DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2008 EXPERIMENTAL CAMPAIGN
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| Title | 388: Type II ELMs and the Evolution of Profiles Between Type I ELMs | ||
| Name: | Tom Osborne ( |
Affiliation: | General Atomics |
| Research Area: | ELM Control & Pedestal Physics | Presentation time: | Not requested | Co-Author(s): | P. Snyder, A. Leonard, J. Yu, J. Boedo, R. Moyer, J. Watkins |
| Description: | In this experiment we study the characteristics and stability properties of Type II ELMs in comparison to the Type I ELM regime. We test the hypothesis that Type II ELMs are pressure gradient driven while Type I ELMs require a peeling current drive. We also reexamine the effect of nonresonant fields on Type II ELMs. This work is motivated by the quest for an alternative to the Type I regime for ITER. | ||
| Experimental Approach/Plan: | We will establish a discharge with low frequency type I ELMs and significant Type II ELMs. We will look differences in filament and mode structure between ELM types on the fast UCSD camera, magnetics and the reciprocating probe. We will look at the effect of nonresonant filed perturbations, which have show strong effects on Type II ELMs in previous experiments, using the I-coil in odd parity. We will also have several repeat shots to gather data from the LiBeam and two view MSE to study the current profile evolution between Type I ELMs to test that hypothesis that the Type I ELM requires a current drive. We will also look at the effects of current ramps on Type I/II ELMs related to this same hypothesis | ||
| Background: | Small Type II ELMs appear near the end of the period between Type I ELMs in most DIII-D discharges. The small size of Type II ELMs compared to Type I ELMs makes their study important ans an alternative to the Type I regime which is unacceptable for ITER. In the rhostar scaling experiment on DIII-D Type II ELMs only appeared in discharges at small rhostar suggesting a favorable scaling to ITER. Discharges with exclusively Type II ELMs have only been obtained on DIII-D when a non-resonant field was applied with the I-coil (odd parity) at moderate collisionality (nustar_ped=1), suggesting a connection between Type II ELMs and non-axisymmetric fields. The fact that that Type II ELMs appear just before a Type I ELM suggests they are driven by a related instability. Typically the pedestal pressure profile reaches steady state well before the Type I ELM. This suggests a hypothesis for Type II ELMs as pressure gradient instabilities while the Type I ELM waits for the pedestal current density to increase to the peeling limit. Since peeling modes typically become more unstable as the pressure gradient decreases this may lead to much larger loss for the current driven Type I ELM. The rhostar scaling experiment and the association of type II ELMs with higher collisionality suggests an alternative hypothesis that the type II ELM is an instability of the foot of the pedestal pressure profile where current may decrease faster than the pressure gradient shutting off second stable access. | ||
| Resource Requirements: | -- | ||
| Diagnostic Requirements: | UCSD fast camera, reciprocating probe, langmuir probes, Divertor IR TV. All profile diagnostics | ||
| Analysis Requirements: | Profiles, kinetic EFIT, ELITE stability, ONETWO simulation of current relaxation between Type I ELMs. | ||
| Other Requirements: | -- | ||
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