DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	376: Role of q and magnetic shear of pedestal formation time and turbulent transport
Name:	Ahmed Diallo adiallo@pppl.gov	Affiliation:	Princeton Plasma Physics Laboratory
Research Area:	Pedestal Structure	Presentation time:	Not requested
Co-Author(s):	R. Groebner, D. Eldon, J. Canik, T. Rhodes, T. Osborne, W. Guttenfelder, and R. Singh	ITPA Joint Experiment :	No
Description:	The goal of this proposal is to investigate the underlying physics mechanism during the density and temperature pedestal formations (e.g., time scales) and to identify the instabilities responsible for edge transport after an ELM crash. Dependencies of pedestal formation time and instability characteristics (amplitude, spectra) for various plasma currents will be investigated to elucidate the role of q and magnetic shear, and the scaling with gradient scale lengths on fast time scale. The experiment will be focused on pedestal profiles within few milliseconds after the ELM crash.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:
Background:	It is largely accepted that the fusion performance is dictated by the pedestal characteristics. More specifically, the pedestal pressure height and width are key parameters for the fusion gain. While the pedestal pressure is key in the edge MHD stability, it is important to independently assess from the edge transport point of view the temperature and density pedestal characteristics during the pedestal formation. An understanding of the pedestal formation is therefore a key issue in developing predictive model and optimizing the pedestal for maximum core fusion. The two recent theories that address the pedestal dynamics are the KBM theory in the EPED model, which postulates the clamping of the pressure pedestal gradient with the onset of KBM instability [SNYDER, PoP 2012]. The onset of the KBM generates a steady-state transport in all channels (particle and heat). One other theory proposes a model on turbulent particle pinch due to ETG in the edge for rapid formation of the pedestal [KAW, IAEA 2012 TH/P4-15]. Clearly to elucidate the physics at play, the next step in experiments is to identify the individual transport mechanisms at play during the pedestal formation to better develop predictive capabilities of particle and heat fluxes in the pedestal. More specifically, several tests will include estimating correlations between the density fluctuations amplitudes as measured from DBS and BES and the density and temperature scale lengths. Variations of these correlations with plasma current (q and magnetic shear) and comparisons with gyrokinetic scaling studies (gradient, q, shear) using the experimental profiles (see example of such studies in Canik et al. IAEA 2012 submitted to Nucl. Fusion 2013) will unambiguously provide information on the instabilities present in the pedestal region during its formation. In summary, we test the pedestal formation before it hits the ballooning limit and characterize the density, temperature, and turbulent fluctuations in the pedestal during the first 10 ms after the ELM crash for three plasma currents. Such characterizations and scaling with plasma current will provide an estimate of the pedestal width to be expected in next-step devices such as ITER and FNSF.
Resource Requirements:	Fast-sweeping reflectometer sampling the pedestal region. Pellet injection capability for ELM triggering, DBS, BES, Thomson with the ability to arbitrary (> 100 microseconds) space the laser pulses. The experiment will require Er measurements (of time scales faster than CER) with DBS (if possible).
Diagnostic Requirements:	All profiles diagnostics. Special attention on the firing of the lasers DBS measurements and reflectometry, BES array centered in pedestal region, and midplane calibrated Dalpha signals.
Analysis Requirements:	Profile analysis, fluctuations analysis. The profile characteristics will provide inputs for GS2 linear and GYRO nonlinear analyses to identify the instabilities, compare with fluctuation data, and identify the transport channels at play during the pedestal formation.
Other Requirements: