Progress towards a predictive model for pedestal height in DIII-D

Groebner, R. J.; Leonard, A.W.; Snyder, P. B.; Osborne, T.H.; Maggi, C. F.; Fenstermacher, M.E.; Petty, C. C.; Owen, L.W.

doi:10.1088/0029-5515/49/8/085037

Cited by 72 publications

(111 citation statements)

References 40 publications

(104 reference statements)

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“…For this purpose, time windows with constant plasma parameters have been selected from a series of reproducible shots. The results show similar trends to those observed on AUG 11 and DIII-D 35,45 . In particular, they indicate that the pedestal pressure gradient may saturate shortly before the ELM onset.…”

Section: Discussionsupporting

confidence: 84%

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Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

Pitzschke

Behn

Sauter

et al. 2011

Plasma Phys. Control. Fusion

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The dynamics of electron temperature and density profiles during quasi-stationary H-mode phases in the TCV tokamak has been investigated for type III and type I ELMs using electron cyclotron heating (ECH) to vary the collisionality. At heating power levels close to the threshold for the L-H transition, ELMs of type III were observed, with an energy loss per ELM below 10% of the total plasma energy. Electron temperature and pressure showed no significant increase during the phase before the ELM crash. ELMs of type I were characterized by a lower repetition rate, but caused fractional energy losses reaching 20%. For this ELM type, a clear increase in pedestal pressure and pressure gradient was observed before the collapse associated with the ELM event. The rapid drop in electron temperature also affected the plasma core explaining the rather large energy losses per ELM. Ideal MHD stability calculations of the edge pedestal with the KINX code showed that high-n ballooning modes restricted the achievable pressure gradient at high collisionality and ELMs of type III. With additional heating and type I ELMs, stability limits were set by medium-n kink-ballooning modes. Comparing the values of the pressure gradients and current densities obtained from experimental data with those of ideal MHD stability calculations showed good agreement. The model, however, needs to account for the radial displacement of the location of maximum pressure gradient during the ELM cycle.

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Section: Discussionsupporting

confidence: 84%

“…Some ELMs were directly triggered when the pressure gradient attained a critical value imposed by ideal MHD 5,44 . However, in other cases it has been observed that the pedestal remains at the pressure limit for several milliseconds before an ELM occurs 31,35 . Fig.…”

Section: A Type I Elmsmentioning

confidence: 99%

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

Pitzschke

Behn

Sauter

et al. 2011

Plasma Phys. Control. Fusion

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“…80, where the pedestal width is plotted as a function of the pedestal poloidal b. 232 The NSTX pedestal width follows a D ¼ c(b h ped ) 1 dependence, which is quite different from the D ¼ c 0 (b h ped ) 0.5 scaling for MAST 230 and DIII-D. 233 The $1.0 exponent scaling observed for NSTX has been shown to be consistent with preliminary analyses using the ballooning critical technique (BCP), 234 which yielded a 0.8 exponent scaling of the pedestal width as shown in Fig. 80.…”

Section: H-mode Pedestal and Edge Localized Modes (Elms)mentioning

confidence: 98%

Recent progress on spherical torus research

Ono

Kaita

2015

Physics of Plasmas

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The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A = R0/a) reduced to A ∼ 1.5, well below the normal tokamak operating range of A ≥ 2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β ∼ 1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era.

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“…In the region of the H-mode pedestal, unlike in the N 2.8 cases, there is a systematic increase of the width of the J BS peak in the pedestal region with increasing q 95 that results primarily from an increase in the width of the electron and ion density gradient profiles. These profile width increases may be the result of increased p [35].…”

Section: Bootstrap Current Densitymentioning

confidence: 99%

Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D

et al. 2011

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Abstract. In order to assess the optimum q profile for discharges in DIII-D with 100% of the current driven noninductively (f NI = 1), the self-consistent response of the plasma profiles to changes in the q profile was studied in high f NI , high N discharges through a scan of q min and q 95 at two values of N . As expected, both the bootstrap current fraction, f BS , and f NI increased with q 95 . The temperature and density profiles were found to broaden as either q min or N is increased.A consequence is that f BS does not continue to increase at the highest values of q min . A scaling function that depends on q min , q 95 , and the peaking factor for the thermal pressure was found to represent well the f BS / N inferred from the experimental profiles. The changes in the shapes of the 2 density and temperature profiles as N is increased modify the bootstrap current density (J BS ) profile from peaked close to the axis to relatively flat in the region between the axis and the H-mode pedestal. Therefore, significant externally-driven current density in the region inside the H-mode pedestal is required in addition to J BS in order to match the profiles of the noninductive current density (J NI ) to the desired total current density (J). In this experiment, the additional current density was provided mostly by neutral beam current drive with the neutral-beam-driven current fraction 40%-90% of f BS . The profiles of J NI and J were most similar at q min 1.35-1.65, q 95 6.8, where f BS is also maximum, establishing this q profile as the optimal choice for f NI = 1 operation in DIII-D with the existing set of external current drive sources.3

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Progress towards a predictive model for pedestal height in DIII-D

Cited by 72 publications

References 40 publications

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

Electron temperature and density profile evolution during the edge-localized mode cycle in ohmic and electron cyclotron-heated H-mode plasmas in TCV

Recent progress on spherical torus research

Optimization of the safety factor profile for high noninductive current fraction discharges in DIII-D

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