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Title	87: farSOL expts at high n/nGW to establish if ITER farSOL will be high-recycling/detached
Name:	Peter Stangeby peter.stangeby@utoronto.ca	Affiliation:	University of Toronto
Research Area:	Divertor & SOL Physics	Presentation time:	Requested
Co-Author(s):	Tony Leonard, Richard Pitts (ITER), Dmitry Rudakov, Clement Wong,	ITPA Joint Experiment :	No
Description:	Motivation: the proposed experiment will provide important information for the ITER decision about PFC choice for targets at the 2ndary divertor at the top (ITER will operate with unbalanced double-null). ITER specs for region outside 2nd separatrix, "farSOL", are based on very little experimental information - particularly for high n/nGW, where most needed, since ITER will use high n/nGW. These plasma conditions apply to the 2nd strike point at the top, where ITER is now considering replacing the Be with W, because of the expected, I.e. ASSUMED, intensity of the plasma-solid interaction there. The present ITER design/planning assumes the farSOL is Sheath-Limited, S-L. For S-L flux tubes, there is ~no drop in plasma temperature along the length of the flux tubes and so the sputtering rate can be high at the solid surfaces which terminate the flux tubes. In ITER there will be two types of farSOL flux tubes involved in the plasma contact with the Be surfaces: divertor-type and limiter-type. The divertor-type is due to the fact that ITER will use unbalanced DN with the 2ndary divertor at the top; this is therefore where the most intense plasma-Be interaction will occur. In addition, limiter-type contact will occur over a much larger fraction of the total Be wall area of 700 m2, albeit at a less intense level, but with potentially major consequences for the erosion-wear rate of the thin Be armour and tritium retention by Be-codeposition . Both types of plasma-Be contact are therefore important in ITER and for both types it is important to know if the farSOL flux tubes involved are in the S-L regime, or in the much more attractive H-R (high-recycling) or DET (detached) regimes, where there is substantial drop of plasma temperature along the length of each flux tube, resulting in reduced sputtering at the Be surfaces at the ends of the flux tubes. There is evidence in DIII-D that even for limiter-type wall contact, the farSOL flux tubes may be in H-R: in DIII-D window-frame expts [Leonard JNM (2007]: "At high density, the baffle probe saturation current is higher than the midplane probe and may indicate .. local recycling." There is a still stronger tendency for divertor-type flux tubes to transition into H-R/DET than limiter-type flux tubes, so DIII-D should study both at high n/nGW. The farSOL at high n/nGW may be H-R/DET (high-recycling/detached) with major practical - positive! - implications for ITER. It is proposed to carry out farSOL expts at high n/nGW in DIII-D using (1) unbalanced DN, and also (2) SN with window-frame, to establish if ITER farSOL is likely to be H-R/DET. In 2008 we carried out unbalanced DN expts (13C-methane injection) to simulate the ITER farSOL, using n/nGW - 0.63. The proposed expts will use the same configuration but raise n/nGW to 0.8 or higher.	ITER IO Urgent Research Task :	Yes
Experimental Approach/Plan:	High power, H-mode. Two densities at similar power, one at highest n/nGW, one 20% lower n/nGW. Two values of dsrsep. IR measurements of the 2ndary target area. (1) Use unbalanced DN with 2ndary divertor at the bottom, where edge diagnosis is better, to study the divertor-type farSOL. (2) Also use LSN with small wall gap to the 'nose' of the upper outer pumping plenum, which creates toroidally symmetrical wall-contact, thus facilitating farSOL analysis using the 'window-frame' method for limiter-type contact farSOL studies.
Background:	Until ~10 years ago it was thought that there was very little plasma contact with the main walls of tokamaks. It is now known that at high n/nGW - where ITER will operate - the total ionic flux to the main walls becomes comparable to the total ionic flux to the primary divertor. This "farSOL" plasma has major implications, direct and indirect, for (i) power loading of the walls, (ii) fuel recycling, (iii) impurity generation by wall sputtering. Direct: due to the ion impact on the walls. Indirect: due to the charge exchange neutral particles that result from the ion-wall contact. Re sputtering, the latter can be more important than the former because the charge exchange neutrals arising from main wall contact can be very energetic, originating from deep in the confined plasma. The vast majority of SOL research to date has focussed on the nearSOL, specifically the narrow power-channel that goes to the primary targets. By contrast the farSOL is still largely unexplored, particularly at high n/nGW. ITER will operate with unbalanced double-null with the 2ndary divertor at the top. Thus the most intense farSOL wall contact in ITER will be divertor-like rather than limiter-like. This increases the probability that the farSOL in ITER will not be in the sheath-limited regime - the usual assumption - but could be in the high-recycling state or even detached. This would have major - largely positive - implications for ITER, certainly for power-loading on the 2ndary targets (the cx fluxes, on the other hand, would increase, although not necessarily the cx sputtering rate). In the present ITER design the 2ndary targets are to be clad with the low-melting metal Be and it is being considered to replace the Be there with W. The latter, however, would have significant potential to cause high W concentrations in the confined plasma. It is therefore important to know if Be targets will be acceptable - which they may be if the 2ndary outer divertor is high-recycling or detached.
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