Magnetic Confinement Devices Research Articles

Preliminary design of KTX magnet system

KTX is a reversed field pinch magnetic confinement device of which the magnet system is designed in ASIPP and USTC. The main parameter of KTX is between RFX and MST. Its magnet system includes the toroidal field (TF) winding and poloidal field (PF) winding (ohmic heating winding and equilibrium field winding), which are less complex than tokamak device due to the fact that a tokamak requires a superconducting system to perform quasi-steady state operation and achieve Q>10. However, the most important part of the magnet system design lies in how to keep the TF magnetic field ripple, as well as any kinds of stray field, to a minimum value. The main design activities of the KTX magnet system are presented as detailed as possible in this paper, and the main activities which have already been completed include magnet coils position and winding, insulation design, plasma modeling prediction, thermal analysis, magnetic field calculations were analyzed and so on. The magnet system design is one of the major activities for KTX device design, which is effective guarantee for the future R&D and manufacture. Besides, the detailed design activities should be continuously optimized and changed based on the results from future R&D and relevant tests.

Assessment of ion cyclotron antenna performance in ASDEX Upgrade using TOPICA

Nuclear fusion in magnetic confinement devices is one of the most challenging application fields for electromagnetics and mechanics. In particular antennas, charged with the task of handling and delivering high power to the plasma, have to operate in a harsh environment with very demanding thermal and mechanical requirements. Within this scenario, the use of tools able to efficiently predict the antenna behaviour is mandatory. In the present work, the TOrino Polytechnic Ion Cyclotron Antenna (TOPICA) code has been used to simulate the real 3D geometry of two launchers, generally operating at 30 MHz and presently installed on the ASDEX Upgrade tokamak. The working frequency belongs to the Ion-Cyclotron range of frequencies that was proven to achieve very efficient plasma heating. Antenna performances are presented in terms of coupled power and RF potentials for different realistic plasma scenarios taken from ASDEX Upgrade

Low-noise broadband differential optoisolator for Langmuir probes

In this article the design of a low-noise broadband optoisolator is discussed. Critical issues for designing the optoisolator together with experimental results are presented. Differential channels and matched optocoupler pair are used to reduce the common mode interference down to an amplitude of less than 1 mV. The optoisolator guarantees a 2 MHz bandwidth with ±4 V peak-peak voltage. The direct current excursion is lower than 25 mV which shows that the optoisolator is very stable. Every optoisolator has its own power supply by rectifying the industry standard power. The 50 Hz noise from the industry power grid is also eliminated. All these characteristics make it a reliable optoisolator for Langmuir probes in the magnetic confinement devices.

Effect of superbanana diffusion on fusion reactivity in stellarators

Fusion reactivity is usually obtained using a Maxwellian distribution. However, energy-dependent radial diffusion can modify the energy distribution. Superbanana diffusion is energy-dependent and occurs in nonaxisymmetric magnetic confinement devices, such as stellarators, because of ripple-trapped particles which can take large steps between collisions. In this paper, the D-T fusion reactivity is calculated using a non-Maxwellian energy distribution obtained by solving the Fokker-Planck equation numerically, including radial superbanana diffusion as well as energy scattering. The ions in the tail of the distribution, with energies larger than thermal, which are most needed for fusion, are depleted by superbanana diffusion. In this paper, it is shown that the D-T fusion reactivity is reduced by tail ion depletion due to superbanana diffusion, by roughly a factor of 0.5 for the parameters used in the calculation.

Determination of the electron density in the tokamak edge plasma from the time evolution of a laser-induced fluorescence signal from atomic helium

The time evolution of a laser-induced fluorescence signal from neutral helium He I in the edge and divertor plasmas in modern magnetic confinement devices is considered. Computations are performed on the basis of a nonstationary collisional-radiative model involving ten singlet and nine triplet states of helium that affect the time evolution of the fluorescence signal. A new method is proposed for determining the electron density from measurements of the time derivative of the profile of the fluorescence spectral line.

Phase-field modeling of thermomechanical damage in tungsten under severe plasma transients

Tungsten is now a primary candidate for plasma facing components in fusion energy systems because of its numerous superior thermophysical properties. International efforts are currently focused on the development of tungsten surfaces that can intercept ionized plasma and pulsed high heat flux in magnetic fusion confinement devices. Thermal shock under transient operating conditions, such as edge localized modes, have experimentally been shown to lead to severe surface and sub-surface damage. We present here a computational multiphysics model to determine the relationship between the thermomechanical loading conditions and the onset of damage and failure of tungsten surfaces. The model is based on thermo-elasto-plasticity constitutive relations, and is developed within the framework of the phase-field method. A coupled set of partial differential equations is solved for the temperature, displacement, and a damage phase fields under severe plasma transient loads. The results clearly show the initiation and propagation of surface and sub-surface cracks as a result of the transient high heat flux. The severity of surface cracking is found to correlate primarily with the magnitude of the near-surface temperature gradient.

Effect of Drift Waves on Plasma Blob Dynamics

Most of the work to date on plasma blobs found in the edge region of magnetic confinement devices is limited to 2D theory and simulations which ignore the variation of blob parameters along the magnetic field line. However, if the 2D convective rate of blobs is on the order of the growth rate of unstable drift waves, then drift wave turbulence can drastically alter the dynamics of blobs from that predicted by 2D theory. The density gradients in the drift plane that characterize the blob are mostly depleted during the nonlinear stage of drift waves resulting in a much more diffuse blob with a greatly reduced radial velocity. Sheath connected plasma blobs driven by effective gravity forces are considered in this Letter and it is found that the effects of resistive drift waves occur at earlier stages in the 2D motion for smaller blobs and in systems with a smaller effective gravity force. These conclusions are supported numerically by a direct comparison of 2D and 3D seeded blob simulations.

Conference Report on the 2nd International Symposium on Lithium Applications for Fusion Devices

The 2nd International Symposium on Lithium Applications for Fusion Devices (ISLA-2011) was held on 27–29 April 2011 at the Princeton Plasma Physics Laboratory (PPPL) with broad participation from the community working on aspects of lithium research for fusion energy development. This community is expanding rapidly in many areas including experiments in magnetic confinement devices and a variety of lithium test stands, theory and modeling and developing innovative approaches. Overall, 53 presentations were given representing 26 institutions from 10 countries. The latest experimental results from nine magnetic fusion devices were given in 24 presentations, from NSTX (PPPL, USA), LTX (PPPL, USA), FT-U (ENEA, Italy), T-11M (TRINITY, RF), T-10 (Kurchatov Institute, RF), TJ-II (CIEMAT, Spain), EAST (ASIPP, China), HT-7 (ASIPP, China), and RFX (Padova, Italy). Sessions were devoted to: I. Lithium in magnetic confinement experiments (facility overviews), II. Lithium in magnetic confinement experiments (topical issues), III. Special session on liquid lithium technology, IV. Lithium laboratory test stands, V. Lithium theory/modeling/comments, VI. Innovative lithium applications and VII. Panel discussion on lithium PFC viability in magnetic fusion reactors. There was notable participation from the fusion technology communities, including the IFE, IFMIF and TBM communities providing productive exchanges with the physics oriented magnetic confinement lithium research groups. It was agreed to continue future exchanges of ideas and data to help develop attractive liquid lithium solutions for very challenging magnetic fusion issues, such as development of a high heat flux steady-state divertor concept and acceptable plasma disruption mitigation techniques while improving plasma performance with lithium. The next workshop will be held at ENEA, Frascati, Italy in 2013.

Gyrokinetic equations for strong-gradient regions

A gyrokinetic theory is developed under a set of orderings applicable to the edge region of tokamaks and other magnetic confinement devices, as well as to internal transport barriers. The result is a practical set equations that is valid for large perturbation amplitudes [qδψ/T=O(1), where δψ=δφ-ν∥δA∥/c], which is straightforward to implement numerically, and which has straightforward expressions for its conservation properties. Here, δφ and δA∥ are the perturbed electrostatic and parallel magnetic potentials, ν∥ is the particle velocity, c is the speed of light, and T is the temperature. The derivation is based on the quantity ɛ≡(ρ/λ⊥)qδψ/T≪1 as the small expansion parameter, where ρ is the gyroradius and λ⊥ is the perpendicular wavelength. Physically, this ordering requires that the E×B velocity and the component of the parallel velocity perpendicular to the equilibrium magnetic field are small compared to the thermal velocity. For nonlinear fluctuations saturated at “mixing-length” levels (i.e., at a level such that driving gradients in profile quantities are locally flattened), ɛ is of the order ρ/Lp, where Lp is the equilibrium profile scale length, for all scales λ⊥ ranging from ρ to Lp. This is true even though qδψ/T=O(1) for λ⊥∼Lp. Significant additional simplifications result from ordering Lp/LB=O(ɛ), where LB is the spatial scale of variation of the magnetic field. We argue that these orderings are well satisfied in strong-gradient regions, such as edge and scrapeoff layer regions and internal transport barriers in tokamaks, and anticipate that our equations will be useful as a basis for simulation models for these regions.

RFX-mod wall conditioning by lithium pellet injection

Plasma–wall interaction is one of the most important issues that present magnetic confinement devices have to face. In the RFX-mod reversed field pinch experiment plasma–wall interaction has become a hard point increasing plasma current up to the RFX-mod maximum design value of 2 MA, since in this case local power deposition can be as high as 10 MW m−2. Since the first wall of RFX-mod is entirely covered by graphite tiles different techniques have been tested to control hydrogen wall influx: He glow discharges cleaning, He discharges at high plasma currents, wall boronization and baking. With the best results obtained by boronization, at high plasma currents all such techniques improve the situation but do not allow a complete and stationary hydrogen influx reduction. Furthermore, in the presence of localized high power load the wall still responds providing very high influxes. In order to improve this situation wall conditioning by lithium has been tested. As a first lithization method to deposit a controllable amount of lithium on the wall, a room temperature pellet injector has been used (maximum pellet diameter of 1.8 mm and maximum length of 5 mm). Lithium coatings with a theoretical thickness of about 10 nm have been applied both to clean graphite tiles and over boronized ones. Lithization demonstrated to be effective in lowering hydrogen wall recycling to a value smaller than that of boronized graphite, with the effect lasting 20–30% more than in the boronized case. Compared with boronization, lithization slightly improves (by about 30%) particle confinement time and also clearly affects edge particle transport providing a lower edge density and more peaked density profiles. Lithization also reduces carbon content by about 10% over boronization but still no clear improvement has been observed in terms of energy confinement. Similar results have been obtained performing lithization over boronized graphite.

Particle Transport in Stochastic Media With Multivariate Gamma Statistics: Analytical Results With Application to Atoms in Tokamaks

Neutral particles (atoms, molecules) play an important role in thermonuclear fusion magnetic confinement devices. At the very edge of the plasma, close to the wall, density fluctuations with amplitude comparable to the mean density have been reported. In this work, we analyze the effect of these fluctuations on the transport of neutral particles, using a multivariate Gamma model to describe the spatial correlations of fluctuations. Analytical results are obtained in 1D for the average neutral particle density in the presence of scattering.

Thermo-mechanical damage of tungsten surfaces exposed to rapid transient plasma heat loads

International efforts have focused recently on the development of tungsten surfaces that can intercept energetic ionized and neutral atoms, and heat fluxes in the divertor region of magnetic fusion confinement devices. The combination of transient heating and local swelling due to implanted helium and hydrogen atoms has been experimentally shown to lead to severe surface and sub-surface damage. We present here a computational model to determine the relationship between the thermo-mechanical loading conditions, and the onset of damage and failure of tungsten surfaces. The model is based on thermoelasticity, coupled with a grain boundary damage mode that includes contact cohesive elements for grain boundary sliding and fracture. This mechanics model is also coupled with a transient heat conduction model for temperature distributions following rapid thermal pulses. Results of the computational model are compared to experiments on tungsten bombarded with energetic helium and deuterium particle fluxes.

Summary of the 19th International Atomic Energy Agency Technical Meeting on ‘Research Using Small Fusion Devices’

This paper presents a summary of recent results reported on several topics on magnetic confinement, dense magnetized plasmas, innovative fusion technology and applications, diagnostic systems and control and data acquisition systems. The main topics covered on the magnetic confinement devices, diagnostics and data acquisition concern the tokamak KTM (Kazakhstan Tokamak for Material testing) for materials research and testing, and IAEA Joint Experiments on small tokamaks. For the dense magnetized plasmas results on development and commissioning of plasma focus devices were reported. The plasmatron VISION I for innovative plasma–wall interaction studies, a lithium divertor for KTM and compact fusion reactors as neutron sources were presented.

The Science of Control

The primary objectives of control are somewhat different from those of much of fusion plasma physics. Magnetic fusion physics has historically focused on understanding the physics of plasmas in magnetic confinement devices, whereas fusion plasma control seeks to capitalize on the understanding already gained to cause the system (fusion device plus plasma) to behave in certain desirable ways. For example, early uses of plasma control in fusion devices had simple goals such as extending the survival of discharges by minimizing plasma-wall interaction or by regulating density. Present applications are primarily aimed at achieving conditions with better potential fusion performance or conditions under which fusion plasmas can be more easily studied. The demanding performance requirements and significant constraints expected on control of future fusion reactors suggest that plasma control is a critical enabling technology for progress toward commercial fusion power. A greater understanding of control techniques for fusion plasmas and a more widespread use of these techniques in existing devices are required in order to develop the solutions needed.

Overview of Studies on Energetic‐Ion‐Driven MHD Instabiities in Stellarator/Helical Plasmas and Comparison with Tokamaks

AbstractSuppression of anomalous radial transport of alpha particles and beam ions due to energetic ion driven MHD in‐stabilities is one of the critical issues toward D‐T burning plasma experiments in toroidal magnetic confinement devices such as tokamaks and stellarator/helical devices. This paper reports the characteristics of energetic ion driven MHD modes such as Alfvén eigenmodes (AEs) and their effects on transport of energetic ions and bulk plasma in the Large Helical Device (LHD) and some different types of stellarator/helical devices. Beneficial impacts of AEs on toroidal plasma confinement are also included. These experimental results on AEs are compared with those in tokamaks, and the important similarities and differences are pointed out that can lead to better understanding of AEs in toroidal plasmas (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Equilibrium properties of the plasma sheath with a magnetic field parallel to the wall

Motivated by the magnetized target fusion (MTF) experiment [R. E. Siemon et al., Comments Plasma Phys. Controlled Fusion 18, 363 (1999)], a systematic investigation of the force balance and equilibrium plasma flows was carried out using analytical theory and the particle-in-cell code VPIC [K. J. Bowers et al., Phys. Plasmas 15, 055703 (2008)] for a one-dimensional plasma sheath with a magnetic field parallel to the wall. Initially uniform full Maxwellian plasma consisting of equal temperature collisionless electrons and ions is allowed to interact with a perfectly absorbing wall. The analysis of the steady-state force balance of the entire plasma as well as its individual components illuminates the roles that the hydrodynamic, magnetic, and electric forces play. In particular, when ρthi<λD, the magnetic force balances the divergence of the pressure tensor. As the magnetic field is decreased, the electric force becomes prominent in areas where quasineutrality breaks, which can be a substantial part of the sheath. Its importance depends on the relation between three parameters, namely, electron and ion thermal Larmor radii and plasma Debye length: ρthe, ρthi, and λD. The relative importance of the electron and ion current in the magnetic or Lorentz force term can be understood through the analysis of the two-fluid force balance. It reveals that the current is carried primarily by the electrons. This is due to the direction of the electric field that helps confine the ions, but not the electrons, which are forced to carry a large current to confine themselves magnetically. In the regimes where the electric field is negligible, the ions also need the current for confinement, but in these cases the divergence of ion pressure tensor is much smaller than that of the electrons. Consequently the ion current is also smaller. The study of the electron and ion flow parallel to the wall clarifies this picture even further. In the regime of strong magnetic field, the particle average velocity parallel to the wall uy is purely diamagnetic. However, since the ion number density is very low near the wall, they do not produce considerable contribution to the current. In the ρthi<λD regime, uy consists of two parts: diamagnetic and E⃗×B⃗ drifts. Since the direction of the former depends on the particle charge while the latter does not (at least to lowest order), the drifts for the electrons add, while for the ions they mostly cancel each other. Although the primary motivation for this research is MTF, the analytical and computational results presented in this paper can also be applicable to the plasma sheath in the conventional magnetic confinement devices, in particular, near the first wall of tokamaks.

Scaling of the plasma sheath in a magnetic field parallel to the wall

Motivated by the magnetized target fusion [R. E. Siemon et al., Comments Plasma Phys. Controlled Fusion 18, 363 (1999)] experiment, a systematic investigation of the scaling of a one-dimensional plasma sheath with a magnetic field parallel to the wall was carried out using analytical theory and the particle-in-cell code VPIC [K. J. Bowers et al., Phys. Plasmas 15, 055703 (2008)]. Starting with a uniform Maxwellian distribution in three-dimensional velocity space, plasma consisting of collisionless electrons, and ions of the same temperature interacts with a perfectly absorbing wall. A much larger ion Larmor radius causes the wall to be charged positively, creating an electric field that tends to repel the ions and attract the electrons, which is the opposite of the conventional Bohm sheath [D. Bohm, Characteristics of Electrical Discharges in Magnetic Fields (McGraw-Hill, New York, 1949)]. This manifests in the form of gyro-orbit modification by this spatially varying electric field, the degree of which is found to intricately depend on the relation between three parameters: electron and ion thermal Larmor radii and plasma Debye length: ρthe, ρthi, and λD. Furthermore, the study of the sheath width scaling through the analysis of the full width at half max of electric field, xEh, elucidates three distinct types of behavior of xEh, corresponding to three different regimes: ρthi<λD, ρthe<λD<ρthi, and λD<ρthe. In addition to the sheath width, the scaling of the wall potential ϕWall, as well as the role of the ion mass and charge Z are investigated. The results of this analytical and computational approach can also be useful in studying the plasma sheath in the conventional magnetic confinement devices, in particular at the first wall of tokamaks.

Beam distribution modification by Alfvén modes

Modification of a deuterium beam distribution in the presence of low amplitude toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in a toroidal magnetic confinement device is examined. Comparison to experimental data shows that multiple low amplitude modes can account for significant modification of high energy beam particle distributions. It is found that there is a stochastic threshold for beam transport, and that the experimental amplitudes are only slightly above this threshold. The modes produce a substantial central flattening of the beam distribution.

Interaction of Mean and Oscillating Plasma Flows Across Confinement Mode Transitions

The interaction of mean plasma flows (driven by equilibrium E×B forces) and oscillating plasma flows - such as zonal flows (ZFs) and geodesic acoustic modes (GAMs) - (radially localized E × B plasma flows generated by non-linear turbulence interactions) is investigated in the ASDEX Upgrade tokamak using Doppler reflectometry. ZFs and GAMs play an important role in magnetic confinement devices by enhancing the velocity shearing of turbulent eddies which reduces turbulent cross-field transport. Although the GAM is universally observed across the plasma edge gradient region of ohmic and additionally heated low confinement L-mode regimes, it is not seen in the high confinement H-mode. Using slow power ramping of the additional ECRH and NBI heating systems the behaviour of the edge GAM, and its interaction with the mean flow shear, i.e. the radial electric field Er profile, across the L to H-mode transition are studied. As the edge Er well deepens with improving confinement, the associated negative Er shear region narrows and the radial extent of the GAM shrinks. The main feature, however, is the movement of spectral power out of the coherent GAM into large amplitude, low frequency random flow perturbations. It is speculated that these flow perturbations may play a role in the triggering of the confinement transition.

3D Isosurface Visualization of Electron Density and Temperature Distribution in a Magnetized Sheet Plasma Ion Source

A 3D visualization of argon and nitrogen-argon plasma electron density (ne) and effective temperature (Teff) distributions were constructed using the I-V curves from Langmuir probe traces at specific discrete positions in the extraction region of a magnetized sheet plasma ion source. Argon and mixed N2-Ar sheet plasmas are characterized using the calculation of the electron energy distribution function (EEDF). By taking the current vs. voltage reading of a single probe in 68 discrete locations of the plasma, a 3-dimensional map of the electron density and electron temperature were constructed. The map was constructed to understand the global condition of the extraction region of the source. The technique can be applied to the determination and understanding of edge plasma parameters in magnetic confinement devices.