[Show abstract][Hide abstract] ABSTRACT: 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.
Physics of Plasmas 05/2013; 20(5). DOI:10.1063/1.4807123 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In tokamak plasmas with low levels of toroidal rotation, the radial electric field Er is a combination of pressure gradient and toroidal and poloidal rotation components, all having similar magnitudes. In order to assess the validity of neoclassical poloidal rotation theory for determining the poloidal rotation contribution to Er, Dα emission from neutral beam heated tokamak discharges in DIII-D (Luxon 2002 Nucl. Fusion 42 614) has been evaluated in a sequence of low torque (electron cyclotron resonance heating and balanced diagnostic neutral beam pulse) discharges to determine the local deuterium toroidal rotation velocity. By invoking the radial force balance relation the deuterium poloidal rotation can be inferred. It is found that the deuterium poloidal flow exceeds the neoclassical value in plasmas with collisionality , being more ion diamagnetic, and with a stronger dependence on collisionality than neoclassical theory predicts. At low toroidal rotation, the poloidal rotation contribution to the radial electric field and its shear is significant. The effect of anomalous levels of poloidal rotation on the radial electric field and cross-field heat transport is investigated for ITER parameters.
[Show abstract][Hide abstract] ABSTRACT: A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where L_{T_{e}}^{-1}=|∇T_{e}|/T_{e} and toroidal rotation were varied, long wavelength (k_{θ}ρ_{s}≲0.4) electron temperature fluctuations exhibit a threshold in L_{T_{e}}^{-1}: below, they change little; above, they steadily increase. The increase in δT_{e}/T_{e} is concurrent with increased electron heat flux and transport stiffness. Observations were insensitive to rotation. Accumulated evidence strongly enforces the identification of the experimentally observed threshold with ∇T_{e}-driven trapped electron mode turbulence.
[Show abstract][Hide abstract] ABSTRACT: DIII-D is using its flexibility and diagnostics to address the critical science required to enable next step fusion devices. Operating scenarios for ITER have been adapted to low torque and are now being optimized for transport. Three ELM mitigation scenarios have been developed to near-ITER parameters. New control techniques are managing the most challenging plasma instabilities. Disruption mitigation tools show promising dissipation strategies for runaway electrons and heat load. An off axis neutral beam upgrade has enabled sustainment of high βN capable steady state regimes. Divertor research is identifying the challenge, physics and candidate solutions for handling the hot plasma exhaust with notable progress in heat flux reduction using the snowflake configuration. This work is helping optimize design choices and prepare the scientific tools for operation in ITER, and resolve key elements of the plasma configuration and divertor solution for an FNSF.
[Show abstract][Hide abstract] ABSTRACT: A database of toroidal momentum transport on five tokamaks, Alcator C-Mod, DIII-D, JET, NSTX and JT-60U, has been constructed under a wide range of conditions in order to understand the characteristics of toroidal momentum transport coefficients, namely the toroidal momentum diffusivity (chi(phi)) and the pinch velocity (V-pinch). Through an inter-machine comparison, the similarities and differences in the properties of chi(phi) and V-pinch among the machines have been clarified. Parametric dependences of these momentum transport coefficients have been investigated over a wide range of plasma parameters taking advantage of the different operation regimes in machines. The approach offers insights into the parametric dependences as follows. The toroidal momentum diffusivity (chi(phi)) generally increases with increasing heat diffusivity (chi(i)). The correlation is observed over a wide range of chi(phi), covering roughly two orders of magnitude, and within each of the machines over the whole radius. Through the inter-machine comparison, it is found that chi(phi) becomes larger in the outer region of the plasma. Also observed is a general trend for V-pinch in tokamaks; the inward pinch velocity (-V-pinch) increases with increasing chi(phi). The results that are commonly observed in machines will support a toroidal rotation prediction in future devices. On the other hand, differences among machines have been observed. The toroidal momentum diffusivity, chi(phi), is larger than or equal to chi(i) in JET and JT-60U; on the other hand, chi(phi) is smaller than or equal to chi(i) in NSTX, DIII-D and Alcator C-Mod. In DIII-D, the ratio -RVpinch/chi(phi) at r/a = 0.5-0.6 is about 2, which is small compared with that in other tokamaks (-RVpinch/chi(phi) approximate to 5). Based on these different observations, parametric dependences of chi(phi)/chi(i), RVpinch/chi(phi) and chi(phi) have been investigated in H-mode plasmas. Across the dataset from all machines, the ratio chi(phi)/chi(i) tends to be larger in low nu(e)* at fixed T-e/T-i and rho(pol)*. An increase in chi(phi) is observed with decreasing n(e) and/or increasing T-e. The pinch number (-RVpinch/chi(phi)) is observed to increase with increasing R/L-ne at both q(95) = 5.5-7.2 and q(95) = 3.7-4.5. Here nu(e)*, nu(pol)*, T-e, T-i, R/L-ne and q(95) are, respectively, the normalized effective electron collision frequency, the normalized ion poloidal Larmor radius, the electron and ion temperatures, the inverse ratio of density scale length, L-ne, to the major radius, R, and the safety factor at the 95% flux surface.
[Show abstract][Hide abstract] ABSTRACT: The first real-time profile control experiments integrating magnetic and kinetic variables were performed on DIII-D in view of regulating and extrapolating advanced tokamak scenarios to steady state devices and burning plasma experiments. Device-specific, control-oriented models were obtained from experimental data and these data-driven models were used to synthesize integrated magnetic and kinetic profile controllers. Closed-loop experiments were performed for the regulation of (a) the poloidal flux profile, Ψ(x), (b) the inverse of the safety factor profile, ι(x)=1/q(x), and (c) either the Ψ(x) profile or the ι(x) profile together with the normalized pressure parameter, β N . The neutral beam injection (NBI), electron cyclotron current drive (ECCD) systems and ohmic coils provided the heating and current drive (H&CD) sources. The first control actuator was the plasma surface loop voltage or current (i. e. the ohmic coil), and the available beamlines and gyrotrons were grouped to form five additional H&CD actuators: co-current on-axis NBI, co-current off-axis NBI, counter-current NBI, balanced NBI and total ECCD power from all gyrotrons (with off-axis current deposition). The control method was also applied on simulated ITER discharges using a simplified transport code (METIS).
[Show abstract][Hide abstract] ABSTRACT: The need for a validated predictive capability of turbulent transport in ITER is now widely recognized. However, to date most validation studies of nonlinear codes such as GYRO (Candy and Waltz 2003 J. Comput. Phys. 186 545) have focused upon low power L-mode discharges, which have significant differences in key dimensionless parameters such as ρ* = ρs/a from more ITER-relevant H-mode discharges. In order to begin addressing this gap, comparisons of the turbulent transport and fluctuations predicted by nonlinear GYRO simulations for a number of DIII-D (Luxon 2002 Nucl. Fusion 42 614) H-mode discharges to power balance analyses and experimental measurements are presented. The results of two H-mode studies are presented in this paper, this first of which investigates the importance of nonlocality at typical DIII-D H-mode ρ* values. Electrostatic global GYRO simulations of H-mode discharges at low and high rotation are shown to predict turbulence and transport levels lower than corresponding local simulations, and which are consistent with or slightly above experimental measurements and power balance analyses, even at 'near-edge' radii where gyrofluid and gyrokinetic models systematically underpredict turbulence and transport levels. The second study addresses the stabilizing effect of a significant density of energetic particles on turbulent transport. The results of local GYRO simulations of low-density QH-mode plasmas are presented, which model the fast beam ion population as a separate, dynamic ion species. The simulations show a significant reduction of transport with this fast ion treatment, which helps to understand previously reported results (Holland et al 2011 Phys. Plasmas 18 056113) in which GYRO simulations without this treatment significantly overpredicted (by a factor of 10 or more) power balance calculations. These results are contrasted with simulations of a high-density, low fast ion fraction QH-mode discharge, which predict transport levels consistent with power balance, regardless of the fast ion treatment.
[Show abstract][Hide abstract] ABSTRACT: The Phase Contrast Imaging (PCI) diagnostic on the DIII-D tokamak has
recently been modified to image density fluctuations near the plasma
mid-radius, thus enabling the investigation of core turbulence. Results
are presented on core fluctuations in experiments exploring ion profile
stiffness [1], i.e. the degree of sensitivity of ion temperature
profiles to heat flux variations. In these experiments, plasmas were
heated by neutral beams (NBI) configured to provide both high and low
input torque; the injected NBI power was varied at constant torque to
evaluate profile stiffness. A preliminary analysis indicates a decreased
stiffness at high rotation in the outer half of the plasma. The toroidal
rotation depends primarly on torque, with little or no dependence on
input power. The amplitude of fluctuations increases with decreasing
rotation, and the power spectra at similar torque have quantitatively
similar shapes and values with little dependence on input power.
Correlation lengths depend neither on torque nor input power. PCI power
spectra and correlation lengths are evaluated and compared to non-linear
gyro-kinetic simulations using the GYRO code.
6pt [1] J.E. Kinsey, et al., Bull. Am. Phys. Soc. 56, 282 (2011).
[Show abstract][Hide abstract] ABSTRACT: Geodesic Acoustic Modes (GAMs) are coherent flows induced by plasma
turbulence that in turn affect the turbulence and turbulent transport.
Recently, in a neutral beam and electron cyclotron heated L-mode plasma
in the DIII-D tokamak, strong GAM oscillations have been observed in
electron temperature fluctuations Te in addition to the
often-observed GAM density fluctuations. The mode frequency is constant
over a radial range (δρ˜0.2), as expected of an
eigenmode, with two different frequencies observed depending upon
radius. Both modes exist at the location where one frequency transits to
another as detected in Te. GAM oscillations in density and
ExB flow peak at far edge (at ρ˜0.9) and have similar profile
shapes. In contrast, the GAM oscillations in Te peak much
deeper into plasma (at ρ˜0.7). After the auxiliary heating
power is turned off for t,> 100 ms, the eigenmode feature evolves
into a continuum. This observation of GAM properties may provide
challenges for existing theories to understand GAMs and plasma
turbulence.
[Show abstract][Hide abstract] ABSTRACT: To improve poloidal rotation measurement capabilities on the DIII-D tokamak, new chords for the charge exchange recombination spectroscopy (CER) diagnostic have been installed. CER is a common method for measuring impurity rotation in tokamak plasmas. These new chords make measurements on the high-field side of the plasma. They are designed so that they can measure toroidal rotation without the need for the calculation of atomic physics corrections. Asymmetry between toroidal rotation on the high- and low-field sides of the plasma is used to calculate poloidal rotation. Results for the main impurity in the plasma are shown and compared with a neoclassical calculation of poloidal rotation.
The Review of scientific instruments 10/2012; 83(10):10D501. DOI:10.1063/1.4728097 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The neutral-beam induced D(α) emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude ∣B∣ from the Stark-split beam emission spectrum, and fast-ion D(α) emission (FIDA) proportional to the beam-injected fast ion density. A comprehensive spectral fitting routine which accounts for all photoemission processes is employed for the spectral analysis. Interpretation of the measurements to determine physically relevant plasma parameters is assisted by the use of an optimized viewing geometry and forward modeling of the emission spectra using a Monte-Carlo 3D simulation code.
The Review of scientific instruments 10/2012; 83(10):10D529. DOI:10.1063/1.4739239 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A synthetic diagnostic has been developed that reproduces the highly structured electron cyclotron emission (ECE) spectrum radiated from the edge region of H-mode discharges. The modeled dependence on local perturbations of the equilibrium plasma pressure allows for interpretation of ECE data for diagnosis of local quantities. Forward modeling of the diagnostic response in this region allows for improved mapping of the observed fluctuations to flux surfaces within the plasma, allowing for the poloidal mode number of coherent structures to be resolved. In addition, other spectral features that are dependent on both T(e) and n(e) contain information about pedestal structure and the electron energy distribution of localized phenomena, such as edge filaments arising during edge-localized mode (ELM) activity.
The Review of scientific instruments 10/2012; 83(10):10E329. DOI:10.1063/1.4733742 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Phase Contrast Imaging (PCI) diagnostic on DIII-D provides a
line-integrated measurement of density fluctuations covering wavenumbers
2 to 30 cm-1. An outer gap scan during QH-mode with
stationary plasma parameters allowed the PCI to sample a large range in
kr/kθ. A narrow peak in turbulence
amplitude is seen near the LCFS. The ExB Doppler shift allows the
location to be determined precisely, showing two distinct regions of
turbulence at 0.5 and 0.2 cm inside the LCFS with kr>0 and
kr<0 respectively, consistent with the expected effects of
shear in the Er well. PCI measurements at 200 kHz show that
kθ=0.8 cm-1 with poloidal correlation
length Lθ=6 cm. Using a simple non-isotropic
turbulence model, we find that kr=3 cm-1 and
Lr=0.5 cm, with n/n˜25% in the pedestal for this
high-kr turbulence. These fluctuations, which are outside the
parameter range accessible to most turbulence diagnostics, are large
enough in amplitude to play a role in setting the pedestal structure.
These PCI observations are qualitatively similar to those made in
ELM-free H-mode and between ELMs suggesting that similar large
kr turbulence may be important.
[Show abstract][Hide abstract] ABSTRACT: In DIII-D, localized electron cyclotron heating (ECH) is used to probe
the critical gradient of, and onset of stiffness in, the electron
temperature Te profile. While keeping the total injected ECH
power constant, the deposition profile was varied to investigate the
relationship between the Te gradient and the electron power
balance heat flux. A critical temperature gradient was observed, above
which both the heat diffusivity and the Te fluctuations
increase sharply. To compare to modeling, efforts have been made to
produce the most realistic equilibrium reconstructions by using kinetic
pressure constraints and motional Stark effect measurements of the local
magnetic pitch. With these reconstructions, the gyrokinetic stability
codes GYRO and TGLF predict that there is a critical Te
gradient, similar to the experimentally observed gradient, above which
electron modes exist and whose growth rates dominate over the ion growth
rates.
[Show abstract][Hide abstract] ABSTRACT: It is desirable to have an ITER H-mode regime that is quiescent to
edge-localized modes (ELMs). ELMs deposit large, localized, impulsive,
surface heat loads that can damage the divertor. One such quiescent
regime with edge harmonic oscillations (EHO) is observed on DIII-D, JET,
JT-60U, and ASDEX-U [1]. The physical mechanisms of EHO are not fully
understood, but linear MHD calculations suggest EHO may be a saturated
kink-peeling mode partially driven by flow-profile shear [2]. We present
preliminary EHO computations using the extended-MHD NIMROD code. The
model incorporates first-order FLR effects and parallel heat flows.
Using reconstructed DIII-D profiles from discharges with EHO, we scan
the ExB and polodial flow profiles and compute linear stability. The aim
is to ascertain the role of the ExB flow shear, as motivated by
experimental results [3], and to compare with theoretical predictions
where the growth rate is enhanced at intermediate wavenumbers and
cut-off at large wavenumbers by diamagnetic effects [4]. Initial
nonlinear computations exploring the EHO saturation mechanism are
presented.[4pt] [1] Phys. Plasmas, v19, p056117, 2012 (and refs.
within).[0pt] [2] Nucl. Fusion, v47, p961, 2007.[0pt] [3] Nucl. Fusion,
v51, p083018, 2011.[0pt] [4] Phys. Plasmas v10, p4405, 2003.
[Show abstract][Hide abstract] ABSTRACT: Experiments on DIII-D have measured the stiffness of electron heat
transport using a new method that combines heat pulse (HP) propagation
and power balance (PB) analysis. Using a single modulated gyrotron, in
addition to 5 cw gyrotrons, the radial profiles of Te
oscillations from the fundamental to the 9^th harmonic are fit to
determine the diffusion (DHP), convection (VHP)
and damping coefficients. The Te gradient is then
systematically scanned by varying the electron cyclotron heating profile
on a shot-by-shot basis using the cw gyrotrons. Numerically integrating
DHP over this scan gives DPB, and the difference
between the diffusive heat flux from DPB and the total
power-balance heat flux determines VPB. The ratio of
DHP to DPB measures the transport stiffness,
defined as the fractional increase in diffusive heat flux divided by the
fractional increase in the Te gradient. In L-mode plasmas, a
sudden increase in electron transport stiffness is seen when the
Te scale length exceeds the theoretically predicted threshold
value. Similar electron transport stiffness is observed with and without
additional NBI.
[Show abstract][Hide abstract] ABSTRACT: A new method of inferring poloidal rotation by combining charge exchange
measurements of impurity tangential rotation on the high- and low-field
side of the magnetic axis has been developed on DIII D [1]. This method
has the advantage of not requiring the calculation of atomic physics
corrections to account for the energy dependent charge exchange cross
section, and it has been used in conjunction with charge exchange
measurements of impurity poloidal rotation from the vertical charge
exchange views to investigate poloidal rotation in DIII-D plasmas.
Measurements of poloidal rotation have been made for a large range of
temperature, toroidal field, and toroidal rotation. In addition,
poloidal rotation has been measured during the formation of an internal
transport barrier, and the dependence of poloidal rotation on normalized
collisionality has been investigated. Comparisons with the neoclassical
theory of poloidal rotation will be made.
6pt [1] C. Chrystal, et al., Rev. Sci. Instrum. 83, 10D501 (2012)
[Show abstract][Hide abstract] ABSTRACT: Comparisons of the bulk deuterium ion toroidal rotation to neoclassical
theory reveal a significant discrepancy. The source of this discrepancy
lies in the prediction of the main-ion poloidal rotation. In low
toroidal rotation plasmas, Er is dominated by the pressure
and poloidal rotation contributions; hence, an accurate determination of
the poloidal flow is required in these conditions. We infer the main-ion
poloidal rotation from measured main-ion toroidal rotation and the
radial force balance relation. Inferred main-ion poloidal flow is
significantly larger in the ion diamagnetic direction than NCLASS
predictions. By experimentally performing scans of the plasma current,
toroidal field and heating mix, the dependence of main-ion toroidal and
poloidal rotation on these parameters can be understood. Comparisons of
main-ion charge exchange recombination measurements of rotation with
Mach probe data and several neoclassical rotation models will be
performed.
[Show abstract][Hide abstract] ABSTRACT: A range of experiments on DIII-D have investigated the role of
externally applied torque and the associated toroidal rotation on
confinement and stability in plasmas with high levels of normalized
fusion performance. In standard H-mode plasmas, the confinement is
initially enhanced by increasing rotation, but saturates at intermediate
levels of rotation. However, the same effect is observed over a wider
range of rotation in advanced inductive plasmas. Both ion and electron
confinement improves with high rotation. Surprisingly, experiments in
quiescent H-mode (QH-mode) plasmas have found the opposite trend, with
improved confinement, performance and reduced turbulence levels at low
rotation. The different behavior suggests that ExB shear, rather than
rotation, is needed for improving confinement. In particular, for these
QH-mode plasmas, it is found that the ExB shear near the edge is
maintained or enhanced with torque from non-resonant magnetic fields,
even at low rotation. In all scenarios, no major difference is observed
in confinement whether the plasma is initiated with high rotation and
slowed down, or formed with low rotation from the beginning.
[Show abstract][Hide abstract] ABSTRACT: Application of static, non-axisymmetric magnetic fields (NAMFs) to
DIII-D plasmas allows sustained quiescent H-mode (QH-mode) operation
under reactor-relevant conditions of beta, collisionality and torque
from neutral beam injection (NBI). QH-mode is an ideal plasma for next
step devices, exhibiting H-mode confinement levels while operating
without edge localized modes at constant density and radiated power.
Peeling-ballooning mode stability theory suggests, and previous studies
confirm, that QH-mode operation requires sufficient radial shear in the
toroidal rotation near the plasma edge. In past experiments, this
rotation shear was predominantly produced by torque from
counter-directed NBI. In recent experiments, co-NBI torque was overcome
by the counter torque due to neoclassical toroidal viscosity (NTV)
produced by the NAMFs. The latest experiments have demonstrated that
sufficient NTV torque can be created using NAMFs produced by coils
outside the toroidal field coil. These new results open a path for
QH-mode utilization in self-heated, burning plasmas, where toroidal
momentum input from NBI will be small or absent.