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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Properties of convective cells generated in magnetized toroidal plasmas

Convective cells for turbulence control, generated by means of biased electrodes, are investigated in the simple magnetized toroidal plasmas of TORPEX. A two-dimensional array of 24 electrodes is installed on a metal limiter to test different biasing schemes. This allows influencing significantly both radial and vertical blob velocities. It is shown that these changes agree quantitatively with the flows deduced from the time averaged potential perturbations induced by the biasing. Detailed measurements along and across the magnetic field provide a rather clear picture of the effect of biasing on time averaged profiles. The biased electrodes produce perturbations of the plasma potential and density profiles that are fairly uniform along the magnetic field. Background flows influence the location where potential variations are induced. The magnitude of the achievable potential variations in the plasma is strongly limited by cross-field currents and saturates at large bias voltages once the electrodes draw electron saturation current. A quantitative discussion on the origin of cross-field currents is presented, considering contributions related with diamagnetic drifts, ion inertia, collisions with neutrals, and viscosity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Multi-energy x-ray imaging and sensing for diagnostic and control of the burning plasma

New diagnostic and sensor designs are needed for future burning plasma (BP) fusion experiments, having good space and time resolution and capable of prolonged operation in the harsh BP environment. We evaluate the potential of multi-energy x-ray imaging with filtered detector arrays for BP diagnostic and control. Experimental studies show that this simple and robust technique enables measuring with good accuracy, speed, and spatial resolution the T{sub e} profile, impurity content, and MHD activity in a tokamak. Applied to the BP this diagnostic could also serve for non-magnetic sensing of the plasma position, centroid, ELM, and RWM instability. BP compatible x-ray sensors are proposed using 'optical array' or 'bi-cell' detectors.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

A novel local equilibrium model for shaped tokamak plasmas

A model is proposed for a local up-down symmetric equilibrium in the vicinity of a specified magnetic surface with given elongation and triangularity. Different from the Miller's model [R. L. Miller et al., Phys. Plasmas 5, 973 (1998)], the derivative of the Shafranov shift in the present model is self-consistently determined. The equilibrium accounts for all the essential features, like the elongation, the triangularity, and the Shafranov shift etc., of a shaped cross section. Hence, it can be used for investigation of radially localized plasma modes, like reversed shear Alfvenic eigenmodes and ballooning mode, etc., and it is also suitable for local equilibrium construction used for flux tube plasma simulations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

{delta}f Monte Carlo calculation of neoclassical transport in perturbed tokamaks

Non-axisymmetric magnetic perturbations can fundamentally change neoclassical transport in tokamaks by distorting particle orbits on deformed or broken flux surfaces. This so-called non-ambipolar transport is highly complex, and eventually a numerical simulation is required to achieve its precise description and understanding. A new {delta}f particle orbit code (POCA) has been developed for this purpose using a modified pitch-angle collision operator preserving momentum conservation. POCA was successfully benchmarked for neoclassical transport and momentum conservation in the axisymmetric configuration. Non-ambipolar particle flux is calculated in the non-axisymmetric case, and the results show a clear resonant nature of non-ambipolar transport and magnetic braking. Neoclassical toroidal viscosity (NTV) torque is calculated using anisotropic pressures and magnetic field spectrum, and compared with the combined and 1/{nu} NTV theory. Calculations indicate a clear {delta}B{sup 2} scaling of NTV, and good agreement with the theory on NTV torque profiles and amplitudes depending on collisionality.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

On the tokamak first wall response to the edge localized mode bursts

The effects of wall outgassing on the H-mode pedestal recovery after edge localized mode (ELM) crash is examined with a simplified model of hydrogen adsorption/desorption processes. The parameters governing the dynamics of the wall response to the ELM crash are found. It is shown that within adopted hydrogen adsorption/desorption model a significant impact of the wall outgassing on the pedestal recovery can be observed for the wall surface temperature range between 500 K and 800 K.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Development of a low-energy and high-current pulsed neutral beam injector with a washer-gun plasma source for high-beta plasma experiments

We have developed a novel and economical neutral-beam injection system by employing a washer-gun plasma source. It provides a low-cost and maintenance-free ion beam, thus eliminating the need for the filaments and water-cooling systems employed conventionally. In our primary experiments, the washer gun produced a source plasma with an electron temperature of approximately 5 eV and an electron density of 5 Multiplication-Sign 10{sup 17} m{sup -3}, i.e., conditions suitable for ion-beam extraction. The dependence of the extracted beam current on the acceleration voltage is consistent with space-charge current limitation, because the observed current density is almost proportional to the 3/2 power of the acceleration voltage below approximately 8 kV. By optimizing plasma formation, we successfully achieved beam extraction of up to 40 A at 15 kV and a pulse length in excess of 0.25 ms. Its low-voltage and high-current pulsed-beam properties enable us to apply this high-power neutral beam injection into a high-beta compact torus plasma characterized by a low magnetic field.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Anisotropic electron temperature measurements without knowing the spectral transmissivity for a JT-60SA Thomson scattering diagnostic

This paper focuses on a method for measuring the electron temperature (T{sub e}) without knowing the transmissivity using Thomson scattering diagnostic with a double-pass scattering system. Application of this method for measuring the anisotropic T{sub e}, i.e., the T{sub e} in the directions parallel (T{sub e Double-Vertical-Line Double-Vertical-Line }) and perpendicular (T{sub e Up-Tack }) to the magnetic field, is proposed. Simulations based on the designed parameters for a JT-60SA indicate the feasibility of the measurements except in certain T{sub e} ranges, e.g., T{sub e Double-Vertical-Line Double-Vertical-Line }{approx} 3.5T{sub e Up-Tack} at 120 Degree-Sign of the scattering angle.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2012

Paraxial Wentzel-Kramers-Brillouin method applied to the lower hybrid wave propagation

The paraxial Wentzel-Kramers-Brillouin (pWKB) approximation, also called beam tracing method, has been employed in order to study the propagation of lower hybrid waves in a tokamak plasma. Analogous to the well-know ray tracing method, this approach reduces Maxwell's equations to a set of ordinary differential equations, while, in addition, retains the effects of the finite beam cross-section, and, thus, the effects of diffraction. A new code, LHBEAM (lower hybrid BEAM tracing), is presented, which solves the pWKB equations in tokamak geometry for arbitrary launching conditions and for analytic and experimental plasma equilibria. In addition, LHBEAM includes linear electron Landau damping for the evaluation of the absorbed power density and the reconstruction of the wave electric field in both the physical and Fourier space. Illustrative LHBEAM calculations are presented along with a comparison with the ray tracing code GENRAY and the full wave solver TORIC-LH.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Simulation of current-filament dynamics and relaxation in the Pegasus Spherical Tokamak

Nonlinear numerical computation is used to investigate the relaxation of non-axisymmetric current-channels from washer-gun plasma sources into 'tokamak-like' plasmas in the Pegasus toroidal experiment [Eidietis et al. J. Fusion Energy 26, 43 (2007)]. Resistive MHD simulations with the NIMROD code [Sovinec et al. Phys. Plasmas 10(5), 1727-1732 (2003)] utilize ohmic heating, temperature-dependent resistivity, and anisotropic, temperature-dependent thermal conduction corrected for regions of low magnetization to reproduce critical transport effects. Adjacent passes of the simulated current-channel attract and generate strong reversed current sheets that suggest magnetic reconnection. With sufficient injected current, adjacent passes merge periodically, releasing axisymmetric current rings from the driven channel. The current rings have not been previously observed in helicity injection for spherical tokamaks, and as such, provide a new phenomenological understanding for filament relaxation in Pegasus. After large-scale poloidal-field reversal, a hollow current profile and significant poloidal flux amplification accumulate over many reconnection cycles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

The nonlinear dispersion relation of geodesic acoustic modes

The energy input and frequency shift of geodesic acoustic modes (GAMs) due to turbulence in tokamak edge plasmas are investigated in numerical two-fluid turbulence studies. Surprisingly, the turbulent GAM dispersion relation is qualitatively equivalent to the linear GAM dispersion but can have drastically enhanced group velocities. As a consequence radially broad ({approx}cm) GAM eigenmodes may form. Those may lead to experimentally observable deviations from the expected scaling of the GAM frequency with the square root of the plasma temperature. In up-down asymmetric geometry, the energy input due to turbulent transport may favor the excitation of GAMs with one particular sign of the radial phase velocity relative to the magnetic drifts. Including the radial gradient of the GAM frequency may lead to periodic bursts of the GAM and the turbulence intensity.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Asymmetric radiative damping of low shear toroidal Alfven eigenmodes

Radiative damping of toroidicity-induced Alfven eigenmodes (TAEs) in tokamaks, caused by coupling to the kinetic Alfven wave (KAW), is investigated analytically in the limit of low magnetic shear. A significant asymmetry is found between the radiative damping of the odd TAE, whose frequency lies above the central TAE gap frequency {omega}{sub 0}, and that of the even TAE, with frequency {omega}<{omega}{sub 0}. For the even TAE, which consists of a symmetric combination of neighboring poloidal harmonics (and therefore has ballooning-type mode structure), the coupling results in two non-overlapping, outgoing fluxes of KAWs that propagate radially away from each other and the TAE localization region. In contrast, the odd TAE consists of an antisymmetric combination of neighboring poloidal harmonics, resulting in anti-ballooning mode structure. For this mode, the KAWs initially propagate towards each other and form an interference pattern in the TAE localization region, resulting in a negligibly small escaping flux and a correspondingly low radiative damping rate. As a result of the up/down asymmetry in radiative damping with respect to the mode frequency, the odd TAE may be destabilized by fusion born alpha particles more easily than the usual, even TAE.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Electron cyclotron current drive modelling with parallel momentum correction for tokamaks and stellarators

The electron cyclotron current drive (ECCD) is described by the solution of a drift-kinetic equation (DKE) with the quasi-linear heating term which is highly localised in phase space. Within the adjoint approach, an equivalent DKE must be solved without this highly localised source, and the ECCD is described by a straightforward convolution in phase space. The parallel momentum correction technique [Maassberg et al., Phys. Plasmas 16, 072504 (2009)] reduces the problem to the solution of a simplified mono-energetic DKE. The mono-energetic distribution functions are calculated by the DKES code [W. I. van Rij and S. P. Hirshman, Phys. Fluids B 1, 563 (1989)]. Parallel momentum correction, requiring only an energy-dependent weighting factor, is applied to these distribution functions allowing for the direct estimation of the EC-driven current. For small yet finite collisionalities, a rather simple model for ECCD is presented. In this approach, the symmetric portion of the electron distribution function with respect to v{sub Parallel-To} (the Ware pinch contribution) is neglected and an 'off-set' only in the passing particle domain adds to the collisionless (anti-symmetric) distribution function. For this approximation, only the mono-energetic parallel conductivity coefficient as function of the collisionality is needed. The impact of small yet finite collisionalities on ECCD is shown for X2- and O2-scenarios at W7-X.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Collisionless kinetic-fluid simulation of zonal flows in non-circular tokamaks

Fluid simulation of linear zonal flow damping is done with a closure model based on the collisionless gyrokinetics [Sugama et al., Phys. Plasmas 14, 022502 (2007)]. Simulation results of residual zonal flow for low radial wavenumbers are compared with theoretical formulas for circular and non-circular tokamaks. The effects of the elongation and the triangularity are shown to be properly treated in the closure model. Effects of initial parallel flows on zonal flow evolution are also clarified. An appropriate choice of the initial parallel flow gives a much higher residual level than the conventional result with no initial parallel flow. Besides, the zonal flow simulations are done with the E Multiplication-Sign B nonlinearity as initial sources, which is evaluated from linear gyrokinetic microinstabilities such as ion temperature gradient modes, trapped electron modes, and electron temperature gradient modes, in order to estimate efficiency of zonal flow generation by the source instabilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Vorticity scaling and intermittency in drift-interchange plasma turbulence

The effects of spatially varying magnetic field strength on the scaling properties of plasma turbulence, modelled by an extended form of Hasegawa-Wakatani model, are investigated. We study changes in the intermittency of the velocity, density, and vorticity fields, as functions of the magnetic field inhomogeneity C=-{partial_derivative} ln B/{partial_derivative}x. While the velocity fluctuations are always self-similar and their scaling is unaffected by the value of C, the intermittency levels in density and vorticity change with parameter C, reflecting morphological changes in the coherent structures due to the interchange mechanism. Given the centrality of vorticity in conditioning plasma transport, this result is of interest in scaling the results of transport measurements and simulations in tokamak edge plasmas, where drift-interchange turbulence in the presence of a magnetic field gradient is likely to occur.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Consequences of plasma rotation for neoclassical tearing mode suppression by electron cyclotron current drive

In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term {Delta}{sub CD}{sup Prime }. We investigate oscillations in {Delta}{sub CD}{sup Prime} originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows that the net result of the oscillations in {Delta}{sub CD}{sup Prime} is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Effect of toroidal rotation on the geodesic acoustic mode in magnetohydrodynamics

Theoretical research on the geodesic acoustic mode (GAM) induced by equilibrium toroidal rotation flow in the tokamak plasmas is approached by using ideal magnetohydrodynamic model. The dispersion relation of the GAM is presented by taking into account magnetic field perturbations. It is shown that {beta} can decrease the frequency of the GAM.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Modification of {Delta}{sup Prime} by magnetic feedback and kinetic effects

Two possible ways of modifying the linear tearing mode index, by active magnetic feedback and by drift kinetic effects of deeply trapped particles, are analytically investigated. Magnetic feedback schemes, studied in this work, are found generally stabilizing for {Delta}{sup Prime }. The drift kinetic effects from both thermal particles and hot ions tend to reduce the power of the large solution from the outer region. This generally leads to a destabilization of {Delta} Prime for the toroidal analytic equilibria considered here.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗