ITPA Joint Experiment Report : DSOL-1

2006 Reports of ITPA/IEA joint multi-machine experiments SOL and Divertor Physics Topical Group

Item# Spokes person DSOL-1 A. Loarte, DSOL-2 V. Philipps, DSOL-3 B. Lipschultz, DSOL-4 A. Loarte, DSOL-5 S Lisgo, DSOL-8 N. Ashikawa, DSOL-9 V. Phillips, DSOL-11 D. Whyte DSOL-1 2 P. Stangeby, DSOL-13 K. Krieger, DSOL-1 4 D. Coster, DSOL-1 5 J. Terry, DSOL-1 6 M.Groth DSOL-18 C. Skinner

Underline: received until Nov. 5 DSOL-1: ITPA-IEA Joint Experiment Report Scaling of Type I ELM energy loss

Topical Group : Divertor and SOL physics Contact Persons : A. Loarte, M. Kempenaars, G. Saibene, M. Beurskens, A.Leonard, T. Osborne, M. Fenstermacher, T. Eich, W. Suttrop, L. Horton Devices involved in the experiment and time period : JET, DIII-D, ASDEX- Upgrade, due to the scheduled shutdown in DIII-D and problems in operations in JET and ASDEX-Upgrade, practically no experimental time was allocated to these experiments in 2006. Description of the Experiment’s Purpose and Goals : The purpose of the experiment is to study ELM losses in discharges for which the pedestal plasmas are similar in the dimensionless sense and to determine their r* scaling. This proposal is directly linked to PEP-2 in which the aim is to investigate the physics processes that determine the pedestal edge gradients by using dimensionless similar discharges in JET, DIII-D and ASDEX- Upgrade. Description of main results : Discharges at d ~0.27 in JET and DIII-D with dimensionally similar pedestal parameters have shown that ELM energy loads are similar for similar pedestal collisionalities but scale with r* differently between JET and DIII-D. While at DIII-D the large r* discharges have a large ELM affected area compared to those at low r* and large normalised ELM energy losses, at JET the ELM affected area and ELM energy losses seem to be independent of r*. At present the reasons for this difference are not clear and further analysis of the low r* experiments at JET has not been possible due to the pedestal gradients in these conditions being too steep for the existing diagnostics at the time. These results were presented at the IAEA 2004 Conference in a joint JET-DIII-D paper by M. Fenstermacher and published in Nuclear Fusion in 2005. Executive Summary : A. Loarte, M. Kempenaars, G. Saibene, M. Beurskens, A.Leonard, T. Osborne, M. Fenstermacher, T. Eich, W. Suttrop, L. Horton. Period 2003-2006. Study ELM energy losses in discharges with identical pedestal plasmas, in dimensionless variables, and their scaling with d and r* in JET, DIII-D and ASDEX Upgrade. DSOL-3: ITPA-IEA Joint Experiment Report Scaling of radial transport

Divertor & SOL Topic Group Contact Persons: Bruce Lipschultz, Tony Leonard, S. Lisgo, A. Kallenbach Devices involved and time period: experimental time on MAST (Spring, Fall, 2006) and C-Mod (12/05 – 9/06). Analysis of data from DIII-D. Experiment’s purpose and Goals: The purpose of this collaboration is better prediction of how radial transport in the SOL scales. We aim to apply the same radial transport analysis to SOL data from a number of experiments for a range of plasmas (some dimensionlessly similar and some not) to try and identify the important scaling parameters.

Principal Results: DIII-D: Data analysis of radial fluxes through separatrix and to limiter surfaces (window frame analysis) done for DIII-D H-mode data. Analysis to appear in J. Nucl. Mater. C-MOD: Expanded operational space of measurements to low opacity SOL, similar to JET. MAST: Magnetic equilibrium developed for window-frame method analysis using dedicated shots. Experiments conducted but data not considered proper quality ASDEX-Upgrade: No data taken. Examining the possibility of using data from another DSOL proposal.

Executive Summary: Progress was made in expanding the database of shots (C-Mod) with analysis planned fort he next year. Equilibria developed for this proposal on MAST. Analysis of DIII-D H-mode data shows that between ELMs the outboard far SOL particle flux increases strongly with density but remains similar to the particle flux across the separatrix; parallel losses in the SOL are made up for by ionization source. During ELMs the far SOL particle flux is increased and becomes localized to the outboard midplane. No data from ASDEX-Upgrade. DSOL-4: ITPA-IEA Joint Experiment Report Comparison of disruption energy balance in similar discharges and disruption heat flux profile characterisation

Topical Group : Divertor and SOL physics Contact Persons : A. Loarte, P. Andrew, V. Riccardo, J. Paley, D. Whyte, G. Counsell, G. Pautasso Devices involved in the experiment and time period : JET, ASDEX Upgrade, MAST and DIII-D. Due to the scheduled shutdown in DIII-D and problems in operations in JET and ASDEX-Upgrade, practically no experimental time was allocated to these experiments in 2006. The work has concentrated on data analysis of the experiments done previously. Description of the Experiment’s Purpose and Goals : The purpose of these experiments is to characterise the energy balance and energy fluxes to plasma facing components during the thermal quench of plasma disruptions in order to determine the scaling of such losses to ITER by comparing results from experiments in devices of various physical sizes. Description of main results : Analysis of existing data in various tokamaks (JET, ASDEX Upgrade, MAST and DIII-D) has shown that the energy of the plasma at the disruption thermal quench is usually much smaller than that of the full performing plasma (with the exception of high  ITB collapses), the power footprint at the divertor is very broad, typically 5-10 times larger than the steady- state power flux and the timescale for energy flux to the divertor scales with machine size but shows a large variability. This affects the expected energy fluxes expected in disruptions in ITER and, in consequence, their lifetime under disruptions. These results were presented at the IAEA 2004 Conference in a joint JET, ASDEX Upgrade, MAST, DIII-D, FTU and TEXTOR paper by A. Loarte. A more detailed evaluation of the temporal dependence of the power flux during disruptions and of the energy fluxes during pre-disruptive events and their consequences for ITER have been presented at the IAEA 2006 Conference in a joint JET, ASDEX Upgrade, MAST, DIII-D, FTU and TEXTOR paper by A. Loarte. Executive Summary : A. Loarte, P. Andrew, V. Riccardo, J. Paley, D. Whyte, G. Counsell, G. Pautasso. Period 2003-2006. Study of energy balance and energy fluxes to plasma facing components during the thermal quench of plasma disruptions in JET, ASDEX Upgrade, MAST and DIII-D. DSOL-8 : ITPA-IEA Joint Experiment Report ICRF Conditioning

Divertor & SOL Topic Group

Contact Persons: N. Ashikawa (LHD), J. Hu (HT-7), V. Philipps (TEXTOR), A. Lyssoivan (ASDEX-Upgrade)

Devices involved and time period: LHD,HT-7, TEXTOR, ASDEX Experiment’s purpose and Goals: The purpose of this collaboration is a development of ICRF wall conditioning method as one of wall conditioning with strong magnetic field in devices with super conducting coil system. We aim to estimate the removal rate of tritium from the wall, determination of effective removal area and establish the operation scenario in ITER.

Principal Results: LHD: Comparison of removal rate during ICC, GDC, and plasma operation in 2005 was reported. (FED(2006).in print). HT-7: This group is focusing for comparison of general ICC operation using He gases and Oxygen operation (PPCF 48 (2006) 807). TEXTOR and ASDEX: To fix a best frequency for ICC, different setting mode was operated (reported in PSI-17, 2006)

Executive Summary: From a making list of removal rate in each devise, a discussion about high energy tail by N. Ashikawa and A. Lyssoivan was stared. To install material samples, N. Ashikawa and J.S. Hu discussed a possibility of installation in HT-7 and LHD, and at present a design of setting samples with sample holder is started. Until 2006 Oct, new experiments were not operated in this year due to limited chance in each device. But in 2006 Nov. new operation is planed in LHD. DSOL-12 : ITPA-IEA Joint Experiment Report Oxygen Wall Cleaning

Divertor & SOL Topic Group Contact Persons: P. Stangeby, V. Phillips, J. Hu, C. Hopf. Devices involved and time period: TEXTOR, HT-7, DIII-D, AUG. 2005-6. Experiment’s purpose and goals: T-retention by co-deposition with C is problematic for ITER. Oxygen cleaning is a potential solution. Principal Results: TEXTOR. (V. Phillips) He/O-GDC and He/O-ICRH have been studied. The inner wall, 6 m2, the largest C-area, was at 390K C. Both methods removed ~2x1019 C/s, which is ~10X lower than the integral C re-deposition rate in TEXTOR, but ~10X higher than the integral C- removal rate by thermal oxidation at 630K. Immediately after the O-GDC, plasma initiation failed; wall cleaning was done in H2-GDC for 66 hours, followed by GDC in He for 30 min, enabling plasma start up but with ~3-5X increased O and C levels. The efficiency of He/O- ICRH can be raised by optimising the duty cycle with ICRH O on and off. C-oxides are mainly released after the ICRF pulses, while the removal during the pulse is limited by the pressure limit at the ICRF antenna; increasing the C-removal rate would require higher pumping speeds. HT-7. (J.S. Hu) Compared O2-baking, O-ICR, 4:1 He/O-ICR and O-GDC all at ~450K. O2- baking ineffective. To remove the retained oxygen on walls: both He-ICR and He-GDC found effective. Plasma operation recovered after a few hrs of plasma discharges. No apparent damage to torus components. Samples cut from limiters tiles were analyzed by SEM and TDS. Before exposure to O-plasmas, the tiles showed apparent deposits; after exposure the deposits were partially removed and deuterium retention was reduced. O-GDC was more effective at removing deposits from limiter front-faces than side-faces. DIII-D. (P. C. Stangeby) Thermal oxidative erosion measurements were made in Toronto of co-deposits on DIII-D divertor tiles. The highest initial D removal and film erosion rates were obtained at 673 K and 79 kPa, the most extreme conditions evaluated. Both the D content and the derived film thickness were reduced by about an order of magnitude in 15 min, corresponding to a film erosion rate of ~7 µm/h. In ITER, if vessel components with co- deposit accumulation could be selectively heated to ~673 K during oxidation at ~80 kPa O2 pressure, then tritium-containing co-deposits of ~70 μm thickness could be eroded in about 10 h. D removal efficiency decreased with increasing B content of the specimens from ~ 98% with ~2% initial B content to ~30% with ~34% initial B content. AUG. (C. Hopf). C-removal in AUG by O2/He GDC. 20 g of C removed in 49 hrs as CO, CO2. Laboratory studies were carried out on the influence of boron on the erosion rate in an oxygen ECR plasma of a-C:B:D films with B/(B+C) ratios between 0 and 1; additional rf bias created ion energies ~60 eV. The C-erosion rate decreased by ~3 orders of magnitude for B/ (B+C) → 0 and 1, while the B erosion rate was almost constant. B may be enriched at the surface due to preferential erosion and subsequently the erosion rate is determined by the physical sputtering yield of the (almost 100%) B top layer. Executive Summary: Plasma-activated oxygen is reported on several devices to be more effective at removing co-deposits than thermal oxygen-baking. All devices report that plasma operation can be recovered relatively quickly. There have been no reports of damage to internal vessel components. Impurities such as B in the co-deposits can greatly reduce removal rates. DSOL-14 : ITPA-IEA Joint Experiment Report Multi-code, multi-machine edge modelling and code benchmarking

Divertor & SOL Topic Group Contact Persons: David Coster, Arne Kallenbach, Xavier Bonnin, Tom Rognlien Devices involved and time period: No machine time Experiment’s purpose and Goals: Benchmark/verify/validate the edge codes currently used to interpret present experiments and predict conditions for ITER

Principal Results: SOLPS/EDGE2D-NIMBUS: Visits by David Coster (DE) and Xavier Bonnin (FR) to JET for 4 weeks; initial runs with D+C produced encouraging results; decided to wait for the release of EDGE2D-EIRENE SOLPS 4.0/SOLPS5.0: a substantial effort is being invested in ensuring that the ITER simulations performed with SOLPS 4.0 can be essentially repeated with SOLPS 5.0 despite the difference in the formulation of the equations and the numerics SOLPS 5.0/SOLPS 6.0: for the no-drifts cases, the two codes produce essentially the same results when the grid adaptation feature of SOLPS 6.0 is disabled UEDGE/SOLPS: underway; David Coster (DE) and Xavier Bonnin (FR) visited Larry Owen (US) in ORNL for two weeks and Tom Rognlien (US) in LLNL for two weeks; substantial progress on the D, no-drifts benchmark has been made and some results should be presented at APS-2006 (Owen) MDSPLUS interface: an interface has been prepared so that UEDGE can save code results to a MDSplus tree in essentially the same format as that used for some years by SOLPS MDSPLUS trees: a number of AUG pulses are now available, and a few from D3D

Executive Summary: Code-code benchmarking is well underway with additional aspects completed. Further comparisons will be facilitated by the completed replacement of NIMBUS by EIRENE. Code-code-experiment comparisons have also started, though more work is anticipated in this area. Additional data in the ITPA standardized format is available from AUG and D3D. DSOL-15: ITPA-IEA Joint Experiment Report Multi-Machine Comparison of Blob Characteristics

Divertor & SOL Topic Group Contact Persons: J. Terry (C-Mod), S. Zweben (NSTX), C. Hidalgo (TJ-II), R. Maqueda (NSTX), O. Grulke, D. D’Ippolito, J. Myra Devices involved and time period: piggyback experiments on multiple devices with a literature search for results from those devices that observe blobs. Possible dedicated experiments on C- Mod, NSTX, and TJ-II Experiment’s purpose and Goals: We desire to compare characteristics and dynamic properties of the blobs/filaments that occur in the SOLs of many devices with the goal of establishing their cause and a model for their formation and dynamics. Principal Results: Blobs have now seen (and studied) in tokamaks, stellarators, RFPs, purely toroidal devices, and linear devices. There has also been considerable theoretical/modeling that now reproduces many of the characteristics and statistical properties of blobs. Executive Summary: Blobs/filaments exist in the edges/SOLs of tokamaks, stellarators, RFPs, and linear devices. Their radial propagation velocity is typically a few percent of the local ion sound speed, Their typical size is 10-50rs. There is general agreement that each is associated with a poloidally-oriented dipole potential structure, which gives rise to a radial ExB drift motion. At present there are two main mechanisms considered to be responsible for the radial dynamics of blobs – one driven by sheath or parallel resistivity conditions and the other based on the nonlinear evolution of interchange motions, without any presumption of plasma sheaths. Both models describe some aspects of what is seen experiment. For example, when comparing various statistical measures of blob structure and intermittency here has been good agreement between experiment and the results of simulations based on the interchange model. Some key references: O. Grulke, et al., PHYSICS OF PLASMAS 13, 012306 2006 O. Grulke, et al., IAEA 2006 presentation S. Zweben, et al., review on SOL turbulence presented at the Workshop on Edge Transport Krakow, Poland 2006, and to be published in PPCF S. Zweben, et al., PHYSICS OF PLASMAS 13, 056114, 2006 O.E. Garcia, et al., Plasma Phys. Control. Fusion 48 (2006) L1–L10 J. R. Myra, et al., Phys. Plasmas 11, 4267 2004 . DSOL-16: : ITPA-IEA Joint Experiment Report Determination of the poloidal fueling profile

Divertor & SOL Topic Group Contact Persons: M.Groth (DIII-D), G.Porter (DIII-D), A. Kallenbach (AUG), D. Coster (AUG), M. Wischmeier (AUG) Devices involved and time period: DIII-D (Sep 2004, March 2005), ASDEX-Upgrade (April 2006)

Experiment’s purpose and Goals: Quiescent Ohmic and L-mode discharges in AUG and DIII-D are to be compared and simulated with the SOLPS and UEDGE codes. Both devices are well-suited for such a comparison because of their similar dimensions and extensive diagnostic suites. The overarching goal of this work is the characterization of the locations of plasma recycling and impurity production, and ultimately, through modeling, the determination of the hydrogen flux across the separatrix and pedestal. Validation of the plasma simulations is obtained against experimental data from edge diagnostics in the two devices. Different core plasma densities are to be achieved to examine attached and detached divertor regimes. Experimental data in AUG is to be obtained prior to the changeover of the divertor plates from carbon (CFC) to Tungsten.

Principal Results: DIII-D: Comprehensive documentation of DIII-D L-mode and ELMy H-mode discharges has been performed in 2004 and 2005. In L-mode, the plasma density was scanned over a range including attached (at n/nGW~0.25) and fully detached (at n/nGW~0.55) divertor operation, while in H-mode significantly higher densities (n/nGW~0.7) and divertor detachment were achieved. Modeling of low-density plasmas with the UEDGE code was carried out showing strongest fueling from the region of the inner X-point.

ASDEX-Upgrade: In April 2006 an extensive set of experimental data from the SOL and pedestal region was obtained in Ohmic plasmas at densities ranging from n/nGW~0.2 to 0.5. The transition from attached to detached divertor operation was documented, and the data were submitted to the AUGDIV database. Preliminary UEDGE modeling was carried out for an Ohmic plasma at n/nGW~0.2.

Executive Summary: Comparison of dimensionally similar boundary plasmas in AUG and DIII-D, and modeling thereof with SOLPS and UEDGE is currently carried out to determine the poloidal fueling profiles in the two devices. Comprehensive sets of experimental data with attached and detachment divertor operation was obtained in AUG (Ohmic plasmas) and DIII- D (L-mode and ELMy H-mode). The AUGDIV database was made available to store data from both devices. Direct comparison of the simulation results was enabled by generating post-processor output in a MDSplus database accessible by the SOLPS and UEDGE working groups.

Experimental proposal for 2007: None. DSOL-18: ITPA-IEA Joint Experiment Report Cross-machine comparisons of pulse-by-pulse deposition.

Divertor & SOL Topic Group

Contact Persons: C. Skinner (NSTX), V. Rohde (AUG)

Devices involved and time period: Initially NSTX and AUG. Additional comparisons with JET, TEXTOR will be established following approval. Data is being recorded continuously. Time period is 2006.

Experiment’s purpose and goals: The aim of experiment is to compare time resolved deposition measurements between different machines to uncover underlying physics and to improve predictive capability for tritium retention and first mirror coating in ITER. The data will be compared to modeling using ERO and BBQ.

Principal Results: NSTX: In 2005 pulse-by-pulse deposition was recorded at three locations on the NSTX vessel wall during 17 weeks of plasma operations and during boronization. The results showed a complex pattern of erosion and deposition. Preliminary analysis of some of the data show strong deposition on the first discharge of a run day with erosion or deposition on subsequent discharges. The erosion/deposition at the upper divertor increased during double-null plasmas (as compared to lower single null). A comparison of high performance lower single null plasmas and ohmic plasmas on the same day showed a changeover from deposition in short duration low energy plasmas to erosion after long high energy plasmas. These results will be presented at the ITPA Div/SOL meeting Jan 9-10 2006 in Shanghai. The data analysis is ongoing and will include comparisons to BBQ code predictions (J. Hogan, ORNL). In 2006 dedicated experiments to check specific hypotheses on the key factors in deposition will be carried out on NSTX.

ASDEX-U: Three qmbs are operating, one on the inner divertor, two on the outer divertor. Data is collected routinely. The erosion / deposition in the outer divertor depends on the plasma scenario. For the 2005/2006 campaign a quartz crystal with a pre existing deposit was installed on the outer divertor qmb to be in a position to record erosion of the pre-existing layer.