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Results are presented from comparisons of modeling and experiment in studies to assess the best choices of safety factor q profile, pressure profile, and discharge shape for high β, steady-state, noninductive advanced tokamak operation in the DIII-D device [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. These studies are motivated by the need for high qminβN to maximize the self-driven bootstrap current while maintaining high toroidal β to increase fusion gain. Modeling shows that increases in the normalized beta βN stable to ideal, low toroidal mode number (n=1,2), instabilities can be obtained through broadening of the pressure profile and use of a symmetric double-null divertor shape. Experimental results are in agreement with this prediction. The general trend is for qminβN to increase with the minimum q value (qmin) although βN decreases as qmin increases. By broadening the pressure profile, βN≈4 is obtained with qmin≈2. Modeling of equilibria with near 100% bootstrap current indicates that operation with βN≈5 should be possible with a sufficiently broad pressure profile. [ABSTRACT FROM AUTHOR]

Using off-axis electron cyclotron current drive (ECCD), self-consistent integrated advanced tokamak operation has been demonstrated on DIII-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159], combining high beta; (>3%) at high q (q[SUBmin] > 2.0) with good energy confinement (H[SUB89] ∼ 2.5) and high noninductive current fraction (f[SUBBS] ∼55%,f[SUBNI] ∼ 90%). Modification of the current profile by ECCD led to internal transport barrier formation even in the presence of type I edge localized modes. Improvements were observed in all transport channels, and increased peaking of profiles led to higher bootstrap current in the core. Separate experiments have shown the ability to maintain a nearly steady-state current profile for up to 1 s with q[SUBmin] > 1.5. Modeling indicates that this favorable current profile can be maintained indefinitely at a higher β[SUBN] using tools available to the near-term DIII-D program. Modeling and simulation have become essential tools for the experimental program in interpreting the data and developing detail plans for new experiments. [ABSTRACT FROM AUTHOR]

A magnetohydrodynamic (MHD) instability which has the characteristics that the displacement phase is in the same direction at all the affected flux surfaces (an interchange-like structure with no phase reversal across a flux surface), has been observed during the L mode in several negative central shear discharges of DIII-D tokamak [J. L. Luxon, P. Anderson, F. Baity et al., Plasma Physics and Controlled Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The instability occurs when the minimum safety factor is around 2.0 and the profile of the safety factor q is deeply reversed in the center. Detailed stability analyses were carried out using standard numerical codes and the high quality magnetic probe, motional Stark effect and electron cyclotron emission (ECE) data. Analysis of the data after the onset shows that the instability has an n = 1 mode number and a growth time of about 400 µs. The electron temperature fluctuations obtained from ECE measurements indicate a localized interchange-like structure early in time, the resistive interchange criterion indicates marginal stability, and ideal mode analyses indicate robust stability with an ideal beta limit of about a factor of 2 higher than the β[sub N] value at the time of onset. Therefore this interchange-parity mode is not an ideal MHD mode. The marginal value of the resistive interchange criterion observed only in discharges with the instability, indicates that this is probably a resistive interchange mode. However, some observed characteristics of the instability may require models beyond the linear resistive interchange theory. [ABSTRACT FROM AUTHOR]

The tearing mode stability of low β plasmas is studied experimentally in the DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak. The linear and nonlinear characteristics of the plasma are measured and compared with theoretical predictions. In contrast to the neoclassical tearing mode, which occurs at high β and is dominated by the bootstrap current effect, in the low β regime the neoclassical bootstrap current effect is minimal. Thus the stability properties are more amenable to comparison with presently available theoretical codes. In the linear phase, the onset of the instability has been found to be predictable from equilibrium reconstruction to agree with the availability of the tearing mode free energy. In the nonlinear phase, the temperature and its fluctuation amplitude and phase have been fitted to the prediction of achieving a perturbed three-dimensional equilibrium with an assumed perturbation eigenfunction. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

For high β, highly shaped plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], the value of the tearing stability index Δ' calculated at a rational surface can be especially sensitive to the pressure and current profiles. Near marginal stability for a global ideal mode, a pole in Δ' exists in equilibrium parameter space, as predicted by analytic theory. The proximity of an equilibrium reconstruction to this pole in parameter space strongly decreases the accuracy of the Δ' calculations. Tearing stability calculations on kinetic equilibrium reconstructions of a series of times in three DIII-D discharges are presented, which indicate that the tearing modes in these discharges are classically unstable at the time of onset. The onset mechanism of two of these discharges (which are in H-mode) is related to the approach of ideal stability boundaries and the occurrence of poles in Δ'. Several ideal modes (sawteeth, edge localized modes, and resistive wall modes) are thought to seed neoclassical tearing modes (NTMs) through forced reconnection, after the ideal mode is unstable. However, tearing modes often appear suddenly and grow quickly without an obvious ideal mode causing a seed island through forced reconnection, which could be explained by this mechanism. This is proposed as an alternative mechanism for the onset of NTMs in tokamaks, which is not incompatible with forced reconnection. [ABSTRACT FROM AUTHOR]

The amplitude and frequency of modes driven in the edge region of tokamak high mode (H-mode) discharges [type I edge-localized modes (ELMs)] are shown to depend on the discharge shape. The measured pressure gradient threshold for instability and its scaling with discharge shape are compared with predictions from ideal magnetohydrodynamic theory for low toroidal mode number (n) instabilities driven by pressure gradient and current density and good agreement is found. Reductions in mode amplitude are observed in discharge shapes with either high squareness or low triangularity where the stability threshold in the edge pressure gradient is predicted to be reduced and the most unstable mode is expected to have higher values of n. The importance of access to the ballooning mode second stability regime is demonstrated through the changes in the ELM character that occur when second regime access is not available. An edge stability model is presented that predicts that there is a threshold value of n for second regime access and that the most unstable mode has n near this threshold. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

One promising approach to maintaining stability of high beta tokamak plasmas is the use of a conducting wall near the plasma to stabilize low-n ideal magnetohydrodynamic instabilities. However, with a resistive wall, either plasma rotation or active feedback control is required to stabilize the more slowly growing resistive wall modes (RWMs). Previous experiments have demonstrated that plasmas with a nearby conducting wall can remain stable to the n=1 ideal external kink above the beta limit predicted with the wall at infinity. Recently, extension of the wall stabilized lifetime τL to more than 30 times the resistive wall time constant τw and detailed, reproducible observation of the n=1 RWM have been possible in DIII-D [Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1986), p. 159] plasmas above the no-wall beta limit. The DIII-D measurements confirm characteristics common to several RWM theories. The mode is destabilized as the plasma rotation at the q=3 surface decreases below a critical frequency of 1?7 kHz (˜1% of the toroidal Alfvén frequency). The measured mode growth times of 2?8 ms agree with measurements and numerical calculations of the dominant DIII-D vessel eigenmode time constant τw . From its onset, the RWM has little or no toroidalrotation (ωmode≤τw-1«ωplasma), and rapidly reduces the plasma rotation to zero. These slowly growing RWMs can in principle be destabilized using external coils controlled by a feedback loop. In this paper, the encouraging results from the first open loop experimental tests of active control of the RWM, conducted in DIII-D, are reported. [ABSTRACT FROM AUTHOR]

The effects of current profile on the ideal ballooning mode for circular and shaped poloidal cross-section plasmas in tokamaks are studied analytically and numerically. The results show that for moderately shaped plasmas the critical normalized beta, βNC, against the ballooning mode increases approximately linearly with the plasma internal inductance li. As the plasma becomes more strongly shaped, this dependence on li becomes weaker, and for a divertor plasma βNC shows a very weak dependence on li for the range of moderate li values considered. [ABSTRACT FROM AUTHOR]

Improvement in both the energy confinement time and the achievable value of normalized beta is obtained by modifying the current density profile from the relatively broad shape obtained in standard tokamak discharges to a more peaked shape. The peaked current profile is produced with either a rapid negative ramp in the total plasma current or a rapid increase in the discharge elongation. Discharges have been obtained with β[sub N]=β/(I/aB)=6% mT/MA simultaneously with total energy confinement time two times the value predicted by L-mode scaling relations. Up to a factor of 1.8 improvement in the normalized thermal energy confinement time, τ[sub th]/I[sub p], has been obtained in both L-mode and H-mode discharges. It is shown that the increase in confinement can be attributed to a local decrease in the thermal diffusivity that is correlated with a local increase in the poloidal magnetic field and the magnetic shear. [ABSTRACT FROM AUTHOR]

Accurate equilibrium reconstruction and detailed stability analysis of a strongly shaped, double-null, βT=11% discharge shows that the plasma core is in the second stable regime to ideal ballooning modes. The equilibrium reconstruction using all the available data (coil currents, poloidal magnetic loops, motional Stark effect data, the kinetic pressure profile, the magnetic axis location, and the location of the two q=1 surfaces) shows a region of negative magnetic shear near the magnetic axis, an outer positive shear region, and a low shear region connecting the two. The inner negative shear region allows a large positive shear region near the boundary, even at low q (q95=2.6), permitting a large outer region pressure gradient to be first regime stable. The inner region is in the second stable regime, consistent with the observed axial beta [βT(0)=44%]. In the low shear region p’ vanishes, consistent with Mercier stability. This is one way to extend the ballooning limit in shaped plasmas while maintaining stability against external kinks. The n=1 analysis shows that the plasma is unstable to an ideal internal mode, consistent with the experimental observations of a saturated internal m/n=1/1 mode. The core plasma pressure, not being limited by ballooning stability, appears to be reaching a local equilibrium limit at the magnetic axis. [ABSTRACT FROM AUTHOR]

The stability of the n=1 ideal kink in DIII-D-like [Fusion Technol. 8, 441 (1985)] configurations is found to depend critically on the details of the current density and pressure profiles. The maximum stable normalized β, β[sub N]=β/(I/aB) (I in MA, a in meters, and B[sub 0] in tesla), is found to increase dramatically as the peak in the pressure gradient is shifted from the central region toward the plasma boundary. Further, for peaked pressure profiles, the β limit is insensitive to the internal inductance, l[sub i], whereas for broad pressure profiles, the β limit depends more strongly on l[sub i] and can increase with l[sub i] almost up to the point where the q profile becomes hollow. With broad pressure profiles and a conducting wall at 1.5 minor plasma radii, kink-stable β[sub N] values of 5.8—significantly above both the Troyon limit and the ballooning limit—have been found. [ABSTRACT FROM AUTHOR]

A theoretical and experimental evaluation of axisymmetric stability and axisymmetric control has led to a modification of the vertical position control in the DIII-D tokamak, which now allows operation to within a few percent of the ideal magnetohydrodynamic (MHD) n = 0 limit. It is found that the onset the departure from rigid shift behavior in D-shaped plasmas limits plasma elongation to 2.5 in DIII-D. The possibility of avoiding the vertical instability in future tokamaks with highly elongated plasmas is discussed. Recent experiments have focused on utilizing this capability for axisymmetric control to construct plasma shapes optimized to increase the achievable beta. Operation near the axisymmetric stability limit allows an increase in the achieved normalized current I[sub p]/aB[sub T], where I[sub p], is the total plasma current, a is the minor radius, and B[sub T] is the toroidal field. Based on stability calculations, an equilibrium was developed to achieve marginal stability simultaneously to axisymmetric, kink, and ballooning instabilities. In the experiment, the shape was altered to higher elongation during the high-beta phase as the current profile broadened. A record high beta for DIII-D of t 1% was achieved. The high-beta phase of the discharge lasted 40 msec, approximately one confinement time. [ABSTRACT FROM AUTHOR]

Low-q (q[sub 95] < 3) double-null divertor discharges with values of the volume-average toroidal beta as high as 9.3% have been operated in the DIII-D tokamak [Fusion Technol. 8, 441 ( 1985 ) ]. In discharges with q[sub 95] ≈ 5, values of β[sub T]/(I/aB) as high as 5 have been obtained. These discharges are shown to be at or below the stability limit to the value of beta for infiniten, ideal ballooning modes. The discharges are significantly below the beta limit for ideal,low toroidal mode number kink modes. The kink mode beta limit is shown to be strongly dependent on the radial profiles of plasma pressure and current. The theoretical beta limit in DIII-D is shown to be in the range β[sub T]/(I/aB) = 4-5 depending on the value of I/aB, and this is consistent with the experiment. High-beta discharges have been operated with ion temperature up to 17 keV. Steady-state, high-beta, low-q operation is demonstrated by a discharge with I/aB = 2.6, q[sub 95] = 2.7, in which β[sub T] > 7% is maintained for 1.5 sec. [ABSTRACT FROM AUTHOR]

Examines high-performance negative central magnetic shear discharges in the DIII-D tokamak. Increases in plasma performance and reactivity resulting from reliable operation of discharges; Noninductive current ranges in the discharges; Toroidal rotation and ion temperature profiles.