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C.C. Petty,
R.J. Jayakumar,
M.A. Makowski,
C.T. Holcomb,
D.A. Humphreys,
R.J. La Haye,
T.C. Luce, P.A. Politzer,
R. Prater,
M.R. Wade,
A.S. Welander
[show abstract]
[hide abstract]
ABSTRACT: Using direct analysis of the motional Stark effect (MSE) signals, an explicit measurement of the 'missing' bootstrap current density around the island location of a neoclassical tearing mode (NTM) is made for the first time. When the NTM is suppressed using co-electron cyclotron current drive, the measured changes in the current profile that restore the bootstrap current are also directly found from the MSE measurements. Additionally, direct analysis of helical perturbations in the MSE signals during slowly rotating 'quasi-stationary' modes shows the first explicit measurement of the deficit in the toroidal current density in the island O-point.
Nuclear Fusion 12/2011; 52(1):013011. · 4.09 Impact Factor
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R. I. Pinsker,
T. C. Luce, P. A. Politzer,
M. Porkolab,
R. H. Goulding,
G. R. Hanson,
P. M. Ryan,
J. C. Hosea,
A. Nagy,
J. R. Wilson,
R. Maggiora,
D. Milanesio,
L. Zeng
[show abstract]
[hide abstract]
ABSTRACT: Fast Waves (FWs) at 60 MHz and 90 MHz are used in DIII‐D for central electron heating and current drive. Coupling of FWs to high‐performance discharges is limited by low antenna loading in these regimes. To extend the application of high‐power FWs to such regimes, methods of increasing the antenna loading in these regimes are needed. A systematic study of loading enhancement techniques has been carried out in DIII‐D, including reduction of the antenna∕plasma distance, gas puffing into the far scrape‐off layer (SOL), and control of other parameters that affect the particle balance in the far SOL. Quantitative understanding of the physics of the loading resistance and its dependence on edge density profiles is demonstrated. The core FW heating efficiency appeared to be ∼100% in the Advanced Inductive regime, consistent with the high first‐pass direct electron absorption of ∼75% that is predicted by the ray‐tracing code GENRAY in this high electron beta regime.
AIP Conference Proceedings. 12/2011; 1406(1):313-320.
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J.R. Ferron,
C.T. Holcomb,
T.C. Luce, P.A. Politzer,
F. Turco,
J.C. DeBoo,
E.J. Doyle,
Y. In,
R.J. La Haye,
M. Murakami,
M. Okabayashi,
J.M. Park,
T.W. Petrie,
C.C. Petty,
H. Reimerdes
[show abstract]
[hide abstract]
ABSTRACT: In order to maintain stationary values of the stored energy and the plasma current in a tokamak discharge with all of the current driven noninductively, the sum of the α-heating power and the power required to provide externally driven current must be equal to the power required to maintain the pressure against transport losses. In a study of high noninductive current fraction discharges in the DIII-D tokamak, it is shown that in the case of present-day tokamaks with no α-heating, adjustment of the toroidal field strength (BT) is a tool to obtain this balance between the required current drive and heating powers with other easily modifiable discharge parameters (βN, q95, discharge shape, ne) fixed at values chosen to satisfy specific constraints. With all of the external power sources providing both heating and current drive, and βN and q95 fixed, the fraction of externally driven current scales with BT with little change in the bootstrap current fraction, thus allowing the noninductive current fraction to be adjusted.
Nuclear Fusion 10/2011; 51(11):113007. · 4.09 Impact Factor
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T.W. Petrie,
T.E. Evans,
N.H. Brooks,
M.E. Fenstermacher,
J.R. Ferron,
C.T. Holcomb,
B. Hudson,
A.W. Hyatt,
T.C. Luce,
C.J. Lasnier,
S. Mordijck,
R.A. Moyer,
T.H. Osborne, P.A. Politzer,
M.E. Rensink,
M.J. Schaffer,
P.B. Snyder,
J.G. Watkins
[show abstract]
[hide abstract]
ABSTRACT: The range in density and collisionality for which resonant magnetic perturbations (RMPs) are effective in suppressing edge-localized modes (ELMs) in the presence of a radiating divertor was found to be modest for representative H-mode plasmas in DIII-D. When deuterium and argon gas injection rates were increased during RMP, both the electron collisionality in the pedestal and the maximum electron pressure gradient (∇Pe,MAX) in the pedestal also increased. As ∇Pe,MAX approached values consistent with the peeling–ballooning stability limit, as determined by edge stability analysis, ELMing activity re-emerged. For cases with the same injected neutral beam power, argon accumulation in the main plasma was greater in the RMP ELM-suppressed cases than in comparable non-RMP ELMing H-mode cases. Reductions in the core concentration of injected argon were observed for both RMP and non-RMP H-mode cases when their respective deuterium injection rates were increased. Although complete ELM suppression in RMP radiating divertor plasmas in DIII-D was only accessible over a limited range in pedestal density and collisionality, significant ELM mitigation with heat flux reduction was possible over a wider range. Comparing RMP radiating divertor discharges after the re-appearance of ELMing activity during gas puffing with a standard ELMing plasma for cases with the same pedestal density reveals that the RMP discharges have (1) lower average electron temperature at the midplane separatrix, implying lower average electron temperature at the divertor target, (2) lower time-averaged peak heat flux and (3) lower transient peak heat flux from ELMs even at the same pedestal collisionality.
Nuclear Fusion 05/2011; 51(7):073003. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: That plasma flow shear has a stabilizing effect on tearing stability is a new insight found from experiments on DIII-D, JET and NSTX. High-order (m/n = 4/3 or 3/2) tearing modes are a key beneficial feature in hybrid scenarios that act to regulate the q-profile to keep q(0) ~ 1 with the absence of large m/n = 1/1 sawteeth. The destabilization of low-order (m/n = 2/1) tearing acts as the limit on achievable beta. Hybrid discharges in DIII-D with 4/3 tearing modes under large co- (to Ip) neutral-beam torque can be run steadily just below the 2/1 tearing beta limit. However, reducing the torque has consequences on both the existing 4/3 tearing mode amplitude and the beta at which the 2/1 tearing mode destabilizes. It is found that flow shear at a rational surface is well correlated with both decreased 4/3 mode amplitude and higher beta 2/1 mode onset. The working physics model is that flow shear is classically stabilizing, i.e. makes the tearing stability index Δ' more negative; this both reduces the amplitude of neoclassical tearing modes and makes mode destabilization more difficult (requiring higher beta). However, a detailed understanding of the effects of flow shear on tearing stability remains a challenge for theory and modelling. The classically stabilizing effect of flow shear in DIII-D (and indeed future larger tokamaks) is in the regime of large magnetic Prandtl and very large Lundquist numbers; this is significant for sorting out which physical effects of flow and flow shear are relevant and would be stabilizing or even destabilizing. Experimental data with applied torque varied from all co- to near-balanced neutral beams in the DIII-D hybrid scenario with 4/3 mode 'regulation' is analysed for 4/3 mode amplitude, 2/1 onset and criticality for 2/1 mode locking. For both existing m/n = 4/3 modes and for the onset of m/n = 2/1 modes, a local flow shear of the order of is found to have a significant stabilizing effect on tearing. In addition, a flow shear effect increasing local viscosity is suggested in order to explain the behaviour of mode locking to the resistive wall.
Nuclear Fusion 04/2011; 51(5):053013. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The paper describes a novel, highly elongated, ellipsoidal shell tokamak (EST) configuration which has favourable high pressure magnetohydrodynamic Mercier and bellooning stability properties for a range of shell geometries. Calculations of the magnetic field (|B|) contours also suggest that an EST can be broadly omnigenous. The considerations leading to the choice of an ellipsoidal shell configuration, the equilibrium and ballooning stability properties and the axisymmetric stability are discussed and the parameters of a sample design for a modest proof-of-principle EST are presented.
Nuclear Fusion 01/2011; 31(3):487. · 4.09 Impact Factor
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E.J. Doyle,
J.C. DeBoo,
J.R. Ferron,
G.L. Jackson,
T.C. Luce,
M. Murakami,
T.H. Osborne,
J.-M. Park, P.A. Politzer,
H. Reimerdes, [......],
T.W. Petrie,
C.C. Petty,
T.L. Rhodes,
M.W. Shafer,
P.B. Snyder,
E.J. Strait,
M.R. Wade,
G. Wang,
W.P. West,
L. Zeng
[show abstract]
[hide abstract]
ABSTRACT: The DIII-D programme has recently initiated an effort to provide suitably scaled experimental evaluations of four primary ITER operational scenarios. New and unique features of this work are that the plasmas incorporate essential features of the ITER scenarios and anticipated operating characteristics; e.g. the plasma cross-section, aspect ratio and value of I/aB of the DIII-D discharges match the ITER design, with size reduced by a factor of 3.7. Key aspects of all four scenarios, such as target values for βN and H98, have been replicated successfully on DIII-D, providing an improved and unified physics basis for transport and stability modelling, as well as for performance extrapolation to ITER. In all four scenarios, normalized performance equals or closely approaches that required to realize the physics and technology goals of ITER, and projections of the DIII-D discharges are consistent with ITER achieving its goals of ≥400 MW of fusion power production and Q ≥ 10. These studies also address many of the key physics issues related to the ITER design, including the L–H transition power threshold, the size of edge localized modes, pedestal parameter scaling, the impact of tearing modes on confinement and disruptivity, beta limits and the required capabilities of the plasma control system. An example of direct influence on the ITER design from this work is a modification of the physics requirements for the poloidal field coil set at 15 MA, based on observations that the inductance in the baseline scenario case evolves to a value that lies outside the original ITER specification.
Nuclear Fusion 06/2010; 50(7):075005. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Understanding the dynamics of plasma startup and termination is important for present tokamaks and for predictive modeling of future burning plasma devices such as ITER. We report on experiments in the DIII-D tokamak that explore the plasma startup and rampdown phases and on the benchmarking of transport models. Key issues have been examined such as plasma initiation and burnthrough with limited inductive voltage and achieving flattop and maximum burn within the technical limits of coil systems and their actuators while maintaining the desired q profile. Successful rampdown requires scenarios consistent with technical limits, including controlled H-L transitions, while avoiding vertical instabilities, additional Ohmic transformer flux consumption, and density limit disruptions. Discharges were typically initiated with an inductive electric field typical of ITER, 0.3 V/m, most with second harmonic electron cyclotron assist. A fast framing camera was used during breakdown and burnthrough of low Z impurity charge states to study the formation physics. An improved “large aperture” ITER startup scenario was developed, and aperture reduction in rampdown was found to be essential to avoid instabilities. Current evolution using neoclassical conductivity in the CORSICA code agrees with rampup experiments, but the prediction of the temperature and internal inductance evolution using the Coppi–Tang model for electron energy transport is not yet accurate enough to allow extrapolation to future devices.
Physics of Plasmas 05/2010; 17(5):056116-056116-8. · 2.15 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Because of the high value of the stored energy in planned ITER plasmas, the safe and controlled termination of ITER discharges is an important aspect of ITER operation. In a series of experiments on the DIII-D tokamak, we have simulated the proposed ITER rampdown for the 15 MA baseline operating scenario. These scaled discharges match the reference scenario (including scaled time dependence) with regard to key parameters such as scaled current, poloidal beta, elongation and internal inductance. The scaled plasma current is reduced to the equivalent of less than 1 MA, well below the 1.4 MA specified for ITER as the maximum allowable for disruptive termination. The plasma shape and position are controlled during rampdown so that the high heat flux zones near the strike points of the separatrix are held within the equivalent of the armoured zones of the ITER divertor; the regulation of the strike-point location is an order of magnitude better than required. Scans of the current rampdown rates indicate that a more rapid rampdown than the ITER reference case may be needed to avoid excessive current in the ITER central solenoid. Rampdown with a full-size plasma was studied, but was found to be unsuitable for ITER because of transitions to ELM-free H-mode with a consequent lack of density control, as well as large excursions in poloidal beta and internal inductance. We find that ELMs play an important role during the H-mode phase of the rampdown, helping to reduce the density as the current is reduced.
Nuclear Fusion 03/2010; 50(3):035011. · 4.09 Impact Factor
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C.C. Petty,
T.E. Evans,
J.C. DeBoo,
B. Hudson,
R.J. La Haye,
T.C. Luce, P.A. Politzer,
S.L. Allen,
E.J. Doyle,
M.E. Fenstermacher,
J.R. Ferron,
A.W. Hyatt,
R.J. Jayakumar,
R.A. Moyer,
T.H. Osborne
[show abstract]
[hide abstract]
ABSTRACT: Large type-I edge localized modes (ELMs) are completely suppressed in hybrid discharges for the first time by applying an edge resonant perturbation (RMP) using an internal coil set with toroidal mode number n = 3. This is an important advance in developing hybrid discharges as a baseline operating scenario for ITER. In these experiments on the DIII-D tokamak, the ELM suppression lasts for ~1 s in plasmas with normalized beta up to βN = 2.5 (volume average beta up to β = 3.4%) and a fusion performance factor as high as , which equals the value for the Q = 10 scenario in ITER. A strong interaction between the 3/2 neoclassical tearing mode and the RMP, which slows the pedestal toroidal rotation, limits the achievable βN and the duration of ELM suppression.
Nuclear Fusion 01/2010; 50(2):022002. · 4.09 Impact Factor
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J. M. Park,
M. Murakami,
C. C. Petty,
T. H. Osborne,
M. A. Van Zeeland,
R. Prater,
T. C. Luce,
M. R. Wade,
N. H. Brooks,
J. C. DeBoo,
J. S. DeGrassie,
J. R. Ferron,
P. Gohil,
R. M. Hong,
A. W. Hyatt,
J. Lohr, P. A. Politzer,
H. E. St John,
W. P. West,
W. W. Heidbrink
[show abstract]
[hide abstract]
ABSTRACT: Neutral beam current drive (NBCD) experiments in DIII-D using vertically shifted plasmas to move the current drive away from the axis have clearly demonstrated robust off-axis NBCD. Time-dependent measurements of magnetic field pitch angles by the motional Stark effect diagnostic are used to obtain the evolution of the poloidal magnetic flux, which indicates a broad off-axis NBCD profile with a peak at about half the plasma minor radius. In most cases, the measured off-axis NBCD profile is consistent with calculations using an orbit-following Monte Carlo code for the beam ion slowing down including finite-orbit effects provided there is no large-scale magnetohydrodynamic activity such as Alfven eigenmodes modes or sawteeth. An alternative analysis method shows good agreement between the measured pitch angles and those from simulations using transport-equilibrium codes. Two-dimensional image of Doppler-shifted fast ion D{sub {alpha}} light emitted by neutralized energetic ions shows clear evidence for a hollow profile of beam ion density, consistent with classical beam ion slowing down. The magnitude of off-axis NBCD is sensitive to the alignment of the beam injection relative to the helical pitch of the magnetic field lines. If the signs of toroidal magnetic field and plasma current yield the proper helicity, both measurement and calculation indicate that the efficiency is as good as on-axis NBCD because the increased fraction of trapped electrons reduces the electron shielding of the injected ion current, in contrast with electron current drive schemes where the trapping of electrons degrades the efficiency. The measured off-axis NBCD increases approximately linearly with the injection power, although a modest amount of fast ion diffusion is needed to explain an observed difference in the NBCD profile between the measurement and the calculation at high injection power.
Physics of Plasmas 09/2009; 16(9). · 2.15 Impact Factor
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T.W. Petrie,
G.D. Porter,
N.H. Brooks,
M.E. Fenstermacher,
J.R. Ferron,
M. Groth,
A.W. Hyatt,
R.J. La Haye,
C.J. Lasnier,
A.W. Leonard,
T.C. Luce, P.A. Politzer,
M.E. Rensink,
M.J. Schaffer,
M.R. Wade,
J.G. Watkins,
W.P. West
[show abstract]
[hide abstract]
ABSTRACT: The effectiveness of the puff-and-pump technique to enrich a seeded impurity in the divertor relative to the core and, thereby, to maximize radiation in the divertor depends sensitively on both the magnetic geometry and the ion B × ∇B drift direction. In the puff-and-pump scenario used here, argon impurities injected into the private flux region are inhibited from accumulation in the core plasma by enhanced plasma flows to the divertor created by a combination of deuterium gas puffing upstream of the divertor targets and particle pumping near the divertor targets. Modelling of single-null, H-mode plasmas with the UEDGE fluid transport code indicates that particle drifts in the scrape-off layer and divertor strongly affect the locations where the argon seed impurity accumulates. It is also found in double-null cases that argon always shows a larger accumulation in the divertor out of which the ion B × ∇B drift is directed, regardless of the divertor into which the argon is injected. Experiments have shown that the degree to which the deuterium gas-puffing rate inhibits the escape of the seed impurity from the divertor(s) depends critically on the direction of the ion B × ∇B drift and on whether the plasma is single-null or double-null. The transition in behaviour from double-null to single-null character during puff-and-pump occurs for |dRsep| = 0.4 cm when the ion B × ∇B drift was pointing away from the dominant divertor. The lowest argon density buildup in the main plasma of any of the configurations studied during puff-and-pump was achieved in single-null plasmas with the ion B × ∇B drift direction away from the divertor.
Nuclear Fusion 05/2009; 49(6):065013. · 4.09 Impact Factor
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C. T. Holcomb,
J. R. Ferron,
T. C. Luce,
T. W. Petrie, P. A. Politzer,
C. Challis,
J. C. DeBoo,
E. J. Doyle,
C. M. Greenfield,
R. J. Groebner, [......],
T. H. Osborne,
J.-M. Park,
R. Prater,
G. D. Porter,
H. Reimerdes,
T. L. Rhodes,
M. W. Shafer,
P. B. Snyder,
A. D. Turnbull,
W. P. West
[show abstract]
[hide abstract]
ABSTRACT: Recent studies on the DIII-D tokamak [
J. L. Luxon, Nucl. Fusion 42, 614 (2002)
] have elucidated key aspects of the dependence of stability, confinement, and density control on the plasma magnetic configuration, leading to the demonstration of nearly noninductive operation for >1 s with pressure 30% above the ideal no-wall stability limit. Achieving fully noninductive tokamak operation requires high pressure, good confinement, and density control through divertor pumping. Plasma geometry affects all of these. Ideal magnetohydrodynamics modeling of external kink stability suggests that it may be optimized by adjusting the shape parameter known as squareness (ζ). Optimizing kink stability leads to an increase in the maximum stable pressure. Experiments confirm that stability varies strongly with ζ, in agreement with the modeling. Optimization of kink stability via ζ is concurrent with an increase in the H-mode edge pressure pedestal stability. Global energy confinement is optimized at the lowest ζ tested, with increased pedestal pressure and lower core transport. Adjusting the magnetic divertor balance about a double-null configuration optimizes density control for improved noninductive auxiliary current drive. The best density control is obtained with a slight imbalance toward the divertor opposite the ion grad(B) drift direction, consistent with modeling of these effects. These optimizations have been combined to achieve noninductive current fractions near unity for over 1 s with normalized pressure of 3.5<βN<3.9, bootstrap current fraction of >65%, and a normalized confinement factor of H98(y,2) ≈ 1.5.
Physics of Plasmas 05/2009; 16(5):056116-056116-9. · 2.15 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: High performance plasmas and daily reference shots (DRSs) with both L-mode and H-mode phases were used to demonstrate the maintenance of good wall conditions over ~7000 s of plasma operation in DIII-D with no intervening boronizations or high temperature bakes during each of the 2006 and 2007 campaigns. High performance discharges with high normalized beta and confinement factor and good density control over the duration of the high-power beam injection period were very repeatable over the course of these campaigns. High performance operation was also demonstrated after a six week entry vent followed by the standard high temperature bake at 350 °C and plasma conditioning, but prior to a boronization. Over the 2006 and 2007 campaigns, the DRS database indicated little to no secular increase in impurity content. Oxygen content and nickel line emission were higher after the entry vent, but were still minor contributors to plasma contamination compared with carbon. Because DIII-D has a plasma facing surface that is >95% graphite, we take this as a demonstration that erosion of boronization films used for wall conditioning will not be a limitation to establishing long-pulse high performance discharges in the new generation of superconducting tokamaks if graphite is used as the primary plasma facing material.
Plasma Physics and Controlled Fusion 03/2009; 51(5):055014. · 2.42 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Analysis of the change in the magnetic field pitch angles during edge localized mode events in high performance, stationary plasmas on the DIII-D tokamak shows rapid (<1 ms) broadening of the current density profile, but only when a m/n=3/2 tearing mode is present. This observation of poloidal magnetic-flux pumping explains an important feature of this scenario, which is the anomalous broadening of the current density profile that beneficially maintains the safety factor above unity and forestalls the sawtooth instability.
Physical Review Letters 01/2009; 102(4):045005. · 7.37 Impact Factor
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P.A. Politzer,
C.C. Petty,
R.J. Jayakumar,
T.C. Luce,
M.R. Wade,
J.C. DeBoo,
J.R. Ferron,
P. Gohil,
C.T. Holcomb,
A.W. Hyatt,
J. Kinsey,
R.J. La Haye,
M.A. Makowski,
T.W. Petrie
[show abstract]
[hide abstract]
ABSTRACT: We report the results of the first experiments on the DIII-D tokamak to examine the dependence of the transport and stability characteristics of ITER hybrid scenario plasmas on the toroidal flow (or rotation) of the plasma. With the new DIII-D capability to independently vary the neutral beam torque and power, the central rotation has been reduced by as much as a factor of 4.6 compared with discharges with unidirectional beams. Although energy confinement decreases and the m/n = 3/2 NTM amplitude increases for low rotation speed, the fusion performance figure of merit, , still exceeds the value required on ITER for Qfus = 10. These observations provide optimism about the projections of the hybrid scenario to low rotation plasmas in ITER, but they also indicate the need for a better understanding of the physics of toroidal rotation in order to project present-day results to future experiments.
Nuclear Fusion 05/2008; 48(7):075001. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The performance (βN ≈ 3, q95 ≈ 4.4, fBS ≈ 0.5, H89 > 2) of hybrid scenario plasmas in DIII-D (Luxon 2002 Nucl. Fusion 42 614) is limited by m/n = 2/1 tearing modes. Unlike conventional plasmas (Hender et al 2004 Nucl. Fusion 44 798), the linear dependence scaling of the global beta for onset of the instability with normalized local ion gyroradius is modified as the n = 1 ideal kink beta limit is approached, suggesting that small island neoclassical tearing mode (NTM) threshold physics does not impose the dominant criterion for NTM stability. The hybrid scenario tends to go unstable just at or below the no wall n = 1 ideal kink beta limit of about 4ℓi. Experimentally 4ℓi decreases with beta as . Thus the 'ceiling' in beta due to coupling of tearing to the ideal kink comes down as beta is increased. Scaling of the tearing unstable beta that combines both NTM threshold physics and a pole in Δ'r due to coupling to the ideal kink is presented.
Nuclear Fusion 01/2008; 48(1):015005. · 4.09 Impact Factor
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T W Petrie,
N H Brooks,
M E Fenstermacher,
J R Ferron,
M Groth,
A W Hytatt,
R J La Haye,
C J Lasnier,
A W Leonard,
T C Luce, P A Politzer,
G D Porter,
M J Schaffer
01/2008;
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R. Prater,
R.J. La Haye,
T.C. Luce,
C.C. Petty,
E.J. Strait,
J.R. Ferron,
D.A. Humphreys,
A. Isayama,
J. Lohr,
K. Nagasaki, P.A. Politzer,
M.R. Wade,
A.S. Welander
[show abstract]
[hide abstract]
ABSTRACT: The m = 2/n = 1 neoclassical tearing mode (NTM) has been observed to strongly degrade confinement and frequently lead to a disruption in high β discharges in DIII-D if allowed to grow to a large size. Stabilization of grown NTMs by the application of a highly localized electron cyclotron current drive (ECCD) at the island location has led to operation at an increased plasma pressure, up to the no-wall kink limit. After the NTM is stabilized by the ECCD, the correct location for the current drive is maintained using information from real-time equilibrium reconstructions which include measurements from the motional Stark effect diagnostic. This same process is used alternatively to prevent the mode from ever growing, leading to performance at the pressure limit in high performance hybrid discharges with β above 4%. Modelling using the modified Rutherford equation shows that the required power is in close agreement with the experimental threshold for the prevention of the 2/1 NTM.
Nuclear Fusion 04/2007; 47(5):371. · 4.09 Impact Factor
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E. A. Lazarus,
T. C. Luce,
M. E. Austin,
D. P. Brennan,
K. H. Burrell,
M. S. Chu,
J. R. Ferron,
A. W. Hyatt,
R. J. Jayakumar,
L. L. Lao, [......],
C. C. Petty, P. A. Politzer,
R. Prater,
T. L. Rhodes,
J. T. Scoville,
W. M. Solomon,
E. J. Strait,
A. D. Turnbull,
F. L. Waelbroeck,
C. Zhang
[show abstract]
[hide abstract]
ABSTRACT: The role of interchange and internal kink modes in the sawtooth oscillations is explored by comparing bean- and oval-shaped plasmas. The n = 1 instability that results in the collapse of the sawtooth has been identified as a quasi-interchange in the oval cases and the internal kink in the bean shape. The ion and electron temperature profiles are followed in detail through the sawtooth ramp. It is found that electron energy transport rates are very high in the oval and quite low in the bean shape. Ion energy confinement in the oval is excellent and the sawtooth amplitude (δT/T) in the ion temperature is much larger than that of the electrons. The sawtooth amplitudes for ions and electrons are comparable in the bean shape. The measured q profiles in the bean and oval shapes are found to be consistent with neoclassical current diffusion of the toroidal current, and the observed differences in q largely result from the severe differences in electron energy transport. For both shapes the collapse flattens the q profile and after the collapse return to q0≳1. Recent results on intermediate shapes are reported. These shapes show that the electron energy transport improves gradually as the plasma triangularity is increased.
Physics of Plasmas 03/2007; 14(5):055701-055701-17. · 2.15 Impact Factor
