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A quasi-neutral blended drift-Lorentz particle model of the field-reversed configuration (FRC) has been developed and implemented in the particle-in-cell code named ANC. A field-aligned mesh and corresponding mesh operations are constructed for solving self-consistent electric fields in FRC geometry. Particle dynamics are described in cylindrical coordinates to allow for cross-separatrix simulation coupling the core and scrape-off layer regions of the FRC. This new model is successfully verified against analytically derived dispersion relations, and FRC turbulence is studied using the blended model for the first time. [ABSTRACT FROM AUTHOR]

Detailed 2D turbulence measurements from the DIII-D tokamak provide an explanation for how resonant magnetic perturbations (RMPs) raise the L-H power threshold P LH [P. Gohil et al., Nucl. Fusion 51, 103020 (2011)] in ITER-relevant, low rotation, ITER-similar-shape plasmas with favorable ion ∇ B direction. RMPs simultaneously raise the turbulence decorrelation rate Δ ω D and reduce the flow shear rate ω shear in the stationary L-mode state preceding the L-H transition, thereby disrupting the turbulence shear suppression mechanism. RMPs also reduce the Reynolds stress drive for poloidal flow, contributing to the reduction of ω shear . On the ∼100 μs timescale of the L-H transition, RMPs reduce Reynolds-stress-driven energy transfer from turbulence to flows by an order of magnitude, challenging the energy depletion theory for the L-H trigger mechanism. In contrast, non-resonant magnetic perturbations, which do not significantly affect P LH , do not affect Δ ω D and only slightly reduce ω shear and Reynolds-stress-driven energy transfer. [ABSTRACT FROM AUTHOR]

We present gyrokinetic simulations with the GENE code addressing the near-edge region of an L-mode plasma in the DIII-D tokamak. At radial position ρ = 0.80, simulations with the ion temperature gradient (ITG) increased by 40% above the nominal value give electron and ion heat fluxes that are in simultaneous agreement with the experiment. This gradient increase is consistent with the combined statistical and systematic uncertainty σ of the charge exchange recombination spectroscopy measurements at the 1.6σ level. Multiscale simulations are carried out with a realistic mass ratio and geometry for the first time in the near-edge. These multiscale simulations suggest that the highly unstable ion temperature gradient (ITG) modes of the flux-matched ion-scale simulations suppress electron-scale transport, such that ion-scale simulations are sufficient at this location. At radial position ρ = 0.90, nonlinear simulations show a hybrid state of ITG and trapped electron modes, which was not expected from linear simulations. The nonlinear simulations reproduce the total experimental heat flux with the inclusion of E × B shear effects and an increase in the electron temperature gradient by ∼23%. This gradient increase is compatible with the combined statistical and systematic uncertainty of the Thomson scattering data at the 1.3σ level. These results are consistent with previous findings that gyrokinetic simulations are able to reproduce the experimental heat fluxes by varying input parameters close to their experimental uncertainties, pushing the validation frontier closer to the edge region. [ABSTRACT FROM AUTHOR]

The L-H transition power threshold (PLH) is found to have a significant dependence on q95, at ne ∼ 3.2 × 1019 m−3 on DIII-D. Comprehensive 2D turbulence and flow measurements in the plasma edge reveal the co-existence of two frequency bands of broadband modes across the L-H transition with higher flow shear at higher q95, which can help explain the linear decrease in PLH as q95 is increased from 3.5 to 4.9. Density fluctuation measurements by beam emission spectroscopy at higher q95 show that the lower frequency band (<20 kHz) of the broadband modes propagates in the ion diamagnetic direction in the lab frame (identified as an ion mode), whereas the higher frequency band (>20 kHz) propagates in the electron diamagnetic direction (identified as an electron mode). The turbulence amplitude at the plasma edge is higher at higher q95, implying a higher drive for a Reynolds stress driven zonal flow. At lower density, ne ∼ 1.5 × 1019 m−3, there is little dependence of PLH on q95. Linear CGYRO kinetic simulation has shown a switch from one mode to two modes at higher q95 at ne ∼ 3.2 × 1019 m−3 compared to lower q95. The ion temperature profile has been shown to strongly impact the dual mode characteristics with the increasing growth rate of the ion mode at a higher edge Ti gradient. The observations suggest that it may be possible to reduce the required input power to trigger the L-H transition for ITER if both the ion and electron modes are similarly driven in the plasma edge. [ABSTRACT FROM AUTHOR]

Gyrokinetic simulations of C-2-like field-reversed configuration (FRC) find that electrostatic driftwaves are locally stable in the core. The stabilization mechanisms include finite Larmor radius effects, magnetic well (negative grad-B), and fast electron short circuit effects. In the scrape-off layer (SOL), collisionless electrostatic drift-waves in the ion-to-electron-scale are destabilized by electron temperature gradients due to the resonance with locally barely trapped electrons. Collisions can suppress this instability, but a collisional drift-wave instability still exists at realistic pressure gradients. Simulation results are in qualitative agreement with C-2 FRC experiments. In particular, the lack of ion-scale instability in the core is not inconsistent with experimental measurements of a fluctuation spectrum showing a depression at ion-scales. The pressure gradient thresholds for the SOL instability from simulations are also consistent with the critical gradient behavior observed in experiments. [ABSTRACT FROM AUTHOR]

A series of DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] low torque quiescent H-mode experiments show that density gradient driven trapped electron mode (DGTEM) turbulence dominates the inner core of H-mode plasmas during strong electron cyclotron heating (ECH). Adding 3.4MW ECH doubles Te/Ti from 0.5 to 1.0, which halves the linear DGTEM critical density gradient, locally reducing density peaking, while transport in all channels displays extreme stiffness in the density gradient. This suggests that fusion α-heating may degrade inner core confinement in H-mode plasmas with moderate density peaking and low collisionality, with equal electron and ion temperatures, key conditions expected in burning plasmas. Gyrokinetic simulations using GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] (and GENE [Jenko et al., Phys. Plasmas 7, 1904 (2000)]) closely match not only particle, energy, and momentum fluxes but also density fluctuation spectra from Doppler backscattering (DBS), with and without ECH. Inner core DBS density fluctuations display discrete frequencies with adjacent toroidal mode numbers, which we identify as DGTEMs. GS2 [Dorland et al., Phys. Rev. Lett. 85, 5579 (2000)] predictions show the DGTEM can be suppressed, to avoid degradation with electron heating, by broadening the current density profile to attain q0 > qmin > 1. [ABSTRACT FROM AUTHOR]

Following the recent remarkable progress in magnetohydrodynamic (MHD) stability control in the C-2U advanced beam driven field-reversed configuration (FRC), turbulent transport has become one of the foremost obstacles on the path towards an FRC-based fusion reactor. Significant effort has been made to expand kinetic simulation capabilities in FRC magnetic geometry. The recently upgraded Gyrokinetic Toroidal Code (GTC) now accommodates realistic magnetic geometry from the C-2U experiment at Tri Alpha Energy, Inc. and is optimized to efficiently handle the FRC's magnetic field line orientation. Initial electrostatic GTC simulations find that ion-scale instabilities are linearly stable in the FRC core for realistic pressure gradient drives. Estimated instability thresholds from linear GTC simulations are qualitatively consistent with critical gradients determined from experimental Doppler backscattering fluctuation data, which also find ion scale modes to be depressed in the FRC core. Beyond GTC, A New Code (ANC) has been developed to accurately resolve the magnetic field separatrix and address the interaction between the core and scrape-off layer regions, which ultimately determines global plasma confinement in the FRC. The current status of ANC and future development targets are discussed. [ABSTRACT FROM AUTHOR]

While operating a magnetic fusion device in H-mode has many advantages, care must be taken to understand and control the release of energy during the H-L back transition, as the extra energy stored within the H-modetransport barrier will have the potential to cause damage to material components of a large future tokamak such as ITER. Examining a scenario where the H-L back transition sequence begins before the E × B shearing layer decays on its own, we identify a long-lived precursor mode that is tied to the events of the H-L sequence and we develop a robust control strategy for ensuring gradual release of energy during the transition sequence. Back transitions in this scenario commonly begin with a rapid relaxation of the pedestal, which was previously shown to be inconsistent with ideal peeling-ballooning instability as the trigger [Eldon et al., Phys. Plasmas 22, 052109 (2015)], despite being otherwise similar to a large type-I Edge Localized Mode(ELM). This so-called transient occurs when the E × B shearing rate ωE×B is significantly larger than the turbulence decorrelation rate ωT, indicating that this is not the result of runaway turbulence recovery. The transient is always synchronous with amplitude and propagation velocity modulations of the precursor mode, which has been dubbed the Modulating Pedestal Mode (MPM). The MPM is a coherent density fluctuation, which, in our scenario at least, reliably appears in the steep gradient region with f≈70 kHz, kθ ≈ 0.3 cm-1, and it exists for ≳100 ms before the onset of back transitions. The transient may be reliably eliminated by reducing toroidal rotation in the co-current direction by the application of torque from counter-injecting neutral beams. The transient in these "soft" H-L transitions is then replaced by a small type-III ELM, which is also always synchronous with the MPM, and MPM shows the same behavior in both hard and soft cases. [ABSTRACT FROM AUTHOR]

Conventional field-reversed configurations (FRCs), high-beta, prolate compact toroids embedded in poloidal magnetic fields, face notable stability and confinement concerns. These can be ameliorated by various control techniques, such as introducing a significant fast ion population. Indeed, adding neutral beam injection into the FRC over the past half-decade has contributed to striking improvements in confinement and stability. Further, the addition of electrically biased plasma guns at the ends, magnetic end plugs, and advanced surface conditioning led to dramatic reductions in turbulence-driven losses and greatly improved stability. Together, these enabled the build-up of a well-confined and dominant fast-ion population. Under such conditions, highly reproducible, macroscopically stable hot FRCs (with total plasma temperature of ~1 keV) with record lifetimes were achieved. These accomplishments point to the prospect of advanced, beam-driven FRCs as an intriguing path toward fusion reactors. This paper reviews key results and presents context for further interpretation. [ABSTRACT FROM AUTHOR]

The H-mode transport barrier allows confinement of roughly twice as much energy as in an L-mode plasma. Termination of H-mode necessarily requires release of this energy, and the timescale of that release is of critical importance for the lifetimes of plasma facing components in next step tokamaks such as ITER. H-L transition sequences in modern tokamaks often begin with a transient outburst which appears to be superficially similar to and has sometimes been referred to as a type-I edge localized mode (ELM). Type-I ELMs have been shown to be consistent with ideal peeling ballooning instability and are characterized by significant (up to ~50%) reduction of pedestal height on short (~1ms) timescales. Knowing whether or not this type of instability is present during H-L back transitions will be important of planning for plasma ramp-down in ITER. This paper presents tests of pre-transition experimental data against ideal peeling-ballooning stability calculations with the ELITE code and supports those results with secondary experiments that together show that the transient associated with the H-L transition is not triggered by the same physics as are type-I ELMs. [ABSTRACT FROM AUTHOR]

A versatile heterodyne Doppler Backscattering (DBS) system is used to measure density fluctuation levels (in the wavenumber range kρs 50), and the toroidal E x B flow velocity in the C-2 Field-Reversed Configuration (FRC). Six tunable frequencies in three waveguide bands (26 GHz ≤ f ≤ 90 GHz) are launched using monostatic beam optics, via a quasi-optical beam combiner/polarizer and an adjustable parabolic focusing mirror (inside the vacuum enclosure) achieving Gaussian beam spot sizes of 3-5.5 cm at the X/O-mode cutoff. The DBS system covers plasma densities of 0.8 x 1013 ≤ ne ≤ 1 x 1014 cm-3, and provides access to the FRC core (up to the ?eld null) and across the FRC separatrix into the scrape-off layer plasma. [ABSTRACT FROM AUTHOR]

Experimental tests of ion poloidal rotation theories have been performed on DIII-D using a novel impurity poloidal rotation diagnostic. These tests show significant disagreements with theoretical predictions in various conditions, including L-mode plasmas with internal transport barriers (ITB), H-mode plasmas, and QH-mode plasmas. The theories tested include standard neoclassical theory, turbulence driven Reynolds stress, and fast-ion friction on the thermal ions. Poloidal rotation is observed to spin up at the formation of an ITB and makes a significant contribution to the measurement of the E... × B... shear that forms the ITB. In ITB cases, neoclassical theory agrees quantitatively with the experimental measurements only in the steep gradient region. Significant quantitative disagreement with neoclassical predictions is seen in the cores of ITB, QH-, and H-mode plasmas, demonstrating that neoclassical theory is an incomplete description of poloidal rotation. The addition of turbulence driven Reynolds stress does not remedy this disagreement; linear stability calculations and Doppler backscattering measurements show that disagreement increases as turbulence levels decline. Furthermore, the effect of fast-ion friction, by itself, does not lead to improved agreement; in QH-mode plasmas, neoclassical predictions are closest to experimental results in plasmas with the largest fast ion friction. Predictions from a new model that combines all three effects show somewhat better agreement in the H-mode case, but discrepancies well outside the experimental error bars remain. [ABSTRACT FROM AUTHOR]

The geodesic acoustic mode (GAM), a coherent form of the zonal flow, plays a critical role in turbulence regulation and cross-magnetic-field transport. In the DIII-D tokamak, unique information on multi-field characteristics and radial structure of eigenmode GAMs has been measured. Two simultaneous and distinct, radially overlapping eigenmode GAMs (i.e., constant frequency vs. radius) have been observed in the poloidal E×B flow in L-mode plasmas. As the plasma transitions from an L-mode to an Ohmic regime, one of these eigenmode GAMs becomes a continuum GAM (frequency responds to local parameters), while the second decays below the noise level. The eigenmode GAMs occupy a radial range of ρ = 0.6-0.8 and 0.75-0.95, respectively. In addition, oscillations at the GAM frequency are observed for the first time in multiple plasma parameters, including ne, Te, and Bθ. The magnitude of Te/Te at the GAM frequency (the magnitude is similar to that of ñe/ne) and measured ne-Te cross-phase (∼140° at the GAM frequency) together indicate that the GAM pressure perturbation is not determined solely by ñe. The magnetic GAM behavior, a feature only rarely reported, is significantly stronger (×18) on the high-field side of the tokamak, suggesting an anti-ballooning nature. Finally, the GAM is also observed to directly modify intermediate-wavenumber ñe levels (kρs ∼ 1.1). The simultaneous temperature, density, flow fluctuations, density-temperature cross-phase, and magnetic behavior present a new perspective on the underlying physics of the GAM. [ABSTRACT FROM AUTHOR]

Since ITER will operate close to threshold and with limited control, the H → L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model [Miki et al., Phys. Plasmas 19, 092306 (2012)], we investigate ELM-free H → L back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number, where Nu=χi,turb/χi,neo. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Prped=Dped/χi,neo. We expect the H-mode to be somewhat more resilient in density than in temperature. [ABSTRACT FROM AUTHOR]

A critical gradient for long wavelength (kθρs<=0.4) electron temperature fluctuations has been observed in an experiment in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)], where below a threshold value of LTe-1=|∇Te|/Te electron temperature fluctuations are constant and above they steadily increase. Above the critical gradient, the electron heat flux inferred by power balance also increases rapidly. Critical gradients are a predicted attribute of turbulence arising from linear instabilities and are thought to be related to transport stiffness. The presented results are the first direct, systematic demonstration of critical gradient behavior in turbulence measurements in a tokamak. The experiment was performed by changing the deposition location of electron cyclotron heating shot-to-shot to locally scan LTe-1 at r/a = 0.6 in L-mode plasmas; rotation was also varied by changing the momentum input from neutral beam injection. Temperature fluctuations were measured with a correlation electron cyclotron emission (CECE) radiometry system. In addition to the CECE measurements, an array of turbulence measurements were acquired to characterize fluctuations in multiple fields and at multiple scales as LTe-1 and rotation were modified: long wavelength (kθρs<=0.5) density fluctuations were acquired with beam emission spectroscopy, the phase angle between electron temperature and density fluctuations was measured by coupling the CECE system and a reflectometer, intermediate scale (kθρs∼0.8) density fluctuations were measured with a Doppler backscattering (DBS) system, and low frequency flows were also measured with DBS. The accumulated measurements and trends constrain identification of the instability responsible for the observed critical gradient to the ∇Te-driven trapped electron mode. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of the low(L) → high(H) transition using a time-dependent, one dimensional (in radius) model which self-consistently describes the time evolution of zonal flows (ZFs), mean flows (MFs), poloidal spin-up, and density and pressure profiles. The model represents the physics of ZF and MF competition, turbulence suppression via E×B shearing, and poloidal flows driven by turbulence. Numerical solutions of this model show that the L→H transition can occur via an intermediate phase (I-phase) which involves oscillations of profiles due to ZF and MF competition. The I-phase appears as a nonlinear transition wave originating at the edge boundary and propagates inward. Locally, I-phase exhibits the characteristics of a limit-cycle oscillation. All these observations are consistent with recent experimental results. We examine the trigger of the L→H transition, by defining a ratio of the rate of energy transfer from the turbulence to the zonal flow to the rate of energy input into the turbulence. When the ratio exceeds order unity, ZF shear gains energy, and a net decay of the turbulence is possible, thus triggering the L→H transition. Numerical calculations indicate that the L→H transition is triggered by this peak of the normalized ZF shearing. Zonal flows act as 'reservoir,' in which to store increasing fluctuation energy without increasing transport, thus allowing the mean flow shear to increase and lock in the transition. A counterpart of the L → I→H transition, i.e., an L→H transition without I-phase, is obtained in a fast power ramp, for which I-phase is compressed into a single burst of ZF, which triggers the transition. Effects of neutral charge exchange on the L→H transition are studied by varying ZF damping and neoclassical viscosity. Results show that the predicted L→H transition power increases when either ZF damping or viscosity increase, suggesting a link between recycling, ZF damping, and the L→H threshold. Studies of fueling effects on the transition and pedestal structure with an emphasis on the particle pinch are reported. [ABSTRACT FROM AUTHOR]

Electron profile stiffness was studied in DIII-D L-mode discharges by systematically varying the heat flux in a narrow region with electron cyclotron heating and measuring the local change produced in ∇Te. Electron stiffness was found to slowly increase with toroidal rotation velocity. A critical inverse temperature gradient scale length 1/LC ∼ 3 m-1 was identified at ρ=0.6 and found to be independent of rotation. Both the heat pulse diffusivity and the power balance diffusivity, the latter determined by integrating the measured dependence of the heat pulse diffusivity on -∇Te, were fit reasonably well by a model containing a critical inverse temperature gradient scale length and varying linearly with 1/LT above the threshold. [ABSTRACT FROM AUTHOR]

An analysis of the dependence of transport on the safety factor profile in high-performance, steady-state scenario discharges is presented. This is based on experimental scans of q95 and qmin taken with fixed βN, toroidal field, double-null plasma shape, divertor pumping, and electron cyclotron current drive input. The temperature and thermal diffusivity profiles were found to vary considerably with the q-profile, and these variations were significantly different for electrons and ions. With fixed q95, both temperature profiles increase and broaden as qmin is increased and the magnetic shear becomes low or negative in the inner half radius, but these temperature profile changes are stronger for the electrons. Power balance calculations show the peak in the ion thermal diffusivity (χi) at ρ=0.6-0.8 increases with q95 or qmin. In contrast, the peak in the electron diffusivity (χe) decreases as qmin is raised from ∼1 to 1.5, and it is insensitive to q95. This is important for fully non-inductive scenario development because it demonstrates that elevated qmin and weak or reversed shear allow larger electron temperature gradients and, therefore, increased bootstrap current density to exist at ρ=0.6-0.8. Chord-averaged measurements of long wavelength density fluctuation amplitudes (ñ) are shown, and these have roughly the same dependence on q-profile as χi. This data set provides an opportunity for testing whether theory based transport models can provide insight into the underlying transport physics of high performance scenarios and if they can reproduce observed experimental trends. To this end, we applied the trapped gyro-Landau fluid (TGLF) code to calculate the linear stability of drift waves and found that the resulting variation of growth rates with q-profile are mostly inconsistent with the observed trends of χi, χe, and ñ with q-profile. TGLF simulations of the temperature profiles consistent with heating sources also have mixed agreement with the measured profiles, such that the simulated electron and ion heat flux in low qmin discharges are too low and heat fluxes in high qmin discharges are too high. [ABSTRACT FROM AUTHOR]

Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric (m=0andn=0) E×B flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Results are presented showing the GAM's observed spatial scales, temporal scales, and nonlinear interaction characteristics, which may have implications for the assumptions underpinning turbulence models towards the tokamak edge (r/a>rsim0.75). Measurements in the DIII-D tokamak [Luxon, Nucl. Fusion 42, 614 (2002)] have been made with multichannel Doppler backscattering systems at toroidal locations separated by 180°; analysis reveals that the GAM is highly coherent between the toroidally separated systems (γ>0.8) and that measurements are consistent with the expected m=0andn=0 structure. Observations show that the GAM in L-mode plasmas with ~2.5-4.5 MW auxiliary heating occurs as a radially coherent eigenmode, rather than as a continuum of frequencies as occurs in lower temperature discharges; this is consistent with theoretical expectations when finite ion Larmor radius effects are included. The intermittency of the GAM has been quantified, revealing that its autocorrelation time is fairly short, ranging from about 4 to about 15 GAM periods in cases examined, a difference that is accompanied by a modification to the probability distribution function of the E×B velocity at the GAM frequency. Conditionally-averaged bispectral analysis shows the strength of the nonlinear interaction of the GAM with broadband turbulence can vary with the magnitude of the GAM. Data also indicate a wavenumber dependence to the GAM's interaction with turbulence. [ABSTRACT FROM AUTHOR]

The multi-field/multi-scale core (ρ ∼ 0.5-0.8) turbulence response to electron cyclotron heating (ECH) of DIII-D Ohmic plasmas is reported for the first time. Long wavelength (low-k) electron temperature (Te/Te) and high-k density turbulence levels (ñe/ne) are observed to strongly increase during ECH. In contrast, low-k and intermediate-k ñe/ne showed little change, whereas the cross-phase between local low-k electron temperature and density fluctuations (αneTe) was significantly modified. The increase in the electron thermal diffusivity determined from power balance is consistent with the increased turbulent transport correlated with the measured increases in low-k Te/Te and high-k ñe/ne. Linear stability analysis using the trapped gyro-Landau fluid (TGLF) model indicates an enhanced growth rate for electron modes [e.g., trapped electron mode (TEM)] at low-k consistent with the observed modifications in Te/Te and αneTe. TGLF also predicts an increase in high-k electron mode growth rates for normalized wavenumbers kθρs > 7, where electron temperature gradient (ETG) modes exist, which is consistent with the observed increase in high-kñe/ne turbulence [ABSTRACT FROM AUTHOR]

Robust validation of predictive turbulent transport models requires quantitative comparisons to experimental measurements at multiple levels, over a range of physically relevant conditions. Toward this end, a series of carefully designed validation experiments has been performed on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] to obtain comprehensive multifield, multipoint, multiwavenumber fluctuation measurements and their scalings with key dimensionless parameters. The results of two representative validation studies are presented: an elongation scaling study performed in beam heated L-mode discharges and an electron heating power scan performed in quiescent H-mode (QH-mode) discharges. A 50% increase in the elongation κ is observed to lead to a ∼50% increase in energy confinement time τe and accompanying decrease in fluctuation levels, qualitatively consistent with a priori theoretical predictions and nonlinear GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] simulations. However, these simulations exhibit clear quantitative differences from experiment in the predicted magnitudes and trends with radius of turbulent fluxes and fluctuation levels which cannot be fully accounted for by uncertainties due to transport stiffness. In the QH-mode study, local nonlinear GYRO simulations that neglect fast ion effects show a similar proportional response to the applied electron cyclotron heating as the experiment, but overpredict the magnitudes of transport and fluctuation levels by a factor of 10 or more. Possible sources of this overprediction, namely nonlocal effects and self-consistent fast beam ions, are identified and discussed. [ABSTRACT FROM AUTHOR]

A novel multichannel, tunable Doppler backscattering (DBS)/reflectometry system has recently been developed and applied to a variety of DIII-D plasmas. Either DBS or reflectometry can be easily configured for use in a wide range of plasma conditions using a flexible quasi-optical antenna system. The multiple closely spaced channels, when combined with other fluctuation diagnostic systems, have opened up new measurements of plasma properties. For example, the toroidal and fine-scale radial structure of coherent plasma oscillations, such as geodesic acoustic modes, have been probed simultaneously in the core of high temperature plasmas by applying correlation analysis between two toroidally separated DBS systems, as well as within the multichannel array. When configured as a reflectometer, cross-correlation with electron cyclotron emission radiometry has uncovered detailed information regarding the crossphase relationship between density and temperature fluctuations. The density-temperature crossphase measurement yields insight into the physics of tokamak turbulence at a fundamental level that can be directly compared with predictions from nonlinear gyrokinetic simulations. [ABSTRACT FROM AUTHOR]

The quasioptical design of a new integrated Doppler backscattering (DBS) and correlation electron cyclotron emission (CECE) system is presented. The design provides for simultaneous measurements of intermediate wavenumber density and long wavelength electron temperature turbulence behavior. The Doppler backscattering technique is sensitive to plasma turbulence flow and has been utilized to determine radial electric field, geodesic acoustic modes, zonal flows, and intermediate scale (k∼1-6 cm-1) density turbulence. The correlation ECE system measures a second turbulent field, electron temperature fluctuations, and is sensitive to long poloidal wavelength (k≤1.8 cm-1). The integrated system utilizes a newly installed in-vessel focusing mirror that produces a beam waist diameter of 3.5-5 cm in the plasma depending on the frequency. A single antenna (i.e., monostatic operation) is used for both launch and receive. The DBS wavenumber is selected via an adjustable launch angle and variable probing frequency. Due to the unique system design both positive and negative wavenumbers can be obtained, with a range of low to intermediate wavenumbers possible (approximately -3 to 10 cm-1). A unique feature of the design is the ability to place the DBS and CECE measurements at the same radial and poloidal locations allowing for cross correlation studies (e.g., measurement of nT cross-phase). [ABSTRACT FROM AUTHOR]

This paper presents new measurements of the cross-phase angle, αneTe, between long-wavelength (kθρs<0.5) density, ñe, and electron temperature, Te, fluctuations in the core of DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak plasmas. The coherency and cross-phase angle between ñe and Te are measured using coupled reflectometer and correlation electron cyclotron emission diagnostics that view the same plasma volume. In addition to the experimental results, two sets of local, nonlinear gyrokinetic turbulence simulations that are performed with the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] are described. One set, called the pre-experiment simulations, was performed prior to the experiment in order to predict a change in αneTe given experimentally realizable increases in the electron temperature, Te. In the experiment the cross-phase angle was measured at three radial locations (ρ=0.55, 0.65, and 0.75) in both a “Base” case and a “High Te” case. The measured cross-phase angle is in good qualitative agreement with the pre-experiment simulations, which predicted that ñe and Te would be out of phase. The pre-experiment simulations also predicted a decrease in cross-phase angle as Te is increased. Experimentally, this trend is observed at the inner two radial locations only. The second set of simulations, the postexperiment simulations, is carried out using local parameters taken from measured experimental profiles as input to GYRO. These postexperiment simulation results are in good quantitative agreement with the measured cross-phase angle, despite disagreements with transport fluxes. Directions for future modeling and experimental work are discussed. [ABSTRACT FROM AUTHOR]

The local value of a/LTe, a turbulence drive term, was modulated with electron cyclotron heating in L-mode discharges on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] and the density and electron temperature fluctuations in low, intermediate, and high-k regimes were measured and compared with nonlinear gyrokinetic turbulence simulations using the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)]. The local drive term at ρ∼0.6 was reduced by up to 50%, which produced comparable reductions in electron temperature fluctuations at low-k. At intermediate k, kθ∼4 cm-1 and kθρs∼0.8, a very interesting and unexpected result was observed where density fluctuations increased by up to 10% when the local drive term was decreased by 50%. Initial comparisons of simulations from GYRO with the thermal diffusivity from power balance analysis and measured turbulence response are reported. Simulations for the case with the lowest drive term are challenging as they are near the marginal value of a/LTe for trapped electron mode activity. [ABSTRACT FROM AUTHOR]

New measurements show that long-wavelength (kθρs<0.5) electron temperature fluctuations can play an important role in determining electron thermal transport in low-confinement mode (L-mode) tokamak plasmas. In neutral beam-heated L-mode tokamak plasmas, electron thermal transport and the amplitude of long-wavelength electron temperature fluctuations both increase in cases where local electron cyclotron heating (ECH) is used to modify the plasma profiles. In contrast, the amplitude of simultaneously measured long-wavelength density fluctuations does not significantly increase. Linear stability analysis indicates that the ratio of the trapped electron mode (TEM) to ion temperature gradient (ITG) mode growth rates increases in the cases with ECH. The increased importance of the TEM drive relative to the ITG mode drive in the cases with ECH may be associated with the increases in electron thermal transport and electron temperature fluctuations. [ABSTRACT FROM AUTHOR]

A novel multichannel Doppler backscattering system has been designed and tested for application on the DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] and National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] fusion plasma devices. Doppler backscattering measures localized intermediate wavenumber (k⊥ρi∼1–4,k⊥∼2–15 cm-1) density fluctuations and the propagation velocity of turbulent structures. Microwave radiation is launched at a frequency that approaches a cutoff layer in the plasma and at an angle that is oblique to the cutoff layer. Bragg backscattering occurs near the cutoff layer for fluctuations with k⊥≈-2ki, where ki is the incident probe wave vector at the scattering location. The turbulence propagation velocity can be determined from the Doppler shift in the return signal together with knowledge of the scattering wavenumber. Ray tracing simulations are used to determine k⊥ and the scattering location. Frequency modulation of a voltage-controlled solid state microwave source followed by frequency multiplication is used to create an array of finely spaced (Δf=350 MHz) frequencies spanning 1.4 GHz. The center of the array bandwidth is tunable within the range of ∼53–78 GHz. This article details the system design, laboratory tests, and presents initial data from DIII-D plasmas. [ABSTRACT FROM AUTHOR]

The deployment of multiple high-resolution, spatially localized fluctuation diagnostics on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] opens the door to a new level of core turbulence model validation. Toward this end, the implementation of synthetic diagnostics that model physical beam emission spectroscopy and correlation electron cyclotron emission diagnostics is presented. Initial results from their applications to local gyrokinetic simulations of two locations in a DIII-D L-mode discharge performed with the GYRO code [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] are also discussed. At normalized toroidal flux ρ=0.5, we find very good agreement between experiment and simulation in both the energy flows and fluctuation levels measured by both diagnostics. However, at ρ=0.75, GYRO underpredicts the observed energy flows by roughly a factor of 7, with rms fluctuation levels underpredicted by a factor of 3. Interestingly, at both locations we find good agreement in the shapes of the radial and vertical density correlation functions and in the shapes of the frequency power spectra. At both locations, the attenuation of the GYRO-predicted fluctuations due to the spatial averaging imposed by the diagnostics’ spot sizes is significant, and its incorporation via the use of synthetic diagnostics is shown to be essential for quantitative comparisons such as these. [ABSTRACT FROM AUTHOR]

A correlation electron cyclotron emission (CECE) diagnostic has been used to measure local, turbulent fluctuations of the electron temperature in the core of DIII-D plasmas. This paper describes the hardware and testing of the CECE diagnostic and highlights the importance of measurements of multifield fluctuation profiles for the testing and validation of nonlinear gyrokinetic codes. The process of testing and validating such codes is critical for extrapolation to next-step fusion devices. For the first time, the radial profiles of electron temperature and density fluctuations are compared to nonlinear gyrokinetic simulations. The CECE diagnostic at DIII-D uses correlation radiometry to measure the rms amplitude and spectrum of the electron temperature fluctuations. Gaussian optics are used to produce a poloidal spot size with wo∼1.75 cm in the plasma. The intermediate frequency filters and the natural linewidth of the EC emission determine the radial resolution of the CECE diagnostic, which can be less than 1 cm. Wavenumbers resolved by the CECE diagnostic are kθ≤1.8 cm-1 and kr≤4 cm-1, relevant for studies of long-wavelength turbulence associated with the trapped electron mode and the ion temperature gradient mode. In neutral beam heated L-mode plasmas, core electron temperature fluctuations in the region 0.5

For the first time, profiles (0.3<ρ<0.9) of electron temperature and density fluctuations in a tokamak have been measured simultaneously and the results compared to nonlinear gyrokinetic simulations. Electron temperature and density fluctuations measured in neutral beam-heated, sawtooth-free low confinement mode (L-mode) plasmas in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] are found to be similar in frequency and normalized amplitude, with amplitude increasing with radius. The measured radial profile of two fluctuation fields allows for a new and rigorous comparison with gyrokinetic results. Nonlinear gyrokinetic flux-tube simulations predict that electron temperature and density fluctuations have similar normalized amplitudes in L-mode. At ρ=0.5, simulation results match experimental heat diffusivities and density fluctuation amplitude, but overestimate electron temperature fluctuation amplitude and particle diffusivity. In contrast, simulations at ρ=0.75 do not match either the experimentally derived transport properties or the measured fluctuation levels. [ABSTRACT FROM AUTHOR]

An omegatron mass spectrometer which can be used to measure the ion species and charge state distribution in a magnetized plasma is described. In the conventional omegatron, ions are formed as the result of gas ionization by a fine electron beam passing through the center of the analyzer along the magnetic field. In the plasma omegatron, the plasma ions are entering the analyzer through a small floating aperture. We employ a biasing technique to reduce the ion velocity along the magnetic field and, thus, achieve improved ion collection and sensitivity. Experiments have been performed to demonstrate the instrument’s operation in the PISCES-A linear plasma device, at a magnetic field B=1.3 kG. Mass spectra have been obtained in hydrogen, helium, and nitrogen plasmas, and typical results are presented. [ABSTRACT FROM AUTHOR]

Studies the dynamics of an ion interacting with an electrostatic wave propagating perpendicularly to a static magnetic field in the presence of elastic ion-neutral scattering. Superharmonic and harmonic perturbations; Quasi-regular or stochastic motion exhibited by the ion in phase space; Accessibility of the entire phase space to the ion irrespective of initial conditions.

Studies the dynamics of a charged particle in a magnetic field perturbed by electrostatic waves propagating transverse to the magnetic field. Dependence of the magnitude of the perturbation and perturbation frequency on the number of perturbating waves; Deviation of the particle motion from cyclotron motion or stochastic motion over phase space.

Reports that a stationary, detached ionization front is observed in an experimentally simulated divertor plasma interacting with a hydrogen gas target. Reduction of the electron temperature at the simulated divertor target; Classical momentum flow parallel to the magnetic field and anomalous cross-field particle transport.

Reports on the first probe measurements of edge turbulence and transport in an neutral beam induced high confinement mode. Strong negative radial electric field observed in H-mode; Transient suppression of normalized ion saturation and floating potential fluctuation levels at the low confinement mode to high confinement mode transition.

The DIII-D tokamak is being upgraded to allow for divertor biasing, baffling, and pumping experiments. This paper gives an overview of the new diagnostics added to DIII-D as part of this advanced divertor program. They include tile current monitors, fast reciprocating Langmuir probes, a fixed probe array in the divertor, fast neutral pressure gauges, and Hα measurements with TV cameras and fiber optics coupled to a high-resolution spectrometer. [ABSTRACT FROM AUTHOR]

The construction and operation of a novel magnetized plasma device are described. The device combines characteristics of both linear mirror and toroidal confinement. It opens up new areas of basic plasma physics research. Examples of experiments on potential formation are presented. [ABSTRACT FROM AUTHOR]

Presents a linear analysis of fluid-type instabilities for a plasma immersed in the strongly sheared electric field. Eigenmode equation and numerical scheme; Effects caused by the fact that the ion drift speed is different from the ExB drift speed; Collisionless Simon-Hoh instability in a strongly double-sheared electric field.