thermodynamic analysis of D-based reactions of interest for the magnetic-confinement fusion applications. The entropy contribution was evaluated via the Sackur–Tetrode equation while the change in enthalpy was considered constant and as corresponding to the Q-value of the fusion reaction. The results of the thermodynamic analysis are compared with nuclear reaction feasibility criteria based on the reaction reactivity. The DT and D3He reactions show a high degree of spontaneity although the second one presents a lower reactivity. An increase in temperature could enhance the reactivity of the D3He reaction at the cost of decreasing its thermodynamic spontaneity. Both branches of the DD reaction are characterized by a much lower thermodynamic spontaneity than that of the DT and D3He reactions. Furthermore, at the temperature of their maximum cross section, the DD reactions exhibit a largely positive change in Gibbs free energy and, therefore, are not spontaneous. At the temperature of magnetic-confinement fusion machines (1.5 × 108 K), among the D-based reactions studied, the DT one exhibits the highest degrees of spontaneity and reactivity.

Model-based electron density profile estimation and control, applied to ITER

In contemporary magnetic confinement devices, the density distribution is sensed with interferometers and actuated with feedback controlled gas injection and open-loop pellet injection. This is at variance with the density control for ITER and DEMO, that will depend mainly on pellet injection as an actuator in feed-back control. This paper presents recent developments in state estimation and control of the electron density profile for ITER using relevant sensors and actuators. As a first step, Thomson scattering is included in an existing dynamic state observer. Second, model predictive control is developed as a strategy to regulate the density profile while avoiding limits associated with the total density (Greenwald limit) or gradients in the density distribution (e.g. neoclassical impurity transport). Simulations show that high quality density profile estimation can be achieved with Thomson Scattering and that the controller is capable of regulating the distribution as desired.

A tutorial on radio frequency sheath physics for magnetically confined fusion devices

Radio frequency (RF) sheaths occur under a wide variety of conditions when RF waves, material surfaces and plasma coexist. RF sheaths are of special importance in describing the interaction of ion cyclotron range of frequency (ICRF) waves with the boundary plasma in tokamaks, stellarators and other magnetic confinement devices. In this article the basic physics of RF sheaths is discussed in the context of magnetic fusion research. Techniques for modelling RF sheaths, their interaction with RF wave fields and the resulting consequences are highlighted. The article is intended as a guide for the early-career ICRF researcher, but it may equally well serve to provide an overview of basic RF sheath concepts and modelling directions for any interested fusion scientist.

Particle resonances in stellarators

Resonances of high energy particles in magnetic confinement devices due to electromagnetic instabilities can strongly modify the particle distribution, leading to a reduction in fusion power and even discharge termination and particle loss to the device walls through an avalanche. The existence of a mode particle resonance depends on the properties of the equilibrium and particle parameters, and their number, location, and density can vary with device design. Recently, the advent of more powerful computing capabilities and advanced theoretical understanding has led to the design of non-axisymmetric devices or stellarators, which could prove to be more advantageous than tokamaks. Stellarators have the advantage of being immune to major disruptions because of the very low plasma current. One of the problems shared by both types of devices is the existence of resonances in particle orbits, which can lead to large amplitude high frequency instabilities and subsequent induced particle loss. We examine the number of resonances, their location, and dependence on particle energy for some stellarator designs.

Potential vorticity transport in weakly and strongly magnetized plasmas

Tangled magnetic fields, often coexisting with an ordered mean field, have a major impact on turbulence and momentum transport in many plasmas, including those found in the solar tachocline and magnetic confinement devices. We present a novel mean field theory of potential vorticity mixing in $\beta$-plane magnetohydrodynamic (MHD) and drift wave turbulence. Our results show that mean-square stochastic fields strongly reduce Reynolds stress coherence. This decoherence of potential vorticity flux due to stochastic field scattering leads to suppression of momentum transport and zonal flow formation. A simple calculation suggests that the breaking of the shear-eddy tilting feedback loop by stochastic fields is the key underlying physics mechanism. A dimensionless parameter that quantifies the increment in power threshold is identified and used to assess the impact of stochastic field on the L-H transition. We discuss a model of stochastic fields as a resisto-elastic network.

Optical tomography of the plasma on the PROTO-SPHERA experiment

PROTO-SPHERA is a magnetic confinement device for controlled fusion experiments which forms a plasma spherical torus not around a metal centerpost (as in spherical tokamaks), but around a plasma centerpost. Thanks to a vacuum vessel in polymethylmethacrylate (PMMA) transparent to visible light, a system of six fast visible light cameras has been implemented around the PROTO-SPHERA experiment so as to perform an optical tomography. To do so, synchronous triggered frames acquisition and exposure time control have been performed using the MARTe2 software framework. Also, a proper calibration/reconstruction of each line of sight for all the cameras has been implemented, so as to compute a 3D representation of the vessel. A 2D tomography inversion of the plasma column equatorial plane has been performed under the consideration that the value of each pixel represents the integrated brightness along its line of sight. The camera acquisition system, the experimental setup and the obtained results will be presented.

Particle resonances in toroidal fusion devices

Resonances of high energy particles in magnetic confinement devices due to electromagnetic instabilities can strongly modify the distribution, leading to a reduction in fusion power and even discharge termination and particle loss to the device walls through avalanche. The existence of a mode particle resonance depends on properties of the equilibrium, particle trajectories, and perturbation mode harmonic content. Resonance location is a function of particle energy and equilibrium field line helicity. Different methods for finding resonance location and energy dependence are developed. The properties of resonances are discussed using examples from magnetic fusion devices. We show that if mode resonances exist at low particle energy, they very likely also exist at high energy, thus modifying high energy beam particles and fusion products. It is possible for a resonance to appear due to mode induced orbit helicity modification when it is forbidden at low mode amplitude.

Analysis on magnetic control stud welding device with the open-close Structure

Arc stud welding is widely used to fix joints like studs, screws, pins, or similar elements on various products. The main defects in the process of large diameter stud welding are arc offset and non-uniform melting. In order to solve the problems, which seriously affect the installation strength of stud, the magnetic confinement device with the open-close structure is used to improve the quality of stud welding. An open-close magnetic confinement device suitable for robotic stud welding system is designed to create rotatable magnetic field. Rotating magnetic field can drive arc deflection. The magnetic field produced by magnetic confinement device is simulated. The distribution and intensity of magnetic field is analyzed. The influence of the design parameters of magnetic confinement device on the magnetic field is studied. According to the practicability and the size limitation of the magnetic confinement device, the appropriate parameters are selected. The prototype of the magnetic confinement device is tested by stud welding. The results show that the open-closed magnetic confinement device designed with reasonable parameters has good inhibitory effect on the arc offset.