Fusion Technol Research Articles

Influence of low-order rational magnetic surfaces on heat transport in TJ-II heliac ECRH plasmas

We study the effect of low-order rational surfaces on electron heat transport in plasmas confined in the TJ-II stellarator (Alejaldre et al 1990 Fusion Technol. 17 131) and heated by electron cyclotron waves. Enhancement of core electron heat confinement is observed when the rational surface is placed in the vicinity of the power deposition zone, either by performing a magnetic configuration scan or by inducing Ohmic current in a single discharge. The key to improving heat confinement seems to be a locally strong positive radial electric field, which is made possible by a synergistic effect between enhanced electron heat fluxes through radial positions around low-order rationals and pump out mechanisms in the heat deposition zone.

Spectral energy transfer and generation of turbulent structures in toroidal plasma

Nonlinear energy cascades in turbulent spectra are studied in the H-1 toroidal heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] using the spectral energy transfer estimation and the amplitude correlation technique. An inverse energy cascade of the spectral energy from the unstable range is shown to be responsible for the generation of the large-scale coherent structures dominating turbulence spectra. Among such structures are zonal flows which are also found to be generated via the inverse cascade. The generation of zonal flows is correlated with the increased strength in the nonlinear energy transfer. The onset of the strongly sheared radial electric field across the low–high (L–H) transition dramatically changes the energy transfer in the spectra and the spectral power of the fluctuations.

Destabilization of fast magnetoacoustic waves by circulating energetic ions in toroidal plasmas

An instability of fast magnetoacoustic waves driven by circulating energetic ions in axisymmetric toroidal plasmas and characterized by frequencies below the ion gyrofrequency is considered. An important role of the l=0 resonance (l is the number of a cyclotron harmonic) in the wave–particle interaction is revealed: It is shown that this resonance considerably extends an unstable region in the space of the pitch-angles of the energetic ions and the wave frequencies. The analysis is carried out for a “slow” instability, which has the growth rate less than the bounce frequency of the energetic ions. Specific examples relevant to the National Spherical Torus Experiment [Spitzer et al., Fusion Technol. 30, 1337 (1996)], where instabilities of this kind were observed, are considered.

Analysis of stable resistive wall modes in a rotating plasma

Measurements of the resistive wall mode (RWM) response to external resonant field pulses yield complete knowledge of the mode characteristics in the parameter range explored. An ideal magnetohydrodynamics model [Garofalo et al., Phys. Plasmas 9, 4573 (2002)] has been generalized to include the effects of plasma rotation and dissipation, and the new model is found to explain quantitatively the experimental observations. Rotation of the RWM with respect to the wall is often described as an essential feature of the mechanism by which plasma rotation stabilizes the RWM. This interpretation of the rotational stabilization of the RWM appears inconsistent with the measurements from recent DIII–D [Luxon and Davis, Fusion Technol. 8, 441 (1985)] experiments. It is found that the theoretically predicted mode rotation with respect to the wall is not needed for stabilization and is only a consequence of torque balance in the absence of magnetic-field nonaxisymmetries.

Degradation of radiatively driven inertial confinement fusion capsule implosions by multifluid interpenetration mixing

The observed degradation of radiatively driven single shell capsule yield with increasing convergence ratio can be quantitatively understood by mixing between the gaseous deuterium-tritium (DT) or deuterium (DD) and the pusher. Calculations using a multifluid interpenetration mix model can replicate the performance of experiments at the NOVA [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] and OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] lasers, using a single fitting parameter, a mixing length ratio. When mix is similarly treated at the ablator–solid DT interface, cryogenic single shell ignition capsules for the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] and the Laser Mega Joule [ P. A. Holstein et al., Laser Part. Beams 17, 403 (1999)] calculate to ignite and burn with gains of several due to isolation of the ignition hot spot from the pusher material.

Integrated predictive modelling simulations of burning plasma experiment designs

Models for the height of the pedestal at the edge of H-mode plasmas (Onjun T et al 2002 Phys. Plasmas 9 5018) are used together with the Multi-Mode core transport model (Bateman G et al 1998 Phys. Plasmas 5 1793) in the BALDUR integrated predictive modelling code to predict the performance of the ITER (Aymar A et al 2002 Plasma Phys. Control. Fusion 44 519), FIRE (Meade D M et al 2001 Fusion Technol. 39 336), and IGNITOR (Coppi B et al 2001 Nucl. Fusion 41 1253) fusion reactor designs. The simulation protocol used in this paper is tested by comparing predicted temperature and density profiles against experimental data from 33 H-mode discharges in the JET (Rebut P H et al 1985 Nucl. Fusion 25 1011) and DIII-D (Luxon J L et al 1985 Fusion Technol. 8 441) tokamaks. The sensitivities of the predictions are evaluated for the burning plasma experimental designs by using variations of the pedestal temperature model that are one standard deviation above and below the standard model. Simulations of the fusion reactor designs are carried out for scans in which the plasma density and auxiliary heating power are varied.

Experimental studies of electrostatic confinement on the intense neutron source-electron device

Theoretical works by Barnes and Nebel [R. A. Nebel and D. C. Barnes, Fusion Technol. 38, 28 (1998); D. C. Barnes and R. A. Nebel, Phys. Plasmas 5, 2498 (1998)] have suggested that a tiny oscillating ion cloud may undergo a self-similar collapse in a harmonic oscillator potential formed by a uniform electron background. By tuning the external radio-frequency electric fields to this naturally occurring mode, it is then possible to heat the ions to obtain very high densities and temperatures simultaneously during the collapse phase of the oscillation through adiabatic compression. However, a major uncertainty in this oscillating plasma scheme is the dynamics and stability of the background electrons in the virtual cathode. Recent work based on the electron fluid equations have demonstrated that the required electron cloud is susceptible to an instability that is analogous to the Rayleigh–Taylor mode present in fluid mechanics [R. A. Nebel and J. M. Finn, Phys. Plasmas 8, 1505 (2001)]. This paper describes an inertial electrostatic confinement device at Los Alamos National Laboratory that is being used to test the electron dynamics in a virtual cathode and will subsequently be used to verify this heating and compression scheme. Results from the device operation will be presented including the formation of deep potential wells and bifurcations in the potential equilibria. A simple model is used to explain this bifurcation.

Low- to high-confinement mode transitions in the National Spherical Torus Experiment

Edge data from plasmas in the National Spherical Torus Experiment (NSTX) [S. Kaye et al., Fusion Technol. 36, 16 (1999)] have been compared to theories of transport suppression that have been used to develop a physics framework for low-confinement (L) to high-confinement (H) mode transitions. The NSTX data were obtained in low aspect ratio (R/a≃1.3) discharges taken from a variety of discharge phases, including L-modes, L-H transitions and H-modes with and without edge localized modes. The comparisons show that the group of points taken just before the L-H mode transition are well mixed with the purely L-mode group to within the measurement uncertainties, indicating that changes in these parameters leading up to the transition are subtle. One of the theory parameters, αMHD=−Rq2 dβt/dr, does show a clear threshold (αMHD=1 to 2) between the H-mode grouping of points and those remaining in the L-mode or taken just prior to the transition. Additionally, there is no evidence for an edge temperature threshold necessary for transitioning into the H-mode. NSTX data indicate further a possible connection between L-H transitions and nonambipolar beam ion losses.

Structure function analysis of long-range correlations in plasma turbulence

Long-range correlations (temporal and spatial) have been predicted in a number of different turbulence models, both analytical and numerical. These long-range correlations are thought to significantly affect cross-field turbulent transport in magnetically confined plasmas. The Hurst exponent, H—one of a number of methods to identify the existence of long-range correlations in experimental data—can be used to quantify self-similarity scalings and correlations in the mesoscale temporal range. The Hurst exponent can be calculated by several different algorithms, each of which has particular advantages and disadvantages. One method for calculating H is via structure functions (SFs). The SF method is a robust technique for determining H with several inherent advantages that has not yet been widely used in plasma turbulence research. In this article, the SF method and its advantages are discussed in detail, using both simulated and measured fluctuation data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. In addition, it is shown that SFs used in conjunction with rescaled range analysis (another method for calculating H) can be used to mitigate the effects of coherent modes in some cases.

Active contours approach for plasma boundary reconstruction

In the current generation of tokamak devices, fusion performance is related to plasma geometric characteristics as elongation and triangularity. Shape diagnostics and control systems are, therefore, essential to plasma operations. It follows that the quality of a tokamak control system depends on the availability of fast and reliable measurements of plasma position and shape. For this purpose, classic reconstruction and control algorithms that have been applied to existing machines refer to the position of a discrete set of points (gaps) around the outermost flux surface [Proc. 11th Symp. Fusion Technol. 2 (1980) 1033; Fusion Technol. 1 (1994) 731]. The paper presents an attempt of producing an integral, more than pointwise description of the plasma boundary using modern techniques, derived from the active vision field. It provides an intuitive and elegant tool for plasma boundary representation and dynamic reconstruction, while widening the point of view over plasma shape diagnostics and control systems. A comparison with the reconstruction code currently available at JET will be presented.

Resistive magnetohydrodynamic transport model for a reversed field pinch. II. Results

The second paper of this series presents a wide range of results and comparisons obtained by numerically solving the resistive magnetohydrodynamic marginal stability model described in paper I [Phys. Plasmas 10, 2330 (2003), preceding paper]. The topics covered include the following. Many simulations have been carried out from which it has been possible to perform a regression analysis leading to macroscopic scaling relations for τE and βp as functions of I, a, n, and the pinch parameter θ. The scaling relations are used to compare with data from the Madison Symmetric Torus (MST) [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] and the Reversed Field Experiment (RFX) [G. Rostagni, Fusion Eng. Des. 25, 301 (1995)], various near-analytic theoretical models, numerical simulations, and an empirical observation first pointed out by Werley concerning the “best” performance of reversed field pinch’s (RFP). The present model, which represents the most comprehensive scaling relations to date, agrees reasonably well with both the limited experimental data available and full three-dimensional nonlinear simulations. It makes more accurate predictions than any of the simpler near-analytic models. Finally, a comparison is made between the projected values for nTτ in an RFP and a tokamak. The scaling is surprisingly similar although the numerical coefficient is about a factor of 30 higher (i.e., more favorable) for the tokamak.

A mechanism for tearing onset near ideal stability boundaries

The prevention of neoclassical tearing modes (NTMs) in tokamak plasmas is a major challenge for fusion. Ideal modes can seed NTMs through forced reconnection, yet in sawtoothing discharges it is not well understood why a particular sawtooth crash seeds a NTM after several preceding sawteeth did not. Also, tearing modes sometimes appear and grow without an obvious ideal mode causing a seed island. Based on theoretical and experimental results a new mechanism for tearing mode onset is proposed and tested which explains these puzzling observations. Tearing stability calculations based on experimental equilibria from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] indicate that tearing modes can be driven unstable by a rapid increase in the linear tearing stability index Δ′ just before onset. Near the ideal kink limit in βN≡β/(I/aBT), Δ′ becomes large and positive as it approaches a pole discontinuity. The relative time scales of the change in Δ′ vs the effects of finite island width will determine the evolution of the island, and the eventual nonlinear state. Theoretical predictions of the onset point, the βN at a specified small island width, and the early evolution characteristics are compared with results from new DIII-D experiments designed specifically to test this hypothesis. Time integration of the island evolution equation shows that these predictions are consistent with the experimental observations. The nonlinear 3D resistive magnetohydrodynamic code NIMROD [A. H. Glasser et al., Plasma Phys. Control. Fusion 41, A747 (1999)] is used to evolve equilibria reconstructed from these experiments in time to provide a more comprehensive prediction of the island evolution during the early nonlinear phase. The simulations are also consistent with the proposed mechanism.

Measurements of vapor flow regimes in liquid metal pools

There is significant ongoing research within the APEX project to develop innovative first wall/blanket concepts capable of efficiently extracting useful thermal energy from magnetic fusion devices that can substantially improve the attractiveness of fusion energy systems. Among the several different ideas are solid and liquid wall concepts. One solid wall concept being considered is the EVOLVE concept. This concept relies on liquid lithium filled tungsten trays to act as a heat transfer and tritium breeding media. In this design, liquid lithium flows into tungsten trays and is brought to the saturation temperature by volumetric neutronic heating of the liquid lithium and tungsten trays. The lithium vapor generated could then be used as the heat transfer media for the thermal power cycle. Key to this design is the vaporization and boiling of the lithium in the presence of a strong magnetic fields (∼7 T). Initial analysis of the boiling phenomena lead to many questions regarding the void fractions and boiling flow regime (void distribution) that may exist in the trays [Fusion Technol. 39 (2001) 810]. Since it is necessary to know the void fraction and flow regime present during the boiling process to determine the neutronic heating and energy removal capability, experiments have been initiated to examine this issue. This paper presents initial experiments and scaling analysis conducted in a new test section, which provides real time X-ray determination of flow regimes and void fractions within a pool of low-density liquid metal (NaK) during gas injection inside a horizontal magnetic field. Data will be presented from a first series of tests with gas injection of volumetric flow rates from 0 to 5 SCFH (0< J g<0.06 m/s) with no field.

Comparing simulation of plasma turbulence with experiment. II. Gyrokinetic simulations

The direct quantitative correspondence between theoretical predictions and the measured plasma fluctuations and transport is tested by performing nonlinear gyrokinetic simulations with the GS2 code. This is a continuation of previous work with gyrofluid simulations [D. W. Ross et al., Phys. Plasmas 9, 177 (2002)], and the same L-mode reference discharge in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] is studied. The simulated turbulence is dominated by ion temperature gradient (ITG) modes, corrected by trapped-electron, passing-electron and impurity effects. The energy fluxes obtained in the gyrokinetic simulations are comparable to, even somewhat higher than, those of the earlier work, and the simulated ion thermal transport, corrected for E×B flow shear, exceeds the experimental value by more than a factor of 2. The simulation also overestimates the density fluctuation level. Varying the local temperature gradient shows a stiff response in the flux and an apparent up-shift from the linear mode threshold [A. M. Dimits et al., Phys. Plasmas 7, 969 (2000)]. This effect is insufficient, within the estimated error, to bring the results into conformity with the experiment.

Thermal–mechanical test on ITER primary first wall mock-ups

In 1998, in the frame of the ITER EDA phase, an European R&D Programme for the Blanket Design was implemented for developing and selecting the materials and the relevant fabrication procedures for manufacturing the shielding modules of the ITER Primary Wall. The fabrication of several Beryllium armored small scale mock-ups, reproducing representative portions of a Primary Wall panels, was also launched (Fusion Technol. (1998) 195). Further experimental activities were also programmed for investigating the thermal–mechanical behavior of these mock-ups at high heat flux and under thermal fatigue tests. In 2001, the ITER European Home Team decided to assign to ENEA a contract for the thermal fatigue testing of six mock-ups aiming at verifying the reliability of the Beryllium/Dispersion Strengthened Copper alloy/Stainless Steel and Beryllium/Precipitation hardened Copper alloy/Stainless Steel joints manufactured by solid Hot Isostatic Pressing (HIP) procedure (Technical Specification for the Thermal Fatigue Tests of Be protected EDA Mock-ups). The paper presents the results of the FEM thermal–mechanical analyses performed by ANSYS code and the progress of the first test campaign.

Compressional Alfvén eigenmode dispersion in low aspect ratio plasmas

A new theory of compressional Alfvén eigenmodes (CAEs) applicable to low aspect ratio toroidal plasmas is developed. The theory is based on the ballooning representation of the poloidal dependence of the perturbed quantities. The theory provides localized CAE solution with discrete eigenfrequencies determined by the Alfvén velocity of the plasma. The eigenmode localization is determined by the effective “potential” well given by the minimum of the magnetic field on a magnetic surface, so that the plasma equilibrium is essential to accurately determine the dispersion of CAEs. The mode structure is localized in both minor radius and poloidal angle directions near the low magnetic field side. Properties of recently observed fast ion beam driven instabilities in MHz frequency range in National Spherical Torus experiments (NSTX) [Kaye, et al., Fusion Technol. 36, 16 (1999)] are consistent with ones of CAEs.

Tearing mode stability studies near ideal stability boundaries in DIII-D

For high β, highly shaped plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], the value of the tearing stability index Δ′ calculated at a rational surface can be especially sensitive to the pressure and current profiles. Near marginal stability for a global ideal mode, a pole in Δ′ exists in equilibrium parameter space, as predicted by analytic theory. The proximity of an equilibrium reconstruction to this pole in parameter space strongly decreases the accuracy of the Δ′ calculations. Tearing stability calculations on kinetic equilibrium reconstructions of a series of times in three DIII-D discharges are presented, which indicate that the tearing modes in these discharges are classically unstable at the time of onset. The onset mechanism of two of these discharges (which are in H-mode) is related to the approach of ideal stability boundaries and the occurrence of poles in Δ′. Several ideal modes (sawteeth, edge localized modes, and resistive wall modes) are thought to seed neoclassical tearing modes (NTMs) through forced reconnection, after the ideal mode is unstable. However, tearing modes often appear suddenly and grow quickly without an obvious ideal mode causing a seed island through forced reconnection, which could be explained by this mechanism. This is proposed as an alternative mechanism for the onset of NTMs in tokamaks, which is not incompatible with forced reconnection.

Quasi-single helicity spectra in the Madison Symmetric Torus

Evidence of a self-organized collapse towards a narrow spectrum of magnetic instabilities in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 (1991)] reversed field pinch device is presented. In this collapsed state, dubbed quasi-single helicity (QSH), the spectrum of magnetic modes condenses spontaneously to one dominant mode more completely than ever before observed. The amplitudes of all but the largest of the m=1 modes decrease in QSH states. New results about thermal features of QSH spectra and the identification of global control parameters for their onset are also discussed.

Edge harmonic oscillations produced by toroidal velocity shear

It is shown that, for a plasma with similar edge density and parallel velocity profiles to that of the Doublet III-D [Fusion Technol. 8, 441 (1985)] tokamak operating in the quiescent double barrier (QDB) regime, the system is unstable to the parallel velocity shear instability. By means of three-dimensional nonlinear simulations, it is found that this system produces turbulence that is radially localized in the vicinity of the parallel velocity gradient and has a toroidal structure. These results are similar to the edge harmonic oscillations (EHOs) that are observed during the QDB discharge. Previously, it was thought that the EHOs were due to a magnetic magnetohydrodynamic mode, yet such structures have not been observed in any one-fluid theoretical models. These results indicate that EHOs may be due to this instability, which can only be realized in a two fluid model.

Simulations of internal transport barrier formation in tokamak discharges using the GLF23 transport model

Results are presented for simulations of tokamak discharges exhibiting internal transport barriers (ITBs) with significant reductions in the core thermal transport using a comprehensive theory-based model for drift-wave transport. The predicted temperature and toroidal velocity profiles from the GLF23 model are compared against the experimental data for twenty-two L- and high-confinement mode (H-mode) ITB discharges from three large tokamaks including DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)], and Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)]. The combined effects of E×B shear and Shafranov shift stabilization of the turbulent transport are essential in reproducing the barriers in the plasma core. Shafranov shift or α-stabilization is found to be an essential ingredient in suppressing the thermal transport due to ion and electron temperature gradient and trapped electron modes that can result in simultaneous electron and ion barriers. Another consequence of α-stabilization is that the power threshold for ITB formation is predicted to decrease for strongly reversed magnetic shear cases in comparison with weakly reversed shear cases.