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This presentation examines the energetic particle transport induced by saturated kink modes and sawtooth crashes in JET deuterium plasmas. It is known that kink mode-resonant transport and phase-space redistribution from sawtooth crashes can drive strong fast ion transport with dependencies on particle pitch and energy. Measurements with JET’s Faraday cup fast ion loss detector array have shown that the internal kink growth phase preceding sawtooth crashes produces substantial fast ion losses. This report will numerically investigate the dominant energetic particle transport mechanism with a detailed examination of the fast ion phase-space dependencies, resonances, orbit topology changes, induced losses, and redistribution associated with the long-lived, resonant, kink mode and non-resonant sawtooth crash. The ORBIT-kick model forms the basis of the transport studies with realistic fast ion distributions produced from TRANSP. A recently created reduced model for sawtooth induced transport is used while the saturated kink modes are modeled with ideal magnetohydrodynamic codes. The simulations were further validated against experiment with a newly developed synthetic Faraday cup fast ion loss detector in addition to scintillator probe and neutron measurements.

Alfvénic instabilities (AEs) are well known to cause enhanced transport of energetic particles (EPs) in fusion devices. Most studies until now have focused on characterizing and understanding AE stability in single-species plasmas heated by neutral beams (NB), where deuterium is typically used as both main plasma species and NB fuel. As the fusion community moves toward fusion reactors that target burning plasma conditions, such as ITER, the single-species picture breaks down. Burning plasmas, which will use a mix of deuterium and tritium (DT) as main fuel, also feature the presence of several supra-thermal fusion products such as alpha particles, protons, helium isotopes and high-energy tritium ions. This work presents the extension of the EP transport kick model implemented in the TRANSP time-dependent tokamak transport code to study the combined effect of multiple EP species on AE stability and, in turn, the response of different EP species to plasma instabilities in terms of their redistribution and losses. Further validation of the enhanced model is planned based on experimental results expected from the JET DT campaign scheduled for 2021, in preparation for ITER plasmas and beyond.

An integrated energetic particle transport model has been constructed in JET plasmas constrained by experimental fast ion loss measurements. The model incorporates a synthetic fast ion loss detector identical to JET’s thin-foil Faraday cup fast ion loss detector array. The loss model combines analyses from the TRANSP and ORBIT-kick codes with enhanced features for producing the synthetic diagnostic. Extensions to the ORBIT code framework allow a full-orbit representation within the vacuum region that can map particles directly to an installed detector geometry. Since synthetic fast ion loss detectors are plagued by weak loss statistics, a novel reverse integration biasing scheme has been implemented to boost computational efficiency. The model is validated against experimental loss measurements induced by long-lived kink modes and is found to be in good agreement. This confirms the development of a fully integrated transport/loss model which can be quantitatively verified against experiment allowing for future validation and predictive studies. The model is particularly useful for more complicated plasma scenarios that involve multiple fast ion species such as JET’s 2021 DT-campaign.

This paper reports the main results of the RFX-mod fusion science activity since the last 2014 IAEA Fusion Energy Conference. The RFX-mod device is characterized by a unique flexibility in terms of accessible magnetic configurations. Axisymmetric and helically shaped Reversed-field pinch equilibria have been studied, along with tokamak plasmas in a wide range of q(a) regimes (spanning from 4 down to 1.2 values). The full range of magnetic configurations in between the two, the so-called ultra low-q ones, has been explored, with the aim of studying specific physical issues common to all equilibria, such as, for example, the density limit phenomenon. The powerful RFX-mod feedback control system has been exploited for MHD control, which allowed to extend the range of experimental parameters, as well as to induce specific magnetic perturbations for the study of 3D effects. In particular, transport, edge and isotope effect in 3D equilibria have been investigated, along with runaway mitigations through induced magnetic perturbations. The first transitions to H-mode in circular and D-shaped tokamak plasmas have been obtained thanks to an active modification of the edge electric field through a polarized electrode. The experiments are supported by intense modelling with 3D MHD, gyrokinetic, guiding center and transport codes. Proposed modifications to the RFX-mod device, which will enable further contributions to the solution of key issues in the roadmap to ITER and DEMO, are also briefly presented.

The design of feedwells in thickeners has a significant influence on the subsequent settling and classification performance. The fluid flow inside tangential entry feedwell models has been investigated in a small-scale physical model of the feedwell and simplified thickener. Laser Doppler velocimetry measurements have been made in feedwells with and without an internal shelf, and dye tracer has been used for flow visualisation. In a tangential entry feedwell without a shelf, the feed spiralled down the inner wall of the feedwell before exiting. With an internal shelf in the feedwell, the feed spiralled above the shelf before slipping over the edge of the shelf and spiralling down the inner wall of the feedwell. The increased spiralling resulted in an increase in the fluid residence time in the feedwell with an internal shelf. A single-phase, three-dimensional computational fluid dynamics model was used to calculate the fluid flow in the model feedwells. The performance of the k –epsilon turbulence model and the differential Reynolds stress turbulence model were evaluated. The numerical predictions of the velocity fields using both turbulence models were in good agreement with the experimental results measured inside the feedwell. The economical k –epsilon turbulence model may be used to assist in the design of feedwells for industrial thickeners.

The Korea Superconducting Tokamak Advanced Research (KSTAR) project is the major effort of the national fusion programme of the Republic of Korea. Its aim is to develop a steady state capable advanced superconducting tokamak to establish a scientific and technological basis for an attractive fusion reactor. The major parameters of the tokamak are: major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 T and plasma current 2 MA, with a strongly shaped plasma cross-section and double null divertor. The initial pulse length provided by the poloidal magnet system is 20 s, but the pulse length can be increased to 300 s through non-inductive current drive. The plasma heating and current drive system consists of neutral beams, ion cyclotron waves, lower hybrid waves and electron cyclotron waves for flexible profile control in advanced tokamak operating modes. A comprehensive set of diagnostics is planned for plasma control, performance evaluation and physics understanding. The project has completed its conceptual design and moved to the engineering design and construction phase. The target date for the first plasma is 2002.

The article reviews the physics of fusion alpha particles and energetic neutral beam ions studied in the final phase of TFTR operation, with an emphasis on observations in reversed magnetic shear (RS) and enhanced reversed shear (ERS) DT plasmas. Energy resolved measurements of the radial profiles of confined, trapped alphas in RS plasmas exhibit reduced core alpha density with increasing alpha energy, in contrast to plasmas with normal monotonic shear. The measured profiles are consistent with predictions of increased alpha loss due to stochastic ripple diffusion and increased first orbit loss in RS plasmas. In experiments in which a short tritium beam pulse is injected into a deuterium RS plasma, the measured DT neutron emission is lower than standard predictions assuming first orbit loss and stochastic ripple diffusion of the beam ions. A microwave reflectometer measured the spatial localization of low toroidal mode number (n), alpha driven toroidal Alfven eigenmodes (TAEs) in DT RS discharges. Although the observed ballooning character of the n = 4 mode is consistent with predictions of a kinetic MHD stability code, the observed antiballooning nature of the n = 2 mode is not. Furthermore, the modelling does not show the observed strong dependence of mode frequency on n. These alpha driven TAEs do not cause measurable alpha loss in TFTR. Other Alfven frequency modes with n = 2-4 seen in both DT and DD ERS and RS discharges are localized to the weak magnetic shear region near qmin. In 10-20% of DT discharges, normal low n MHD activity causes alpha loss at levels above the first orbit loss rate.

In RFX-mod high current experiments the plasma frequently reaches the Single Helical Axis (SHAx) state, in which the core electron temperature assumes a helical shape and significant gradients appear. The plasma magnetic topology is strongly helical in the core while it is almost axisymmetric at the edge, thanks to the active control of the edge radial field. The weak helical field produced by the plasma profoundly affects the shape of the equilibrium going from 2D to 3D, therefore three-dimensional tools are being developed for the description of these states. On one hand the SHEq code, based on a perturbative approach, is used to determine the magnetic field topology. On the other hand, the VMEC code has been modified for the helical RFP, in order to describe the SHAx states by using tools developed by the Stellarator community. The effect of three-dimensional fields on transport is also being actively investigated. The magnetic shear is found to be correlated with regions with reduced transport: an investigation on the effect of 3D fields on transport mechanisms underlying such barriers is being performed by adopting several approaches.

The method of Kruskal-Newton diagrams for the solution of differential equations with boundary layers is shown to provide rapid intuitive understanding of layer scaling and can result in the conceptual simplification of some problems. The method is illustrated using equations arising in the theory of pattern formation and in plasma physics.

Contamination of soil, water, and air with toxic chemicals is a serious and on going problem facing the world today. Hazardous compounds, such as polycyclic aromatic hydrocarbons (PAHs), pentachlorophenol (PCP), polychlorinated biphenyls (PCBs), 1,1,1-trichloro-2,2-bis(4chlorophenyl)ethane (DDT), and trinitrotoluene (TNT) are persistent in the environment and are known to have carcinogenic and mutagenic effects. Removing these pollutants from the environment in an ecologically responsible, safe, rapid, and cost-effective way is a priority for land management agencies. Bioremediation, using microbial organisms, is one way to achieve this target. Extensive laboratory studies have shown the capability of various organisms to remediate contaminated soil and water. More research, however, to determine the applicability and practicability of utilizing these microorganisms in contaminated field sites needs to be achieved. This review and case study presents an evidence of successful bioremediation of PCP and related dioxins using fungal-based technology.

Recently reported numerical results for axisymmetric devices with low aspect ratio A found radial transport enhanced over the expected neoclassical value by a factor of 2 to 3. In this paper, we provide an explanation for this enhancement. Transport theory in toroidal devices usually assumes large A, and that the ratio B{sub p}/B{sub t} of the poloidal to the toroidal magnetic field is small. These assumptions result in transport which, in the low collision limit, is dominated by banana orbits, giving the largest collisionless excursion of a particle from an initial flux surface. However in a small aspect ratio device one may have B{sub p}/B{sub t} {approx} 1, and the gyroradius may be larger than the banana excursion. Here, we develop an approximate analytic transport theory valid for devices with arbitrary A. For low A, we find that the enhanced transport, referred to as omniclassical, is a combination of neoclassical and properly generalized classical effects, which become dominant in the low-A, B{sub p}/B{sub t} {approx} 1 regime. Good agreement of the analytic theory with numerical simulations is obtained.

Calculations of collisional thermal and particle diffusivities in toroidal magnetic plasma confinement devices order the toroidal gyroradius to be small relative to the poloidal gyroradius. This ordering is central to what is usually referred to as neoclassical transport theory. This ordering is incorrect at low aspect ratio, where it can often be the case that the toroidal gyroradius is larger than the poloidal gyroradius. We calculate the correction to the particle and thermal diffusivities at low aspect ratio by comparing the diffusivities as determined by a full orbit code (which we refer to as omni-classical diffusion) with those from a gyroaveraged orbit code (neoclassical diffusion). In typical low aspect ratio devices the omni-classical diffusion can be up to 2.5 times the calculated neoclassical value. We discuss the implications of this work on the analysis of collisional transport in low aspect ratio magnetic confinement experiments.

We have started to examine the reactor potential of quasi-axisymmetric (QA) stellarators with an integrated approach that includes systems evaluation, engineering considerations, and plasma and coil optimizations. In this paper, we summarize the progress made so far in developing QA configurations with reduced alpha losses while retaining good MHD stability properties. The minimization of alpha losses is achieved by directly targeting the collisionless orbits to prolong the average resident times. Configurations with an overall energy loss rate of {approx}10% or less, including collisional contributions, have been found. To allow remotely maintaining coils and machine components in a reactor environment, there is a desire to simplify to the extent possible the coil design. To this end, finding a configuration that is optimized not only for the alpha confinement and MHD stability but also for the good coil and reactor performance, remains to be a challenging task.

Compact optimized stellarators offer novel solutions for confining high-beta plasmas and developing magnetic confinement fusion. The 3-D plasma shape can be designed to enhance the MHD stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low-aspect ratio, high beta-limit, and good confinement of advanced tokamaks. Quasi-axisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4-4.4 and average elongation of approximately 1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassical-tearing modes for beta > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at beta = 4% (the rest is from the coils), thus the equilibrium is much less nonlinear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of ''reversed'' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks.more » Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.« less

Absorption of fast Alfven waves by the minority fundamental ion-cyclotron resonance, coupled with finite banana width physics, generates torque distributions and ultimately rotational shear layers in the bulk plasma, even when the toroidal wavenumber k(subscript ''phi'') = n/R of the fast wave vanishes (n=0) and cyclotron absorption introduces no angular momentum nor canonical angular momentum [F.W. Perkins, R.B. White, P.T. Bonoli, and V.S. Chan, Phys. Plasmas 8 (2001) 2181]. The present work extends these results to travelling waves with non-zero n where heating directly introduces angular momentum. Since tokamak fast-wave antennas have approximately one wavelength per toroidal field coil, the toroidal mode number n lies in the range n = 10-20, independent of machine size. A zero-dimensional analysis shows that the rotation rate arising from direct torque is comparable to that of the rotational shear layer and has the same scaling. Nondimensional rotation profiles for n = (-10, 10) show modest changes from the n = 0 case in the expected direction. For a balanced antenna spectrum, the nondimensional rotational profile (averaged over n = -10, 10) lies quite close to the n = 0 profile.

A general method for including various collisional effects, such as the drag and diffusion of test particles due to background plasmas, the effect of particle source and sink, and the like-particle Coulomb collisions, is presented. The marker density g is generally unknown along the particle trajectory, and its evaluation depends on the way particles are initially loaded and new particles are injected into the simulation. The method is demonstrated for the problem of the nonlinear evolution of the Toroidicity Induced Alfven eigenmode, driven by energetic {alpha} particles. The saturation amplitude is found to scale with the collision rate in a way as predicted by theory.

In recent Deuterium-Tritium experiments on the Tokamak Fusion Test Reactor (TFTR), both the Pellet Charge Exchange (PCX) and the alpha Charge Exchange Recombination Spectroscopy ({alpha}-CHERS) diagnostics indicate that sawtooth oscillations can cause significant broadening of the fusion alpha radial density profile. The authors investigate this sawtooth mixing phenomenon by applying a Hamiltonian guiding center approach. A model of time evolution of the Kadomtsev-type sawtooth is constructed. The presence of more than one mode in the nonlinear stage of the sawtooth crash is necessary to cause significant broadening of the alpha density profile. Use of numerical equilibria allows us to perform detailed comparisons with TFTR experimental data. The results are in reasonable agreement with {alpha}-CHERS and show a broadening of alpha particles similar to that seen in PCX measurements.

The threshold for stochastic transport of high energy trapped particles in a tokamak due to toroidal field ripple is calculated by explicit construction of primary resonances, and a numerical examination of the route to chaos. Critical field ripple amplitude is determined for loss. The expression is given in magnetic coordinates and makes no assumptions regarding shape or up-down symmetry. An algorithm is developed including the effects of prompt axisymmetric orbit loss, ripple trapping, convective banana flow, and stochastic ripple loss, which gives accurate ripple loss predictions for representative Tokamak Fusion Test Reactor and International Thermonuclear Experimental Reactor equilibria. The algorithm is extended to include the effects of collisions and drag, allowing rapid estimation of alpha particle loss in tokamaks.

This report discusses the following topics: toroidal Alfven eigenmode induced ripple trapping, and high-{beta} disruption in TFTR.

The interaction of high energy particles with an Alfven eigenmode is investigated self-consistently by using a realistic kinetic dispersion relation. All important poloidal mode numbers and their radial mode profiles as calculated with the NOVA-K code are included. A Hamiltonian guiding center code is used to simulate the alpha particle motion. The numerical simulations include particle orbit width, nonlinear particle dynamics and the effects of the modes on the particles. Modification of the particle distribution leading to mode saturation is observed. Particle loss is limited to devices in which the alpha particle gyro radius is a significant fraction of the minor radius.

The interaction of alpha particles with a toroidicity induced Alfven eigenmode is investigated self-consistently by using a kinetic dispersion relation. All important poloidal harmonics and their radial mode profiles are included. A Hamiltonian guiding center code is used to simulate the alpha particle motion. The simulations include particle orbit width, nonlinear particle dynamics and the effects of the modes on the particles. Modification of the particle distribution leading to mode saturation is observed. There is no significant alpha particle loss.

Transport in a magnetic field configuration near stochastic threshold is investigated. The waiting time distribution near magnetic islands, and the size distribution of these islands are identified as the two fundamental functions which determine both the short time subdiffusive behavior, and the regular diffusive regime. These time and length distributions are studied, the mean square displacement is expressed in terms of these functions, and the result is compared to direct numerical simulation.

This report discusses long-wavelength microinstabilities in toroidal plasmas and alpha particle destabilization of TAE modes in TFTR DT Experiments.

Heat transport in a stochastic magnetic field configuration is shown to be nonlocal. Collisional transport processes, in such a disordered media, cannot always be reduced to a standard diffusion process, and the concept of a diffusion coefficient is meaningless for a wide range of typical tokamak parameters. In the nonlocal regime the relaxation of a gradient is described by an integral equation, involving a nonlocal propagator. This propagator is calculated, and the relation to previous results is elucidated. 15 refs.

Electrostatic collisionless drift waves in a sheared magnetic field have been studied extensively with use of particle-code simulations. When the shear is strong, we have found that the plasma is dominated by spatially uncorrelated small-amplitude oscillations with frequencies close to those prescribed by the local dispersion relation. With a weaker shear, spatially correlated large-amplitude waves with a well-defined frequency have been observed. Theoretically predicted eigenmodes are not the dominant feature of the fluctuation spectrum.

A graphics robot simulator designed for an IBM PC/XT/AT or PS/2 personal computer is presented. The simulator is a terminate-and-stay resident (TSR) program that runs in the background and intercepts commands that would normally go to the robot controller through the COM1 serial communication device. With the use of the simulator, students can develop and test robot control programs offline without a physical robot present, using the language of their choice. The status of the simulated robot is available through a 3-D graphics display and a one-line text window, each of which can be activated from the keyboard or from within a user program. Data files are used to specify the robot to be simulated and the environment or workcell within which it is to operate. Currently supported robots include the Rhino XR-3 educational robot and the Adept One and Inteledex 660 industrial robots. The workcell features an overhead camera and objects that can be sensed by the camera and manipulated by the robot. >

The solution of ordinary differential equations by the means of Phase Integral Methods is eminently suited for an interactive computational system with real time graphics: We describe a code devised for this purpose and illustrate its use.