Session GP9: Poster Session III: Laser and Beam-Driven Acceleration; DIII-D Tokamak I; General Tokamak; Field Reversed Configurations and Spheromaks; Mini-Conference: Integrated, Multiphysics, High-Performance Computations for Magnetic Fusion Research

Rayleigh-Taylor instability of two-specie laser-accelerated foils

When an ultra intense circularly polarized laser pulse irradiates an ultra thin film, a monoenergetic ion beam is produced with characteristics well suited for applications in science and medicine. Upon laser incidence, the electrons in the foil are pushed via the ponderomotive force to the foil rear; the charge separation field then accelerates ions. In the accelerating frame the ions are trapped in a potential well formed by the electrostatic and inertial forces. However, their energy spectrum can be quickly degraded by the Rayleigh-Taylor (RT) instability. Stabilization in the case of a two-specie foil is the subject of this poster. First, we use a 1D particle-in-cell (PIC) simulation to establish an equilibrium state of the two-specie foil in the accelerating frame. Next we perturb this equilibrium and analytically investigate the 2D RT instability. Analytical results are compared with 2-D simulations. We also investigate parametrically various effects on the RT growth rate. The protons completely separate from the carbons, and although the vacuum-carbon interface remains unstable, the large spatial extent of the carbon layer prevents perturbations from feeding through to the proton layer. The monoenergetic proton beam is shown to persist beyond the conclusion of the laser pulse interaction. [1] T.P. Yu, A. Pukhov, G. Shvets, and M Chen, Phys. Rev. Lett. (in press)   

Relativistic Rayleigh Taylor Instability of Radiation Pressure Driven Foil

Laser acceleration of protons has drawn vigorous attention due to their application in cancer treatment and other areas. The scheme that has emerged very promising for the generation of mono-energy ions is the Radiation Pressure Acceleration (RPA) of laser illuminated thin foils. A circularly polarized laser impinged on a submicron thick foil of overdense plasma exerts a ponderomotive force on electrons, pushing them forward and producing a space charge separation. The electric field created by this charge distribution accelerates the ions while the electrons are driven by the difference of radiation pressure force and space charge force. The entire foil is then accelerated stably in a double layer mechanism. The foil can only be accelerated until that Rayleigh-Taylor Instability (RTI) becomes a dominant factor and decomposes the foil into separate cusps. We present RTI mechanism as a single fluid model, by calculating the ponderomotive force of the input pulse in a relativistic circumstance and demonstrating that RTI will force the foil to form cusps after a few waveperiods, preventing the energy gain of monoenergetic ions to increase endlessly. By defining a proper saturation time while the cusps form, we show the scaling of the maximum energy gain to the input pulse amplitude. Comparison with 2D PIC simulation are also presented.   

Efficient energy transfer from laser to proton beam in an intense short pulse laser foil interaction

A remarkable improvement is demonstrated in the energy conversion efficiency from laser to protons by particle simulations in a laser-foil interaction. The total laser-proton energy conversion efficiency becomes 16.7{\%} in an optimized multi-hole target and in this case the laser substrate-Al-ion energy transfer is 38.7{\%}. When an intense short-pulse laser illuminates the thin foil target, the foil electrons are accelerated around the target by the intense laser. The hot electrons generate a strong electric field, which accelerates the foil protons, and the proton beam is generated. In our previous study, we found that multi-hole thin-foil target was efficient for the energy conversion from laser to protons [Phys. Rev. E 78, 046401 (2008)], and the energy conversion efficiency from laser to protons was 9.3{\%}. In this paper 2.5-dimensional particle-in-cell simulations clarify the role of the target hole thickness and depth in the laser-proton energy conversion. The optimized multi-hole foil target provides a remarkable increase in the laser-proton energy conversion efficiency.   

Ion acceleration from near-critical density aerogel and foam targets

Interaction of an intense laser pulse with near-critical density plasma makes a channel both in electron and then in ion density. The propagation of a laser pulse through such a channel is connected with the acceleration of electrons in the wake of a laser pulse and generation of strong moving electric and magnetic fields in the propagation channel. Upon exiting the plasma the magnetic field generates a quasi-static electric field that accelerates and collimates ions from a thin filament formed in the propagation channel. Two-dimensional Particle-in-Cell simulations show that a 100 TW laser pulse tightly focused on a near-critical density target is able to accelerate protons up to an energy of 250 MeV [1]. We present the experimental results on ion acceleration from silica aerogel targets with density of 40-100 mg/cm$^{3}$ and CHO foam targets with density of 3-45 mg/cm$^{3}$ using 100 TW, 30 fs laser pulses focused to intensities of 10$^{22}$ W/cm$^{2}$ at normal incidence. Detailed 2D-PIC computer modeling of these interactions will also be presented and compared to the experimental data. [1] S.S. Bulanov et al. PoP \textbf{17}, 043105 (2010). *Morehouse College   

Relativistic transparency and non-axisymmetry of laser-accelerated ion beams from the Break-Out Afterburner

In the Break-Out Afterburner (BOA) ion acceleration mechanism [1], an ultraintense, ultrahigh contrast laser interacts with a nano-scale, solid-density target, which expands as the electrons under the laser spot heat to relativistic temperatures. When the electron density drops below the relativistic critical density, the target turns transparent and a period of enhanced ion acceleration, called the Break Out Afterburner, ensues. A large (tens of TeV), longitudinal electric field forms that co-moves with the target ions. A defining features of the BOA, as seen in VPIC kinetic plasma simulations and observed in experiments at the LANL Trident laser facility, is that the ion beams form as a pair of lobes with density and energy possessing maxima in the direction orthogonal to the laser polarization. This paper will focus on analytic theory explaining how these lobes form as a consequence of subtle effects of the laser ponderomotive force. [1] Yin et al. \textit{Laser and Part. Beams} \textbf{24}, 2, 291 (2006).   

Studies of spectral modification in LWFA and filament formation

Intense short laser pulses propagating in nonlinear media suffer spectral modification. For example, in laser wake field acceleration (LWFA) the pulse spectrum is modified by the presence of a large amplitude plasma wave. In the case of pulse propagation in air the spectrum is modified by the nonlinear response and initialization of the air molecules. The laser pulse is modeled (as in WAKE) using the envelope approximation, which assumes the spectrum is narrow. In practice the spectrum can be broad. Here we investigate limitations to and extensions of the envelope approximation in the modeling of LWFA and filament formation in air.   

Numerical methods for the 1D quasistatic wave equation

We studied the numerical and analytical dispersion relations of the full and reduced 1D quasistatic wave equation in moving window coordinates. The goal is to identify appropriate numerical methods in terms of numerical dispersion relations calculated from discrete transforms of the representative difference equations. Comparing the numerical and actual dispersion relations illustrates a source of phase error, and the accuracy of the group velocities is important for correct wave propagation. Each method yields a particular dispersion relation, and we have made a survey of stability conditions for all the reasonable numerical methods used to solve the wave equation. The dispersion relation of the reduced wave model is disparate from that of the full wave model, but for any one wave number we can adjust the coordinate velocity to reconcile the dispersion relations for the forward-propagating modes. We compare our findings in the case of a linear plasma and the vacuum.   

The Physics of Reduced Models of Intense Laser-Plasma Interactions

Full-fluid models (wherein the only approximation is the assumption of hydrodynamics) can be computationally intensive in the parameter regimes relevant to advanced accelerator concepts, thus there is interest in evaluating the physics content of reduced descriptions. We present detailed comparisons of the predictions of these models for the case of long propagation of a resonant laser pulse. This configuration is important as it is the primary candidate for a multi-GeV accelerator stage and is the subject of active experimental investigation by a number of groups. Accurate handling of the laser propagation over long distances (order of a thousand plasma oscillations) is essential to assessing the performance of a putative accelerator module. We examine the differences in physics content of these models in regard to laser energy transport, generation of plasma waves, and the role of the wave action adiabatic invariant. We study various numerical concerns such as resolution, convergence, dissipation, dispersion, and computational resource requirements. In particular, we find that proper numerical treatment of the wave operator (reduced or full) is essential to correctly capture the evolution of the laser field.   

Fluid simulations of GeV scale laser plasma accelerator experiments

The cold, relativistic fluid algorithm of the parallel VORPAL framework [1] is used to simulate GeV scale laser plasma accelerator stages in the quasilinear regime. Simulations conducted in a Lorentz-boosted frame are compared with results using appropriately scaled physical parameters [2]. Both approaches offer speed-up which can exceed 1000x for meter-scale interaction lengths; however, boosted frame simulations can correctly treat betatron oscillations of the laser pulse and the accelerated bunch, which is important for accurate treatment of the beam emittance. The parallel 1D, 2D and 3D fluid algorithm in VORPAL has been shown to agree with PIC for laser-plasma simulations in the quasilinear regime, it is free of particle noise, and it can be used together with a PIC representation of the accelerated bunch. We will address the difficult question of preserving ultra-low beam emittance. Also, progress on implementation of a warm, relativistic fluid model [3] will be presented. \\[4pt] [1] Nieter and Cary, JCP (2004).\\[0pt] [2] Cormier-Michel et al., AAC (2008).\\[0pt] [3] Schroeder and Esarey, PRE (2010).   

Simulation of meter-scale laser wakefield stages using an envelope model

Simulation of laser-plasma accelerator (LPA) stages is computationally intensive due to the disparate length scales involved. The next generation of LPA experiments, such as those at the upcoming BELLA facility, will extend stage length to $\mathord{\sim}1\,\mathrm{m}$---over one million laser wavelengths. This makes explicit PIC simulations prohibitively expensive. We can substantially improve the performance of LPA simulations by modeling the envelope evolution of the laser field rather than the field itself, allowing for much coarser grids. Here we describe the model and its implementation in the parallel VORPAL framework. We present rigorous benchmarks, showing second-order convergence and accurate group velocity. We also show excellent agreement with scaled explicit simulations for LPA parameters relevant for meter-scale stages, while attaining orders of magnitude speedup. In addition, we describe techniques and trade-offs involved in resolving the laser fields into the depletion regime. We then present use of this method to simulate 10 GeV stages at full scale, for design relevant to future BELLA experiments.   

Modeling laser wakefield accelerators in a Lorentz boosted frame

Modeling of laser-plasma wakefield accelerators in an optimal frame of reference is shown to produce 4-6 orders of magnitude speed-up in calculations from first principles of stages in the 10 GeV-1 TeV energy range, in agreement with the maximum obtainable speedups that are predicted by theory. Obtaining these speedups requires mitigation of a high-frequency instability that otherwise limit effectiveness using a mix of established and novel numerical techniques, and solutions for handling data input and output in a relativistically boosted frame of reference. Using these techniques, agreement at the percentage level is demonstrated between simulations using different frames of reference, with speedups reaching two orders of magnitude, for a 0.1 GeV class stages. The method is then shown to enable direct, efficient full-scale modeling of deeply depleted laser-plasma stages in the 10 GeV-1 TeV range, verifying the scaling of plasma accelerators to very high energies, providing designs for experiments on new lasers such as BELLA.   

Single particle dynamics in ultra intense laser fields including radiation cooling

Under extreme acceleration, charged particles can radiate strongly and the corresponding radiation damping/cooling can become important. This occurs when the radiated energy in a typical oscillation period (e.g. cyclotron period or laser period) is comparable to mc$^{2}$. In particular, under the presence of ultra high static fields or high intensity lasers the motion of particles in the ultrarelativistic regime can be severely affected by radiation damping. Using a single particle dynamics code and the Osiris 2.0 framework, and including radiation cooling, we have examined and identified the different qualitative regimes for single electron interaction with counter- and co-propagating ultra-intense lasers fields. For conditions where the radiation cooling is important, qualitative differences arise as compared with the scenarios where radiation cooling is absent; this is identified not only in the particle phase space trajectories, but also on the net velocity imparted to the counter-propagating electrons, and the possibility of cooling particle beams.   

A Basic Analysis of Electrons Crossing a Plasma Wave and or Electromagnetic Wave

Simulations examining the interaction of a HeNe laser and a 5 to 50 keV electron beam are being presented. Laser intensity and beam current are a couple of the parameters that will be adjusted within the models. The hope of the study is to produce a novel way of measuring electron beam properties and the residual laser. We also examine the crossing of a plasma wave and electron beam (5 to 50 keV) at different angles. Beam current, beam length, and plasma wave magnitude are a few of the parameters that can be varied. The models should give results on how the varying of the previously mentioned parameters has on the beam path, speed, and trapping.   

Ion response to laser plasma electron acceleration in the blow-out regime

The ion response to relativistic electron bunches in the so called ``bubble'' or blow-out regime of a laser-plasma accelerator is modelled using numerical simulations. In response to the strong fields of the accelerated electrons the ions form a central filament along the laser axis that can be compressed to densities two orders of magnitude higher than the initial particle density. A theory and a simple model of the filament formation and its subsequent expansion are proposed. The long time ion evolution could provide a unique diagnostic of laser plasma electron accelerators. It is also shown that in the case of a sharp rear plasma-vacuum interface the ions can be accelerated by a combination of three basic mechanisms: charge separation field, longitudinal inductive field and electrostatic field at the tip of the ion filament.   

Electron self-injection and acceleration into an evolving plasma ``bubble"

We propose a new approach to injecting plasma electrons into a full blow-out (or bubble) region of laser wakefield accelerator in order to generate monochromatic high-energy electron beams. It is shown analytically and by two types of PIC simulations (VLPL and WAKE) that self-injection of the background plasma electrons into the quasi-static plasma bubble can be caused by slow temporal expansion of the bubble. The sufficient condition for the plasma electron trapping is that its Hamiltonian in the co-moving with the bubble frame must decrease by at least mc$^2$. We will demonstrate that the necessary condition for electron injection is more relaxed, and dependent on the plasma density and details of the bubble potential. We will also demonstrate that there is a minimum bubble expansion rate necessary for trapping and discuss its dependence on the details of the accelerating field near the tail of the bubble. We also demonstrate that a bubble whose expansion stops after injection generates a monoenergetic electron bunch, as electrons that are injected late in the expansion equilibrate in energy with those that are injected early. [1] S. Y. Kalmykov, S. A. Yi, V. Khudik, and G. Shvets, Phys. Rev. Lett. 103, 135004 (2009). [2] S. A. Yi, V. Khudik, S. Y. Kalmykov, and G. Shvets, Plasma Phys. Control. Fusion (in press, 2010). This work is supported by the US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945.   

Electron self-injection due to a plasma density downramp and gas ionization in a plasma wakefield accelerator in the blowout regime

We study self-injection into a plasma wakefield accelerator (PWFA) in the blowout regime analytically and through particle-in-cell (PIC) simulations. We propose a new injection mechanism into a plasma wakefield accelerator, where growth of the blowout region is enabled through a slow decrease in background plasma density along the direction of propagation. Deepening of the potential well due to this growth causes a reduction of electron Hamiltonian in the co-moving frame. This reduction depends on the shape of the blowout region, its growth rate, and impact parameter of the electron. When the reduction is greater than $mc^2$ [1,2], the electron becomes trapped inside the bubble. We demonstrate this effect using analytic expressions for the bubble potentials [3], and estimate plasma density gradients, and beam charge and size required for injection. We also apply the injection criterion to electron trapping through gas ionization. This work is supported by the US DOE grants DE-FG02-04ER41321 and DE-FG02-07ER54945. [1] S. Kalmykov, S.A. Yi, V. Khudik, and G. Shvets, {\it Phys. Rev. Lett.} {\bf 103}, 135004 (2009). [2] S.A. Yi, V. Khudik, S. Kalmykov, and G. Shvets, {\it Plasma Phys. Contr. Fus.}, in press. [3] W. Lu, C. Huang, M. Zhou, M. Tzoufras et al., {\it Phys. Plasmas} {\bf 13}, 056709 (2006).   

Using Photon Accleration to Visualize Bubble Evolution in a LWFA

Diagnosing relativistic plasma structures, such as the bubble of an LWFA, presents a considerable challenge to numerical simulations. We present an approximative technique, photon acceleration [1], which has received little attention despite its versatility. Photon acceleration can effectively simulate techniques such as Fourier Domain Holography [2], techniques necessary to achieve a more complete understanding of a bubble's structure. Furthermore, photon acceleration provides intuitive visualization of local frequency shift, as well as light trapping and deflection by a bubble. We will discuss the extensions to photon acceleration required to handle FDH, the most important being the inclusion of field phase. Finally, we will use the photon acceleration framework to uncover information about bubble evolution, which has recently been shown to be critical to electron self-injection [3]. \\[4pt] [1] Mendonca, J.T., ``Theory of Photon Acceleration''\\[0pt] [2] Dong, Reed et al., ``Formation of Optical Bullets in Laser-Driven Plasma Bubble Accelerators''\\[0pt] [3] Kalmykov, Yi et al., ``Electron Self-Injection and Trapping into an Evolving Plasma Bubble''   

Self and Controlled Injection in Multi-dimentional Wakefield Driven by Laser or Charged Particle Beams

In plasma based accelerators (LWFA and PWFA), the methods of injecting high quality electron bunches into the accelerating wakefield is of utmost importance for various applications. A through understanding of how the injection occurs in both self and controlled scenarios is therefore important and needed. To simplify this understanding, we start from single particle motion in an arbitrary traveling wave electromagnetic structure (e.g., wakefields driven by non-evolving drivers), and obtained the general conditions for trapping to occur. We then compare this condition with high fidelity PIC simulations through advanced particle and field tracking diagnostics. Numerous numerical convergence test were performed to ensure the correctness of the simulations. The agreement between theory and simulations clarifies the role played by driver evolution on injection, and a physical picture of injection first proposed in Ref.[1] is confirmed through simulations. Several ideas, including ionization assisted injection, for achieving high quality controlled injection were also explored and some simulation results relevant to current and future experiments will be presented. [1] W. Lu et al., PRSTAB 10, 061301, 2007   

Characterization of plasma wake excitation and particle trapping in the nonlinear bubble regime

We investigate the excitation of nonlinear wake (bubble) formation by an ultra-short ($k_pL \sim 2$), intense ($e A_{\rm laser}/mc^2 > 2$) laser pulse interacting with an underdense plasma. A detailed analysis of particle orbits in the wakefield is performed by using reduced analytical models and numerical simulations performed with the 2D cylindrical, envelope, ponderomotive, hybrid PIC/fluid code {\small INF\&RNO}, recently developed at LBNL. In particular we study the requirements for injection and/or trapping of background plasma electrons in the nonlinear wake. Characterization of the phase-space properties of the injected particle bunch will also be discussed.   

Effect of phase and frequency variation on laser driven wakefield acceleration

Combining several laser beams can be one of the solutions to achieve ultra high intensities in future state of the art laser facilities (e.g. ELI and HiPER). However, slight mismatches in the laser parameters (e.g. frequency, phase, pulse width, etc.) of these beams can result into a laser with enhanced bandwidth and pulse duration, and asymmetric transverse and longitudinal profiles. The study of laser wakefield acceleration (LWFA) driven by such lasers becomes crucial for future applications. In this work we consider the effect of phase, frequency and pulse duration mismatch on LWFAs. We find that the injection longitudinal position, and self-injected charge can be tuned by the longitudinal chirp of the laser pulse. Moreover, the transverse injection position may be controlled by a suitable chirp of the laser wave number on the transverse direction, and may lead to off-axis injection. Our results are supported by PIC simulations using the Osiris 2.0 framework, and with analytical estimates.   

Nonlinear phase velocity of relativistic plasma waves driven by intense lasers

The nonlinear phase velocity of a plasma wave driven by a relativistically intense short-pulse laser propagating in a cold underdense plasma is investigated. The nonlinear laser intensity transport velocity, the plasma wave phase velocity, and the evolution of the plasma wave amplitude are calculated and compared to solutions of Maxwell equations coupled to the plasma fluid. The plasma wave phase velocity is shown to be approximately the laser intensity velocity in the linear regime, and is significantly reduced in the nonlinear regime owing to laser evolution. Laser evolution (frequency red-shifting and pulse steepening) is shown to further decrease the nonlinear phase velocity as the laser propagates. In a laser-plasma accelerator, the plasma wave phase velocity determines the dephasing length of the plasma accelerating structure, and therefore the energy gain of the accelerated particle beam.   

Ionization-induced injection in laser-plasma accelerators

Ionization injection into a laser plasma accelerator is studied analytically and by particle-in-cell (PIC) simulations. Details of the injection mechanism, and the dependence of electron injection number and beam quality on the gas and laser parameters are analyzed. Simulations show low energy spread beams can be generated using a short region of gas mixture (H and N) to trap electrons, followed by a region of pure H, that is injection-free, for acceleration. Effects of gas mix parameters, including species, concentration, and length of the mixture region, on the final electron injection number and beam quality are studied. Regimes where injection number linearly increases with the gas length or saturates are found. In the linearly increasing regime, the final beam energy spread is found to be proportional to the gas length. Laser polarization effects on injection number and final electron emittance are studied. Two-dimensional PIC simulations have been used to study the ionization injection process in the bubble regime. With proper injection parameters, tens of pC, mono-energetic electron beams with energy spread less than 1\% can be produced in a mixed gas.   

Betatron X-ray spectra from a laser wakefield accelerator using ionization injection

Electron beams typically produced by the HERCULES laser wakefield accelerator can be characterized as being of relatively high charge ($\sim$100 pC) in femtosecond duration, quasi-monoenergetic bunches. Oscillations of these electrons in the electromagnetic fields of the plasma bubble cavity created by laser driven ponderomotive expulsion can lead to extremely bright sources of X-rays in the 5-100 keV energy range. Such configurations are proposed as future sources of radiation for a number of applications, as their compact size and potential low cost is highly attractive, and may enable such facilities to be more widely available to the scientific, medical and engineering communities. To help understand and explore this phenomena, experimental measurements of X-ray spectra from laser wakefield accelerated electrons that are ionization-injected will be presented in comparison to X-ray spectra resulting from self-injection across various plasma densities and laser powers.   

A high-repetition rate LWFA for studies of laser propagation and electron generation

Advances in ultrafast optics today have enabled laser systems to deliver ever shorter and more intense pulses. When focused, such laser pulses can easily exceed relativistic intensities where the wakefield created by the strong laser electric field can be used to accelerate electrons. Laser wakefield acceleration of electrons holds promise for future compact electron accelerators or drivers of other radiation sources in many scientific, medical and engineering applications. We present experimental studies of laser wakefield acceleration using the $\lambda$-cubed laser at the University of Michigan -- a table-top high-power laser system operating at 500~Hz repetition rate. The high repetition rate allows statistical studies of laser propagation and electron acceleration which are not accessible with typical sub-$0.1$~Hz repetition rate systems. In addition, we compare the experiments with particle-in-cell simulations using the code OSIRIS.   

Plasma parameters of a capillary plasma channel for laser guiding

Optical guiding of an ultrashort and intense laser pulse with long interaction length is important in many applications such as laser-driven plasma accelerators. To overcome this limitation, a waveguide which has a refractive index profile like an optical fiber is required [1]. We demonstrated the production of an optical waveguide in a capillary discharge-produced plasma using a cylindrical capillary. Plasma parameters of its waveguide were characterized by use of both a Normarski laser interferometer and a hydrogen plasma line spectrum. A space-averaged maximum temperature of 3.3 eV with electron densities of the order of $10^{17}$ cm$^{-3}$ was observed at a discharge time of 150 ns and a maximum discharge current of 200 A. An ultrashort, intense laser pulse was guided by use of this plasma channel. \\[4pt] [1] T. Higashiguchi $et$ $al$., Rev. Sci. Instrum. 81, 046109 (2010).   

Simulations of Slow Capillary Discharges for BELLA

Capillary plasma channels are used to extend the propagation distance of relativistically intense laser pulses for laser plasma acceleration [1], and axial density modulation has been used to stabilize injection at LBNL. Channel formation is a complex process in which a gas is ionized via a slow discharge, and subsequently stabilized by a capillary wall via heat transfer. Here we describe simulations using a multi-species, 2-temperature plasma model to study the effects of electrical and thermal conduction, species diffusion, and externally-applied magnetic fields on this process for present experiments and to plan m-scale capillaries at reduced densities for the BELLA laser. These radially-symmetric simulations, performed with the 1D cylindrical code SCYLLA from LBNL, resolve the radial behavior of the plasma within the capillary but do not accurately describe dynamics near the ends of the capillary or near gas feed slots or jets. To understand these regions, we present results of simulations using the 3-dimensional hydrodynamics code HYDRA from Lawrence Livermore National Laboratory. We discuss work in progress on a multi-dimensional plasma model that leverages results from these simulations. References: [1] W. Leemans et al., Nat. Phys. 2, 696 (2006)   

Plasma Dynamics of Capillary Discharges for the BELLA project

Capillary discharges to form a meter-scale plasma waveguide are important for 10 GeV scale laser plasma accelerator experiments on the BELLA laser in progress at Lawrence Berkeley National Laboratory. We present simulation results of capillary plasma properties, including radial density and temperature profiles, using the Nautilus code. An effect known to play a dominant role is the transfer of heat from the plasma to the capillary wall. We present benchmark results for heat transfer modeling with Nautilus in the regime of interest to capillary discharges. We also discuss the relative importance of diffusion, Ohm's law, and applied solenoidal fields on the radial profiles needed for experiments. For instance, some previous models estimate applied solenoidal fields could increase on-axis temperatures by roughly a factor of two, and we compare with these estimates. Finally, we compare radial profile results with other simulation results and with recent measurements made at LBNL.   

Positron driven plasma wakefields

The LHC is producing high-energy, high-charge proton bunches (1e11 protons at 1-7 TeV each) that could be used to accelerate ``witness'' electron bunches to TeV range eneregies via a plasma wakefield accelerator (PWFA). Simulations [1] suggest that a proton ``drive'' bunch is able to excite large wakefields if the bunch size is on the order of 100 $\mu$m; however, the LHC paramters are currently on the 1 cm scale. SLAC'S FACET is able to supply positorn bunchs with the ideal parameters for driving a PWFA. Although at lower energy (2e10 positrons at 23 GeV each), initial simiulations in QuickPIC show that the physics of a positron drive bunch is very similar to that of a proton drive bunch. Differences in the physics arise from the mass difference: slower dephasing but faster transverse bunch evolution. Other considerations include driver head erosion and purity of the wakefield ion column. The physics of positive drivers for PWFA and the viability of this scheme for future high-energy colliders will be investigated at SLAC's FACET.\\[4pt] [1] Caldwell, et al. Nature Physics 5, 363 (2009).\\[0pt] [2] C.H. Huang, et al., J. Comp. Phys., 217(2), 658, (2006).   

Resonant Excitation of Plasma Wakefields

The resonant excitation of plasma wakefields by a train of equidistant electron bunches can lead to large energy loss and energy gain. The bunch train can in principle also be tailored to produce a large transformer ratio R ($>>$2). The energy gain by a witness bunch can in principle reach R times the drive bunch energy and high energy bunches can be produced with low energy drivers. The physics of these situations is tested with low energy beams at the Brookhaven National Laboratory Accelerator Test Facility. The plasma density is varied in a capillary discharge and the resonance is observed when the relativistic wave plasma period is equal to the drive bunch spacing. Differences in energy loss and gain between the bunch train and the initial long bunch are also observed. Detailed experimental results will be presented.   

Transferring the Energy of Hadron Beams to Lepton Beams via Plasma Wakes

Hadron beams ($p^-$ \& $p^+$) exist at Fermilab and CERN could be used to drive high gradient plasma wakefields for accelerating trailing lepton ($e^-$ \& $e^+$) beams. We consider what would be possible if the existing hadron beams could be compressed and if existing beams excite wakes via self-modulation instabilities. A compressed $p^-$ beam drives an identical wake as an electron beam [1] with the same current. However, for this case dephasing (not pump depletion) limits the acceleration length. Simulation results show that a witness electron bunch can gain more than 600 GeV in a 1 TeV p$^-$ beam driven PWFA during 50 meters acceleration. For the $p^+$ beam, driving a similar wake by using a short $p^+$ beam for accelerating electrons has been proposed recently [2]. Although $p^+$ beam available at CERN is much longer, a train of short bunches may be generated through self-modulation as the long bunch propagates in the plasma [3]. Preliminary simulation results for such interactions will be presented. [1] I. Blumenfeld et al., Nature 445, 741 (2007) [2] A. Caldwell et al., Nature Phys. 5, 363 (2009) [3] N. Kumar et al., Phys. Rev. Lett. 104, 255003 (2010)   

Simulations of two-bunch Plasma Wake Field Accelerator on FACET

Previous experiments on FFTB at SLAC demonstrated that short electron bunches can produce accelerating gradient of 50 GeV/m over one meter[1]. These experiments provided the science case for the new FACET facility which will have 23 GeV high current beams. Two-bunch PWFA experiment has a second bunch appropriately loaded into the wake of the first bunch so that the second bunch maintains a narrow energy spread. Simulation results show that in possible two bunch scenarios the first bunch (with less current than that in the FFTB case) still can generate a meter long plasma column via field ionization with a density around $5\times10^{16}cm^{-3}$ if a gas with lower ionization threshold is used. The trailing beam can gain $\sim$10 GeV with a very narrow energy spread. The energy gain can be increased to 25 GeV by using a pre-ionized plasma . The possibility of using a partially ionized pre-plasma instead of the fully pre-ionized plasma is also discussed.\\[4pt] [1] I. Blumenfeld et al., Nature 445, 741 (2007).   

Comparison of Plasma Wake Fields between WAKE and QUICKPIC

Simulation of Plasma Wake Field Acceleration (PWFA) over long distances requires efficient algorithms for computation of the wake fields. The codes WAKE (2D) and QUICKPIC (3D) achieve efficiency by making the quasi-static approximation. The QUICKPIC simulations of PWFA have been carefully benchmarked against the full PIC code OSIRIS, with speed ups of 100-1000 over OSIRIS. However, for axisymmetric beams a full 3D code is not necessary and the 2d axisymmetric code WAKE, which has recently been modified to simulate self-consistent evolution of the beam driver particles, can be used. We compare the results from WAKE and QUICKPIC. The two codes currently use different choices for the set of quasi-static equations to be solved. Differences in the accuracy and efficiency arising from these choices are also explored. The synergistic use of 2D axisymmetric (WAKE) and 3D (QUICKPIC) codes will provide an important tool to simulate upcoming PWFA experiments.   

An Overview of Recent DIII-D ELM Control Experiments

Recent ELM control experiments in DIII-D have focused on expanding our understanding of urgent ITER physics issues. Results from these experiments are contributing to an integrated ITER ELM control program involving both ELM suppression and mitigation using resonant magnetic perturbation (RMP) fields and pellet pacing. For example, RMPs applied before the H-mode transition with $q_{95}=3.5$, the ELM suppression operating point, increase in the L-H power threshold by 40\% while no significant effects are seen when operating above the ELM suppression resonant window at $q_{95}=4.1$. Peak energy transients on the divertors, due to RMP mitigated ELMs, are reduced by a factor of 8 compared to typical type-I ELMs when operating outside the resonant ELM suppression window. Deuterium fueling pellets injected from the high-field side of the discharge sometimes trigger ELMs while those injected from the low-field side do not. Suppression of the first ELM after the H-mode transition is obtained by controlling the density rise during the initial H-mode ELM free period. An overview of our recent results is presented.   

Compatibility of RMP ELM Suppression with Radiating Divertor in DIII-D

The integration of edge localized mode (ELM) $\it{suppression}$ using resonant magnetic perturbations (RMPs) with radiating divertor operation is explored. Moreover, during ELM $\it{mitigation}$ experiments, we find that radiating divertors with the RMP coils activated produce both higher levels of radiated power from the divertor and SOL/edge plasma regions ($\sim$30\% higher) and significant reductions in peak heat flux from ELMs at the divertor targets ($\sim$30-40\% lower) than comparable non-RMP \hbox{H-mode} discharges at the same density. These results build on the theoretical and experimental progress made previously in identifying the underlying physics involved in two distinct areas, i.e., puff-and-pump radiating divertor [1] and ELM suppression using RMPs [2]. \vskip6pt \noindent [1] T.W. Petrie, et al., Nucl. Fusion $\bf{49}$ (2009) 065013. \par\noindent [2] T.E. Evans, et al., Nucl. Fusion $\bf{48}$ (2008) 024002.   

Plasma Response to Complex External Magnetic Perturbations

The dependence of the plasma response to external magnetic perturbations consisting of an unknown intrinsic external error field and a known and controlled applied external field is studied theoretically by constructing various response models. For a rotating dissipative plasma, the plasma behaves nearly ideally in the linear regime. The response is relatively weak for a low beta plasma. In the quasi-linear regime, the response connects to the development of magnetic islands within the plasma. The relationship of the modeled response to that observed in DIII-D is studied. Possibility of application of this model in determination of intrinsic error field in ITER is explored.   

Non-axisymmetric Response Calculations Using a Non-ideal Two-fluid Model

The linear plasma response to externally applied non-axisymmetric fields is calculated for several DIII-D discharges. The response is computed using the initial-value, finite-element code \hbox{M3D-C1}. Calculations include viscosity, Spitzer resistivity, two-fluid effects, and rotation. Both the closed- and open-field line regions are included in the calculations, with the open-field line region treated as low-density, low-temperature plasma rather than a vacuum. The response is calculated both in the presence of a conducting wall and a resistive wall, with the resistive-wall response calculated by coupling \hbox{M3D-C1} to the VACUUM code. Results of these calculations are compared with ideal-MHD calculations for the same discharges, as well as with experimental measurements.   

Modeling Experimental Changes in Particle Transport from Resonant Magnetic Perturbations (RMPs) Using SOLPS5

We use a modulated gas puff to investigate the changes in particle transport as a result of Resonant Magnetic Perturbations in L- and H-mode discharges on DIII-D. We observed density pump-out for the first time in diverted L-mode discharges on DIII-D. In these L-mode discharges, the gas puff penetration is reduced during the RMP pulse whereas in H-mode the gas puff penetrates into the core in RMP ELM suppressed plasmas. The experimental values for D and V are input to the SOLPS5 code and the resulting density profile is compared to experiments, to validate the experimentally observed changes in particle transport. Finally, we add the neutral source terms to improve the experimental transport analysis.   

Effects of 3D Magnetic Perturbations on DIII-D Reconstructed Equilibria

Non-axisymmetric perturbation magnetic fields are routinely applied in \hbox{DIII-D} experiments to study and control MHD modes. The effects of the perturbation fields on the background 2D equilibrium are studied using a set of recent \hbox{DIII-D} single- and double-null \hbox{H-mode} discharges with varying amount of \hbox{I-coil} perturbations. The reconstructed vertical positions of the plasma separatrix boundary at the \hbox{DIII-D} Thomson diagnostic viewing chord are compared against those estimated from the measured edge electron temperature profiles. For lower single-null and double-null plasmas, the reconstructed vertical boundary locations generally agree with those estimated from the edge temperature profiles within $\sim$1 cm. For upper single-null plasmas, the differences in the Thomson boundary locations tend to be greater. The effects of the perturbation fields on the 2D reconstructed equilibria are also being analyzed using a set of 3D codes TRIP3D without and \hbox{MARS-F} and VMOM3D with plasma response. Details will be presented.   

IMFIT Integrated Modeling Applications Supporting Experimental Analysis: Multiple Time-Slice Kinetic EFIT Reconstructions, MHD Stability Limits, and Energy and Momentum Flux Analyses

This presentation summarizes several useful applications provided by the IMFIT integrated modeling framework to support \hbox{DIII-D} and EAST research. IMFIT is based on Python and utilizes modular task-flow architecture with a central manager and extensive GUI support to coordinate tasks among component modules. The kinetic-EFIT application allows multiple time-slice reconstructions by fetching pressure profile data directly from MDS+ or from ONETWO or PTRANSP. The stability application analyzes a given reference equilibrium for stability limits by performing parameter perturbation studies with MHD codes such as DCON, GATO, ELITE, or PEST3. The transport task includes construction of experimental energy and momentum fluxes from profile analysis and comparison against theoretical models such as MMM95, GLF23, or TGLF.   

Effect of Rotation and Rotation Shear on Stability of RWM in DIII-D Tokamak Using MARS

It was established that RWM can be stabilized by fast plasma rotation (at a few percent of the Alfv\'en frequency). The experiment in \hbox{DIII-D} and \hbox{JT-60U} observed mode stabilization at plasma rotation frequency lower than that predicted by the fluid theory. To understand the mechanism in this work, the plasma in \hbox{DIII-D} with a low rotation and surrounded by an external resistive wall is revisited. Based on fluid [1] and kinetic [2] MHD models, the effect of plasma rotation and shear flow on RWM in \hbox{DIII-D} is studied numerically with the MARS code. Using different wall parameters and plasma rotation profiles, the numerical results of the growth rates with different wall locations are obtained. From analysis of the numerical results, the linear stability of RWM with these lower plasma rotations in DIII-D is studied and compared with previous results. \vskip6pt \noindent [1] M.S. Chu, et al., Phys. Plasmas $\bf 2$, 2236 (1995). \par\noindent [2] Y.Q. Liu, et al., Phys. Plasmas $\bf 15$, 112503 (2008).   

Comparison of a Plasma Transport Analysis and Simulations in DIII-D and HL-2A ECH/ECCD H-mode Discharges,

ECH and ECCD are one of the important heating and current drive technique in ITER and present tokamaks. In particular, energy and particle transport in ECH/ECCD tokamak plasma are two main research topics. In this presentation, experimental results for HL-2A and \hbox{DIII-D} ECH experiments are compared. ONETWO transport power balance analysis shows that ion energy fluxes before and after ECH turn-on are similar. However, electron energy fluxes are significantly different. After ECH turn-on, electron energy flux increases significantly in the plasma volume outside of the ECH heat location. Preliminary analysis using GLF23 and TGLF transport models indicates that the energy fluxes before and after ECH turn-on is similar. ONETWO, GLF23 and TGLF are also used to analyze and simulate \hbox{H-mode} experiments on \hbox{HL-2A}. The electron energy flux, ion energy flux, electron temperature, ion temperature and electron density after \hbox{L-H} transition are analyzed.   

ELM Pacing Using Modulated Magnetic Field Perturbations

Experiments have been conducted on \hbox{DIII-D} to investigate the viability of using modulated magnetic field perturbations as a tool for pacing ELMs. It is found that the ELMs are entrained with twice the modulation frequency. When applied to plasmas operating near the \hbox{L-H} power threshold with naturally low ELM frequency, the modulated fields result in a clear redistribution of the divertor heat flux loads, with large infrequent ELMs replaced with more rapid, smaller sized ELMs. However, more detailed analysis has revealed that this change in ELM character appears to be a direct result of the density reduction (so-called ``density pumpout\rq\rq) associated with the fields. More specifically, the reduced density tends to reduce the \hbox{L-H} power threshold, so at fixed input power, we move further away from the power threshold, which is known to reduce ELM size.   

Improving Diamagnetic Flux Temporal Resolution to Measure ELM Energy Loss

When an ELM occurs in a tokamak, a substantial loss of stored thermal energy can occur in a very short time, resulting in a change in the plasma diamagnetism. A diamagnetic loop is therefore an attractive diagnostic for characterizing the change in energy during ELMs. A loop external to the vessel can be used but it is bandwidth-limited by the vessel wall, therefore the signal is severely attenuated above 40 Hz in \hbox{DIII-D}. The temporal resolution can be improved by combining the (slow) diamagnetic signal with a properly scaled internal (fast) toroidal $B_{\rm T}$ signal. The results agree with finely-spaced EFIT equilibrium reconstructions to within 10\% before each ELM, but the diamagnetic calculation often shows up to twice the drop in energy at the ELM. The $B_{\rm T}$ signal reveals the magnetic change completes in 0.5 ms or less with occasional dynamics above 10 kHz. This improved temporal resolution allows comparison of phenomenology in natural vs. pellet-triggered ELMs, and also effects of partial ELM suppression under resonant magnetic perturbation.   

Developing and Testing the EPED Pedestal Model

The EPED model has been developed to predict the pressure at the top of the tokamak edge transport barrier (or ``pedestal height\rq\rq), which strongly impacts fusion performance. EPED is based on two fundamental and calculable constraints, criticality to (A) peeling-ballooning and (B) kinetic ballooning modes. The constraints are calculated using MHD and gyrokinetic stability codes, leading to a model that is simple and yet first principles in the sense that nothing is fit to observations. The model is tested against observation on several devices, including dedicated experiments on \hbox{DIII-D} and \hbox{C-Mod}, and prediction and optimization of ITER are discussed.   

Turbulence Studies at the Top of the Pedestal

The confinement of tokamak plasmas dramatically improves during a high performance mode of operation (\hbox{H-mode}). An \hbox{H-mode} plasma typically has steep temperature and density gradients (a barrier) near the edge of the plasma, but the question remains as to what limits the gradients in the \hbox{H-mode} barrier. Two possibilities are electron temperature gradient (ETG) and kinetic ballooning mode (KBM) turbulence. A key parameter that drives the ETG mode is $\eta_{\rm e}$, the ratio of the electron density scale length to the electron temperature scale length. It is shown that qualitative changes in $\eta_{\rm e}$ at the top of the pedestal correlate in time with the occurrence of ELM-free phases in some high-$\beta$ \hbox{DIII-D} discharges. The effect this change in $\eta_{\rm e}$ has on turbulent fluxes is presented.   

Evolution of Edge Pedestal Transport Between ELMs

Measured profiles of density, temperatures, rotation velocities and radial electric field in several time bins between edge localized modes (ELMs) in a H-mode DIII-D discharge have been interpreted within the context of the particle, momentum and energy balance constraints to elucidate the evolution of transport within the edge pedestal between ELMs. The evolution of the toroidal angular momentum transfer rate, the particle diffusion coefficient and pinch velocity, and the electron and ion thermal diffusivities were inferred from the measured data. The measured data are generally consistent with the particle, momentum and energy balance constraints, within the limitations imposed by the resolution of the CER data.   

Paleoclassical Model of Pedestal Structure

Predictions are developed for the structure of plasma parameter profiles of \hbox{H-mode} pedestals in transport quasi-equilibrium in tokamak plasmas. They are based on assuming paleoclassical radial plasma transport processes dominate throughout the pedestal. The natural level of paleoclassical density transport is large in the pedestal compared to edge fueling due to neutral recycling. Thus, in this model the pedestal density profile is determined not by edge source fueling but rather by the density profile needed for the outward paleoclassical diffusive flux to be nearly balanced by the inward paleoclassical pinch flow. Specific predictions are given for the electron temperature and density gradients, profiles and magnitudes in the pedestal. The transition into ETG-driven anomalous radial electron heat transport in the core plasma determines the height of the electron pressure pedestal. Also, the profile of the toroidal plasma rotation in the pedestal is predicted. Model predictions are found to agree quantitatively (within a factor of 2) with the interpretive transport results obtained for the 98889 \hbox{DIII-D} pedestal [1]. \vskip6pt\noindent [1] J.D. Callen et al., Nucl. Fusion \textbf{50}, 064004 (2010).   

Automated EPED Pedestal Stability Computations Within the IMFIT Framework

The determination of pedestal stability boundaries using a combination of the MHD, transport and stability codes, EFIT, ONETWO/GCNMP, ELITE, and TOQ is a tedious and complex task that is currently performed manually. The IMFIT framework was designed to automate such tasks as much as possible. Here we present the current work on an algorithm for both the IMFIT interface and the modified, mixed (Robin) boundary conditions often required for the transport equations in order to successfully carry out EPED type stability calculations in an autonomous manner. Code interface issues are effectively treated through the development of an IMFIT master state file that allows communication between the necessary physics components. Several applications to \hbox{DIII-D} discharges are presented.   

Closed-Loop Simulation of Model-Based Current Profile Control with the \hbox{DIII-D} Plasma Control System

Current profile control has proven to be a critical requirement for advanced operating scenarios with improved confinement and possible steady-state operation. Limitations exhibited by non-model-based controllers tested at \hbox{DIII-D} motivated the design of model-based controllers that account for the dynamics of the $q$ profile evolution. A control-oriented model of the current profile evolution in \hbox{DIII-D} was recently developed and used to design both open-loop and closed-loop control schemes. In this work, we report on the design and implementation of these advanced model-based controllers in the \hbox{DIII-D} Plasma Control System (PCS) and on the evaluation of these controllers by connecting the PCS to a simulation of the current profile evolution represented by a magnetic diffusion equation.   

2D Soft X-ray System for Imaging Magnetic Topology in the Pedestal Region on DIII-D

A new tangential 2D Soft \hbox{X-ray} Imaging System (SXRIS) is designed to examine the edge magnetic island structure in the lower \hbox{X-point} region of \hbox{DIII-D}. Plasma shielding and/or amplification of applied resonant magnetic perturbations (RMPs) may play a role in the suppression of edge localized modes. The SXRIS will aid in determining the 3D magnetic structure due to applied RMPs. A synthetic diagnostic calculation based on 3D SXRIS emissivity estimates calibrated against NSTX data, shows a signal-to-noise ratio of 10 with \hbox{1 cm} resolution for a 25 ms integration time. Impurity puffing is expected to increase the SNR further. Image inversion is required but is an ill-posed problem, requiring symmetry assumptions such as constant emission along field lines. Advanced inversion methods are examined in the context of noise, spatial sensitivity, and symmetry assumptions. Forward modeling is used to compare 3D equilibria, e.g. from SIESTA, and simulated images to examine the non-ideal plasma response.   

Characterization of Neoclassical Transport in the Plasma Edge

An extensive characterization of neoclassical transport in the plasma edge is studied numerically using the NEO code, which solves a hierarchy of drift-kinetic equations based on an expansion in $\rho_{\ast\rm i}$. The code includes the self-consistent coupling of electrons and multiple ion species, fully general geometry, including up-down asymmetry effects, and full rapid toroidal rotation effects. The validity of standard local neoclassical transport theory in the \hbox{H-mode} edge is assessed for typical \hbox{DIII-D} plasmas via solution of the higher-order drift-kinetic equations, and the influence of finite-orbit-width effects on the non-local neoclassical energy transport and flows is explored. Preliminary results indicate that only a weak finite-orbit-width effect is found due to the steep gradients and thus the $\delta f$ formulation is valid for most of the pedestal. Extended studies of the influence of impurities and orbit-squeezing effects on the edge transport are presented. Limitations of analytic theories, such as the Chang-Hinton theory and the Sauter model, are also studied for realistic experimental parameters.   

How Accurate is Analytic Theory of Neoclassical Ion Transport?

The idea underlying analytic theory of neoclassical transport is pervasive in plasma physics. The pioneering work of Rosenbluth [1] employs a variational principle to justify the dominance of pitch angle scattering in the limit of small inverse aspect ratio $\delta$ and $\nu_\ast \ll 1$. We derive [2] the same result using the method of matched asymptotic expansions. Because of the complexity of the Fokker-Planck operator, the accuracy of the analytic approach is not easily ascertained. We have written a code to solve the linearized drift kinetic equation with the exact Fokker-Planck operator, using expansion of the distribution function in Legendre/Laguerre polynomials in velocity space and Fourier decomposition in poloidal angle. Using the code, we find that while the analytic method is justified, its numerical accuracy requires extreme values of the parameters $\delta$ and $\nu_\ast$. A rather large number of terms in the expansion are also found necessary in these parameter ranges. \vskip6pt \noindent [1] M. Rosenbluth, et al., Phys. Fluids \textbf{15}, 116 (1972). \par\noindent [2] S.K. Wong \& V.S. Chan, Phys. Rev. E \textbf{67}, 066406 (2003).   

Neoclassical Rotation Theory for Toroidal and Poloidal Rotation Velocities Using Miller Equilibrium Analytical Flux Surface Geometry

Rotation of tokamak plasmas is not only of intrinsic interest for understanding transport but also is important for the stabilization of tokamak plasmas and other reasons. The neoclassical viscosity depends on the poloidal dependence of various quantities, which in turn depend on the poloidal dependence of the magnetic geometry, among other things. The objective of this research is to derive the neoclassical toroidal and poloidal rotation theory for tokamaks using the more accurate representation of the equilibrium flux surface geometry given by ``Miller equilibrium flux surface model\rq\rq. The Miller model improves earlier flux surface models by taking into account the shifted centers $R_0(r)$ of the flux surfaces (Shafranov shift), the elongation $\kappa$, and triangularity $\delta$, thus more accurately describing the actual flux surfaces in tokamak plasmas.   

Impurity Poloidal Rotation in DIII-D Under Low Toroidal Field Conditions

Predictive understanding of plasma transport is a long-term goal of fusion research. This requires testing models of plasma rotation including poloidal rotation. The present experiment was motivated by recent poloidal rotation measurements on NSTX which show that the poloidal rotation of C$^{+6}$ is much closer to the neoclassical value than results in larger aspect ratio machines such as TFTR, DIII-D and JET working at higher toroidal field $B_{\rm T}$. We investigated whether the difference in aspect ratio (1.44 on NSTX vs 2.7 on DIII-D) could explain this. We performed a poloidal rotation experiment in DIII-D under conditions which matched, as best possible, those in the NSTX experiment; we matched plasma current (0.65 MA), on-axis $B_{\rm T}$ (0.55 T), minor radius (0.6 m), and outer flux surface shape as well as the density and temperature profiles. DIII-D results from this work show reasonable agreement with neoclassical theory. Accordingly, the different aspect ratio does not explain the previously mentioned difference in poloidal rotation results.   

Tracking of Current and Rotation Profile Evolution in the \hbox{DIII-D} Tokamak via System Identification

Transport theories produce nonlinear models based on partial differential equations (PDEs) whose complexity often renders them difficult to use for control design. As an alternative, data-driven modeling techniques involving system identification have the potential to obtain practical, low-complexity, dynamic models for the control of plasma systems. The plasma dynamics is first assumed to be governed by a tractable model with unknown and to-be-estimated transport coefficients. After discretizing both in space and time, the system states and to-be-estimated coefficients are combined into an augmented state vector. The resulting nonlinear state-space model is used for the design of an extended Kalman filter that provides real-time estimates not only of the system states but also of the unknown transport coefficients required by the current and rotation feedback controllers.   

Divertor Heat Flux Control Scaled to High Power

The control of divertor target heat flux is examined as a function of input power during \hbox{H-mode} in \hbox{DIII-D}. The profile of heat flux parallel to the magnetic field as a function of distance from the target plate is inferred from power balance measurements of surface heat flux and radiated power. The 2D profile of $n_{\rm e}$ and $T_{\rm e}$ from divertor Thomson scattering allow interpretation of the parallel transport processes of electron conduction and plasma convective flow. The parallel plasma transport is found to transition from conduction to convection with Mach=1 flow starting at 15 eV. Convective transport increases the volume of divertor plasma with conditions for high radiative dissipation. Increasing input power leads to higher density for transport of the higher heat flux at the same temperature and convective velocity. Implications for divertor heat flux control at reactor level power densities is examined.   

Comparison of Measured Heat Flux Profiles with UEDGE Simulations of \hbox{H-mode} Discharges in DIII-D

On \hbox{DIII-D} we have performed a series of experiments designed to compare the upstream Thomson and midplane probe measurements of $T_{\rm e}$ with the downstream divertor heat flux width. Experimentally, we observe little correlation of heat flux width with the upstream temperature gradient scale length, contrary to simple two-point models. In order to determine what processes control the heat flux width, we model selected discharges with UEDGE. Preliminary analysis shows that the drifts are important in this regard and to a lesser extent impurity recycling. Without the drifts the electron temperature at the target plate is high and the density low, leading to a small-radiated power fraction. The drifts increase the plasma density at the plate and lower the temperature and bring the radiated power fraction into closer alignment with the experimentally measured power.   

Application of OEDGE to Transport Coefficient Extraction in \hbox{DIII-D} Joule Milestone Discharges

The OEDGE modeling code is used to extract estimates of radial transport coefficients from a series of \hbox{DIII-D} experiments designed to assess the divertor heat flux dependence on operational parameters. OEDGE is being used to analyse five ELMy \hbox{H-mode} discharges in which the plasma current was varied from \hbox{0.5 MA} to 1.48 MA while other parameters were held constant. Estimates of the effective experimental $\chi_\perp$ and $\rm D_\perp$ in the outer SOL for these discharges are determined. This process requires using experimental diagnostic data and onion-skin models (OSM) to reconstruct a plasma solution. Langmuir probe measurements of $n_{\rm e}$ and $T_{\rm e}$ and infrared measurements of target heat flux were used to determine input profiles of $n_{\rm e}$, $T_{\rm e}$ and $T_{\rm i}$ to be used in the plasma reconstruction. At the 5 mm outer midplane surface, typical extracted $\rm D_\perp$ values are 0.05 m$^2$/s while $\chi_\perp$ is 0.15 m$^2$/s. All discharges showed an increase of extracted $\chi_\perp$ with radius.   

Strong Suppression of Net Erosion of Graphite at Divertor Targets Due to Prompt Local Deposition Caused by the Strong Electric Field of the Magnetic Pre-sheath

A new analysis is presented of the effective thickness of the magnetic pre-sheath, $L_{MPS}$, for prompt deposition due to the strong \hbox{E-field} in the MPS. The gross erosion rate at divertor strike points in future devices such as ITER, FDF, etc. will be extremely high. It is long recognized, however, that net erosion can be much smaller, at least for high-Z PFCs such as W, due to prompt deposition which occurs when the ionization distance of the sputtered neutral, $\lambda_{iz}$, is much less than its ion larmor radius, $\rho_Z$. This latter process tends not to be effective for low-Z PFCs like C; however, prompt local deposition can still occur if $\lambda_{iz} < L_{MPS}$, which is of order $\rho_{DT}$. It is shown that $L_{MPS}$ is a actually a function of the sputtered neutral energy, $E_0$, e.g. $L_{MPS} \sim 6 \rho_{DT}$ for $E_0=0.4 kT_e$. It is shown that for plasma conditions typical of divertor strike locations in ITER and FDF that the net erosion of graphite can be expected to be much less than gross erosion.   

Thermo-Oxidation Experiments in the \hbox{DIII-D} Tokamak

To evaluate the effectiveness of removing carbon co-deposits and trapped deuterium from tokamak surfaces by thermo-oxidation and to demonstrate the recovery of high performance plasmas, two independent thermo-oxidations of the \hbox{DIII-D} tokamak were performed. Graphite tiles containing $^{13}$C rich co-deposits and various witness samples were installed on passively heated, flange-mounted platforms for the first exposure. Plasma operations were recovered and followed by a series of $^{13}$C-seeded plasma shots. A second thermo-oxidation was performed, followed by a removal of select wall and divertor tiles. Results show a decrease in co-deposited $^{13}$C and D content on precharacterized tiles, and a temperature dependent uptake of D on stainless steel witness samples. Preliminary results, as well as operational and experimental details of the thermo-oxidation exposures are presented.   

In situ FTIS Measurement of Thermo-oxidation in DIII-D

Carbon plasma-facing surfaces in the ITER divertor will chemically erode and produce co-deposits that contain trapped tritium. Direct monitoring of these gases is important as retained tritium will limit fusion reactor operation. These erosion products have distinct optical absorption bands that can be measured by Fourier Transform Infrared Spectroscopy (FTIS). Methods of in-situ removal will also require new diagnostics to monitor deuterium and tritium release. At the end of the 2009/2010 experimental campaign, a thermo-oxidation experiment was performed in the DIII-D tokamak. Thermo-oxidation removes both carbon and deuterium from co-deposits and produces CO, CO$_{2}$, and D$_{2}$O. Real-time molecular absorption measurements were made on the~tokamak utilizing a novel technique employing FTIS and existing optical pathways. Initial results of the measurements and future applications of the diagnostic will be discussed.   

Main Ion Charge Exchange Spectroscopy for Ion Temperature and Rotation at DIII-D

Charge exchange (CX) spectroscopy using impurities is the standard diagnostic technique for ion temperature, impurity density and velocity. However, neoclassical theory predicts the rotation of the main ions (deuterons) and impurities are not necessarily the same. Accurate measurement of the bulk ion rotation is required for comparison with theory to assess the rotation stabilization of MHD modes, and to develop predictions for future devices where differential impurity and main-ion rotation can be significant. A prototype main-ion diagnostic is operational on \hbox{DIII-D} that measures the $D_\alpha$ emission from direct CX with the beams and halo emission, and has two unique sightlines viewing co- and counter-$I_p$ neutral beams. The atomic processes that contribute to thermal $D_\alpha$ emission are simulated with a Monte-Carlo based beam injection and halo diffusion code. Comparison between main-ion and carbon temperature and velocity measurements will be presented.   

Preliminary Measurements with the Ultra-Fast Charge Exchange Recombination Spectrometer (UF-CHERS) on DIII-D

A dual-channel, high throughput (1 mm$^2$-ster), high efficiency, customized spectrometer (UF-CHERS) has been deployed at \hbox{DIII-D} to measure ion temperature and parallel velocity fluctuations through detection of the beam-excited CVI charge exchange line near \hbox{529 nm} at \hbox{0.25 nm} resolution. Turbulence-relevant time resolution of \hbox{1 $\mu$s} is achieved with cooled avalanched photodiode detectors (APDs). Initial measurements obtained during the 2010 \hbox{DIII-D} experimental run campaign demonstrate detection of the charge exchange and edge emission at near photon-noise-limited performance, although signal levels are lower than estimated. The Doppler broadened spectrum, time evolution, radial variation, and cross correlation with BES-measured density fluctuations from \hbox{L-mode} discharges are presented to assess signal levels and noise characteristics. Hints of GAM-induced temperature fluctuations are observed. Upgrades to the optics and detectors for improved signal-to-noise are evaluated.   

Improved Spectral Fitting Models for the \hbox{B-Stark} Diagnostic at DIII-D

Recent results are presented from the B-Stark diagnostic installed on the \hbox{DIII-D} tokamak. This diagnostic provides measurements of the magnitude and direction of the internal magnetic field. The \hbox{B-Stark} system is a version of a motional Stark effect (MSE) diagnostic based on the relative line intensities and spacing of the Stark split $D_\alpha$ emission from injected neutral beams. Improvements to the spectral fitting model are presented, including the addition of an analytical model for $D_\alpha$ emission from the fast-ion distribution. We discuss the accuracy of using in-situ beam-into-gas calibrations to find the beam emission line profiles, the viewing direction and the transmission properties of the collection optics. We also present results of efforts to improve the determination of the beam emission line profiles. Finally, the magnetic field measured with the \hbox{B-Stark} system is compared to values found from plasma equilibrium reconstructions (EFIT) and the MSE polarimetry system on \hbox{DIII-D}.   

Time Dependent Solution for the He I Line Ratio Electron Temperature and Density Diagnostic in TEXTOR and DIII-D

We developed a time dependent solution for the \hbox{He I} line ratio diagnostic. Stationary solution is applied for L-mode at TEXTOR. The radial range is typically limited to a region near the separatrix due to metastable effects, and the atomic data used. We overcome this problem by applying a time dependent solution and thus avoid unphysical results. We use a new \hbox{R-Matrix} with Pseudostates and Convergence Cross-Coupling electron impact excitation and ionization atomic data set into the Collisional Radiative Model (CRM). We include contributions from higher Rydberg states into the CRM by means of the projection matrix. By applying this solution (to the region near the wall) and the stationary solution (near the separatrix), we triple the radial range of the current diagnostic. We explore the possibility of extending this approach to \hbox{H-mode} plasmas in \hbox{DIII-D} by estimating line emission profiles from electron temperature and density Thomson scattering data.   

An LIF Diagnostic for Measurement of the Neutral Deuterium Profile in the SOL and Pedestal of DIII-D

A two-photon absorption, laser-induced fluorescence (TALIF) diagnostic is being designed to measure the neutral deuterium profile in the plasma scrapeoff layer and pedestal, near the divertor \hbox{X-point}. Spatially resolved data are desired over the region $0.95-1.0$ in normalized $\rho$, to address how neutral ionization affects the pedestal density profile. Additionally, profile data are sought in the scrape-off layer (SOL) outside the separatrix to understand the in/out asymmetry observed in the onset of detachment. Several geometries of lasers and collection optics will be presented which permit simultaneous detection of the fluorescent signal for each firing of a pulsed, ultraviolet laser capable of 10 Hz repetition rate.   

High Resolution Pedestal Thomson Scattering System at \hbox{DIII-D}

Predicting the performance of the next generation of fusion devices requires detailed and precise understanding of the \hbox{H-mode} pedestal characteristics. The development and validation of theoretical models predicting the pedestal height and width is necessary to increase confidence on the extrapolation to burning plasmas. Validating these models requires accurate, high resolution measurements in the region. Accordingly, the \hbox{DIII-D} Thomson scattering system is being upgraded to include high-resolution measurements of the edge of the plasma. Preliminary investigation indicates that sub-centimeter radial resolution will be achievable with limited changes to the existing optics. Details of the upgrade will be presented, including a physical model of the mechanical assembly, an optical model of the system, light collection efficiency, and a physics model to estimate the expected errors in the temperature measurement.   

Upgraded Thomson Scattering System at \hbox{DIII-D}

The \hbox{DIII-D} Thomson scattering system is undergoing a significant upgrade. Four new \hbox{1 Joule}, \hbox{50 Hz} ND:YAG lasers are being installed at \hbox{DIII-D}. These lasers will significantly increase the measurement frequency when they are added to the current set of eight \hbox{0.5 Joule}, \hbox{20 Hz} lasers. The increased laser power will also improve measurement accuracy. Installation of the new lasers required an expansion of the Thomson laser room, a replacement of the Thomson laser control system and an upgraded laser path to the \hbox{DIII-D} vessel. The upgrade plan and status as well as plans for the 2011 run campaign will be presented.   

Bandwidth Doubling Upgrade for DIII-D Electron Cyclotron Emission Imaging Diagnostic System

A dual-array Electron Cyclotron Emission Imaging (ECEI) system recently installed on the DIII-D tokamak has generated exciting time-resolved images of electron temperature profiles and fluctuations. In its current configuration, this double down-conversion heterodyne system has a total of 320 channels (20 vertical by 16 radial), with each array spanning an intermediate frequency (IF) range of 2 GHz to 9.2 GHz or approximately 15 cm of radial coverage. New ECEI electronics are being developed which will greatly increase the radial coverage of each array by extending the instantaneous IF coverage to a minimum of 2 to 15 GHz while simultaneously increasing the number of radial channels of each array to 16. Details regarding three different frequency extender approaches under investigation will be presented together with test results from each approach. The impact of this ECEI upgrade on planned MHD and turbulence experiments and studies on DIII-D will also be addressed. *Work supported by U.S. DOE Grants DE-FG02-99ER54531 and DE-AC02-76CHO307, POSTECH, and by NWO and the Association EURATOM-FOM.   

Progress in rapid tomography for the COMPASS tokamak

After its reinstallation in Prague, the COMPASS tokamak has completed its first year of operation. In this period, among others the first set of Soft X-ray, bolometric and visible light profile cameras has been installed and commissioned. Three more sets (under construction) shall be installed at different ports of the same toroidal sector allowing for plasma tomography with a prospective of real-time processing. A dedicated tomography algorithm for the planned setup was developed, including set-up of the contribution matrix and speed optimization. The proposed code is based on robust and validated post-processing tomography algorithm with a potential to ensue and refine a simplified real-time version. The code implements rapid Minimum Fisher Regularization with optional unisotropic smoothing constrained by magnetic flux surfaces. Reconstruction on simulated data (phantom plasma emissivity models) provided quantitative evaluation of the tomography performance in the designed diagnostic set-up.   

Modification of Edge Current Profile and Drift-Alfven Mode Suppression in SINP-TOKAMAK by Biased Electrode at the Edge Region

Experiments with biased electrode inserted in the edge region have been carried out to study the physics behind improve plasma confinement in the SINP-Tokamak, an iron-core tokamak with major and minor radii of 30 and 7.5 cm, respectively. Previously improved confinement with modification of edge current density profile was reported\footnote{J. Ghosh, R. Pal, P. K. Chattopadhyaya and D. Basu, Nuclear Fusion \textbf{47}, 331 (2007)} in its very low edge safety factor (1 $<$ q$_{a} \quad <$ 2) operation. The same experiment has been extended now in normal q$_{a }(\sim $ 5 to 7) operational regime of the tokamak. Improvement of plasma confinement is also observed in this case with nearly similar results. Introducing small magnetic and Langmuir probes carefully in the edge region the edge plasma current density profile is seen to be modified as before. Interestingly, analysis of fluctuation measurements in the probes indicates suppression of drift-Alfven mode by biased electrode leading to better confinement. Detailed experimental results will be presented in this paper.   

Experimental studies of ECRH/ECCD effects on Tearing Mode stability using the new TCV real-time control system

While ECRH/ECCD will be the main method for suppressing NTMs in ITER, some aspects of the effect of localised heating and current drive on the mode stability need further investigation. Using the new digital TCV control system we are able to fully exploit the flexibility of the TCV multi-beam EC system for detailed studies of tearing mode formation and suppression. An event-based real-time controller is used, which reacts to the appearance of Tearing Modes by varying the EC injection angles and power. The power can also be modulated with variable phase with respect to the island, allowing to study the effect of heating in the island O or X point. We present not only the experimental results but also a practical demonstration of the use of the new TCV control system. Due to its simple design and implementation, the session leader can rapidly react to changes between shots and change the controller behaviour as the progress of the experimental session requires.   

Feedback Control of (2,1) Mode in q$_edge\sim$2 Tokamak Plasma in RFX-MOD

RFX-mod is the largest reversed field pinch (R=2 m, a=0.46 m, plasma current up to 2 MA). Thanks to its great flexibility it can be operated as a 150 kA, 1 s pulse length tokamak. The real-time control system, based on 192 coils independently driven, is used therefore to actively control MHD stability also in such tokamak plasmas. Preliminary results on active control of a current-driven (2,1) mode in a ohmic discharge with ramping current, in presence of a resistive shell, will be presented. In these discharges qedge ramps down to $\sim$2, when the (2,1) mode is destabilized and then feedback controlled. Feedback control allows for stable operation at q$_edge\sim$2, which would be otherwise not possible. Future plans for an extended tokamak campaign in 2011, dedicated to MHD active control and to the interaction of external non axis-symmetric magnetic with the plasma will also be discussed.   

Multiple-applications of Accelerated Compact Toroid Injection for MFE

The CTIX experiment has explored the potential applications of launching a fast moving magnetized compact toroid for Magnetic Fusion experiments. These applications include central fueling of a MFE device such as tokamaks, stellarators, etc. At present, the UC Davis CTIX accelerator has achieved densities at mid to upper $10^{15}$ per cc, at speeds reaching over 200 km/sec. In order to meet the parameters of even larger fusion devices, the technology of the accelerator needs to incorporate the latest plasma wall interaction findings. As a result of the next step in CT development, UC Davis will be collaborating with the Fusion Technology group at Sandia National Laboratory in Livermore California. We will be designing new plasmas facing electrodes that can reduce electrode impurities and increase electrode lifetime. In addition to producing high density CTs, we will include the updated conical compression results from our previous installed drift section compressor. In addition of the MFE applications, the ability to enhance the CT density, fields as well as speed can be useful to other fusion areas such as MIF, etc.   

Triggering Edge Localized Modes through Lithium Dust Injection

Edge Localized Modes (ELMs) of low amplitude should have the beneficial effect of transporting impurities away from the core plasma, without reducing the plasma stored energy, thus improving the performance of a Tokamak fusion device. In past experiments deuterium pellets have been injected into the DIII-D Tokamak, successfully triggering ELMs, and ITER is considering using deuterium pellets injected by a gas gun to trigger ELMs. Here, a new apparatus for injecting packets of lithium powder into a Tokamak at a frequency of greater than 100Hz, with the hope of triggering ELMs, was designed, built, and tested in a small vacuum chamber. The apparatus drops a thin sheet of lithium powder of diameter 40 micrometers to 500 micrometers onto a rotating paddle wheel, which propels packets of the lithium forward at greater than 20m/s. A fast framing camera was used to measure the velocity and spatial distributions of the particles leaving the paddle wheel.   

Optical Simulation Methods for Studying Gaussian Beam Propagation in the Microwave Imaging Reflectometry in KSTAR

The microwave imaging reflectometry (MIR) is a new innovative plasma diagnostic system that can form images of the density fluctuations at the cut-off layer in the plasma. To design the optical system for the KSTAR MIR system, it is essential to understand the diffraction pattern of the probing waves at the cut- off layer. Two commonly used methods are compared in this poster; the FDTD simulation solving the Maxwell's equations exactly on a rectangular grid and analytical solution from the phase screen model, with the cut-off layer modeled as a reflecting surface of infinite conductivity. In both cases, the target surface is bent circularly to match the curvature of an incoming probe beam with embedded corrugation simulating the density fluctuation. Differences between two methods are apparent for shorter target wavelength and larger corrugation amplitude. A systematic comparative study will cover a wide range of the corrugation parameters such as wave number and corrugation amplitude in this poster.   

Experimental Study of Diffracted Beam Patterns of Microwave Imaging Reflectometry

Verification of diffracted beam pattern of the Microwave Imaging Reflectometry (MIR) is important for reconstructing the density fluctuation at the cut-off layer in the plasma. The diffraction patterns of the expanded Gaussian probe beam, reflected from a corrugated metallic target with a matching curvature, is compared with those of the Gaussian beam used in conventional reflectometry from the same target. The measured diffracted beam patterns are compared with the two simulation methods (FDTD method and analytic method based on the phase screen model). Finally, characteristic of the images of the corrugated target surface at the imaging plane is studied for various target conditions.   

Investigation of the Sawtooth Physics using ECE Imaging system on KSTAR

There have been reports on partial crash events of the sawtooth oscillation but the details such as the reconnection process and heat transport during these events were not addressed at all. Further analysis from the ECEI data (TEXTOR) provided a detailed 2D information on the internal m=1 tearing mode during the postcursor phase. In this study, the time scale of the postcursor oscillation appears to be correlated with the toroidal rotation of the plasma/core current density which is controlled by the heating beam power. It requires a systematic investigation in order to understand the causality of the various crash patterns. A new improved ECE Imaging system and available heating sources (NBI, ECH and ICRF) on KSTAR will be employed to study this subject.   

Reconstruction of Plasma Equilibria and Projected Stabilization of Global MHD Modes in KSTAR

Experimental equilibria of the KSTAR tokamak with plasma current up to 0.34 MA were reconstructed using EFIT. Vessel currents were included by fitting estimated values based on loop voltage measurements and effective resistances from 2 and 3-D vacuum model calculations including a double-walled vessel with large port penetrations and passive stabilizers. Active and passive stabilization of global MHD instabilities for operation above the no-wall beta limit is also projected. The stabilization is applied using a set of segmented internal coils called in-vessel control coils (IVCCs). Passive stability of the resistive wall mode and power requirement for its active stabilization are investigated including conductive casing structures covering the IVCC, and noise effects. The potential for ELM mitigation by resonant magnetic perturbations is also examined by using the TRIP3D code. Favorable configurations of the IVCC based on the Chirikov parameter are determined using a combination of all IVCCs (midplane and off-midplane coils) with a dominant n = 2 field configuration.   

The Research Progress of the J-TEXT Tokamak

In 2004, the TEXT-U tokamak was disassembled and shipped to China, and later on settle down in Huazhong University of Science and Technology. The machine was renamed as the Joint TEXT (J-TEXT) tokamak and obtained its first plasma in 2007. The typical J-TEXT Ohmic discharge was performed in the limiter configuration with the main parameters as follows: major radius R=1.05 m, minor radius a=0.27m, toroidal magnetic field B$_{\rm{T}}$=2.2T, plasma current I$_{\rm{p}}>$200kA, line-averaged density n$_{\rm{e}}\sim$ 2-3 $\cdot$ 10$_{19}$/m$^3$, and electron temperature T$_{\rm{e0}}\sim$ 700eV. Up till now, a few diagnostic systems used to facilitate routine operation and experimental studies were designed and developed. Benefiting from these diagnostic tools, the observation of MHD activities and the statistical analysis of disruption events were done. And measurements of the electrostatic fluctuations in the edge region and conditional analysis of the intermittent burst events near the LCFS were also made as well. The preliminary results will be presented in detail in the meeting.   

HBT-EP Research Program: Advancing Active Mode Control

HBT-EP (http://www.seas.columbia.edu/apam/hbtep/) active mode control research is advancing ITER and fusion relevant modular feedback control coil configurations and their impact on kink mode rigidity, advanced control algorithms, and the effects of plasma rotation on MHD mode stability. This poster our plans to use our recently enhanced active mode control facility (i) to quantify external kink dynamics and multimode response to applied magnetic perturbations, (ii) to develop and understand the relationship between control coil configuration, conducting and ferritic wall configuration, and active feedback control effectiveness, and (iii) to explore advanced feedback control algorithms and internal feedback control coil configurations that are ITER and reactor relevant. Together with our successful high-speed multiple-input-output (MIMO) digital control system, an improved capability from the VALEN 3D feedback modeling code, and a very highly-instrumented control wall, we aim to optimize the use of modular feedback coils to control instability growth near the ideal wall stabilization limit, answer critical questions about the role of plasma rotation in active control of the Resistive Wall Mode (RWM) and the Ferritic Resistive Wall Mode (FRWM), and improve the performance of control systems used in fusion experiments.   

High-Resolution, High-Accuracy Magnetic Measurements for Multi-Mode MHD Instability Control

Accurate measurements of multimode plasma magnetic response are important for quantifying the effects of a variety of MHD phenomena. A new array of 216 poloidal and radial magnetic sensors has been designed, installed, and calibrated on the HBT-EP tokamak to measure multi-mode MHD dynamics. High accuracy of these magnetic measurements on HBT-EP is accomplished using both detailed bench and in-situ ``copper plasma'' calibration techniques. The in-situ calibration was carried out with current ring sources installed into the vacuum chamber, near the location of the plasma core, as well as the equilibrium poloidal and toroidal field coils, and special in vacuum chamber calibration dipole coils. The calibration procedure is based on a linearized least squares algorithm to verify the location and orientation of each individual sensor based upon magnetic measurements using the various current ring sources along with detailed position measurements using a combination of custom and ROMER coordinate measurement machines. The accuracy of this calibration procedure, the effects of eddy currents, system detection limits, and initial multi-mode plasma response measurements will be presented. Supported by U.S. DOE Grant DE-FG-02-86ER53222   

Control of Static and Dynamic Non-axisymmetric Magnetic Fields in HBT-EP

A major research goal of the HBT-EP tokamak is to control multimode non-axisymmetric magnetic perturbations. Even small non-axisymmetic equilibrium field errors in tokamaks can have a dramatic impact on plasma performance, and reducing these field errors can improve plasma behavior. We report progress and plans to minimize non-axisymmetric perturbations through (i) precision alignment of the equilibrium coils, conducting wall segments, and 120 control coils, and (ii) active control of the 3D plasma boundary using an upgraded real-time digital control system. Tokamak metrology is accomplished using both a radial measurement arm built on a rotating central axis of HBT-EP and a ROMER coordinate measuring machine provided through collaboration with PPPL. Metrology results are used to account for asymmetries of sensors and control coils during resistive wall mode studies. Dynamic control and analysis of non-symmetric fields excited from both unstable resistive wall modes and from pulsed external perturbations will be accomplished by a high-throughput active control system. Details of the tokamak metrology, analysis of field errors produced by small misalignments of the equilibrium field coils, and plans for active control will be presented. Supported by U.S. DOE Grant DE-FG02-86ER53222.   

Creating a Stochastic Magnetic Edge in HBT-EP

The newly-installed control wall in the HBT-EP tokamak (http://www.seas.columbia.edu/apam/hbtep/) are able to apply radial fields more than an order magnitude larger than our previous studies and large enough to allow study of the plasma response associated with driven tearing modes, the onset of island overlap, and the creation of a stochastic edge. Understanding plasma response effects while applying fields able to create vacuum island overlap are particular interest for ELM mitigation.\footnote{Evans, \textit{et al.}, Nuclear Fusion, {\bf 48}, 024002 (2008).} Control RMP fields will be applied in HBT-EP using both static and rotating fields to study the effects of the relative rotation between the applied field and the plasma frame. Additionally, we have demonstrated the ability to strongly alter the plasma rotation with a biased probe, and we will be installing a ``zero-net-turns'' plasma shaping coil to increase the magnetic shear at the plasma edge. These experimental ``knobs'' will allow a detailed exploration of the magnitude of plasma shielding currents and the impact of stochastic fields on the plasma response and related efforts of mode control.   

High-Speed, High-Power Active Control Coils for HBT-EP

We report the performance of a newly installed high-speed, high-power active control system for the application of non-symmetric magnetic fields and the study of rotating MHD and resistive wall modes in the HBTEP tokamak. The new control system consists of an array of 120 modular control coils and 40 solid-state, high-power amplifiers that can apply non-symmetric control fields that are more than 10 times larger than previous studies in HBT-EP and exceed 5{\%} of the equilibrium poloidal field strength. Measurements of the current and field response of the control system are presented as a function of frequency and control coil geometry, and these demonstrate the effectiveness of the system to interact with both growing RWM instabilities and long-wavelength modes rotating with the plasma. We describe a research plan to study the interaction of both kink and tearing mode fluctuations with applied static and rotating magnetic perturbations while systematically changing the plasma rotation with a biased molybdenum electrode inserted into the edge plasma.   

Investigation of Multi-Mode Plasma Response in HBT-EP

The new measurement and control system of the HBT-EP tokamak was designed to investigate multi-mode plasma dynamics under a variety of plasma and control coil configurations. We aim to identify optimal control coil configurations and quantify the flexibility available for designing and configuring active feedback systems. Experimental data will also be used to systematically validate the VALEN multi-mode modeling code and thereby facilitate the transfer of all results to next-step devices like ITER. In this poster we present first computational and experimental results. The TokaMac and DCON codes are used to find equilibria with favorable multi-mode kink stability properties. We show that the installation of ``zero-net-turns'' shaping coils provides experimental control of the plasma response by reducing the external kink resonance of limited discharges and changing the coupling of $n=1$ and $n=2$ RWMs. Experiments were aimed at reproducing the computed equilibria and exciting the desired multi-mode responses with different control coil configurations. Actual equilibria are reconstructed using 216 magnetic sensors and pressure profile information. Measured mode dynamics are compared to simulations.   

Plans for the Study and Control of the Ferritic Resistive Wall Mode on HBT-EP

A leading candidate for low activation, high neutron radiation resistant damage materials required in a DT fusion power plant is a broad class of ferritic steel alloys. Unlike low magnetic permeability stainless steel alloys routinely employed in present day toroidal fusion energy experiments, these candidate ferritic steel alloys have significant ferromagnetic properties even though the strong toroidal magnetic fields in a fusion power plant would drive the ferritic steel structural material into magnetic saturation. In addition to the introduction of 3D magnetic field errors onto the plasma equilibrium, the residual ferromagnetism attracts magnetic flux perturbations associated with long wavelength MHD modes and provides an additional ferritic destabilization of the beta driven resistive wall kink mode -- the FRWM. These ferritic wall destabilization effects have not yet been observed experimentally in toroidal confinement geometry, but were observed recently in a linear line-tied pinch device [Bergerson 2008]. Implementation scenarios, their design, and the installation of ferritic wall elements needed to illustrate the key physical effects of the FRWM on the HBT-EP experiment will be presented and discussed. Supported by U.S. DOE Grant DE-FG-02-86ER53222.   

Measurement and Interpretation of MHD Instabilities in HBT-EP with NIMROD

The newly installed fully instrumented control wall in the HBT-EP tokamak has more than 200 high-speed magnetic sensors enabling unprecendented coverage for the measurement of both the linear and nonlinear evolution of tokamak MHD instabilities. This poster presents progress and plans for the detailed validation of the NIMROD code [1] to pressure and current-driven instabilities in HBT-EP. First steps include parameterization of the time evolution of external and internal kink instabilities in model HBT-EP equilbria that show the spatial structures of instabilities having growth rates comparable to experimental observations. We also discuss future plans for the study of the nonlinear evolution of observed instabilities in time-evolving equilibria, comparison of simulation mode structure with observations, and the use of models for the conducting wall surrounding HBT-EP plasmas. Supported by US DOE Grant: DE-FG02-86ER53222 \\[4pt] [1] C.R. Sovinec, et al., J. Comp. Phys., 195, 355 (2004).   

Formation of high temperature field reversed configurations in Tri Alpha Energy's C-2 System

Tri Alpha Energy's C-2 experimental system is a linear device consisting of two separate multi-coil theta-pinch formation sections and a central confinement vessel. Two high-beta compact toroids (CT's) are formed by sequential firing of the main reversal coils in the formation sections. This 'dynamic' formation technique results in CT's with Mach numbers greater than 1. Collision and thermalization of the CT kinetic energy, predominantly into the ion channel with ion temperature of 400 -- 500 eV, results in high temperature field reversed configurations (FRC's) in the confinement vessel. Strong flux amplification may also occur during the merging process. A discussion of the formation section, dynamic formation parameters and the resultant merged CT and FRC characteristics will be presented.   

Overview of the Flux-Coil-Generated Field-Reversed Configuration

A Flux-Coil-Generated Field-Reversed Configuration (FRC) device has been assembled with the goal of producing an FRC dominated by large-orbit ions. The inner (flux) coil is used to pre-ionize the working gas and then accelerate the plasma into an FRC while the outer (limiter) coil provides the axial bias field. Two chords of infrared interferometry at 12 and 20 cm radii measure the electron density. Spectroscopy monitors NII ($\lambda $ = 568 nm) at an 18-cm radial chord to measure ion temperature and rotational energy. Three magnetic probe arrays are located outside of the chamber, through the midplane, and at the axial end of the FRC. During the more quiescent portion of the FRC, the magnetic field-null r$_{0}\sim $ 20-24 cm, the separatrix r$_{s}\sim $ 27-30 cm, the field-reversal $\Delta $B $\sim $ 300-500 G, the electron density n$_{e}\sim $ 2-9$\times $10$^{13}$ cm$^{-3}$, the ion temperature T$_{i}\sim $ 20-40 eV and the ion rotational energy E$_{i\theta }\sim $ 7-19 eV. Ongoing neutral particle analyzer studies suggest even higher rotational energy. The FRC plasma lifetime is $\tau \sim $ 60-100 $\mu $s.   

Direct Measurement of Impurity Transport Coefficients in an FRC

An optical tomography diagnostic is being developed that will be used to make a direct measurement of the time and space resolved density profile of an impurity species in a Field-Reversed Configuration (FRC). Localization of the impurity will be controlled by means of a puff valve. This will allow for accurate temporal resolution up to within a few millimeters of the nozzle. The tomographic system consists of sixteen collimated chords. They are fanned in such a way as to cover about 30{\%} of the FRC at the midplane. The signal of each chord will be piped out to an individual Photo-Multiplier Tube (PMT) equipped with a narrow band pass filter which is centered at the wavelength of the desired spectral line of the impurity species. An inversion method, such as linear regularization, will be used to generate an image of the density profile from these data. Once this has been accomplished the convective and diffusive transport coefficients will be determined by further analysis. Preliminary investigation has verified that when Argon is used as the impurity it emits enough light to be measured when it comprises 0.1{\%} of the total backfill.   

Near-Real-Time 3-D Reconstructions of FRC Plasmas

Results from the C-2 experiment at Tri Alpha Energy, Inc. have demonstrated that the plasmas found in merging FRC experiments are not always nicely behaved, thus making simplistic analysis techniques inappropriate. A new method has been developed that treats the reconstruction process as the optimization of a parametric model. In our case C-2 plasmas are approximated by a 14-parameter model; the Genetic Algorithm (GA) is used to perform optimizations of this model constrained primarily by bolometer data, and typically converges in just a few seconds, i.e. near-real-time 3-D reconstructions. The specific details of this novel approach to the realistic analysis of FRC plasmas will be the primary focus of this presentation along with a few results from its application to the C-2 experiment.   

Modeling of Dynamic FRC Formation

We have developed a 2-D resistive MHD code, Lamy Ridge, to simulate the entire FRC formation process in Tri Alpha's C2 device, including initial formation, translation, merging and settling into equilibrium.~ Two FRC's can be created simultaneously, and then translated toward each other so that they merge into a single FRC.~ The code couples the external circuits around the formation tubes to the partially ionized plasma inside.~ Plasma and neutral gas are treated as two fluids.~ Dynamic and energetic equations, which take into account ionization and charge exchange, are solved in a time advance manner.~ The geometric shape of the vessel is specified by a set of inputs that defines the boundaries, which are handled by a cut-cell algorithm in the code.~ Multiple external circuits and field coils can be easily added, removed or relocated through individual inputs.~ The design of the code is modular and flexible so that it can be applied to future devices.~ The results of the code are in reasonable agreement with experimental measurements on the C2 device.   

Electron and ion temperature profiles during merging of two FRC plasmas

Plasma temperature profiles are measured in the C-2 field reversed configuration (FRC) experiment during the coaxial merging of two FRCs plasmas. Electron temperatures are measured using a nine spatial point Thomson scattering system and ion temperature profiles are estimated from multi-chord ion Doppler spectroscopy. Results are used to refine MHD simulations performed using the LamyRidge code.   

Plasma Rotation Profile Measurements in C2 FRC

Field Reversed Configuration (FRC) holds a promising future for commercializing thermonuclear power. However, n=2 rotational instability appears virtually in every FRC plasma and degrades their performance. The exact mechanism to generate and accelerate the rotation is still a puzzle. In this presentation, we attempt to investigate this puzzle by measuring the rotation profile in C2's core plasma, scrape-off layer, and exhaust region with Mach probe and Doppler spectrometer.   

Measurement of electron density profiles in the C-2 merging FRC experiment

Plasma electron density profiles and profile evolutions in the C-2 field reversed configuration (FRC) experiment are measured by a 6-channel two-color laser interferometer. Measured profiles from translated FRC plasmas are compared with MHD simulation results from the LamyRidge code. Profile changes at the merging process will be investigated. Profile dependence on the FRC parameters such as external magnetic field strength will be examined. Density measurements from a 9-point Thomson scattering diagnostic and a triple probe will be compared.   

Internal Magnetic Field Measurement on C-2 FRC Plasma

Three-axis internal magnetic probes are being developed to measure simultaneously $B_{z}$, $B_{t}$ and $B_{r}$ of a field-reversed configuration (FRC) plasma from the geometric axis ($r$=0) to outside of the separatrix in C-2. The probe assembly consists of 30 commercial chip-inductors (10 different radial positions of each field-component with 5 cm apart), OD$\sim $0.25'' stainless-steel tube with 5-mil wall for the vacuum boundary, and interlocking Boron-Nitride jackets as a plasma facing material. In C-2, it is important to understand field-structure of FRC plasma during translation and colliding two-FRCs in the confinement section as well as the equilibrium/quiescent phase. With 6-chord interferometry located in the midplane of C-2, the internal structure of FRC can be compared and discussed by using a rigid-rotor profile model for the field and the density of FRCs. The preliminary result of internal field measurements will be presented at the meeting as well as the detailed probe design.   

Excluded flux measurements of long-lived field-reversed configuration plasmas

Diamagnetic measurements of field-reversed configuration (FRC) plasmas produced in the C-2 experiment are obtained with a linear array of 19 magnetic probes positioned along the length of the stainless steel confinement vessel. The excluded flux axial profile of the FRC, which approximates the separatrix shape, is calculated from this probe data under the assumptions of vessel wall flux conservation, coaxial symmetry, and negligible plasma pressure outside the separatrix. While initially valid, it is found that these assumptions break down towards the end of the $\sim$ 1 ms FRC lifetime. Magnetic probes placed on the outside of the confinement vessel detect significant flux leakage through the vessel wall. Both radiation bolometer measurements and additional magnetic probes around the circumference of the vessel indicate that FRC motion off of the central axis increases with time. In addition, axial shrinkage causes large uncertainties in estimating the separatrix length and volume. The errors caused by these non-ideal effects are quantified and methods for correction are discussed.   

TAE neutron detectors and some physics results

Several neutron detectors are part of the comprehensive diagnostics set deployed in the C-2 TAE FRC machine [1]. They include a pair of very sensitive $^{3}$He counters, and a pair of plastic and ZnS scintillators. These detectors perform reliably and provide valuable insights into the properties of TAE FRC plasmas. A novel HW and SW detection scheme has been used for counting neutron pulses from the $^{3}$He detectors. Estimates of the total neutron emission from the C-2 device, and the corresponding ion temperature will be presented. The collision of two dynamically formed Deuterium FRCs leaves a distinct signature on the fast plastic neutron scintillator. \\[4pt] [1] M. W. Binderbauer~\textit{et all}, Phys.Rev.Lett. \textit{in print}   

Neutral Particle Transport in Field Reversed Configurations

As far as we know, there has been no study on the neutral particle transport in the Field Reversed Configurations (FRC). The presence of neutral gas impacts, e.g., the charge-exchange (cx) of the energetic ion population. In this work we will present three computational tools to study the neutral particle transport in an FRC: In the first approach we couple an MHD code with a neutral fluid-- Lamy Ridge --, in the second approach we apply the modified Monte Carlo particle code -- DEGAS2 -- to study the neutral transport in a pseudo-equilibria, and the third approach uses the GTNEUT and GTNEUT-upgrade codes, which are based on the calculation of transmission and escape probabilities and balancing fluxes with a pseudo-equilibrium. A good agreement between DEGAS2 and GTNEUT and GTNEUT-upgrade has been made. Separately, we will present a reasonable agreement between DEGAS2 and Lamy Ridge. In addition, using DEGAS2 we show the neutral density profiles of atomic and molecular deuterium, as well as D$\alpha $ profiles and its comparison to the experimental results.   

Field-reversed configuration experiments with lithium-coated vessel walls

In C-2, two high-beta plasmas are merged to produce a field-reversed configuration (FRC) plasma. A system of evaporators has been installed on the C-2 experiment's confinement chamber in order to produce a solid lithium coating on the plasma facing wall. The lithium evaporator system is capable of depositing up to 80 mg/min over the $\sim 20$ m$^{2}$ inner surface of the confinement chamber. The effect of the lithium coating is investigated using internal (including a 6-chord interferometer, multi-point Thomson scattering, and a D$_{\alpha}$ array) and edge (including fast ion gauges, magnetic probes, and triple probe) diagnostics. Preliminary data shows a decrease in D$_{\alpha}$ emission indicative of a drop in wall recycling.   

Merging Formation of Large-Size Field-Reversed Configurations with the Assistance of Neutral Beam Injection

The effect of energetic beam ions on oblate Field-Reversed Configurations (FRCs) was studied experimentally in the TS-4 plasma merging device. An important question is whether plasma flows and large ion gyroradii generated by beam ions can stabilize the global modes essential to high-$s$ (plasma size normalized by ion gyroradius) FRCs. We developed a new high-power pulsed Neutral Beam Injection (NBI) with a washer gun plasma source, achieving beam power up to 0.6 MW (15 kV, 40 A). The Monte Carlo simulation of tangential co-current NB injection indicates that the beam ions are trapped between the magnetic axis and the separatrix. A new finding is that two merging high-$s$ spheromaks with opposite helicities relax into the largest scale FRC with its poloidal flux as high as 10 mWb. On the other hand, they did not relax to an FRC without the assistance of NBI. These facts suggest ion kinetic effects essential to FRC equilibrium and stability. Effect of high energy beam on parameters $s$ or $S^*$ of FRCs will be a key to solve the unknown FRC stability.   

Flow and Fluctuations in the Colorado FRC Experiment

We present the latest results from the Colorado Field-Reversed Configuration (FRC) Experiment. The FRC is formed by merging counter-helicity spheromaks and the diagnostic suite places emphasis on the investigation of fluctuations and flows. Analysis of the fluctuation spectrum using a multi-point (16 positions x 3 axes) magnetic diagnostic is underway. Dispersion relations are extracted for coherent waves by using a cross-spectral density correlation technique. Observations during merging include signatures of magnetosonic, L-mode, and ion-cyclotron waves. In order to highlight flow-driven processes, we have designed and constructed a two-point biasing probe for driving bulk E x B flows at subsonic to supersonic speeds. We present first results from the use of the biasing probe.   

Experimental Results from Initial Operation of Plasma Injector 1

General Fusion has begun operation of its first full-scale plasma injector, designed to accelerate high density spheromak plasmas into the compression chamber of a proposed MTF reactor. The geometry of Plasma Injector 1 (PI-1) is that of a two stage coaxial Marshal gun with a conical converging accelerator electrodes, similar in shape to the MARAUDER device, while pulsed power is applied in the same configuration as the RACE device. PI-1 is 5 meters in length 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/compression stage is 4 m long and tapers to a final outer diameter of 40 cm. PI-1 is now operating at 1 MJ of total capacitor power, which will be doubled again before it reaches its design parameters. Diagnostics include 3 interferometer chords, 21 magnetic probes (2 axis poloidal/toroidal), 13 fast photodiode chords, as well as one Thomson scattering chord, a visible light survey spectrometer, and a Langmuir triple probe. Electrode voltage and current are also monitored. So far spheromaks of poloidal flux exceeding 100 mWb have been formed in the expansion region, and spheromaks of 40-50 mWb have been formed and accelerated out the end of the accelerator into a flux conserving target chamber. Expansion region densities are typically $\sim 5 \times 10^{14}cm^{-3}$, while conditions in the target chamber have reached $n_e \sim 10^{16}cm^{-3}$, and lifetimes of 300 $\mu s$.   

Recent results on the TCS-Upgrade device

The Translation, Confinement, and Sustainment Upgrade (TCSU) device is a facility to form and sustain field-reversed configurations (FRC) in quasi-steady state using rotating magnetic fields (RMF). Results from operation with internal flux rings and an additional Ti gettering campaign are reported. Several new diagnostics have been installed including a 90-channel three-axis (30 radial positions) internal magnetic field probe, a multi-point Thomson scattering system, and a Langmuir probe. A broad range of RMF frequencies, from 85 kHz -- 240 kHz, and field configurations have been investigated with a full diagnostic set and results will be reported. Results from operation with odd-parity RMF antennas, which should close field lines, [S.A. Cohen and R.D. Milroy, Physics of Plasmas \textbf{7}, 2539, (2000)] will also be reported.   

Three-Axis Magnetic Field Measurements in the TCSU RMF Current Drive Experiment

Detailed magnetic measurements of Field-Reversed Configurations (FRC) from the Translation Confinement and Sustainment Upgrade (TCSU) experiment will be presented. A new 3-axis probe with 90 windings that can simultaneously measure B$_{r}$, B$_{\theta }$, and B$_{z}$ at 30 radial positions has been installed. This probe is axially translatable and can be moved to any axial position, allowing for a full r-z map of the magnetic field by taking multiple repeatable shots. The probe is BN clad with a 5 mm outside diameter, is UHV compatible, and can be baked to 200 degrees C. Data from this new probe will be combined with data from the current 48 winding 2-axis probe that measures B$_{\theta }$, and B$_{z}$ as a function of radius at the axial midplane. Measurements and analysis will be presented, and a comparison will be made with 3D NIMROD simulations of RMF current drive in the TCSU device [R.D. Milroy, C.C. Kim and C.R. Sovinec, ``Extended MHD simulations of FRC formation and sustainment with RMF current drive,'' Phys. Plasmas, \textbf{17}, 062502 (2010)].   

Results from TCSU Thomson Scattering Diagnostic

A Thomson scattering system has been installed on the Translation, Confinement, and Sustainment Upgrade (TCSU) experiment at the University of Washington to make direct point measurements of electron temperature and density. In this experiment a Rotating Magnetic Field (RMF) is applied to form a Field Reversed Configuration (FRC). Magnetic field and interferometer measurements, combined with an assumption of pressure balance have led to plasma density and temperature estimates of (Te+Ti) $\sim $ 150 eV and a density of about 5$\times $10$^{18}$. The Thomson scattering system uses a single pulse ruby laser and observes five spatial points on the bottom half section of the 40 cm radius in the TCSU device. Five General Atomics (GA) polychromators, each equipped with three PPPL pre-amplifiers, are used to analyze the scattered light. All signals are digitized by ADCs (LeCroy 2250L) and sent to the server for further analysis. The system design, calibration, and electron temperature estimates will be presented.   

Hybrid equilibrium with an end-loss ion distribution with solutions for an FRC

Hybrid equilibria (fully kinetic ions + warm fluid electrons) are found for an end-loss ion distribution. The distribution is expressed in terms of the two constants of motion (Hamiltonian $H$, and canonical angular momentum $P_{\theta})$ of an axisymmetric equilibrium, both of which depend on velocity \textit{and} space coordinates. The distribution function is the solution of a simplified Fokker-Planck equation in which ion collisions cause the ions to diffuse toward the unconfined region in $H-P_{\theta}$ space. Moments of the distribution give the macroscopic parameters of the ion distribution, e.g. local density and flow, all of which are explicit analytic functions of the magnetic flux function \textit{$\psi $}, electrostatic potential \textit{$\phi $}, and the radius coordinate. The electron temperature function (of \textit{$\psi $}) is prespecified. With these, and Ampere's law a complete system of equations is in hand that can be solved by relaxation methods. Solutions are presented for field-reversed configurations. The pressure and density profiles are not extraordinary, but the ion flow has large flow shear near the separatrix. These results are applied to the FRC data compendium to infer collision frequencies in experiment and compare them with those for common turbulent transport mechanisms. Implications of the large flow shear on global stability will be discussed.   

Pulsed High Density Experiment in Double Ended Merging and Compression

The results from the initial FRC dynamic formation and translation experiments on PHD attained target parameters with equilibrium temperatures of 300 eV or 15 mWb of flux. A long FRC equilibrium phase was observed after reflection from a downstream mirror. The next phase requires rapid formation of a stationary FRC at higher flux. Based on the success of the dynamic formation and merging of two FRCs [1], the PHD experiment has been modified to a double ended merging and compression system. The previous PHD source section has been split to create two 1.25 m long, 0.8 m diameter, source sections. The FRCs will be injected into a compression section that is 2.8 m long with a 0.4 m diameter for merging and compression experiments. A 0.4 MJ bank is used initially for flux compression. This same bank was designed to be used for future metal and plasma liner experiments as well. Operating at full power the flux compression of the FRC is expected to produce a range of plasma parameters with T $\sim $ 0.5-2 keV, and 2-8x10$^{21}$m$^{-3}$. \\[4pt] [1] G. Votroubek, J. Slough, S. Andreason, C. Pihl, J. of Fus. Energy, Vol. 27, No. 1-2, pg. 123 (2007).   

High Flux FRC Facility for Stability, Confinement, and ITER Divertor Studies

In order to advance the FRC concept into a more fusion-like regime, the existing RPPL facility at the University of Washington will be modified to take full advantage of the new FRC formation methodology of dynamic formation and merging of FRCs. This method has been shown to provide appreciable increases in the key parameters of ion temperature, poloidal flux and FRC lifetime. A key goal of the high flux FRC facility (HFF) will be to form FRCs with poloidal fluxes sufficiently large to fully confine high energy ion orbits ($\sim $ 10 keV). Rapid injection of plasma with a highly directed energy will be investigated as a method to provide this kinetic component early in the formation phase to maintain FRC stability. The HFF will also be used to make significant contributions towards solving critical problems that hinder the tokamak concept. The FRC exhaust flow will be directed to targets at energy density levels equivalent to those expected in ITER ELM activity and disruptions for materials studies. The FRC experiments will also provide a valuable test bed for code validation in a high beta regime, with large two-fluid effects, plasma flows and an energetic minority species.   

Effects of External Magnetic Filed on the Coupling between RF Antenna and FRC Plasma

In FRC plasma driven by rotating magnetic field (RMF), the coupling between RF antenna and plasma is important to achieve high efficiency of plasma current drive. With the recent development of RF power measurement at Prairie View (PV) rotamak, a series of experiments are carried out to investigate the effects of external magnetic fields on the coupling. By using a toroidal magnetic field, the penetration of RMF is significantly improved due to the excitation of rotating whistler wave; therefore, the RF coupling is enhanced and plasma current is increased and even doubled in comparison with the FRC regime for the same RF power. By using magnetic shaping coils which provide axial magnetic field, even though the penetration of RMF surprisingly becomes worse, the RF coupling is substantially improved due to the increase of plasma elongation, and hence plasma current is boosted from 2 kA to 5.2 kA.   

Kinetic Behaviors of Energetic Ions in Oblate Field-Reversed Configuration formed by Plasma Merging

Energetic ions are keys to improve stability and confinement of the field-reversed configuration (FRC) plasma. During the merging formation of an oblate FRC, the reconnection outflow generates sheared toroidal flow whose direction is determined by the polarity of the merging two spheromaks. The experimental results show that the decay rate and the electron density profile of the oblate FRC are strongly affected by the direction of this toroidal flow. When the toroidal flow is anti-parallel to the plasma current on the outside of the magnetic axis, the electron density peaks near the plasma edge and the FRC suffers from rapid decay. Numerical calculation of the fast ion's trajectory shows that most of the ions in this case drift toward outboard side and escape from the plasma region in a short period, indicating that the differences in the decay rate and the density profile are mainly induced by the preferential particle loss of the energetic ions.   

Progress on the Princeton field-reversed-configuration-2 (PFRC-2) device

Research goals of the PFRC-2 device, an RMF-heated FRC being built with ARRA funding, are to attain ion and electron heating to keV energies and improved energy confinement. To reach these goals requires increases in device parameters, particularly plasma radius, axial magnetic field, pulse duration, and heating power, from which improvements in plasma behavior, such as reduction of drift parameter, turbulence level and neutral hydrogen density, should derive. Herein we describe the technical methods being implemented to achieve the increases in device parameters, physics reasons why the improvements are expected, and diagnostics that will be used to ascertain whether they have.   

HIT-SI Progress and Plans

Experiments in the Helicity Injected Torus-Steady Inductive (HIT-SI) spheromak have yielded improved current amplification and a new understanding of the injector-spheromak interaction. The HIT-SI experiment investigates steady inductive helicity injection with the aim of forming and sustaining a high-beta equilibrium in a spheromak geometry using two semi-toroidal injectors. In each injector, the toroidal flux and induced loop voltage are sinusoidally oscillated in phase at a frequency of 5.8 kHz, generating a DC spheromak. Operations with unequal helicity injection rates between the two injectors produced the highest spheromak current (38 kA), and current amplification (I$_{\mbox{\scriptsize tor}}/$I$_{\mbox{\scriptsize inj}}$ $\approx$ 2) to date. Single-injector operations establish a preferred direction of generated spheromak current for each injector depending on the sign of the injected helicity and its orientation relative to the confinement volume. Since the HIT-SI injectors are mounted on opposite sides of the confinement volume, they prefer to drive opposing spheromak currents. A new experimental configuration with three high-frequency injectors on the same side of the confinement volume is being developed to take advantage of this new understanding. Higher frequency operations (14.7 kHz) have been tested using the present injector configuration with encouraging results. Work supported by USDoE.   

Comparison of Edge Magnetic Activity in the HIT SI Experiment to Numerical Simulations

An array of surface magnetic probes embedded in the HIT-SI spheromak flux conserver resolves plasma dynamics from 10~Hz~--~200~kHz. Amperian loops formed by sub-arrays at toroidal angles of 0\r{ }, 45\r{ }, 180\r{ }, and 225\r{ } allow non-axisymmetric toroidal plasma currents to be measured, capturing both injector and spheromak currents. Unipolar toroidal currents indicate formation and sustainment of a spheromak. The surface magnetic probe array provides an extensive and non-perturbative set of measurements for comparison with numerical models to further describe the activity seen in the experiment. A 3D Taylor-state solver developed by the PSI-Center computes the HIT-SI equilibrium as a superposition of independent injector and spheromak equilibria. Comparison of experimental results to the Taylor equilibrium at different injector driving frequencies shows general agreement of global magnetic field oscillations, but also regions of significant disagreement. Comparisons to calculated non-uniform $\lambda $ ($\lambda =\mu _{o}$j/B) equilibria with injector $\lambda $ closer to experimental values will also be presented.   

Results of inductive helicity-injected current drive on the HIT-SI spheromak at 15 kHz

The Helicity Injected Torus-Steady Inductive (HIT-SI) uses two semi-toroidal injectors to inductively inject helicity into a confinement volume with a bowtie cross-section. Improved spheromak formation and sustainment on HIT-SI has been achieved by increasing the oscillating frequency of the injectors to 15 kHz from 5.8 kHz. Higher frequency operations form higher spheromak currents at lower plasma density and are less disruptive to the equilibrium spheromak. Oscillations in electron density at the injector frequency, as measured by an FIR interferometer, are smaller during higher frequency operations. A three-pronged probe containing arrays of 3D pickup coils, inserted to the magnetic axis, measures the internal magnetic field profiles. Comparisons of the internal magnetic field profiles to the calculated minimum energy Taylor state show agreement for longer periods of time during high frequency operations. Time-resolved spectroscopy, observing discrete wavelengths from the VUV to visible spectrum, will also be presented.   

Observations Supporting Electron Hyper-Viscosity Current Drive in the HIT-SI Spheromak

Observations from HIT-SI are presented which support the model of electron hyper-viscosity current drive. In the model, current is driven across closed flux surfaces by electron dynamo action. An electron velocity gradient, combined with perturbations in the magnetic field near the closed flux surface, result in a current-driving viscous-like drag on the electrons in the closed flux region and an anti-current-driving drag in the adjacent externally-driven region. Magnetic Field alignment calculations for the HIT-SI geometry predict regions where current drive should occur; bolometric data show increased radiation from these regions and ion Doppler spectroscopy observations of bulk ion flows are consistent with the model. A preferred spheromak current direction in HIT-SI is consistent with the model because electrons exiting the injector drive current more effectively than electrons entering the injector. Plans for further study, including a new high-speed camera diagnostic on HIT-SI, are also presented. Work supported by USDoE.   

Design of a Polarimeter for Non-invasive Internal Magnetic Field Measurements on the HIT-SI Spheromak

The Helicity Injected Torus -- Steady Inductive (HIT-SI) program investigates helicity injection current drive for magnetic confinement. The installation of a non-perturbative diagnostic of the internal magnetic field in HIT-SI discharges would allow the important ability to measure current-, $q$- and \textit{$\lambda $}-profiles. Between motional Stark effect and polarimetric methods, the latter is more feasible on HIT-SI and is being pursued. This diagnostic introduces a probe beam at millimeter wavelengths, and the beam's polarization is modified by the spheromak plasma as it propagates; measurement of this effect yields the density-weighted, line-integrated magnetic field parallel to the propagation. The diagnostic will also make an interferometric measurement of the electron density. The measurements will help to validate MHD codes running at the PSI-Center. This research is supported in part by an appointment to the U.S. Department of Energy Fusion Energy Postdoctoral Research Program administered by the Oak Ridge Institute for Science and Education.   

Flexible Tank Circuit Design and Digital Feedback Control Implementation for HIT-SI

Current drive in the HIT-SI spheromak has been primarily due to relaxation of injected helicity.~The ability to couple the injected helicity's perturbation to the plasma during time scales comparable to the natural behavior of the plasma has been unattainable with previous hardware. Only recently, through novel tank circuit and digital feedback control upgrades, has this new regime of operation, including higher injector flux operation, and multiple available driving~frequencies been realized.~A tank has been designed and added to the flux circuit so that in combination with digital control, the machine will operate near resonance while avoiding large phase shifts during plasma loading. The Analog~Devices Blackfin micro-controller-Linux-based digital feedback system has also been improved. Presently, multiple ADC input channels are now available for feedback algorithms, and latency has been improved down to 3 microseconds. These improvements allow for cycle-to-cycle phase control between injector flux and loop voltage circuits.~Details of the changes to HIT-SI circuits, control systems, and preliminary results will be presented.   

Nimrod Simulations of HIT-SI Plasmas

We present NIMROD simulation studies of current-drive, magnetic reconnection and relaxation behavior of the HIT-SI experiment. HIT-SI (Steady Inductive Helicity Injected Torus) is a spheromak that uses two semi-toroidal injectors to provide steady inductive helicity injection (SIHI). SIHI produces and sustains a spheromak by generating poloidal flux using relaxation current drive. Because NIMROD can only model axi-symmetric geometries, the helicity injectors of the experiment are modeled as flux ($\psi_{\rm{inj}}$) and current ($I_{\rm{inj}}$) boundary conditions by applying a tangential electric field at the top and bottom of the tank. The tangential electric field provides both the voltage drop needed to drive the injector current and the loop voltage to bring the injector flux in/out of the equilibrium region. A highly resistive thin edge-layer approximates the insulating walls of the experiment and turns the injectors into current sources. Our simulations use a zero $\beta$ resistive MHD model with uniform density. The Lundquist number S ($\equiv\sqrt{\frac{\mu_{0}}{\rho}}\frac{B}{2\pi R \eta \lambda_{sp}^{2}}$) is 22 and injector lambda ($\lambda_{\rm{inj}}\equiv\mu_{0}I_{\rm{inj}}$/$\psi_{\rm{inj}}$) is 30. Here $\rho$ and $\eta$ are the plasma density and resistivity, R is the magnetic axis. To date, our results show little relaxation and nearly zero plasma current during injector operation whereas current amplification up to a factor of 2 is observed in the experiment.   

Improvements to the PSI-TET Equilibrium Code and Ideal MHD Equilibria in HIT-SI

Recent improvements to the PSI-TET equilibrium code have been made to increase solution accuracy and speed. Implementation with a hybrid MPI/OpenMP model allows for efficient solution of large system sizes utilizing current and future MPP systems. A mimetic discretization on a 3D tetrahedral mesh with geometric multigrid solvers is employed. The code solves for solutions to the ideal MHD equilibrium equation mu0*j=lambda*B in arbitrary 3D geometry. Lambda is allowed to vary across flux surfaces but must be constant in stochastic regions. Field line tracing is used to identify the location of the separatrix and magnetic axis. A fixed lambda profile, specified as a function of a flux surface variable, is used. Equilibria in HIT-SI have been computed for the homogenous (spheromak) and inhomogeneous (injector) fields separately for experimental comparison. Combined equilibria of interest with spatially variable lambda and injector driving have also been computed for HIT-SI. Equilibria in HIT-SI will be presented for Taylor states and states with spatially varying lambda and injector driving. Solver scalability for MPI and Hybrid approaches will also be presented.   

Kinking central column and plasma flows generated during coaxial helicity injection current drive in spherical torus

MHD relaxation phenomena such as plasmoid ejection, helical kinks, magnetic reconnection, and flows are observed in helicity injection experiments as well as in solar flares and astrophysical jets. Comprehensive understanding of the relaxation mechanism underlying the physics in the helicity injection system is of fundamental importance for both laboratory and space plasmas. The kinking central open column and subsequent plasma flows during coaxial helicity injection current drive in spherical torus are investigated by 3-D nonlinear MHD simulations. Flow plots on the poloidal and toroidal cross sections reveal that as a result of a magnetic reconnection caused by the plasma with the $n$=1 helical kink distortion, the toroidal flow ($\sim $ 37 km/s) is driven in the opposite direction to toroidal current $I_{t}$, but in the same direction as the \textbf{\textit{E}}$\times $\textbf{\textit{B}} plasma rotation induced by an applied voltage. This toroidal reconnection flow expands toward the outboard side and occupies almost the entire toroidal cross section, causing the increase in $I_{t}$. In the next stage, the toroidal reconnection flow remains at the inboard, while intense new reversed toroidal flow is induced at the outboard. Then this reversed flow which may be associated with the anti-dynamo effect is enhanced and the toroidal reconnection flow and $I_{t}$ is gradually suppressed.   

Flux amplification and sustainment of ST plasmas by multi-pulsed coaxial helicity injection on HIST

The Helicity Injected Spherical Torus (HIST) device has been developed towards high-current start up and sustainment by Multi-pulsed Coaxial Helicity Injection (M-CHI) method. Multiple pulses operation of the coaxial plasma gun can build the magnetic field of STs and spheromak plasmas in a stepwise manner. So far, successive gun pulses on SSPX at LLNL were demonstrated to maintain the magnetic field of spheromak in a quasi-steady state against resistive decay [1]. The resistive 3D-MHD numerical simulation [2] for STs reproduced the current amplification by the M-CHI method and confirmed that stochastic magnetic field was reduced during the decay phase. By double pulsed operation on HIST, the plasma current was effectively amplified against the resistive decay. The life time increases up to 10 ms which is longer than that in the single CHI case (4 ms). The edge poloidal fields last between 0.5 ms and 6 ms like a repetitive manner. During the second driven phase, the toroidal ion flow is driven in the same direction as the plasma current as well as in the initial driven phase. At the meeting, we will discuss a current amplification mechanism based on the merging process with the plasmoid injected secondly from the gun. [1] B. Hudson \textit{et al.}, Phys. Plasmas Vol.15, 056112 (2008). [2] Y. Kagei\textit{ et al.}, J. Plasma Fusion Res. Vol.79, 217 (2003).   

Study of Plasma Shape Control and Current Drive

An equilibrium control system has been installed at the Prairie View (PV) rotamak. This system consists of three coils wound over the chamber surface and connected to a programmable current source. The coils are used to control both plasma shape and boundary poloidal magnetic flux. The effect of equilibrium control system on plasma current has been examined in two series of experiments: the field-reversed configuration (FRC), and spherical tokamak (ST) configuration with a steady toroidal magnetic field. For a given 200 kW RF power, plasma current is boosted from 2.1 kA to 5.2 kA when the shaping coils are energized with a total DC current around 550 A. The boost of plasma current is mainly due to the increase of plasma elongation and thus the enhancement of coupling between RF antenna and plasma. Plasma discharge is disrupted when the current in the shaping coils is above 600 A. In FRC regime, the disruption is related to the excitation of n = 1 radial shift mode; while in ST regime, no instability mode is observed, the disruption is due to the poor coupling.   

Calculation of two-fluid resonant modes in spheromaks

Numerical computation is applied to investigate two-fluid effects on resonant modes in spheromaks using the NIMROD code [C.R. Sovinec et. at., Phys. Plasmas 10(2003)]. Earlier whole-device simulations of SSPX show that MHD stability has a strong influence on confinement during the sustained decay phase [E.B. Hooper et. al., POP 15, 032502 (2008)]. Recent computations of spheromak equilibria in a cylindrical domain with prescribed peaked pressure profiles show ideal interchange behavior. A moderate reduction of growth rate $(10-70\%)$ for intermediate toroidal mode numbers $(n=16\sim 20)$ is observed when two-fluid effects are included [E.C. Howell and C.R. Sovinec, APS 2009]. Here, we consider more realistic pressure and safety-factor profiles from 3D self-consistent nonlinear MHD simulations. Linear analyses of axisymmetric equilibria reconstructed from the simulations are performed, and growth rates calculated using both ion gyroviscosity and a two fluid Ohm's law are compared with resistive MHD results.   

Three-Dimensional, Nonlinear MHD Simulations of Spheromak Merging and Plasma Relaxation

The HYM (Hybrid MHD) code has been used to perform three-dimensional, nonlinear MHD simulations of co- and counter-helicity spheromak merging. Extensive comparisons are made between the simulation results and experimental results from the Swarthmore Spheromak Experiment (SSX). Remarkable agreement is observed in both cases. The analysis of the simulation data is aided by high-quality three-dimensional visualizations that are rendered using the VisIt software package. The co-helicity merging scenario is particularly interesting because the spheromaks are arranged so that they merge in a highly non-axisymmetric fashion. During this process, the spatial profile of the Taylor eigenvalue $\lambda$ is observed to evolve from a highly non-uniform profile to a nearly-flat profile that represents a relaxed, non-axisymmetric Taylor eigenstate. Additional simulations are used to examine the effects of line-tying and finite plasma pressure on this relaxation process.   

Latest results from coupled core-edge simulations of pedestal buildup in the DIII-D tokamak using the FACETS code

We present simulations of H-mode pedestal buildup in the DIII-D tokamak using the FACETS code with particular emphasis on the ELM free regions of shots 98889, 118897 and 140417. The core region of the tokamak is simulated using a parallel, nested-iteration based core solver using a combination of anomalous (GLF23, MMM95 or TGLF model) and neoclassical (NCLASS or Chang-Hinton) fluxes. Sources are provided from either NUBEAM simulations or from interpretive ONETWO calculations. Magnetic equilibrium is taken from an experimental reconstruction and is held fixed or varied kinematically during the simulation. The edge region of the tokamak is simulated using the fluid code UEDGE. The transport coefficients used in UEDGE are held constant in time but are allowed to vary spatially. Coupling is achieved by exchanging fluxes and values at the core-edge interface. Electron and ion temperatures and plasma density are evolved and compared to experimental results.   

Coupled Kinetic-MHD Simulations of Divertor Heat Load with ELM Perturbations

The effect of Type-I ELM activity on divertor plate heat load is a key component of the DOE OFES Joint Research Target milestones for this year. In this talk, we present simulations of kinetic edge physics, ELM activity, and the associated divertor heat loads in which we couple the discrete guiding-center neoclassical transport code XGC0 with the nonlinear extended MHD code M3D using the End-to-end Framework for Fusion Integrated Simulations, or EFFIS. In these coupled simulations, the kinetic code and the MHD code run concurrently on the same massively parallel platform and periodic data exchanges are performed using a memory-to-memory coupling technology provided by EFFIS. The M3D code models the fast ELM event and sends frequent updates of the magnetic field perturbations and electrostatic potential to XGC0, which in turn tracks particle dynamics under the influence of these perturbations and collects divertor particle and energy flux statistics. We describe here how EFFIS technologies facilitate these coupled simulations and discuss results for DIII-D, NSTX and Alcator C-Mod tokamak discharges.   

Implicit core-edge coupling in FACETS

In integrated simulations, implicit coupling can be used to put component codes in a self-consistent state at the end of every time step. With explicit coupling, self-consistency is only achieved in the limit of infinitely short time step, or at steady state. From the perspective of an implicit coupler, each component code is simply a vector-valued function (typically nonlinear) of a vector argument, where the components of the argument vector are the code input variables and the value vector consists of the output variables. Implicit coupling then becomes the solve of a nonlinear system of equations. A good nonlinear solver should minimize the number of function evaluations, each corresponding to a code run and therefore very numerically expensive. We have implemented implicit component coupling in FACETS using either Picard iteration or a quasi-Newton scheme with the Jacobian computed using the hypersecant approximation, which requires no extra function evaluations. Results are presented for implicit coupling of core and edge transport components.   

Progress Towards a Coupled Kinetic Plasma - Neutral Transport Code

To provide a kinetic neutral simulation capability for the Center for Plasma Edge Simulation, a subroutine interface to the DEGAS~2 Monte Carlo neutral transport code has been implemented and coupled into the XGC neoclassical particle transport code. The DEGAS 2 routine simulates collisions of kinetic neutrals with a fluid plasma background provided by XGC; a complementary collision routine in XGC handles plasma particle collisions with a fluid neutral background. The procedure used to couple the codes has been designed so that the mass exchange rate in plasma-neutral collisions is the same in the two routines in a statistical sense; this is demonstrated in a practical calculation. However, the corresponding energy exchange rates differ noticeably (e.g., $25 \%$) due to the velocity dependence of the charge exchange cross section and the non-Maxwellian character of both the ion and atom distribution functions. The coupled codes should still be useful for scientific investigations of neoclassical transport in diverted tokamaks and of the role of neutral transport in the plasma edge since the associated error in XGC's global energy conservation is $< 1 \%$. Nonetheless, approaches to ensuring complete energy and momentum conservation are being assessed and will be discussed.   

Integrated Plasma Simulation of Lower Hybrid Current Drive Modification of Sawtooth in Alcator C-Mod

Experiments were performed in Alcator C-Mod, where the onset time for sawteeth was delayed significantly (up to 0.5 s) relative to ohmically heated plasmas, through injection of off-axis LH current drive power [1]. In this poster we discuss simulations of these experiments using the Integrated Plasma Simulator (IPS) [2], through which driven current density profiles and hard x-ray spectra are computed using a ray tracing code (GENRAY) and Fokker Planck code (CQL3D) [3], that are executed repeatedly in time. The background plasma is evolved in these simulations using the TSC transport code with the Porcelli sawtooth model [4]. \\[4pt] [1] C. E. Kessel \textit{et al}, Bull. of the Am. Phys. Soc. \textbf{53}, Poster PP6.00074 (2008). \\[0pt] [2] D. Batchelor \textit{et al}, Journal of Physics: Conf. Series \textbf{125}, 012039 (2008). \\[0pt] [3] R. W. Harvey and M. G. McCoy, Proc. of the IAEA Tech. Comm. Mtg. on Sim. and Mod. of Therm. Plasmas, Montreal, Canada (1992). \\[0pt] [4] S. C. Jardin \textit{et al}, Journal Comp. Phys. \textbf{66}, 481 (1986).   

Parallelizing Time Using Parareal As Implemented by the SWIM IPS

A challenge for time-dependent simulations is to make effective use parallel computers--time cannot be directly parallelized. The Parareal algorithm [Lyons] does parallelize in time, and may run with a shorter wall-clock time at the expense of computer cycles. Parareal iteratively couples a coarse, fast serial solver with a fine, (much) slower solver that runs time slices in parallel. The algorithm specifies how to couple the two solvers and (always) converge to a solution. How efficiently it converges depends on the choice of the coarse solver. This workflow is implemented within the IPS framework to test the use of Parareal in speeding up drift wave simulations. The IPS resource manager is key to the implementation. Tests using the Lorenz system have been successful, and investigations for the drift wave code BETA. [Newman] are now underway [Samaddar]. [Newman] D.E. Newman, et al., Phys. Plasmas 1, 1592 (1994). [Lyons] J.L Lyons, et al., C. R. Acad. Sci. Paris 332, 661 (2001). [Samaddar] D. Samaddar, et al., Journal of Computational Physics 229, 6558 (2010).   

First results of coupled IPS/NIMROD/GENRAY simulations

The Integrated Plasma Simulator (IPS) framework, developed by the SWIM Project Team, facilitates self-consistent simulations of complicated plasma behavior via the coupling of various codes modeling different spatial/temporal scales in the plasma. Here, we apply this capability to investigate the stabilization of tearing modes by ECCD. Under IPS control, the NIMROD code (MHD) evolves fluid equations to model bulk plasma behavior, while the GENRAY code (RF) calculates the self-consistent propagation and deposition of RF power in the resulting plasma profiles. GENRAY data is then used to construct moments of the quasilinear diffusion tensor (induced by the RF) which influence the dynamics of momentum/energy evolution in NIMROD's equations. We present initial results from these coupled simulations and demonstrate that they correctly capture the physics of magnetic island stabilization [Jenkins et al, PoP {\bf 17}, 012502 (2010)] in the low-beta limit. We also discuss the process of code verification in these simulations, demonstrating good agreement between NIMROD and GENRAY predictions for the flux-surface-averaged, RF-induced currents. An overview of ongoing model development (synthetic diagnostics/plasma control systems; neoclassical effects; etc.) is also presented. Funded by US DoE.   

Validation of inputs in multiphysics simulations

In multiphysics simulations involving integration of originally separately developed components, there are several opportunities for error in the specification. Both syntactic, set, and semantic validity are of concern as well as inter-component consistency. Syntactic validity requires that formatting of the input files to the components are correct, such as proper syntax in XML files or proper spacing and ordering in text files lacking markup. Set validity is the requirement that inputs conform to the type and range or set of values permitted. Semantic validity is the assertion that inputs are sensical and logically internally consistent - that is they belong to the set of possible intended inputs. These semantic constraints are sometimes explicit in the codes, but are more often unstated or unenforced. Possibly included in semantic validity is consistency between component inputs so that inputs that are assumed to be the same actually are. We describe the application of XML technologies to these problems: XML schema for set consistency, XForms for manipulating the inputs in the form of an XML instance coupled with XML style sheet transforms to ensure correct input syntax, and XML ontologies to enforce semantic constraints.   

Database-driven web interface automating gyrokinetic simulations for validation

We are developing a web interface to connect plasma microturbulence simulation codes with experimental data. The website automates the preparation of gyrokinetic simulations utilizing plasma profile and magnetic equilibrium data from TRANSP analysis of experiments, read from MDSPLUS over the internet. This database-driven tool saves user sessions, allowing searches of previous simulations, which can be restored to repeat the same analysis for a new discharge. The website includes a multi-tab, multi-frame, publication quality java plotter Webgraph, developed as part of this project. Input files can be uploaded as templates and edited with context-sensitive help. The website creates inputs for GS2 and GYRO using a well-tested and verified back-end, in use for several years for the GS2 code [D. R. Ernst et al., Phys. Plasmas 11(5) 2637 (2004)]. A centralized web site has the advantage that users receive bug fixes instantaneously, while avoiding the duplicated effort of local compilations. Possible extensions to the database to manage run outputs, toward prototyping for the Fusion Simulation Project, are envisioned. Much of the web development utilized support from the DoE National Undergraduate Fellowship program [e.g., A. Suarez and D. R. Ernst, http://meetings.aps.org/link/BAPS.2005.DPP.GP1.57.]   

Experience using Plasma States in Multiphysics Simulations

The ``Plasma State'' was developed originally in the SWIM SciDAC as a software resource for data exchange in multiphysics tokamak simulation. Commonly used time dependent data such as axisymmetric tokamak MHD equilibrium flux surface geometry, plasma temperature and density profiles, and profiles of sources of heat, momentum, particles, and current, as well as time invariant data such as machine description, shot configuration, and plasma species lists, are all shared between physics model components using ``Plasma State'' objects. The simple, flat structure of these objects, along with rich metadata, I/O and interpolation services, has made them relatively easy to use; as a result of this, their use has spread beyond the SWIM SciDAC into FACETS, TGYRO, and other tokamak community research applications. This poster reviews Plasma State design, current usage and experience, with consideration of potential implications for future high performance integrated modeling.   

A comparison of data interoperability approaches of fusion codes with application to synthetic diagnostics

As various efforts to integrate fusion codes proceed worldwide, standards for sharing data have emerged. In the U.S., the SWIM project has pioneered the development of the Plasma State, which has a flat-hierarchy and is dominated by its use within 1.5D transport codes. The European Integrated Tokamak Modeling effort has developed a more ambitious data interoperability effort organized around the concept of Consistent Physical Objects (CPOs). CPOs have deep hierarchies as needed by an effort that seeks to encompass all of fusion computing. Here, we discuss ideas for implementing data interoperability that is complementary to both the Plasma State and CPOs. By making use of attributes within the netcdf and HDF5 binary file formats, the goals of data interoperability can be achieved with a more informal approach. In addition, a file can be simultaneously interoperable to several standards at once. As an illustration of this approach, we discuss its application to the development of synthetic diagnostics that can be used for multiple codes.   

Progress Towards a Rad-Hydro Code for Modern Computing Architectures LA-UR-10-02825

We are entering an era of high performance computing where data movement is the overwhelming bottleneck to scalable performance, as opposed to the speed of floating-point operations per processor. All multi-core hardware paradigms, whether heterogeneous or homogeneous, be it the Cell processor, GPGPU, or multi-core x86, share this common trait. In multi-physics applications such as inertial confinement fusion or astrophysics, one may be solving multi-material hydrodynamics with tabular equation of state data lookups, radiation transport, nuclear reactions, and charged particle transport in a single time cycle. The algorithms are intensely data dependent, e.g., EOS, opacity, nuclear data, and multi-core hardware memory restrictions are forcing code developers to rethink code and algorithm design. For the past two years LANL has been funding a small effort referred to as Multi-Physics on Multi-Core to explore ideas for code design as pertaining to inertial confinement fusion and astrophysics applications. The near term goals of this project are to have a multi-material radiation hydrodynamics capability, with tabular equation of state lookups, on cartesian and curvilinear block structured meshes. In the longer term we plan to add fully implicit multi-group radiation diffusion and material heat conduction, and block structured AMR. We will report on our progress to date.   

Kinetic simulations of plasma sheath with parallel to the wall magnetic field

Plasma-wall interactions can play an important role in plasma transport and confinement in tokamaks or Magneto-Inertial Fusion (MIF), where one of the approaches is to use an imploding metal liner to compress magnetized target plasma to thermonuclear temperatures. Since for the MIF applications the magnetic field is parallel to the liner surface, the ions, with their large gyro-radii, positively charge the wall. This creates a strong ExB shear flow which can cause turbulence and influence transport. Here we report on progress of the simulation studies of plasma sheath turbulence using a state-of-the-art VPIC [1] code. Baseline calculations have confirmed the possibility of establishing a quiescent plasma sheath in 1D for a flat liner surface[2,3]. However, in higher dimensions, these self-consistent plasma and field parameters do not always result in a stable sheath. In this work we present the analysis of a 2D equilibrium and quantify its stability characteristics for two cases, with Debye length being intermediate between electron and ion thermal Larmors and Debye length being much smaller than electron Larmor. [1] K. J. Bowers, et al. Phys. Plasmas 15, 055703 (2008). [2] N. Krasheninnikova, et al. Phys. Plasmas 17 057103 (2010). [3] N. Krasheninnikova, et al. Phys. Plasmas 17 063508 (2010).