Session CO7: Plasma Material Interface; Alternate Magnetic Confinement

Theoretical model of ``fuzz'' growth

Recent more detailed experiments on tungsten irradiation with low energy helium plasma, relevant to the near-wall plasma conditions in magnetic fusion reactor like ITER, demonstrated (e.g. see Ref. 1) a very dramatic change in both surface morphology and near surface material structure of the samples. In particular, it was shown that a long (mm-scale) and thin (nm-scale) fiber-like structures filled with nano-bubbles, so-called ``fuzz,'' start to grow. In this work theoretical model of ``fuzz'' growth [2] describing the main features observed in experiments is presented. This model, based on the assumption of enhancement of creep of tungsten containing significant fraction of helium atoms and clusters. The results of the MD simulations [3] support this idea and demonstrate a strong reduction of the yield strength for all temperature range. They also show that the ``flow'' of tungsten strongly facilitates coagulation of helium clusters and the formation of nano-bubbles.\\[4pt] [1] M. J. Baldwin, et al., J. Nucl. Mater. \textbf{390-391} (2009) 885;\\[0pt] [2] S. I. Krasheninnikov, Physica Scripta \textbf{T145 }(2011) 014040;\\[0pt] [3] R. D. Smirnov and S. I. Krasheninnikov, submitted to J. Nucl. Materials.   

Impact of melt-layer ejection from metallic first wall on tokamak plasmas

At present, all-metallic tokamak first wall is preferred over carbon composite materials for next generation fusion devices, such as ITER, due to favorable thermo-physical and chemical properties of metals in fusion plasma environment. However, recent experiments demonstrate that surface of metallic components, including tungsten ones, under high transient heat load pertinent to next step tokamaks can melt and eject molten material into fusion plasma in form of droplets or fine spray [1]. The ejected material can be a source of impurity contamination of fusion plasmas and even in some cases cause discharge termination, as was observed recently on LHD. In this work, we investigate impact of ejection of beryllium droplets of various sizes on ITER-like plasmas using coupled dust-plasma edge transport code DUSTT/UEDGE [2]. Different ejection scenarios are modeled, including intermittent and prolonged ejection of molten material at the top, midplane and divertor poloidal locations in ITER. Using the modeling we assess modifications of the plasma profiles, radiation power losses, and impurity particle fluxes to the plasma core produced by various quantities of the ejectile. Critical amounts of the different materials ejected, which can lead to discharge termination, are evaluated.\\[4pt] [1] J.W. Coenen, et al., Nucl. Fusion \textbf{51} (2011) 113020;\\[0pt] [2] R.D. Smirnov, et al., J. Nucl. Mater. \textbf{415} (2011) S1067.   

A thermo-electric-driven flowing liquid lithium limiter/divertor for magnetic confined fusion

The concept of using a liquid metal, especially liquid lithium, as the plasma facing surface may provide the best path forward toward reactor designs. A liquid PFC can effectively eliminate the erosion and thermal stress problems compared to the solid PFC while transferring heat and prolong the lifetime limit of the PFCs. A liquid lithium surface can also suppress the hydrogen isotopes recycling and getter the impurities in fusion reactor. The Lithium/metal infused trench (LiMIT) concept successfully proved that the thermoelectric effect can induce electric currents inside liquid lithium and an external magnetic field can drive liquid lithium to flow within metallic open trenches. IR camera and thermocouple measurements prove the strong heat transfer ability of this concept. A new flowing lithium system with active control of the temperature gradient inside the lithium trenches and back flow channels has been designed. TEMHD driven liquid lithium run steady state and pulsed for a few seconds of high heat flux ($\sim $15MW/m$^{2})$ has been used to investigate the transient reaction of the flowing lithium. A similar tray is scheduled to be tested in HT-7, Hefei, China as a limiter in Sept. 2012. Related movies and analysis will be shown.   

Three-dimensional modeling of the thermoelectric MHD problem of the LIMIT liquid lithium divertor for fusion devices

Flowing liquid lithium is a promising technique for the continuous heat removal from plasma-facing components in fusion devices. In ITER-like conditions, the divertor has to handle stationary fluxes of the order 10 MW/m$^{2}$; heat fluxes even bigger occur during H-mode-related instabilities and disruptions. The Lithium-Metal Infused Trenches (LIMIT) concept, proposed at University of Illinois, offers a viable and self-adaptive solution, thanks to the use of a thermoelectric MHD drive of liquid lithium inside elongated metal trenches. We present a 3D finite-element-based model for the solution of the TEMHD. The continuity of mass, momentum, energy and current are solved together with the generalized constitutive laws of thermoelectricity. The numerical results show that TE currents are generated at the interface between the two metals; under the action of the toroidal magnetic field, the resulting JxB force pushes the liquid lithium along the channels. The force acts mainly at the interface, where the Hartmann and the fluid boundary layers are present, developing early turbulence and fluid bi-shaped macrostructures on the velocity field. The stability of the method is discussed, together with further developments toward turbulent average of the convective noise.   

Simulation of non-Maxwellian electron velocity distribution functions in the divertor region

A single Maxwellian distribution is typically used in fluid-based simulations of scrape-off layer (SOL) plasmas, but recent experimental results indicate that kinetic effects in the SOL may be significant enough to call this approach into question. We performed particle-in-cell simulations of electron velocity distribution function (EVDF) for typical divertor plasma parameters. Simulations show that due to insufficient collisionality and large temperature gradients near the wall, the EVDF is non-Maxwellian and its energetic tail has temperature larger than the bulk of cold electrons. In the limit of no recycling and high electron temperature, the EVDF becomes strongly anisotropic, the sheath potential nearly vanishes and thus electrons are practically not confined and ions are not accelerated towards the walls. This strong modification of EVDF leads to the fact that the heat transmission factor of the sheath can reach a low value of about 2, compared to the conventional theoretical value of 7.   

Formation and Acceleration Physics on Plasma Injector 1

Plasma Injector 1 (PI-1) is a two stage coaxial Marshal gun with conical accelerator electrodes, similar in shape to the MARAUDER device, with power input of the same topology as the RACE device. The goal of PI-1 research is to produce a self-confined compact toroid with high-flux (200 mWb), high-density (3x10$^{16}$ cm$^{-3})$ and moderate initial temperature (100 eV) to be used as the target plasma in a MTF reactor. PI-1 is 5 meters long and 1.9 m in diameter at the expansion region where a high aspect ratio (4.4) spheromak is formed with a minimum lambda of 9 m$^{-1}$. The acceleration stage is 4 m long and tapers to an outer diameter of 40 cm. The capacitor banks store 0.5 MJ for formation and 1.13 MJ for acceleration. Power is delivered via 62 independently controlled switch modules. Several geometries for formation bias field, inner electrodes and target chamber have been tested, and trends in accelerator efficiency and target lifetime have been observed. Thomson scattering and ion Doppler spectroscopy show significant heating ($>$100 eV) as the CT is compressed in the conical accelerator. B-dot probes show magnetic field structure consistent with Grad-Shafranov models and MHD simulations, and CT axial length depends strongly on the lambda profile.   

Impact of helical boundary conditions in MHD modeling of RFP and tokamak plasmas

Helical boundary conditions imposed by the active control system of the RFX-mod device provide a handle to govern the plasma dynamics in both RFP and Ohmic tokamak discharges [1]. By applying an edge radial magnetic field with proper helicity, it is possible to increase the persistence of the spontaneous helical RFP states at high current,and to stimulate them also at low current or high density. Helical BCs even allow to access helical states with different helicity than the spontaneous one [2]. In Ohmic tokamak operation at $q(a)<2$, the presence of the 2/1 RWM reduces the sawtoothing activity of the 1/1 internal kink, which takes a stationary snake-like character instead. Many of these features are qualitatively reproduced in 3D nonlinear MHD modeling. We study the impact of helical BCs on the MHD dynamics in both RFP and tokamak with two successfully benchmarked numerical tools, SpeCyl and PIXIE3D [3]. We recover the bifurcation from a sawtooth to a snake solution when imposing a 2/1 BC in the tokamak case and we interpret this as a toroidal/nonlinear coupling effect. We show that the bifurcation is more easily stimulated with a 1/1 BC.\\[4pt] [1] P. Piovesan, invited talk this meeting\\[0pt] [2] M. Veranda et al EPS-ICPP Conference (2012) P4.004\\[0pt] [3] D. Bonfiglio et al Phys. Plasmas (2010)   

Experimental Observation of Microtearing Modes in the RFX-mod Reversed Field Pinch Plasma

The results of the experimental analysis of quasi-coherent magnetic activity during quasi single helicity (QSH) states of a reversed field pinch plasma are presented. A system of in vessel coils, measuring the fluctuations of the three components of the magnetic field at the edge of the RFX-mod plasma column, allows to determine the spectral properties of such fluctuations with good resolution both in terms of frequency and wavelength. Quasi-coherent modes, propagating in the plasma with the electron diamagnetic velocity with associated wavelengths of the order of the ion Larmor radius, are observed to be correlated with the steep temperature gradients forming in the plasma. A comparison of the experimental results with the predictions of dedicated gyrokinetic calculations suggests an interpretation of the electromagnetic instabilities as microtearing modes.   

Recent High-Energy Resolution NPA Measurements on MST RFP Plasmas

Ion distribution and temperature measurement have been made on the Madison Symmetric Torus (MST) using the Florida A{\&}M University compact neutral particle analyzer (CNPA). The CNPA is a low energy (0.34-5.2 keV), high-energy resolution (25 channels) neutral particle analyzer, with a radial view on MST. Majority ion temperatures from Rutherford scattering diagnostic and neutral fluxes for plasma ranging in currents between 200 -- 600 kA have been used to constrain the ion distribution function. Ion distributions have also been studied during standard and neutral beam injection (NBI) plasmas. Early results have shown that NBI shots at 25 keV have an ion distribution tail in the 2.0 to 5.2 keV range, which shows the capability of detecting the fast ions as they slow down on the thermal background and indicating sufficiently good confinement for heating the plasma.   

Turbulent Parameter Evolution in Madison Symmetric Torus RFP Plasmas

Using Fourier analysis and chaos theory, the turbulent parameters have been used to characterize turbulence in many different plasma systems. The Fourier components measure the characteristic frequency that is associated with instabilities that drive turbulence, the amount of energy associated with turbulence and the rate at which that energy moves between scales. The chaos components measure the complexity and volatility of the fluctuations. The Madison Symmetric Torus provides a plethora of plasma regimes to study turbulence and its associated transitions. Magnetic field fluctuations measurements have been made during the ramp-up, sawtooth crash, and equilibrium phases of a standard discharge, along with the increased confinement period during poloidal pulse current drive (PPCD). While the Fourier components of the turbulent parameters are independent of plasma current, the chaotic components show that the complexity and volatility are dependent on both plasma current and density.