High Density H-mode 13 C-Tracer Experiments in DIII-D

1. ITPA DSOL Task Group MeetingToronto 6-9 Nov 2006 High Density H-mode 13C-Tracer Experiments in DIII-D P.C. Stangebya, S.L. Allenb, J.A. Boedoc, N.H. Brooksd, K. Christiee, C. Chrobake, J.D. Eldera, M. Fenstermacherb, R.J. Groebnerd, M. Grothb, C.J. Lasnierb, A.G. McLeana, A.W. Leonardd, S. Lisgoa, G.D. Porterb, M.E. Rensinkb, T.D. Rognlienb, D.L. Rudakovc, J. Sheae, W.R. Wamplerf, J.G. Watkinsf, W.P. Westd, D.G. Whytee,g a University of Toronto b Lawrence Livermore National Laboratory C University of California San Diego d General Atomics e University of Wisconsin-Madison f Sandia National Laboratory g Massachusetts Institute of Technology UNIVERSITY OF TORONTO Institute for Aerospace Studies

2. Principal Initial Findings (1) • 13CH4 puffed toroidally symmetrically at top of LSN, NBI-heated, high density, detached, ELMy H-mode repeat shots in DIII-D , “ITER-relevant conditions”. Simulates wall source of net-eroded material – carbon or other elements. • Substantial 13C deposition found on the tile faces on the walls of the inner divertor and Private Flux Zone. • Divertor plasma very cold, dense → the first hypothesis that we OEDGE-model-tested and report on here: 13C was convected by fast flow along inner SOL, then experienced volume recombination at/near the inner divertor entrance, with deposition occurring as neutrals. Further hypothesized that the 13C-deposits in the divertor were not significantly disturbed by ongoing plasma exposure, including ELMs.

3. Principal Initial Findings (2) • Next hypothesis to be OEDGE-model-tested: that ELMs caused some erosion and re-deposition of 13C. • In any case, any disturbance, if present, did not cause the sort of massive migration of co-deposits to remote regions that occurred in the JET DTE1 experiment and it appears that the 13C that entered the divertor remained on the divertor tiles. • The associated co-deposited tritium may thus be in soft deposits on surfaces having good access for T-recovery, e.g. by disruption flash heating, oxygen discharge-cleaning, laser heating, flash-lamp heating, etc.

4.  BxB Upper single null: <ne> ~3x1019 m-3 RCP At the crown carbon ions are entrained in the deuteron SOL flow of M ~ 0.5 via frictional coupling Te ~ 10 eV: VD+ ~ 15 km s-1 VC+  15 km s-1 CH  BxB Inner target CII spol M. Groth J.A. Boedo

5. Substantial 13C-Deposition found in Divertor • 37% of the 13C was found on front faces of tiles in the inner divertor region and the private flux zone, PFZ. • Figure also shows results from an earlier, similar experiment which used low density L-mode discharges where half as much 13C was injected in total. • The most significant difference in the 13C deposition pattern was that in the detached H-mode experiment there was substantial deposition on the PFZ wall, ~ ¼ of total injected. W.R.Wampler D.G. Whyte

6. Co-deposits stayed in divertor. • ~half the injected 13C is accounted for on the front faces of 7 tile-rows in the divertor (~37%) and front faces of 3 tile-rows near the injection point (~10%). • The inner wall tiles have not been measured with PIGE yet, but in the earlier L-mode 13C experiment in DIII-D, PIGE analysis found ~40% (with large uncertainties) of the injected 13C there. • It therefore appears that the ongoing plasma exposure, including ELMs, did not significantly disturb the divertor deposits by eroding and re-depositing the 13C – in any case, not enough to have caused the sort of massive migration of co-deposits to remote regions, as occurred in the JET DTE1 experiment - which did not use a single plasma condition, as here, but was the result of an entire campaign – and also strike-point sweeping. • Tile gap deposits have not been measured yet.

7. OEDGE and UEDGE code modeling • The interpretive OEDGE code is being used with ad hoc, adjustable input assumptions for: (a) parallel flow speed in SOL, and (b) a radial pinch (in +R-direction), in an attempt to identify what is happening. • The first-principles UEDGE code is being used to try to understand why it is happening.

8. OEDGE code: both a fast parallel SOL flow and a radial pinch implied • Fig shows that both a parallel flowand a radial pinch are required in order to replicate anything at all similar to the deposition pattern measured. • In the OEDGE modeling, most of the 13C deposited as neutrals formed by volume recombination in the cold, dense detached plasma in the inner divertor and the PFZ, Private Flux Zone. J.D. Elder

9. OEDGE code : radial pinch (+R-direction) ~10 m/s in inner SOL is implied • The methane breaks up to create 13C-ions too far out in the SOL to be able to replicate the observed deposition. • A radial pinch is needed in OEDGE to move the 13C-ions toward separatrix in order for deposition to occur where seen. • Figure illustrates effect of assuming different radial pinches in OEDGE analysis. (M|| = 0.3 was fixed.) • A radial pinch of ~10 m/s ~replicates the measured deposition pattern of the 13C, both shape and magnitude. J.D. Elder

10. UEDGE reproduces multiple diagnostics in the divertor and main chamber SOL simultaneously L-mode • UEDGE predicts Te,ISP ~ 1.5 eV, consistent with measurements in inner divertor • Inner strike point D/D ~ 0.15 • Lack of CII emission in the inner leg • ExB drifts play an important role in obtaining low Te,ISP • Omitting drifts raises Te,ISP from 1.5 eV to 3 eV • Less consistent with experiment! ExB G.D. Porter, M. Groth

11. UEDGE: 13C deposition at inner and outer target is strongly dependent on D+ flow in main SOL L-mode Outer plate • Main chamber SOL flow and 13C deposition inconsistent with low Te,ISP Te,ISP = 1.5 eV Inner plate Te,ISP = 3 eV Inner plate Outer plate

12. Potentially important implications for tritium recovery. That the co-deposits seem to stay approximately where they are initially formed, at least for detached divertor conditions, might have important implications for tritium retention: absence of migration of the tritium to remote locations that may be less accessible for tritium recovery.

13. ….however, still many more questions than answers • Is the condition we used in DIII-D ITER-relevant - quite strongly detached with rather weak, frequent ELMs? • Had it been possible to continue for many more of the same shots, would large scale migration have occurred, as in JET DTE1? • If there had been a cavity on the inside containing water-cooled structures, as in JET → large scale migration? • Had the target been hotter → large scale migration? • When we find the still-missing ~ 1/2 of the 13C, will that indicate large scale migration to some location? • When we measure the 13C in the tile gaps, will this indicate large scale migration?

14. Tritium in ITER: we don’t know what will happen • “ITER-relevant conditions” in present devices can result in radically different deposition behaviour…. • …with radically different implications for (a) T-retention, (b) T-recovery. • Implies extrapolation from present devices not reliable. • Extrapolation steps, e.g. total energy input, are too large. • Extrapolation requires knowing how to scale. We don’t. • Codes can’t explain present co-deposition therefore can’t predict ITER. • Extrapolation to DT-ITER needs to be based on H/D-ITER.