The Study of Fueling Method and Particle Confinement Improvement

Several high performance tokamak operation regimes have been achieved experimentally in the experiments with the peaked density profiles. The regimes include the improved Ohmic confinement in ASDEX, the pellet enhanced performance mode in Alcator-C, and the super-shot mode in TFTR. In these regimes, peaked core density profiles are always existent, and almost always go with the internal transport barriers, these barriers generally produced by sheared radial electric field. In addition to enhance confinement, the peaked density profile is also needed for the optimized fusion reaction rate and alpha heating power in tokamak plasma, and combined peaked density profile and peaked temperature profile, would make the ignition condition easy obtained. It is desirable to seek and analyze the density profile control schemes that effectively lead to density profile peaking in particle transports experimental investigation.     

Poloidal rotation dynamics, radial electric field, and neoclassical theory in the jet internal-transport-barrier region

Results from the first measurements of a core plasma poloidal rotation velocity (upsilontheta) across internal transport barriers (ITB) on JET are presented. The spatial and temporal evolution of the ITB can be followed along with the upsilontheta radial profiles, providing a very clear link between the location of the steepest region of the ion temperature gradient and localized spin-up of upsilontheta. The upsilontheta measurements are an order of magnitude higher than the neoclassical predictions for thermal particles in the ITB region, contrary to the close agreement found between the determined and predicted particle and heat transport coefficients [K.-D. Zastrow, Plasma Phys. Controlled Fusion 46, B255 (2004)]. These results have significant implications for the understanding of transport barrier dynamics due to their large impact on the measured radial electric field profile.     

Two-dimensional structure and particle pinch in tokamak H mode

Two-dimensional structures of the electrostatic potential, density, and flow velocity near the edge of a tokamak plasma are investigated. The model includes the nonlinearity in bulk-ion viscosity and turbulence-driven shear viscosity. For the case with the strong radial electric field (H mode), a two-dimensional structure in a transport barrier is obtained, giving a poloidal shock with a solitary radial electric field profile. The inward particle pinch is induced from this poloidal asymmetric electric field, and increases as the radial electric field becomes stronger. The abrupt increase of this inward ion and electron flux at the onset of L- to H-mode transition explains the rapid establishment of the density pedestal, which is responsible for the observed spontaneous self-reorganization into an improved confinement regime.     

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E _r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E _r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma.The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime.The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates ( 5×10⁶ s^-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements.During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.     

Effects of Applied DC Radial Electric Fields on Particle Transport in a Bumpy Torus Plasma

The toroidal ring of plasma contained in the NASA Lewis Bumpy Torus may be biased to positive or negative potentials approaching 50 kilovolts by applying DC voltage to twelve or fewer midplane electrode rings. The electric fields, which are responsible for raising the ions to high energies by ExB/B2 drift, then point radially outward or inward. The profiles of plasma number density are observed to be flat or triangular across the plasma diameter. The absence of a second derivative in the density profile, combined with the flat electron temperature profiles which are observed, implies that the radial transport processes are not diffusional in nature and are dominated by the strong radial electric fields which are applied to the plasma. Evidence from a paired comparison test shows that the plasma number density and confinement time can increase more than an order of magnitude if the electric field acting along the minor radius of the toroidal plasma points inward, relative to the values observed when the electric field points radially outward. Some characteristic data taken under nonoptimized conditions yielded the highest plasma number density (2.7 × 1011/cc on axis) and the longest particle containment times (1.9 milliseconds) observed so far in this experiment.     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

Observation of toroidal flow antiparallel to the <E(r) x B(straight theta)> drift direction in the hot electron mode plasmas in the compact helical system

A toroidal flow antiparallel to the drift direction is observed in the hot electron mode plasmas when a large positive electric field and a sharp electron temperature gradient are sustained inside the internal transport barrier in the Compact Helical System. This toroidal flow reaches up to 5x10(4) m/s at the plasma center, and it is large enough to reverse the toroidal flow driven by a tangentially injected neutral beam. These observations clearly show the plasma favors flow in the minimum nablaB direction at the transport barrier.     

Experimental studies on E×B drift rotational transform andconfinement in the bumpy torus NBT-1M

Experimental studies on E×B drift rotational transform for toroidal plasma confinement were carried out using a bumpy torus NBT-1M. An inward radial electric field was observed, which induced E×B poloidal precession and formed closed drift surfaces in a conventional ECH configuration. However, the confinement was degraded by the plasma convective loss due to the asymmetric potential profile and core electrostatic fluctuations. By the use of a combination of two frequencies for ECH (18 GHz and 8.5 GHz), we could reduce both the potential asymmetry and the fluctuation. In this operation, the plasma confinement by E×B drift rotational transform was demonstrated more clearly than that in the conventional single frequency operation     

Instationary Electric Fields,Ion Transport and Strong Density Fluctuations in Tokamaks with Dominant Anomalous Electron Transport

A common explanation is given for ion transport and strong broadband density fluctuations in tokamaks as a result of large anomalous electron transport near dominant magnetic surfaces (resp. in small magnetic islands). The main mechanism is local density flattening connected with an anomalous electron transport induced instationary radial electric field, which forces the ions via polarization drift to follow the electrons. For the density flattening process an exact solution of the time-dependent diffusion equation for a linear initial profile over the island width is used. From this we also derive an expression for a temporal growing radial electric field. This positive field reaches its maximum at the density plateau. Strong viscous diffusion or instability-induced transport between high and low electric field regions may now reverse the density flattening. Therefore relaxation oscillations result which may also explain the observed strong density and potential fluctuations in tokamaks. Several details of recent measurements of impurity ion behaviour and density fluctuations in tokamaks may be better explained with the theory given here.     

Initial Measurements of Plasma Potential in the Core of the MST Reversed Field Pinch with a Heavy Ion Beam Probe

Measurement of the plasma potential in the core of MST marks both the first interior potential measurements in an RFP, as well as the first measurements by a Heavy Ion Beam Probe (HIBP) in an RFP. The HIBP has operated with (20-110) keV sodium beams in plasmas with toroidal currents of (200-480) kA over a wide range of densities and magnetic equilibrium conditions. A positive plasma potential is measured in the core, consistent with the expectation of rapid electron transport by magnetic fluctuations and the formation of an outwardly directed ambipolar radial electric field. Comparison between the radial electric field and plasma flow is underway to determine the extent to which equilibrium flow is governed by E×B. Measurements of potential and density fluctuations are also in progress.Unlike HIBP applications in tokamak plasmas, the beam trajectories in MST (RFP) are both three-dimensional and temporally dynamic with magnetic equilibrium changes associated with sawteeth. This complication offers new opportunity for magnetic measurements via the Heavy Ion Beam Probe (HIBP). The ion orbit trajectories are included in a Grad-Shafranov toroidal equilibrium reconstruction, helping to measure the internal magnetic field and current profiles. Such reconstructions are essential to identifying the beam sample volume locations, and they are vital in MST's mission to suppress MHD tearing modes using current profile control techniques. Measurement of the electric field may be accomplished by combining single point measurements from multiple discharges, or by varying the injection angle of the beam during single discharges.The application of an HIBP on MST has posed challenges resulting in additional diagnostic advances. The requirement to keep ports small to avoid introducing magnetic field perturbations has led to the design and successful implementation of cross-over sweep systems. High levels of ultraviolet radiation are driving alternative methods of sweep plate operation. While, substantial levels of plasma flux into the HIBP diagnostic chambers has led to the use of magnetic plasma suppression.     

Stochasticity from external magnetic field measurements

To determine whether or not magnetic field lines inside a tokamak plasma are stochastic we need the Fourier coefficients of any perturbing radial field inside the plasma. Usually what is measured with magnetic pick-up coils is the root mean square poloidal field outside the plasma. Although no unique transformation is available, we present a model which allows an interpretation of the measured (external) root mean square field in terms of the internal Fourier harmonics. The results are applied to particular TEXT discharges, and suggest a link between magnetic stochasticity and an increasing (more positive) radial electric field, as measured with a heavy ion beam probe     

Nonlinear Upper Hybnd Drift Waves for ? ? B?0 in the Vlasov-Maxwell Approximation

Upper hybrid drift waves are found as a special solution to a Vlasov-Maxwell plasma which has a longitudinal electric field and a perpendicular uniform magnetic field. A single-species plasma with a constant-density mobile neutralizing background supports spatially varying disturbances that oscillate at the upper hybrid frequency. The general functional dependences of the electric field, the plasma number density, and the one-particle distribution function for the special case are found from more general Vlasov-Maxwell equations invariant under a Lie group point transformation. The one-particle distribution function for the plasma is a function of the Liouville invariant, which is the energy in the generalized Bernstein-Greene-Kruskal (BGK) reference frame, and the momentum in the drift direction.     

Influence of radial electric field on trajectories of plug potential bounce ion in the tandem mirror

Bouncing ions between the plug potentials play an important role in improvement of the axial confinement in the tandem mirror. We examined the influence of the radial electric field on the trajectories of the ions passed through the anchor cell with nonaxisymmetric magnetic configuration on the assumption that the shape of the magnetic flux tube was shifted from the shape of the equipotential surface of the plasma at the mirror throats of the anchor cells. The discrepancy between the shapes enhanced the radial drift of the bounce ion. Radial potential profile of the core plasma was controlled by adjustment of the radially separated endplate potentials, and it was found that the flattened radial potential profile was effective for the decrease of the radial drift. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002.     

低杂波控制电流分布与改善约束研究

在HT-7超导托卡马克装置上利用低杂波电流驱动有效地控制了等离子体电流分布,并使等离子体约束性能改善。数值模拟与硬X射线测量结果均表明,低杂波的发射功率谱、纵场和等离子体密度对改变等离子体电流分布有明显的影响。在优化低杂波电流驱动实验参数的条件下,等离子体密度、温度分布发生了理想的变化。在电子和离子温度分布上出现了内部输运垒,同时等离子体的能量约束时间和粒子约束时间均有提高。     

Effect of E × B electron drift and plasma discharge in dc magnetron sputtering plasma

Study of electron drift velocity caused by Etimes B motion is done with the help of a Mach probe in a dc cylindrical magnetron sputtering system at different plasma discharge parameters like discharge voltage, gas pressure and applied magnetic field strength. The interplay of the electron drift with the different discharge parameters has been investigated. Strong radial variation of the electron drift velocity is observed and is found to be maximum near the cathode and it decreases slowly with the increase of radial distance from the cathode. The sheath electric field, E measured experimentally from potential profile curve using an emissive probe is contributed to the observed radial variation of the electron drift velocity. The measured values of the drift velocities are also compared with the values from the conventional theory using the experimental values of electric and magnetic fields. This study of the drift velocity variation is helpful in providing a useful insight for determining the discharge conditions and parameters for sputter deposition of thin film.     

Measurement of fluctuation-induced particle fluxes in the shadow of the diaphragm of the FT-2 tokamak

Movable multielectrode probes are used in an experimental study of fluctuations in particle density and electric field, their spectral and correlation characteristics, and the poloidal-radial distributions of fluctuation-induced drift fluxes in the shadow of the diaphragm of the FT-2 tokamak. The spatial regions in which these fluxes are particularly intensive at different stages of the discharge are determined. The part played by fluctuation-induced fluxes in the peripheral radial particle transport is estimated and found to be substantial. Zh. Tekh. Fiz. 67, 48–54 (April 1997)     

Confinement physics for thermal, neutral, high-charge-state plasmas in nested-well solenoidal traps

A theoretical study is presented which indicates that it is possible to confine a neutral plasma using static electric and solenoidal magnetic fields. The plasma consists of equal temperature electrons and highly stripped ions. The solenoidal magnetic field provides radial confinement, while the electric field, which produces an axial nested-well potential profile, provides axial confinement. A self-consistent, multidimensional numerical solution for the electric potential is obtained, and a fully kinetic theoretical treatment on axial transport is used to determine an axial confinement time scale. The effect on confinement of the presence of a radial electric field is explored with the use of ion trajectory calculations. A thermal, neutral, high-charge-state plasma confined in a nested-well trap opens new possibilities for fundamental studies on plasma recombination and cross-field transport processes under highly controlled conditions.     

Plasma confinement in tokamaks with robust torus

We present a non-linear symplectic map that describes the alterations of the magnetic field lines inside the tokamak plasma due to the presence of a robust torus (RT) at the plasma edge. This RT prevents the magnetic field lines from reaching the tokamak wall and reduces, in its vicinity, the islands and invariant curve destruction due to resonant perturbations. The map describes the equilibrium magnetic field lines perturbed by resonances created by ergodic magnetic limiters (EMLs). We present the results obtained for twist and non-twist mappings derived for monotonic and non-monotonic plasma current density radial profiles, respectively. Our results indicate that the RT implementation would decrease the field line transport at the tokamak plasma edge.     

介质壁加速腔加速结构优化

为了在介质壁加速器中增大轴向加速电场, 提高加速梯度的同时抑制径向电场对束包络的扩张, 提出了在每个加速电极上添加金属栅网结构。采用基于粒子云网格方法的电磁粒子模拟软件对不加栅网与添加栅网的电极结构进行了数值仿真, 分析了不同结构下加速管道中的电场分布和束包络变化。通过实验对比了两种不同结构下经过相同的加速长度获得的粒子能量。结果表明:添加金属栅网结构相对于不加栅网的金属小孔式结构, 轴向加速电场强度提高20%, 同时径向电场得到有效抑制;栅网结构下, 被加速的粒子束在自由漂移空间中的径向发散基本得到抑制;在相同的加速长度下加速H3+粒子, 栅网结构得到的能量增益提高了一倍。