DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
Print this page

Print this page
Title	370: Particle pinch in pedestal
Name:	Richard Groebner groebner@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Pedestal Structure	Presentation time:	Not requested
Co-Author(s):	J. Boedo, T. Leonard, T. Osborne	ITPA Joint Experiment :	No
Description:	Look for evidence of an inward particle pinch in the pedestal by using reciprocating Langmuir probe to look for an inward fluctuation-driven particle flux.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	Develop H-mode plasma which is benign as possible for the reciprocating Langmuir probe. This means no beams (thus ECH heating or Ohmic H-mode), low density (thus low current) and low temperature (thus as low as possible Bt). Develop an H-mode with as long an ELM-free phase as possible (which will probably not be long under these circumstances). Insert probe as far as possible into pedestal to measure profile of turbulent-driven particle flux. We want to see if this flux is inwards, and if so, how far into the pedestal the flux is inwards.
Background:	There a number of reasons why it is important to obtain an understanding of the physics processes which govern the density profile. For instance, 1) ITER needs to know what the fuelling requirements are to build up the necessary density pedestal; 2) Control of the density profile appears to be part of the physics for why some ELM-suppression regimes work. Thus control of the density pedestal may be a tool to optimize ELM control. 3) Control of the pedestal density is predicted by EPED to be a tool that would enable optimization of the pedestal height. The actual physics that forms the density pedestal structure is not well understood. In the past, some of us have thought that the neutral penetration depth determined the width. However, we now have a number of reasons why we suspect that this is not the full story and that details of particle transport also play a role. In particular, we suspect that there is an inward particle pinch in the pedestal that helps build up the density and also to push the density barrier further into the plasma. We have made attempts to infer the presence of a pinch via different experimental means, particularly by producing plasmas with dense scrape-off layers, which would limit neutral fuelling of the pedestal. However, our studies have not been definitive and it looks to be very difficult to indirectly infer a pinch. It would be much better to directly measure an inward particle flux, and the reciprocating Langmuir probe is capable of making these measurements. In fact, there have been reports in the past of inward-directed particle fluxes observed in H-mode on Langmuir probes. The goal of this experiment would be to fill in gaps from past work and attempt to get a clear measurement of an inward particle flux under plasma conditions which were as benign to the probe as possible.
Resource Requirements:	DIII-D tokamak, ECH heating, neutral beam blips for diagnostic purposes.
Diagnostic Requirements:	Reciprocating Langmuir probe, TS, CER
Analysis Requirements:	Analysis of Langmuir probe data to produce particle fluxes. The results could be compared to particle fluxes computed with a code, such as TGLF.
Other Requirements:	--