Plasma Confinement Properties Research Articles

Impact of the mass isotope on plasma confinement and transport properties in the HL-2A tokamak

The impact of the mass isotope on plasma confinement and transport properties has been investigated in Ohmically-heated hydrogen and deuterium plasmas in the HL-2A tokamak. Experimental results show that under similar discharge parameters the deuterium plasma has better confinement and lower turbulent transport than the hydrogen one, and concomitantly, it is found that the magnitude of geodesic acoustic mode zonal flows, the tilting angle of the Reynolds stress tensor and the turbulence correlation lengths are all larger in the edge region of the deuterium plasma. The results provide direct experimental evidence on the importance of the nonlinear energy coupling between ambient turbulence and zonal flows for governing the isotope effects in fusion plasmas.

Optimal shape of stellarators for magnetic confinement fusion

We are interested in the design of stellarators, devices for the production of controlled nuclear fusion reactions alternative to tokamaks. The confinement of the plasma is entirely achieved by a helical magnetic field created by the complex arrangement of coils fed by high currents around a toroidal domain. Such coils describe a surface called “coil winding surface” (CWS). In this paper, we model the design of the CWS as a shape optimization problem, so that the cost functional reflects both optimal plasma confinement properties, through a least square discrepancy, and also manufacturability, thanks to geometrical terms involving the lateral surface or the curvature of the CWS.We completely analyze the resulting problem: on the one hand, we establish the existence of an optimal shape, prove the shape differentiability of the criterion, and provide the expression of the differential in a workable form. On the other hand, we propose a numerical method and perform simulations of optimal stellarator shapes. We discuss the efficiency of our approach with respect to the literature in this area.

Confinement properties of L-mode plasmas in ASDEX Upgrade and full-radius predictions of the TGLF transport model

The properties of L-mode confinement have been investigated with a set of dedicated experiments in ASDEX Upgrade and with a related modelling activity with the transport code ASTRA and the quasi-linear turbulent transport model TGLF–SAT2, with boundary conditions at the separatrix. The values at the boundary have been set by the two-point model for the electron temperature, with the ion temperature proportional to the electron temperature by a constant factor, and the electron density set by a constant fraction of the volume averaged density. The influx of neutrals has been set through a feedback procedure which ensures that in the simulation the same particle content as in the experiment is obtained. The sensitivity of the results under considerable variations in the choice of the boundary conditions has been investigated and found to be limited. The predictions of this full-radius modelling set-up have been compared to experimental results covering a scan in electron cyclotron resonance heating (ECRH) power in both hydrogen and deuterium plasmas, a plasma current scan with fixed magnetic field, under both ECRH and neutral beam injection heating, an increase in plasma density with constant ECRH power in hydrogen plasmas, as well as variations of the fraction of electron and ion heating at approximately constant total heating power, as well as a change of main ion from deuterium to hydrogen. The ASTRA-TGLF predictions have been found to reproduce all of the experimentally explored dependences with relatively good accuracy, providing evidence, for the first time to our knowledge, that the main properties of L-mode confinement can be reproduced by conventional full-radius transport modelling with a quasi-linear turbulent transport model. Evidences of largest disagreement, although usually not exceeding the 20%, have been found at high electron heating power, where TGLF underpredicts the electron and particularly the ion thermal stored energies, and in the current dependence of confinement, which, in electron heated conditions, is predicted to be weaker than in the experiment.

Nonlinear burn control in ITER using adaptive allocation of actuators with uncertain dynamics

ITER will be the first tokamak to sustain a fusion-producing, or burning, plasma. If the plasma temperature were to inadvertently rise in this burning regime, the positive correlation between temperature and the fusion reaction rate would establish a destabilizing positive feedback loop. Careful regulation of the plasma’s temperature and density, or burn control, is required to prevent these potentially reactor-damaging thermal excursions, neutralize disturbances and improve performance. In this work, a Lyapunov-based burn controller is designed using a full zero-dimensional nonlinear model. An adaptive estimator manages destabilizing uncertainties in the plasma confinement properties and the particle recycling conditions (caused by plasma–wall interactions). The controller regulates the plasma density with requests for deuterium and tritium particle injections. In ITER-like plasmas, the fusion-born alpha particles will primarily heat the plasma electrons, resulting in different electron and ion temperatures in the core. By considering separate response models for the electron and ion energies, the proposed controller can independently regulate the electron and ion temperatures by requesting that different amounts of auxiliary power be delivered to the electrons and ions. These two commands for a specific control effort (electron and ion heating) are sent to an actuator allocation module that optimally maps them to the heating actuators available to ITER: an electron cyclotron heating system (20 MW), an ion cyclotron heating system (20 MW), and two neutral beam injectors (16.5 MW each). Two different actuator allocators are presented in this work. The first actuator allocator finds the optimal mapping by solving a convex quadratic program that includes actuator saturation and rate limits. It is nonadaptive and assumes that the mapping between the commanded control efforts and the allocated actuators (i.e. the effector model) contains no uncertainties. The second actuator allocation module has an adaptive estimator to handle uncertainties in the effector model. This uncertainty includes actuator efficiencies, the fractions of neutral beam heating that are deposited into the plasma electrons and ions, and the tritium concentration of the fueling pellets. Furthermore, the adaptive allocator considers actuator dynamics (actuation lag) that contain uncertainty. This adaptive allocation algorithm is more computationally efficient than the aforementioned nonadaptive allocator because it is computed using dynamic update laws so that finding the solution to a static optimization problem is not required at every time step. A simulation study assesses the performance of the proposed adaptive burn controller augmented with each of the actuator allocation modules.

Role of wave-particle resonance in turbulent transport in toroidal plasmas

A clear understanding of wave-particle interaction and associated transport mechanisms of different particle species in the drift wave instabilities is important for accurate modeling and predictions of plasma confinement properties in tokamaks. In particular, the roles of linear resonance and nonlinear scattering in turbulent transport need to be delineated when constructing reduced transport models. First-principle, global gyrokinetic simulations find that electron particle and heat transport decreases to a very low level, while ion heat transport level has no dramatic change when wave-particle resonance is suppressed in the collisionless trapped electron mode (CTEM) turbulence. On the other hand, ion heat transport in the self-consistent ion temperature gradient (ITG) turbulence simulation is qualitatively similar to that in the test-particle simulation using the static ITG turbulence fields. These simulation results show that electron transport is primarily driven by the wave-particle resonance in the CTEM turbulence, and the ion transport is mostly driven by the nonlinear wave-particle scattering in both the CTEM and ITG turbulence.

Temperature response of laser heated emissive probe materials under vacuum and free atmospheric conditions

Precise temporal and spatial knowledge of plasma potential has been a challenging task for decades. Gradient in values of plasma potential govern local electric fields providing insight into many other bulk plasma properties like particle drifts, confinement, transport barriers etc and plays a crucial role in determining stability of magnetically confined high temperature plasmas. In high temperature devices like tokamaks, plasma tends to develop edge bifurcations and results in edge transport barriers, which are a key tool for enhancing the plasma confinement properties in magnetic fusion devices, which in turn requires knowledge of plasma potential. Conventional emissive probes (CEPs) in high temperature magnetically confined plasmas are not advisable owing to their inherent properties and tokamak parameters like high magnetic field, ultra-high vacuum pressure etc as well as tokamak geometry. A new type of emissive probe is becoming popular in recent times in such devices called the laser heated emissive probe (LHEP). Mostly, LaB6 and graphite are used as a LHEP tip owing to their inherent properties of thermal conductivity, low work function, high emissivity, higher lifetime etc. Similar with LaB6 in its mechanical and electrical properties, CeB6 is emerging as a promising candidate for LHEP. CeB6 is a better electron emitter than graphite and LaB6 at comparatively low power due to its lower work function. In this work, the heating dynamics of LaB6 and CeB6 heated by a CW CO2 laser with maximum power of 55 W have been reported. Theoretical and simulation models using Matlab and ANSYS have been developed to understand and explain the temperature gaining process of the probes. Simulation results are further validated by comparing them with experimentally measured data using an infrared camera.

Effect of permanent magnets on plasma confinement and ion beam properties in a double layer helicon plasma source

The work described in this article was carried out to investigate how permanent magnets (PM) affect the plasma confinement and ion beam properties in an inductively coupled plasma which expands from a helicon source. The cylindrical plasma device Njord has a 13 cm long and 20 cm wide stainless steel port connecting the source chamber and the diffusion chamber. The source chamber has an axial magnetic field produced by two coils, with magnetic field lines expanding into the diffusion chamber. Simulations have shown that the field lines leaving the edge of the source hit the port wall, causing a loss of electrons in this section. In the experiments performed in this work, PMs were added around the port walls near the exit of a plasma source and the effect was investigated experimentally by means of a retarding field energy analyser probe. The plasma potential, ion density and ion beam parameters were estimated, and the results with and without the PMs were compared. The results showed that the plasma density in the centre can in some cases be doubled, and the density at the edges of the plasma increased significantly with PMs in place. Although the plasma potential was slightly affected, and the beam velocity dropped by ${\sim}$ 10 %, the ion beam flux increased by a factor of 1.5.

Confinement and manipulation of electron plasmas in a multicell trap

Plasma dynamics and transport are studied experimentally in a multicell Penning-Malmberg trap. The goal is to develop methods for accumulation and long-term confinement of larger numbers of charged particles (e.g., positrons) than is presently possible. In this scheme, the particles constitute non-neutral plasmas which are confined separately in a parallel array of storage cells. Experiments are presented in which pure electron plasmas are transferred from a large-diameter “master cell” trapping region into four smaller, parallel “storage cells,” three of which are offset from the magnetic symmetry axis. The physics of the transfer process, as well as the confinement properties of plasmas in the storage cells, is discussed. We show that plasmas can be transferred into the storage cells and held there for up to a day or more using the rotating wall technique, provided that the plasma radius is sufficiently small compared to that of the cell wall. Experiments regarding the confinement of plasmas with kilovolt space charge are discussed. Recommendations are provided for future efforts with high-capacity multicell traps.

High density experiments in TCV ohmically heated and L-mode plasmas

Recent experiments have been performed on the Tokamak à configuration variable (TCV) to investigate the confinement properties of high density plasmas and the mechanism behind the density limit. In a limiter configuration with plasma elongation and triangularity the operational density range has been extended up to 0.65 of the Greenwald density at kA () and even to the Greenwald value at low plasma current kA (). A transition from the linear to the saturated ohmic confinement regime is observed at high density . A further density increase leads to sawtooth stabilization and is accompanied by a decrease of the energy and particle confinement times. The development of the disruption at the density limit was preceded by sawtooth stabilization. It is shown that electron cyclotron heating leads to the prevention of sawtooth stabilization and then to the increase of the density limit value.

Publisher's Note: “Momentum transport in the vicinity of q min in reverse shear tokamaks due to ion temperature gradient turbulence” [Phys. Plasmas 21, 012302 (2014)

Publisher's Note: “Momentum transport in the vicinity of <i>q</i> <i>min</i> in reverse shear tokamaks due to ion temperature gradient turbulence” [Phys. Plasmas 21, 012302 (2014)

Inclusion of Pressure and Flow in the KITES MHD Equilibrium Code

One of the simplest self-consistent models of a plasma is single-fluid magnetohydrodynamic (MHD) equilibrium with no bulk fluid flow under axisymmetry. However, both fluid flow and non-axisymmetric effects can significantly impact plasma equilibrium and confinement properties: in particular, fluid flow can produce profile pedestals, and non-axisymmetric effects can produce islands and stochastic regions. There exist a number of computational codes which are capable of calculating equilibria with arbitrary flow or with non-axisymmetric effects. Previously, a concept for a code to calculate MHD equilibria with flow in non-axisymmetric systems was presented, called the KITES (Kyoto ITerative Equilibrium Solver) code [D. Raburn and A. Fukuyama, Plasma Fusion Res. 7, 240318 (2012)]. Since then, many of the computational modules for the KITES code have been completed, and the work-in-progress KITES code has been used to calculate non-axisymmetric force-free equilibria [D. Raburn and A. Fukuyama, Proceedings of the 9th EPS Conference on Plasma Physics, Stockholm, Sweden (2012)]. Additional computational modules are required to allow the KITES code to calculate equilibria with pressure and flow. Here, the authors report on the approaches used in developing these modules and provide a sample calculation with pressure.

Stellerators

Stellarators are toroidal devices where the required rotational transform of the magnetic field lines is generated by external field coils and not via an induced net toroidal plasma current. This confinement scheme has the advantages that, in principle, steady-state plasma operation is possible and that it does not have to brace itself against disruptions of a toroidal plasma current. At the cost of having to give up toroidal symmetry the properties of the stellarator field can be tailored to suit reactor needs. Research focuses on the plasma confinement properties of different stellarator fields and investigates the problems arising when one extrapolates to reactor parameters.

0D model of magnetized hydrogen–helium wall conditioning plasmas

In this paper the 0D description of magnetized toroidal hydrogen–helium RF discharges is presented. The model has been developed to obtain insight into the ICRF plasma parameters, particle fluxes to the walls and the main collisional processes, which is especially relevant for the comprehension of RF wall conditioning discharges. The 0D plasma description is based on the energy and particle balance equations for nine principal species: H, H+, H2, , , He, He+, He2+ and e−. It takes into account (1) elementary atomic and molecular collision processes, such as excitation/radiation, ionization, dissociation, recombination and charge exchange, and elastic collisions, (2) particle losses due to the finite dimensions of the plasma volume and confinement properties of the magnetic configuration, and particle recycling, (3) active pumping and gas injection, (4) RF heating of electrons (and protons) and (5) a qualitative description of plasma impurities. The model reproduces experimental plasma density dependences on discharge pressure and coupled RF power, both for hydrogen RF discharges (ne ≈ (1–5) × 1010 cm−3) and for helium discharges (ne ≈ (1–5) × 1011 cm−3). The modeled wall fluxes of hydrogen discharges are in the range of what is estimated experimentally: ∼1019–1020 m−2 s−1 for H atoms, and ∼1017–1018 m−2 s−1 for H+ ions. It is found that experimentally evidenced impurity concentrations have an important impact on the plasma parameters, and that wall desorbed particles contribute largely to the total wall flux.

Overview of TJ-II experiments

This paper presents an overview of experimental results and progress made in investigating density control using Li-coating, transport and L–H transitions in TJ-II. The Li-coating changes drastically the plasma–wall interaction, decreasing the recycling, and enlarges substantially the operational range of the device delaying the appearance of radiative collapse that happens for higher densities, which permits confinement properties of much denser plasmas to be studied. Moreover, L–H mode transition has only been achieved after Li-coating in TJ-II. The effect of rational surfaces on heat transport is studied showing a decrease in heat diffusivity close to their position, and it is also seen that rational surfaces located in the edge make L–H transition easier. TJ-II findings provide a new guideline for understanding the trigger mechanism of the L–H transition pointing out the importance of low frequency fluctuating sheared E × B flows. The properties of fast-ion confinement are described as well as the effects of impurities on radiation profiles, showing two types of profiles the bell and the dome shape, the latter being more prone to radiative collapse.

Dynamic decoupling and multi-mode magnetic feedback for error field correction in RFX-mod

Magnetic field errors can have a significant impact on the confinement properties of magnetized fusion plasmas. In the RFX-mod reversed-field pinch (Sonato et al 2003 Fusion Eng. Des. 33 161) a significant error field is produced during the current ramp by the eddy currents induced in the 3D wall structures, such as the gaps and some large portholes, by the temporal variation of the vertical magnetic field. A set of 192 magnetic sensors and 192 active coils allowed accurate identification of the error field spatiotemporal pattern and its correction. The correction scheme combines pre-programmed current waveforms and multi-mode magnetic feedback. The pre-programmed currents were computed with the dynamic decoupling algorithm developed in Soppelsa et al (2008 Fusion Eng. Des. 83 224). This accounts for the mutual interaction between different feedback coils and magnetic sensors, which is affected by the frequency-dependent response of the 3D wall structures to external magnetic fields. At the same time, multi-mode magnetic feedback is applied to the main error field harmonics. During the current ramp, multiple tearing modes are normally phase-locked and produce a toroidally localized deformation of the plasma column that tends to grow where the error fields are larger. With error field correction, this deformation does not grow at preferred positions, thus avoiding the plasma–wall interaction being too localized there. In general, the decoupling approach used in this work may find applications in other machines.

Impact of heating and current drive mix on the ITER hybrid scenario

Hybrid scenario performance in ITER is studied with the CRONOS integrated modelling suite, using the GLF23 anomalous transport model for heat transport prediction. GLF23 predicted core confinement is optimized through tailoring the q-profile shape by a careful choice of current drive actuators, affecting the transport due to the predicted dependence of the turbulence level on the absolute q-profile values and magnetic shear. A range of various heating and current drive choices are examined, as are different assumptions on the pedestal height. The optimum q-profile shape is predicted to be one that maximizes the ratio of s/q throughout the bulk of the plasma volume. Optimizing the confinement allows a minimization of the plasma density required in order to achieve a defined target fusion power of 350 MW. A lower density then allows a lower total current (Ip) at the same Greenwald fraction (fG), thus aiding in maintaining q > 1 as desired in a hybrid scenario, and in minimizing the flux consumption. The best performance is achieved with a combination of NBI and ECCD (e.g. 33/37 MW NBI/ECCD for a scenario with a pedestal height of 4 keV). The q-profile shape and plasma confinement properties are shown to be highly sensitive to the positioning of the ECCD deposition. Comparisons with the lower performing cases where some or all of the ECCD power is replaced with LHCD or ICRH are shown (e.g. 33/20/17 MW NBI/ECCD/LHCD or NBI/ECCD/ICRH). The inclusion of LHCD reduces confinement due to deleterious shaping of the q-profile, and the inclusion of ICRH, particularly in a stiff model, does not lead to significantly increased fusion power and furthermore does not contribute to the non-inductive current fraction. For the optimum NBI/ECCD current drive mix, the predictions show that a satisfactory ITER hybrid scenario (Pfus ∼ 350 MW, Q ⩾ 5, qmin close to 1) may be achieved with Tped ⩾ 4 keV. In addition, predicted performance sensitivity analysis was carried out for several assumed parameters, such as Zeff and density peaking.

Stellarators

Stellarators are toroidal devices where the required rotational transform of the magnetic field lines is generated by external field coils and not via an induced net toroidal plasma current. This confinement scheme has the advantages that, in principle, steady-state plasma operation is possible and that it does not have to brace itself against disruptions of a toroidal plasma current. At the cost of having to give up toroidal symmetry the properties of the stellarator field can be tailored to suit reactor needs. Research focuses on the plasma confinement properties of different stellarator fields and investigates the problems arising when one extrapolates to reactor parameters.

3D nonlinear MHD simulations of ultra-low q plasmas

Magnetohydrodynamic (MHD) phenomena occurring in the ultra-low safety factor (ULq) configuration are investigated by means of 3D nonlinear MHD simulations. The ULq configuration, a screw pinch characterized by the edge safety factor qedge in the interval 0 < qedge < 1, is the intermediate state between the tokamak and the reversed field pinch. This numerical study, based on the simple frame of the visco-resistive pressureless MHD model, shows that ULq plasmas have the natural tendency to select discrete qedge values which are about the major rational numbers, suggesting plasma self-organization. Similar behaviour is observed in experimental ULq discharges, like those recently obtained exploiting the flexibility of the RFX-mod device. The transition of qedge from a major rational number to the next one occurs together with the development of a kink deformation of the plasma column, whose stabilization yields a nearly axisymmetric state with a rather flat q profile. Numerical simulations also show that it is possible to sustain either of the two conditions, namely, the saturated kink helical configuration and the axisymmetric one, by forcing qedge at a suitable value. Finally, the effects of this MHD phenomenology on the confinement properties of ULq plasmas are discussed.

Stellarators

Stellarators are toroidal devices where the required rotational transform of the magnetic field lines is generated by external field coils and not via an induced net toroidal plasma current. This confinement scheme has the advantages that, in principle, steady-state plasma operation is possible and that it does not have to brace itself against disruptions of a toroidal plasma current. At the cost of having to give up toroidal symmetry the properties of the stellarator field can be tailored to suit reactor needs. Research focuses on the plasma confinement properties of different stellarator fields and investigates the problems arising when one extrapolates to reactor parameters.

Stellarators

Stellarators are toroidal devices where the required rotational transform of the magnetic field lines is generated by external field coils and not via an induced net toroidal plasma current. This confinement scheme has the advantages that, in principle, steady-state plasma operation is possible and that it does not have to brace itself against disruptions of a toroidal plasma current. At the cost of having to give up toroidal symmetry the properties of the stellarator field can be tailored to suit reactor needs. Research focuses on the plasma confinement properties of different stellarator fields and investigates the problems arising when one extrapolates to reactor parameters.