DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 50: Test of a theoretical model for the irreducible minimal pedestal width scaling | ||
| Name: | Richard Groebner ( |
Affiliation: | General Atomics |
| Research Area: | ELM Control & Pedestal Physics | Presentation time: | Requested | Co-Author(s): | -- |
| Description: | Measure the width of the density pedestal while performing variations of pedestal ion temperature, toroidal magnetic field and plasma current to test a theory of the pedestal width. This theory predicts that the width of the density pedestal is proportional to sqrt (Ti Mi) / Bt, which implies a toroidal gyroradius scaling. Although the aim of this experiment is to test a specific model for pedestal width, it would also provide information for a wider class of models, based on ExB shear suppression ideas, which predict that pedestal width should scale with the toroidal gyroradius. | ||
| Experimental Approach/Plan: | Vary the toroidal magnetic field Bt over as wide a range as possible, while keeping the pedestal ion temperature and density as constant as possible. Perform measurements during ELM-free phase. Then, vary the pedestal ion temperature over as wide a range as possible with Bt fixed and density held as constant as possible. After this scan is completed, perform Ip scan at fixed Bt. Use breathing to obtain improved pedestal profiles for ions and electrons. Measure the electron density profile and its width in the pedestal. Compare the width to the theoretical formula. | ||
| Background: | C.S. Chang and colleagues have developed a theoretical expression for the width of the H-mode density pedestal under the assumption that the ion thermal and particle transport are entirely caused by collisions, orbit loss and banana squeezing/expansion (due to ExB shearing) (IAEA 2004). This theory predicts that the width of the density pedestal, as obtained from a tanhfit, is proportional to sqrt (Mi Ti) / Bt, where Mi is the ion mass, Ti is the pedestal ion temperature and Bt is the toroidal magnetic field. DIII-D pedestal data, obtained under ELM-free conditions, have been used for initial testing of this prediction. A best fit to these data showed the measured width going as (Ti)^0.4 and (Bt)^-0.6; these dependences are close to those predicted by the theory. The comparison with theory can be improved significantly by obtaining single parameter scans of ion temperature and toroidal field with other parameters held as constant as possible. A variation of current with fixed Bt is also proposed in order that the variation of pedestal width with ion poloidal gyroradius can be examined. | ||
| Resource Requirements: | 6 NBI sources
Cryopumps | ||
| Diagnostic Requirements: | Thomson
CER CO2 interferometer | ||
| Analysis Requirements: | Select averaging times during the discharges; these time ranges must be ELM-free or inter-ELM. Use Osborne's python codes to fit pedestal profiles for Ti and ne. Obtain tiped and newid for available discharges and examine scaling relationship between tiped, newid and bt. | ||
| Other Requirements: | -- | ||
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