DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 405: Role of pedestal on global confinement in hybrid scenario: extend beta range with AUG shape | ||
| Name: | Costanza Maggi ( |
Affiliation: | IPP Garching |
| Research Area: | ELM Control & Pedestal Physics | Presentation time: | Not requested | Co-Author(s): | Richard Groebner |
| Description: | Study the relation between pedestal and global confinement in hybrid scenario as the input power (beta) is increased.The main physics goal is to determine if there is evidence of saturation of the pedestal beta as the input power is increased or if the pedestal beta continues to increase with power and total beta. A beta scan over as large a range as possible is desirable, for instance a factor of 2, but this has not yet been achieved. These experiments (following a proposal to ROF 2005) were performed in Spring 2007 and were successful. However, on the first experimental day we could not complete the power scan in the AUG shape due to frequent cut-outs of beam sources 330. This meant that we could not achieve stable operation at betaN>2.5. In addition, in the discharge (#128213) where betaN ~2.7 was achieved over a short time interval, no Thomson data could be collected. It would be highly desirable to be able to complete this power scan and obtain pedestal data also at high betaN in the AUG shape, since at present we are missing some information on the effect of shape on pedestal pressure at high betaN. | ||
| Experimental Approach/Plan: | Plasma shape: AUG hybrid shape as in shots #128198 >> 128213 - early heating scheme. This requires the AUG patch panel.
Requested betaN: repeat first part of power scan performed in March 2007 and add points at higher betaN: betaN=1.5 (#128206), 2.0 (#128198) , 2.5 (#128200), 2.7 for comparison with DIII-D shape (#128245), 2.9 for comp. with DIII-D shape (#128249), ~ 3.3 or max achievable, depending on available beam power. Plasma density: 4x 10^19 m-3, as in the discharges performed on 26.03.07. | ||
| Background: | Experiments were performed in Spring 2007 at DIII-D to study the role of the pedestal on global confinement in hybrid discharges and to closely compare the pedestal in hybrid discharges of ASDEX Upgrade and DIII-D. The experiments were focussed on the study of the variation of the edge transport barrier (ETB) with power (beta) at fixed q95 ~ 4.5 and density (~ 4x 10^19 m-3) in two different shapes: a low triangularity (AUG) and a high triangularity (DIII-D) shape. The experiments were successful, except for the missing data at high beta in the AUG shape, and excellent pedestal data were collected using outer plasma sweeps to improve the spatial resolution of the Thomson scattering and CER diagnostics in the edge region. Analysis of the experiments shows that at a given betaN the confinement factor as well as the pedestal beta, betaN,PED, are higher in the higher triangularity shape. However, a different variation of the pedestal top pressure with power was observed in the two shapes: whereas betaN,PED increased with power (beta) in the AUG shape (up to betaN=2.5), it remained constant or even decreased with total betaN in the higher triangularity DIII-D shape (max total betaN=2.9). These data imply that the plasma is not stiff (as shown clearly by the kinetic profiles, especially Ti) in the DIII-D shape: the increase in confinement is due to an increase in core enrgy at essentially constant pedestal energy. The results of this power scan are at odds with other power scans in DIII-D and with previous analysis of the pedestal in hybrid discharges [1], which showed pPED to increase with power roughly as pPED~ P_NET^0.31, although the improved confinement at high betaN was due to improved core confinement. This difference is not yet understood, but analysis of the data is ongoing. The power scan in the AUG shape shows a variation of betaN,PED with power similar to what found in AUG up to betaN=2.5. Unfortunately, due to technical reasons, the high beta range of the scan is missing in this shape.
Further experiments are being proposed in improved H-modes on ASDEX Upgrade for 2008 to continue the characterization of the response of the ETB structure to input power (including high resolution measurements of the ion ETB). [1] C.F. Maggi et al., Nucl. Fusion 47 (2007) 535. | ||
| Resource Requirements: | Machine time : 1/2 day | ||
| Diagnostic Requirements: | ne, Te profiles from Thomson scattering
ni, Ti, vtor profiles from CER particular emphasis on pedestal region, therefore also outer plasma sweeping for improved spatial resolution at the edge MSE | ||
| Analysis Requirements: | Thomson, CER data profiles | ||
| Other Requirements: | -- | ||
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