Scaling of ELM and H-mode Pedestal Characteristics in ITER Shape Discharges in the DIII-D Tokamak PDF Download
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Languages : en
Pages : 7
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Languages : en
Pages : 7
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Languages : en
Pages : 4
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The character of edge localized modes (ELMs) and the height of the edge pressure pedestal in DIII-D tokamak H-mode discharges have been modified by varying the discharge shape (triangularity and squareness) and the safety factor, increasing the edge radiation, and injecting deuterium pellets. Changes in the ELM frequency and amplitude, and the magnitude of the edge pressure gradient, and changes in the calculated extent of the region of access to the ballooning mode second stability regime are observed.
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The structure and scaling of the H-mode pedestal are examined for discharges in the DIII-D tokamak. For typical conditions, the pedestal values of the ion and electron temperatures T[sub i] and T[sub e] are comparable. Measurements of main ion and C[sup 6+] profiles indicate that the ion pressure gradient in the barrier is 50%--100% of the electron pressure gradient for deuterium plasmas. The magnitude of the pressure gradient in the barrier often exceeds the predictions of infinite-n ballooning mode theory by a factor of two. Moreover, via the bootstrap current, the finite pressure gradient acts to entirely remove ballooning stability limits for typical discharges. For a large dataset, the width of the pressure barrier[delta] is best described by the dimensionless scaling[delta]/R[proportional-to] ([beta][sub pol][sup ped])[sup 0.4] where ([beta][sub pol][sup ped]) is the pedestal value of poloidal beta and R is the major radius. Scalings based on the poloidal ion gyroradius or the edge density gradient do not adequately describe overall trends in the data set and the propagation of the pressure barrier observed between edge-localized modes. The width of the T[sub i] barrier is quite variable and is not a good measure of the width of the pressure barrier.
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Category : Nuclear fusion
Languages : en
Pages : 324
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Through comparison of experiment and ideal magnetohydrodynamic (MHD) theory, modes driven in the edge region of tokamak H-mode discharges[Type I edge-localized modes (ELMs)] are shown to result from low toroidal mode number (n) instabilities driven by pressure gradient and current density. The mode amplitude and frequency are functions of the discharge shape. Reductions in mode amplitude are observed in discharge shapes with either high squareness or low triangularity where the low-n 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.
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Languages : en
Pages : 17
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A regime of very high confinement (VH-mode) has been observed in neutral beam-heated deuterium discharges in the DIII-D tokamak with thermal energy confinement times up to [approx]3.6 times that predicted by the ITER-89P L-mode scaling and 2 times that predicted by ELM-free H-mode thermal confinement scalings. This high confinement has led to increased plasma performance, n[sub D] (0)T[sub i](0)[tau][sub E] = 2 [times] 10[sup 20] m[sup [minus]3] keV sec with I[sub p] = 1.6 MA, B[sub T] = 2.1 T, Z[sub eff] [le] 2. Detailed transport analysis shows a correspondence between the large decrease in thermal diffusivity in the region 0.75 [le] [rho] [le] 0.9 and the development of a strong shear in the radial electric field in the same region. This suggests that stabilization of turbulence by sheared E [times] B flow is responsible for the improved confinement in VH-mode. A substantial fraction of the edge plasma entering the second regime of stability may also contribute to the increase in confinement. The duration of the VH-mode phase has been lengthened by feedback controlling the input power to limit plasma beta.
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Languages : en
Pages : 4
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Introduction. A regime of very high confinement (VH-mode) has been observed in divertor discharges in DIII-D. The VH-mode, first seen following the initial boronization of the DIII-D vessel in 1991, exhibits total energy confinement a factor of 2.5 to 3.5 greater than that predicted by the ITER89-P L-mode scaling relation. Also, confinement of thermal energy alone is greater than 1.6 times that of the JET/DIII-D H-mode scaling and in many cases has exceeded twice that amount. VH-mode is observed during a long (≤0.8 sec) ELM-free phase of the discharges. At the beginning of the ELM-free period, the plasma appears to be in H-mode, with confinement near that predicted by the JET/DIII-D scaling. In the usual H-mode, confinement is observed to decrease or remain constant over time. In the present discharges, confinement has been observed to remain nearly constant for up to hundreds of milliseconds, after which the behavior sharply deviates from H-mode as the confinement begins to increase over time. This increase in confinement continues until the occurrence of a beta- related ([beta]>2.8I/aB) global MHD event, which rapidly decreases the plasma stored energy with a temperature reduction across the entire profile. Magnetic measurements indicate that at least in some cases, this event includes both an internal n = 1 mode and a more localized high-n mode near the edge. After this event, the plasma relaxes into an ELMing H-mode phase. As a consequence of the boronization, the plasmas in these discharges are unusually clean, with very low radiated power. In previous H-mode discharges, the radiated power increased during the ELM-free, sometimes reaching levels comparable with the input power if the ELM-free period was long enough. Also, Z{sub eff}is constant or decreasing over the length of the discharge, with a central value of ≈1. It is noted that most of the energy in these discharges is thermal energy, with ≤10% contained in fast ions.
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Languages : en
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Improving confinement and beta limits simultaneously in long-pulse ELMy H-mode discharges is investigated. The product[beta][sub N]H[sub 98y] serves as a useful figure-of-merit for performance, where[beta][sub N][triple-bond][beta]/(I/aB) and H[sub 98y] is the ratio of the thermal confinement time relative to the most recent ELMy H-mode confinement scaling established by the ITER confinement database working group. In discharges with q[sub 0][approximately] 1 (no sawteeth) and discharges with q[sub min]> 1.5 and negative central magnetic shear, [beta][sub N][approximately] 2.9 and H[sub 98y][approximately] 1.4 are sustained for up to 2 s. Although peaked profiles are observed, steep internal transport barriers are not present. Further increases in[beta][sub N] in these discharges is limited by neoclassical tearing modes (NTM) in the positive shear region. In another recently developed regime, [beta][sub N][approximately] 3.8 and H[sub 98y][approximately] 1.8 has been sustained during large infrequent ELMs in non-sawtoothing discharges with 1[sub 0][approximately] 1. This level of performance is similar to that obtained in ELM-free regimes such as VH-mode. The limitation on[beta][sub N] and pulse length in these discharges is also the onset of NTMs.
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Category : Controlled fusion
Languages : en
Pages : 504
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Languages : en
Pages : 21
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The characteristics of the H-mode are studied in discharges with varying triangularity and squareness. The pressure at the top of the H-mode pedestal increases strongly with triangularity primarily due to an increase in the margin by which the edge pressure gradient exceeds the ideal ballooning mode first stability limit. Two models are considered for how the edge may exceed the ballooning mode limit. In one model [1], access to the ballooning mode second stable regime allows the edge pressure gradient and associated bootstrap current to continue to increase until an edge localized, low toroidal mode number, ideal kink mode is destabilized. In the second model [2], the finite width of the H-mode transport barrier, and diamagnetic effects raise the pressure gradient limit above the ballooning mode limit. We observe a weak inverse dependence of the width of the H-mode transport barrier, [Delta], on triangularity relative to the previously obtained [3] scaling [Delta] ∞ ([beta]{sub P}{sup PED})12. The energy loss for Type I ELMs increases with triangularity in proportion to the pedestal energy increase. The temperature profile is found to respond stiffly to changes in T{sup PED} at low temperature, while at high temperature the response is additive. The response of the density profile is also found to play a role in the response of the total stored energy to changes in the W{sup PED}.
