Toroidal Plasma Device Research Articles

Gradient-driven turbulence in Texas Helimak

We investigate the turbulence level dependence on plasma profiles in experiments in Texas Helimak, a toroidal basic plasma device, with long stable electron cyclotron resonant heating (ECRH) discharges and great flexibility to alter the equilibrium magnetic field. A large set of Langmuir probes is used to obtain the turbulence level and also the plasma radial profiles for several magnetic field intensities with the same safety factor and field line pitch profiles. As a consequence of the ECRH heating, changing the toroidal magnetic field, the equilibrium density profiles are radially displaced. For all the analyzed discharges, with constant magnetic field curvature and shear profiles, we verify that the plasma turbulence has a critical dependence on the equilibrium density profile. Namely, radial regions with negative density radial gradient, i.e., in the opposite direction of the magnetic curvature, present high turbulence level. By properly comparing the turbulence radial profiles with the density peak position, we show that the negative density gradient is the main cause of high amplitude turbulence, in agreement with predictions for ideal interchange modes. Furthermore, intermittence analysis shows that the extreme events (bursts) contribution for the probability density functions (PDFs) is also related to the relative position with respect to the density peak, and that the turbulence level enhancement is likely due to the increase in burst occurrence.

Validation of edge turbulence codes in a magnetic X-point scenario in TORPEX

Transport processes around the magnetic X-point of tokamaks, such as turbulence and mean-field drifts, are scarcely understood. The assessment of the capability of turbulence codes to quantitatively reproduce these dynamics has been hampered by limitations in computational power and available experimental data. In this paper, we present a rigorous validation of full-scale simulations of a newly developed X-point scenario in the basic toroidal plasma device TORPEX, performed with the four state-of-the-art codes FELTOR, GBS, GRILLIX, and STORM. High-resolution Langmuir probe array measurements of various time-averaged and fluctuating quantities and across the entire cross section of TORPEX show that this X-point scenario features the key ingredients of X-point dynamics, such as small-scale fluctuations and background drifts. The codes are able to qualitatively reproduce some characteristics of the time-averaged fields, such as the ion saturation current profiles at mid-height, the plasma up-down asymmetry, and the blob trajectories. A quantitative agreement is found for the background E × B velocity pattern, while the fluctuation levels are generally underestimated typically by factors of 2 or more, and thus, background fluxes are found to dominate over turbulent ones in simulations. The sensitivity of the simulation results on the plasma collisionality and on the position of the sources is tested in GBS, showing a mild effect on the overall quantitative agreement with the experiment. Overall, this validation reveals the challenges to reproduce the plasma dynamics near an X-point and provides a clear path to a quantitative and computationally relatively inexpensive assessment of future developments in turbulence codes.

Evolution of Heavy Ion Beam Probing from the Origins to Study of Symmetric Structures in Fusion Plasmas

The overview discusses development of the unique fusion plasma diagnostics—Heavy Ion Beam Probing (HIBP) in application to toroidal magnetic plasma devices. The basis of the HIBP measurements of the plasma electric potential and processing of experimental data are considered. Diagnostic systems for probing plasma in tokamaks TM-4, TJ-1, TUMAN-3M and T-10, stellarators WEGA, TJ-II and Uragan-2M are presented. Promising results of the HIBP projects for various existing modern machines, such as TCV, TCABR, MAST, COMPASS, GLOBUS-M2, T-15 MD and W7-X and the international fusion tokamak reactor ITER are given. Results from two machines with similar size and plasma parameters, but with different types of the magnetic con-figuration: axisymmetric tokamak T-10 and helically symmetric stellarator TJ-II are compared. The results of studies of stationary potential profiles and oscillations in the form of quasimonochromatic and broadband fluctuations, turbulent particle flux, fluctuations of density and poloidal magnetic field are presented. The properties of symmetric structures—zonal flows and geodesic acoustic modes of plasma oscillations as well as Alfvén Eigenmodes excited by fast particles from neutral beam injection heating are described. General trends in the behavior of electric potential and turbulence in magnetized fusion plasmas are revealed.

An Overview of the Hybrid Illinois Device for Research and Applications Material Analysis Test-stand (HIDRA-MAT)

The Hybrid Illinois Device for Research and Applications (HIDRA) at the University of Illinois at Urbana-Champaign is a toroidal plasma device that enables fusion plasma-material interaction testing with both stellarator and tokamak plasmas. HIDRA’s long-pulse steady state stellarator plasmas provide a testbed for plasma facing component (PFC) plasma exposures. The HIDRA Material Analysis Test-stand (HIDRA-MAT) is a material characterization module attached to HIDRA that is being designed and fabricated to include thermal desorption spectroscopy and laser induced breakdown spectroscopy systems for in-vacuo PFC characterization. A specialized rotatable sample holder positions the sample for liquid metal droplet application from a liquid metal droplet injector on HIDRA-MAT. Early experiments look to investigate the effect liquid lithium has on porous tungsten samples’ retention of H, D, and He after plasma exposure. Preliminary results from a dual residual gas analyzer system show the ability to differentiate D2 and He in HIDRA-MAT. This work aims to advance the understanding of liquid metal PFCs and further the design and development of new fusion PFCs and technologies.

Statistical properties of intermittent bursts in the Texas Helimak

We present investigations of intermittent turbulence in the Texas Helimak, a simple toroidal plasma device in which the turbulence properties are modified by applying a bias voltage. The analyzed turbulence presents high density bursts, detected by Langmuir probes measuring ion saturation current fluctuations. The turbulent time series is reproduced by a synthetic signal model that has bursts with the same temporal profile and random amplitudes, plus a low amplitude fluctuating stochastic background. Using this model, we identify two burst regimes, observed according to the radial position and external bias: in the first regime, the bursts occur in random instants, leading to a Poisson distribution of the time interval between bursts, while in the second regime, the time interval between large bursts is correlated and modeled by a Gamma distribution. Furthermore, we use the shape parameter k that measures the correlation between occurrence times of successive bursts to characterize the burst regime in most of the low field side. We find that in the region described by the second regime, the k values increase with positive applied bias.

Physics design for a lithium vapor box divertor experiment on magnum PSI

The lithium vapor box divertor is a potential solution for power exhaust in toroidal confinement devices. The divertor plasma interacts with a localized, dense cloud of lithium vapor, leading to volumetric radiation, cooling, recombination, and detachment. To minimize contamination of the core plasma, lithium vapor is condensed on cool (300–400°C) baffles upstream of the detachment point. Before implementing this in a toroidal plasma device with a slot divertor geometry, we consider an experiment with a scaled baffled-pipe geometry in the high-power linear plasma device Magnum-PSI. Three 15 cm-scale open cylinders joined by 6 cm diameter ‘nozzles’ are positioned on the plasma beam axis upstream of a target. The central box may be loaded with several tens of grams of lithium, which can be evaporated at 650°C to produce a vapor predicted, using a simple plasma-neutral interaction model, to be dense enough to cause volumetric detachment in the plasma. The power delivered to the target and box walls as measured by increases in their temperatures after a 10 s plasma pulse can be compared to determine the effectiveness of the vapor in detaching the plasma. Direct Simulation Monte Carlo simulations are performed to estimate the flow rates of lithium vapor between the boxes and to estimate the trapping of H2 delivered by the plasma in the boxes, which could inadvertently lead to detachment. Details of the geometry, simulations, and possible diagnostic techniques are presented.

Decomposing magnetic field measurements into internally and externally sourced components in toroidal plasma devices

The magnetic field on any closed surface can be separated into the component produced by currents in the volume enclosed by the surface and the component produced by currents external to the surface. This decomposition was first discovered by Gauss in 1839, and “Gauss' Algorithm” was used to show that the terrestrial field is sourced from inside the Earth. This magnetic source-decomposition can be used in magnetic confinement fusion to distinguish plasma fields from the fields of external sources using standard magnetic measurements, when no conductors reside in the region between the plasma and the magnetic sensors (otherwise, the internal source is not the plasma alone). Here, Gauss' Algorithm is extended from spherical geometry to cylindrical geometry to decompose fields in the cylindrical tokamak. Demonstrations of the decomposition on DIII-D discharges include the measurement of a rotating tearing mode separate from the induced wall eddy currents and the recovery of a vacuum measurement in the presence of an externally applied 3D field. Anticipated axisymmetric and non-axisymmetric applications, and the requisite diagnostics, are discussed. It is concluded that the ITER magnetic diagnostic design is well suited for many important axisymmetric and non-axisymmetric applications. Future work will extend this decomposition to include fields from halo currents and to generalize it to arbitrary toroidal geometries.

Nonlocal transport in toroidal plasma devices

Collisional particle transport is examined in several toroidal plasma devices in the presence of perturbations typical of modes leading up to a disruption, of saturated tearing modes, or of unstable Alfvén modes. The existence of subdiffusive transport for electrons is found to occur in some cases at very low mode amplitudes and to also exist even for ions of high energy. Orbit resonances can produce long time correlations and traps for particle trajectories at perturbation amplitudes much too small for the orbits to be represented as uniformly chaotic. The existence and nature of subdiffusive transport is found to depend on the nature of the mode spectrum and frequency as well as the mode amplitudes.

Internal transport barriers in plasmas with reversed plasma flow

Evidences of internal particle transport barriers have been observed in plasma discharges with reversed plasma flow. To investigate the influence of the radial electric field profile on these barriers, we apply a drift wave map that describe the plasma particle transport and allows the integration of particle drift in the presence of a given electrostatic turbulence spectrum. With this procedure we show that transport barriers due to the shearless flow invariant lines are created inside the plasma. Moreover, by varying the radial electric field profile, we observe the formation and destruction of internal transport barriers constituted by shearless invariant lines, as well as its effects on the transport in the map's phase space. Applicability of our results are discussed for the Texas Helimak, a toroidal plasma device in which the radial electric field can be changed by application of bias potential.

On energetic-particle excitations of low-frequency Alfvén eigenmodes in toroidal plasma

It is well-known that, within the ideal magnetohydrodynamic (MHD) description, there exist two low-frequency Alfvén continuous spectra in toroidal plasma devices, such as tokamaks. The corresponding three accumulation frequencies are the beta-induced Alfvén eigenmode (BAE) frequency, the ion-sound wave (ISW) frequency, and the zero frequency accumulation point at vanishing parallel wave number, k∥ = 0. To form localized discrete eigenmodes, the plasma with a normal magnetic shear must be ideal MHD unstable. The zero-frequency branch then corresponds to the ideal MHD unstable discrete mode, while the BAE and ISW discrete eigenmode frequencies could be significantly shifted away from the respective accumulation frequencies. Energetic-particle (EP) effects can be analyzed and understood as an effective potential well via the generalized fishbone linear dispersion relation. In particular, it is demonstrated that, for an ideal MHD stable plasma, EPs could play the roles of both discretization and destabilization, and their effect is generally non-perturbative. The theoretical results further predict that EPs preferentially excite the BAE branch over the ISW branch. The zero-frequency branch, meanwhile, becomes the well-known fishbone dispersion relation, giving rise to energetic-particle modes. Extensions to the case of reversed magnetic shear as well as the kinetic effects will also be discussed.

Development of REBCO-Based Magnets for Plasma Physics Research

The accessible performance range for most magnetic confinement plasma physics devices expands markedly with increasing magnetic flux density. The MIT Plasma Science and Fusion Center is investigating the use of high-temperature superconductors as a low cost means to significantly enhance the performance characteristics of small to moderate scale devices. Our initial investigation emphasized the no-insulation winding technique as a means to produce highly stable dc magnets for devices operating at moderate to high magnetic flux density. We present design, manufacture, and test results for a double pancake coil wound from a 500 m length of 12 mm wide REBCO tape. The coil provides on-axis magnetic flux density within an 8 cm clear bore in excess of 0.5 T when operated in liquid nitrogen and in excess of 6 T when operated in liquid helium. The ultimate aim of the program is to develop conduction-cooled HTS coil modules that can be used for both linear and toroidal plasma devices.

Thomson scattering of plasma turbulence on PSI-2

The turbulent transport in the edge of fusion reactors can be conveniently simulated by linear plasma devices with long duty cycles. At high input power steady state plasma discharges at PSI-2 exhibit intermittent fluctuations similar to the edge of toroidal plasma devices. As their influence on erosion predictions is obscured by time-averaged measurements a time-resolved Thomson scattering setup is installed and tested at PSI-2. Aided by a fast CMOS camera and conditional averaging a time resolution of 3 µs was achieved for temperature and density profile evolution in Argon and Deuterium discharges. A 40 kHz, m = 1 oscillation with 50 µs coherence time and 1 eV electron temperature amplitude was identified in Argon, while in Deuterium intermittent events were associated with a 4 eV rise in edge temperature and a 10% reduction of bulk density.

On a Spectral Analysis and Modeling of Non-Gaussian Processes in the Structural Plasma Turbulence*

The paper presents an empirical approach to the analysis of broadband Fourier-spectra of the low-frequency structural plasma turbulence based on a priori assumptions concerning the number of processes and the Gaussian form of the spectral components. This type of turbulence in toroidal plasma devices is described by the mathematical model of non-stationary continuous-time random walk, namely compound doubly stochastic Poisson process or compound Cox process. Measurements of low-frequency (in ranges of frequencies to 5 MHz) long-wave (3..6 cm) fluctuations spectra were obtained at the plasma edge in the L-2M stellarator by the technique of Doppler reflectometry. Under some experimental conditions, the efficiency of the methodology was demonstrated. The spectra were successfully decomposed into a few components. There are more than two harmonics in broadband Fourier-spectra besides a harmonic connected with poloidal rotation of the plasma. Those additional harmonics correspond to fluctuations rotating in the directions of electron and ion drift in toroidal plasma devices. In all modes it was possible to reveal the components corresponding to the poloidal rotation of the plasma (which could be defined by a radial electric field) and phase velocity of the two types of structural turbulence (which could be defined by electron gradient and ion gradient instabilities). The suggested description of probabilistic and spectral characteristics of low-frequency plasma turbulence allow us to formulate the correct problem of characterization of structural turbulence by a system of stochastic differential equations. The equations of the system should involve stochastic processes with densities in the form of mixtures of probability distributions. Such a comprehensive approach assumes a correct comparison of different structural turbulence models (caused by drift dissipative, ion-sound, gradient instabilities, etc.) with characteristics of the obtained stochastic processes.

Burst propagation in Texas Helimak

We present investigations of extreme events (bursts) propagating in the Texas Helimak, a toroidal plasma device in which the radial electric field can be changed by application of bias. In the experiments analyzed, a large grid of Langmuir probes measuring ion saturation current fluctuations is used to study the burst propagation and its dependence on the applied bias voltage. We confirm previous results reported on the turbulence intermittency in the Texas Helimak, extending them to a larger radial interval with a density ranging from a uniform decay to an almost uniform value. For our analysis, we introduce an improved procedure, based on a multiprobe bidimensional conditional averaging method, to assure precise determination of burst statistical properties and their spatial profiles. We verify that intermittent bursts have properties that vary in the radial direction. The number of bursts depends on the radial position and on the applied bias voltage. On the other hand, the burst characteristic time and size do not depend on the applied bias voltage. The bias voltage modifies the vertical and radial burst velocity profiles differently. The burst velocity is smaller than the turbulence phase velocity in almost all the analyzed region.

3D toroidal physics: Testing the boundaries of symmetry breakinga)

Toroidal symmetry is an important concept for plasma confinement; it allows the existence of nested flux surface MHD equilibria and conserved invariants for particle motion. However, perfect symmetry is unachievable in realistic toroidal plasma devices. For example, tokamaks have toroidal ripple due to discrete field coils, optimized stellarators do not achieve exact quasi-symmetry, the plasma itself continually seeks lower energy states through helical 3D deformations, and reactors will likely have non-uniform distributions of ferritic steel near the plasma. Also, some level of designed-in 3D magnetic field structure is now anticipated for most concepts in order to provide the plasma control needed for a stable, steady-state fusion reactor. Such planned 3D field structures can take many forms, ranging from tokamaks with weak 3D edge localized mode suppression fields to stellarators with more dominant 3D field structures. This motivates the development of physics models that are applicable across the full range of 3D devices. Ultimately, the questions of how much symmetry breaking can be tolerated and how to optimize its design must be addressed for all fusion concepts. A closely coupled program of simulation, experimental validation, and design optimization is required to determine what forms and amplitudes of 3D shaping and symmetry breaking will be compatible with the requirements of future fusion reactors.

Nondiffusive transport regimes for suprathermal ions in turbulent plasmas.

The understanding of the transport of suprathermal ions in the presence of turbulence is important for fusion plasmas in the burning regime that will characterize reactors, and for space plasmas to understand the physics of particle acceleration. Here, three-dimensional measurements of a suprathermal ion beam in the toroidal plasma device TORPEX are presented. These measurements demonstrate, in a turbulent plasma, the existence of subdiffusive and superdiffusive transport of suprathermal ions, depending on their energy. This result stems from the unprecedented combination of uniquely resolved measurements and first-principles numerical simulations that reveal the mechanisms responsible for the nondiffusive transport. The transport regime is determined by the interaction of the suprathermal ion orbits with the turbulent plasma dynamics, and is strongly affected by the ratio of the suprathermal ion energy to the background plasma temperature.

A simple experimental method to determine magnetic field topology in toroidal plasma devices.

Estimation of the parallel wavenumber in plasma devices finds wide applications such as determining the nature of instabilities. This task is often challenging, especially in toroidal magnetic configurations. In the present work, a simple yet effective method of achieving accurate probe-alignment along the magnetic field lines is demonstrated in a simple magnetized toroidal device BETA (Basic Experiments in Toroidal Assembly). The alignment was achieved by aligning each probe to a tiny localized plasma source. Such an alignment is necessary for determining the parallel wavenumber precisely. The probe-alignment was confirmed further from the measurements in the plasma and the corresponding parallel wavenumber was found to be in good agreement with the analytical predictions.

Electrostatic turbulence intermittence driven by biasing in Texas Helimak

We investigate changes in the intermittent sequence of bursts in the electrostatic turbulence due to imposed positive bias voltage applied to control the plasma radial electric field in Texas Helimak [K. W. Gentle and H. He, Plasma Sci. Technol. 10, 284 (2008)]—a toroidal plasma device with a one-dimensional equilibrium, magnetic curvature, and shear. We identify the burst characteristics by analyzing ion saturation current fluctuations collected in a large set of Langmuir probes. The number of bursts increase with positive biasing, giving rise to a long tailed skewed turbulence probability distribution function. The burst shape does not change much with the applied bias voltage, while their vertical velocity increases monotonically. For high values of bias voltage, the bursts propagate mainly in the vertical direction which is perpendicular to the radial density gradient and the toroidal magnetic field. Moreover, in contrast with the bursts in tokamaks, the burst velocity agrees with the phase velocity of the overall turbulence in both vertical and radial directions. For a fixed bias voltage, the time interval between bursts and their amplitudes follows exponential distributions. Altogether, these burst characteristics indicate that their production can be modelled by a stochastic process.

Propagation of shear Alvén waves in two-ion species plasmas confined by a nonuniform magnetic field

Ray tracing calculations are performed for shear Alfvén waves in two-ion species plasmas in which the magnetic field varies with position. Three different magnetic topologies of contemporary interest are explored: a linear magnetic mirror, a pure toroidal field, and a tokamak field. The wave frequency is chosen to lie in the upper propagation band, so that reflection at the ion-ion hybrid frequency can occur for waves originally propagating along the magnetic field direction. Calculations are performed for a magnetic well configuration used in recent experiments [S. T. Vincena et al., Geophys. Res. Lett. 38, L11101 (2011) and S. T. Vincena et al., Phys. Plasmas 20, 012111 (2013)] in the Large Plasma Device (LAPD) related to the ion-ion hybrid resonator. It is found that radial spreading cannot explain the relatively low values of the resonator quality factor (Q) measured in those experiments, even when finite ion temperature is considered. This identifies that a damping mechanism is present that is at least an order of magnitude larger than dissipation due to radial energy loss. Calculations are also performed for a magnetic field with pure toroidal geometry, without a poloidal field, as in experiments being planned for the Enormous Toroidal Plasma Device. In this case, the effects of field-line curvature cause radial reflections. A poloidal field is included to explore a tokamak geometry with plasma parameters expected in ITER. When ion temperature is ignored, it is found that the ion-ion hybrid resonator can exist and trap waves for multiples bounces. The effects of finite ion temperature combine with field line curvature to cause the reflection point to move towards the tritium cyclotron frequency when electron temperature is negligible. However, for ITER parameters, it is shown that the electrons must be treated in the adiabatic limit to properly describe resonator phenomena.

Termination of a Magnetized Plasma on a Neutral Gas: The End of the Plasma

Experiments are performed at the Enormous Toroidal Plasma Device at UCLA to study the neutral boundary layer (NBL) between a magnetized plasma and a neutral gas along the direction of a confining magnetic field. This is the first experiment to measure plasma termination within a neutral gas without the presence of a wall or obstacle. A magnetized, current-free helium plasma created by a lanthanum hexaboride (LaB6) cathode terminates entirely within a neutral helium gas. The plasma is weakly ionized (ne/nn∼1%) and collisional λn≪Lplasma}. The NBL occurs where the plasma pressure equilibrates with the neutral gas pressure, consistent with a pressure balance model. It is characterized by a field-aligned ambipolar electric field, developing self-consistently to maintain a current-free termination of the plasma on the neutral gas. Probes are inserted into the plasma to measure the plasma density, flow, temperature, current, and potential. These measurements confirm the presence of the ambipolar field and the pressure equilibration model of the NBL.