Particle Confinement Research Articles

Design and simulation of the parameters affecting the ion beam characteristics from a penning ion source

Today, many different kinds of ion sources have been developed. One of the most important ones is the penning or PIG ion source. Due to its simplicity of structure and longer lifetime, these ion sources are more widely considered than other ones. In this paper, design and simulation of the parameters affecting the ion beam characteristics from a penning ion source are performed using the CST software. Among the various structures of the permanent magnet around the source, the circular arrangement is found to be better for the confinement of particles inside the source. The electric potential of the anode, the cathode and the extraction electrode are found to be 15+, -500 and -3000 Volt, respectively. The decelerator and accelerator electrodes are used as ion source extraction systems. In addition, in the beam transport systems, high efficiency focusing in Einzel lens is more than that of immersion lens.

Charged particle dynamics near an X-point of a non-symmetric magnetic field with closed field lines

Understanding particle drifts in a non-symmetric magnetic field is of primary interest in designing optimized stellarators in order to minimize the neoclassical radial loss of particles. Quasisymmetry and omnigeneity, two distinct properties proposed to ensure radial localization of collisionless trapped particles in stellarators, have been explored almost exclusively for magnetic fields with nested flux surfaces. In this work, we examine radial particle confinement when all field lines are closed. We then study charged particle dynamics in the special case of a non-symmetric vacuum magnetic field with closed field lines obtained recently by Weitzner & Sengupta (Phys. Plasmas, vol. 27, 2020, 022509). These magnetic fields can be used to construct magnetohydrodynamic equilibria for low pressure. Expanding in the amplitude of the non-symmetric fields, we explicitly evaluate the omnigeneity and quasisymmetry constraints. We show that the magnetic field is omnigeneous in the sense that the drift surfaces coincide with the pressure surfaces. However, it is not quasisymmetric according to the standard definitions.

Evaluation of tritium burnup fraction for CFETR scenarios with core-edge coupling simulations

A key mission for the next-step fusion tokamak device China Fusion Engineering Test Reactor (CFETR) is demonstrating tritium self-sufficiency, which requires a sufficiently high tritium burnup fraction (fburnup) in order to match a practically achievable tritium breeding ratio (TBR) with the blanket design constraints. Core-edge coupling simulations are performed to investigate the dependence of fburnup on different controlling parameters for CFETR scenarios. Core plasma profiles with a range of pedestal densities are simulated by consistent iterative calculations of equilibrium, transport, auxiliary heating and current drives within the OMFIT framework. The core-SOL integrated COREDIV code is then used to evaluate fburnup with the OMFIT modelled core plasma parameters as input. According to the simulations, fburnup can be effectively increased with a higher pedestal density on account of the fusion power increasing faster than the fueling source required to maintain steady-state. Higher can also increase the fburnup due to increase of fuel recycling. Deeper pellet fueling deposition and lower ratio of particle to thermal diffusivities D/χ can both increase the effective particle confinement time and thus fburnup. However, the effect of helium and other impurities (Ar and W) is shown to reduce fburnup for comparable impurity and main ion transport. Based on our analysis, using present pellet fueling technology, achieving fburnup > 3% for CFETR will be very challenging. This is a lower limit for the required TBR (>1) to match the achievable TBR for tritium self-sufficiency. Furthermore, our study suggests that if fueling can penetrate deeper than r/a < 0.8 under optimistic conditions, the required burnup fraction could be attainable. The modelling results thus provide important suggestions and implications for the optimization of CFETR scenarios and development of advanced fueling systems.

Non-adiabatic reflection of particles in a multipole plasma trap configuration

Charged particles incident on a region of time-varying, spatially inhomogeneous electric field may be reflected back toward the region of weaker field. This effect is used for adiabatic particle confinement in charged particle traps, but it may also be subject to non-adiabatic reflection that increases the particle energy. The electrons of a plasma in a multipole plasma trap may be heated by this effect. This work explores the energy gain of electrons in such a device with parameters relevant to laboratory experimentation, and it presents in one representative case an average 10% energy gain after a single reflection.

Physics analysis of results of stochastic and classic stellarator coil optimization

In this paper we follow up on the results from our previous publication (Lobsien et al 2018 Nucl. Fusion 58 106013), where it was found that stellarator coil design optimization can be substantially improved by using a stochastic optimisation approach. In that paper performance was quantified by lower (better) and more narrow (more robust) distributions of the penalty functions at the end of the optimisations. Here, we evaluate and compare the various coil sets of the previous paper but seek a verification and deeper understanding of the physics performance by replacing the relatively simple penalty function estimate with more accurate ones from state-of-the-art calculations of MHD stability, neoclassical transport in the -regime, fast particle confinement, and gyrokinetic behavior. The investigation shows that stochastic stellarator coil optimization generally outperforms the earlier non-stochastic stellarator optimization, also when using these more accurate metrics, generally confirming and quantifying the better performance. We do find some discrepancies, indicating that the penalty function does not represent a physics performance optimum perfectly. For example, as pointed out by others before us, the depth of the magnetic well is not a sufficiently good proxy for MHD stability, and the neoclassical transport can be significantly reduced in configurations that have relatively high field errors and therefore high penalty values. Thus, our work points to areas where better physics model inside the optimisation loop are needed, than what is currently represented by our penalty function.

Characterization of argon plasma in a variable multi-pole line cusp magnetic field configuration

This paper presents a detail characterization of argon plasma confined by a multi-pole line cusp magnetic field (MMF) over a large cylindrical volume (1 m axial length and 40 cm diameter) (Patel et al 2018 Rev. Sci. Instrum. 89 043510) and various magnetic field scaling with magnet current obtained from the magnetic field simulation in the vacuum. From the experimental results, it has been observed that in this field configuration the confinement of the primary electrons increases and leak width (plasma escaping through the cusp) of plasma decreases with increasing the magnetic field. As a result the mean density, particle confinement time and the stability of the plasma increase with increasing magnetic field values. In addition to this, it has been also observed that the radial uniformity of the plasma density across the magnetic field explicitly depends on the magnetic field values of MMF. Moreover, the nature of turbulence in a quiescent argon plasma has been identified.

Local axisymmetry-breaking–induced transition of trapped-particle orbit and loss channels in quasi-axisymmetric stellarators

The transition of trapped-particle orbit topologies has been investigated in quasi-axisymmetric (QA) configurations, such as the Chinese First Quasi-axisymmetric Stellarator (CFQS). It is found that the axisymmetry-breaking phenomenon in QA configurations is of great significance at some specific locations, which could easily induce blocked particles to transit into localized particles. A novel aspect is presented to interpret the transition mechanism of trapped-particle orbit topologies in this paper, i.e., as the amplitudes of non-axisymmetric field increase along the radius direction, the region of large toroidal inhomogeneity is gradually generated, which makes the length of the trapped-particle trajectory substantially short, and hence, may restrict particles to a single helical field period. Meanwhile, at such locations the “pseudo-axisymmetric” field results in coupling of the maximum radial drift and the minimum poloidal drift, which enables the transition of trapped-particle orbit topologies considerably and forms specific loss channels, degrading plasma confinement. These results may shed light on the optimization of QA configurations via avoidance of such coupling with respect to energetic particle confinement. Moreover, this work is also relevant to the generation of inhomogeneity of particle flux deposition on the devertor plates.

Energy and particle confinement in JET H-mode plasma

This work describes the behaviour of the global energy and particle confinement on JET observed in a massive database of H-mode plasmas covering almost the whole lifetime of JET operations, both with the carbon and metal wall. The analysis is focused on type I ELMy H-modes in stationary phases. It is shown that plasma density in that regime is determined mainly by the plasma current, edge safety factor, triangularity of the last closed flux surface and the hydrogenic isotope mass. Behaviour is consistent for the whole database regardless of divertor configuration or the plasma facing materials. On average, thermal energy confinement time in JET with the carbon wall (JET-C) is accurately predicted by the IPB98(y,2) scaling. For JET with the ITER-like wall (JET-ILW), the energy confinement is found to be lower than expected from the scaling. The difference is found to be much stronger in a divertor configuration with the outer strike point at a vertical target and the pumping throat at the private flux. Observed lower confinement in JET-ILW can partially be attributed to the additional operational constraints of the metal wall machine, i.e. the avoidance of heavy impurity accumulation via additional gas dosing, but not fully. The isotope effect on the energy confinement in the Meff = 1–2 range is found to be the same in JET-C and JET-ILW, thus is independent of the wall material, if correlation with plasma density is accounted for. The effect of the toroidal magnetic field on the confinement is between zero and slightly negative. Triangularity has a generally favourable effect on the energy confinement, but the magnitude changes across the database. The effect of triangularity on plasma density is always much stronger than on energy confinement however, therefore plasmas with high triangularity are in general more dense and colder than at lower triangularities. The work described in this paper is completed under the EUROfusion global confinement database project, and the data shown here will be available to EUROfusion collaborators shortly. The data are also either already included in the international H-mode confinement database, or will be in the future updated version.

Poly-N-isopropylacrylamide Nanogels and Microgels at Fluid Interfaces.

The confinement of colloidal particles at liquid interfaces offers many opportunities for materials design. Adsorption is driven by a reduction of the total free energy as the contact area between the two liquids is partially replaced by the particle. From an application point of view, particle-stabilized interfaces form emulsions and foams with superior stability. Liquid interfaces also effectively confine colloidal particles in two dimensions and therefore provide ideal model systems to fundamentally study particle interactions, dynamics, and self-assembly. With progress in the synthesis of nanomaterials, more and more complex and functional particles are available for such studies. In this Account, we focus on poly(N-isopropylacrylamide) nanogels and microgels. These are cross-linked polymeric particles that swell and soften by uptaking large amounts of water. The incorporated water can be partially expelled, causing a volume phase transition into a collapsed state when the temperature is increased above approximately 32 °C. Soft microgels adsorbed to liquid interfaces significantly deform under the influence of interfacial tension and assume cross sections exceeding their bulk dimensions. In particular, a pronounced corona forms around the microgel core, consisting of dangling chains at the microgel periphery. These polymer chains expand at the interface and strongly affect the interparticle interactions. The particle deformability therefore leads to a significantly more complex interfacial phase behavior that provides a rich playground to explore structure formation processes. We first discuss the characteristic "fried-egg" or core-corona morphology of individual microgels adsorbed to a liquid interface and comment on the dependence of this interfacial morphology on their physicochemical properties. We introduce different theoretical models to describe their interfacial morphology. In a second part, we introduce how ensembles of microgels interact and self-assemble at liquid interfaces. The core-corona morphology and the possibility to force these elements into overlap upon compression results in a complex phase behavior with a phase transition between microgels with extended and collapsed coronae. We discuss the influence of the internal particle architecture, also including core-shell microgels with rigid cores, on the phase behavior. Finally, we present new routes for the realization of more complex structures, resulting from multiple deposition protocols and from engineering the interaction potential of the individual particles.

Fusion &amp;alpha; loss due to toroidal field ripple perturbation in CFETR

The key issue of burning plasma in tokamak is the loss and confinement of the energetic particles, and one of the loss channels is the perturbation due to discrete toroidal field coils. The work investigated the α ripple loss of CFETR hybrid scenario with plasma transportation code TRANSP/NUBEAM and guiding center code ORBIT, which have considered the initial α distribution, classical steady state slowing down distribution and other models. The results of the work can give the information of the lost particles, including the lost position, fraction and localized heat load on the wall. The study reveals the heat load on CFETR is about 0.13 MW/m2, which is below the safety threshold of ITER, 0.5 MW/m2, and the paper also gives the direction to reduce the α ripple loss.

Measurement of Some Argon Plasma Parameters Glow Discharge Under Axial Magnetic Field

This paper investigates the characteristics some of argon plasma parameters of glow discharge under axial magnetic field. The DC power supply of range (0-6000) V is used as a breakdown voltage to obtain the discharge of argon gas. The discharge voltage-current (V-I) characteristic curves and Paschen’s curves as well as the electrical conductivity were studied with the presents of magnetic field confinement at different gas pressures. The magnetic field up to 25 mT was obtained using four coils of radius 6 cm and 320 turn by passing A.C current up to 5 Amperes. Spectroscopic measurements are employed for purpose of estimating two main plasma parameters electron temperature (Te) and electron density (ne). Emission spectra from positive column (PC) zone of the discharge have been studies at different values of magnetic field and pressures at constant discharge currents of 1.5 mA. Electron temperature (Te) and its density are calculated from the ratio of the intensity of two emission lines of the same lower energy levels. Experimental results show the abnormal glow region characteristics (positive resistance). Breakdown voltage versus pressure curves near the curves of paschen and decrease as magnetic field increases due to magnetic field confinement of plasma charged particles. Also the electrical conductivity increases due to enhancing magnetic field at different gas pressures. Both temperature density of electron and the intensities of two selected emission lines decrease with increasing pressure due decreasing of mean free path of electron. Electron density increase according to enhancing magnetic field, while the intensity of emitting lines tends to decrease.

The MHD Stability of Global Modes and Fast Particle Confinement in a Quasi-Axisymmetric Tokamak–Stellarator Hybrid

The compatibility of global MHD modes stability and good collisionless confinement of fast particles is studied in a quasi-axisymmetric tokamak–stellarator hybrid. It is shown that both of these requirements can be satisfied at a relative plasma pressure up to $$\left\langle \beta \right\rangle $$ ~ 0.02.

Probing Intermittent Motion of Polymer Chains in Weakly Attractive Nanocomposites

In this study, we investigate the motion of polymer segments in polymer/nanoparticle composites by varying nanoparticle (NP) volume fractions. By studying the probability distribution of segment displacement, segment trajectory, and the square displacement of segment, we find the intermittent motion of segments, accompanied with the coexistence of slow and fast segments in polymer nanocomposites (PNCs). The displacement distribution of segments exhibits an exponential tail, rather than a Gaussian form. The intermittent dynamics of chain segments is comprised of a long-range jump motion and a short-range localized motion, which is mediated by the weakly attractive interaction between NP and chain segment and the strong confinement induced by NPs. Meanwhile, the intermittent motion of chain segments can be described by the adsorption-desorption transition at low particle loading and confinement effect at high particle loading. These findings may provide important information for understanding the anomalous motion of polymer chains in the presence of NPs.

Rheological and structural characterization of organo-hectorite dispersions: Influence of the organoclay loading

The complex behavior of organoclay dispersions is influenced by the structure and the intermolecular interactions between clay platelets. The confinement of the clay particles is seen to be the main reason of this complex behavior because of the presence of surfactants with long alkyl chains on clay surfaces. The rheological properties of organo-hectorite (OHt) dispersed in gasoil were studied using a controlled stress rheometer. Commercial organoclay surfaces were modified with dimethyldialkyl quaternary ammonium cations. It was found that all the dispersions exhibit yield stress with a shear thinning behavior. Flow curves were correlated using both the Herschel-Bulkley phenomenological model and the Carreau-Yasuda structural model, in order to describe, respectively, the macroscopic and the microstructural behaviors of the dispersions (3 to 10 wt%). The parameters of the Carreau-Yasuda model showed the existence of a critical concentration of 6 wt%. Furthermore, frequency sweep tests showed that the dispersions start to have a fully developed gel behavior with a more pronounced solid-like structure at loadings greater than the critical concentration.

Isotope effects on energy, particle transport and turbulence in electron cyclotron resonant heating plasma of the Large Helical Device

Positive isotope effects have been found in electron cyclotron resonant heating plasma of the Large Helical Device (LHD). The global energy confinement time (τE) in deuterium (D) plasma is 16% better than in hydrogen (H) plasma for the same line-averaged density and absorption power. The power balance analyses showed a clear reduction in ion energy transport, while electron energy transport does not change dramatically. The global particle confinement time (τp) is degraded in D plasma; τp in D plasma is 20% worse than in H plasma for the same line-averaged density and absorption power. The difference in the density profile was not due to the neutral or impurity sources, but rather was due to the difference in the transport. Ion scale turbulence levels show isotope effects. The core turbulence (ρ = 0.5–0.8) level is higher in D plasma than in H plasma in the low collisionality regime and is lower in D plasma than in H plasma. The density gradient and collisionality play a role in the core turbulence level.

Stellarator equilibria with reactor relevant energetic particle losses

Stellarator configurations with reactor relevant energetic particle losses are constructed by simultaneously optimizing for quasisymmetry and an analytically derived metric ( $\unicode[STIX]{x1D6E4}_{c}$ ), which attempts to align contours of the second adiabatic invariant, $J_{\Vert }$ with magnetic surfaces. Results show that with this optimization scheme it is possible to generate quasihelically symmetric equilibria on the scale of ARIES-CS which completely eliminate all collisionless alpha particle losses within normalized radius $r/a=0.3$ . We show that the best performance is obtained by reducing losses at the trapped–passing boundary. Energetic particle transport can be improved even when neoclassical transport, as calculated using the metric $\unicode[STIX]{x1D716}_{\text{eff}}$ , is degraded. Several quasihelically symmetric equilibria with different aspect ratios are presented, all with excellent energetic particle confinement.

Linear confinement of spin-0 relativistic quantum particles in (1+2)-dimensions Gürses space–time

In this work, we investigate the linear confinement of spin- 0 relativistic quantum particle in the backgrounds of (1+2)-dimensions Gürses space–time (M. Gürses, Class. Quantum Grav. 11, 2585 (1994)). We solve the Klein–Gordon equation with a linear scalar potential in the considered framework, and evaluate the energy eigenvalues and corresponding eigenfunctions, in detail.

Experimental analysis of the particle transport in the magnetized plasma column with an application to the helicon discharge

Different transport mechanisms in a magnetized radio frequency plasma discharge in the IShTAR device are compared. The total cross field particle transport systematically shows the best agreement with the turbulent diffusion. Also, the ion mobility dominated transport could substantially contribute to radial losses. The relative role of parallel and perpendicular losses in the overall particle confinement is also compared. The total perpendicular particle losses are comparable or even larger than the parallel ones, imposing a practical limitation on achieving high density plasma simply combining several helicon antennae sequentially arranged along the glass tube. For the same reason, reducing the diameter of a source tube, in an attempt to achieve a higher plasma density, could be not very efficient when perpendicular particle loss is dominant.

Development of a Faraday cup fast ion loss detector for keV beam ions.

The development and testing of a Faraday cup fast-ion loss detector capable of measuring sub 100 keV particles is documented. Such measurement capabilities play an important role in the assessment of particle confinement of nuclear fusion experiments. The detector is manufactured using thin-film deposition techniques, building upon previous work using discrete foils. This new manufacturing method allows the form factor of the sensor to become that of essentially a microchip. Analysis of the diagnostic response is performed using Monte-Carlo particle simulations. These simulations show peaks in the detector response at 40 and 70 keV. The sensor is then tested in a tunable linear accelerator capable of accelerating protons from 20 to 120 keV. The detector response was found to be well matched to simulations. Improvements to the design to facilitate robustness are discussed.

Reduced energetic particle transport models enable comprehensive time-dependent tokamak simulations

Time-dependent integrated simulations through codes such as TRANSP are becoming an indispensable tool for the interpretation of existing experiments and predictions of optimized scenarios. For many practical cases, quantitative simulations need to include the effect of plasma instabilities on the evolution of a tokamak discharge. An example is the degradation in energetic particle (EP) confinement induced by instabilities, which in turn affects important source terms for heating, non-inductive current, and momentum in a simulation. The reduced-physics kick model provides phase-space resolved transport probability matrices to TRANSP that are used to account for enhanced EP transport by instabilities in addition to neoclassical transport. The model has recovered the measured Alfvén eigenmode (AE) spectrum on NSTX, NSTX-U and DIII-D, and has reproduced details of phase-space resolved fast ion diagnostic data measured on DIII-D for EP-driven modes and tearing modes. In general, the kick model has proven the potential of phase-space resolved EP simulations to unravel details of EP transport for detailed theory/experiment comparison and for scenario planning based on optimization of neutral beam injection parameters. In this work, the extension of the kick model to low-frequency instabilities such as tearing modes and fishbones, in addition to AEs, is assessed. The goal is to enable TRANSP simulations that retain the main effects of multiple types of instabilities through a common framework. Results from the NSTX/NSTX-U and DIII-D tokamaks show that the extension to multi-mode scenarios can expand the range of applicability of the model for more reliable, quantitative integrated simulations.