Status of measurement requirements for the ITER divertor

Status of measurement requirements for the ITER divertor 1 R. A. Pitts, 2 G. Vayakis, 2 A. Costley with thanks for comments to A. Kukushkin, D. Whyte 1 Centre de Recherches en Physique des Plasmas, Association EURATOM-Confédération Suisse, École Polytechnique Fédérale de Lausanne, CH-1015 Switzerland 2 ITER-IT, Naka, Japan

Recent history of these measurement requirements 13/10/2000: A. Costley gives review of measurement of ITER divertor plasma and target parameters at SOL and Divertor Physics Expert Group, Garching, - specified (at that time) and estimated measurement performance. 09/07/2001: A. Costley gives update on divertor measurement requirements at the 14th Divertor and SOL Expert Group meeting in Naka on the basis of some feedback following the Garching meeting. Presentation given in the form of 6 discussion points in key areas requiring attention. Action item on R. A. Pitts at the Naka meeting to propose a minimum set of requirements on behalf of the SOL and Divertor group, addressing each of the discussion points. 15/11/2001: G. Vayakis makes a presentation at the first ITPA diagnostics meeting in St. Petersburg using as a base a document transmitted to G. Vayakis and A. Costley containing feedback on the discussion points.

This process has led to a set of modified requirements and some actions on the ITPA Diagnostics Group. Aim here is to summarise these measurement specifications for the ITPA Divertor Group following the lines of the previous presentation by G. Vayakis. This is in preparation for the debate to be held in a joint session of the Divertor and Diagnostics ITPA groups on the first morning of the 2nd Diagnostics ITPA meeting: Monday March 4, 2002. Apologies in advance for some repetition of previous material - there are some new members in the group.

Six key discussion points where minimum measurement requirement uncertain * 1 Is the proposed spatial resolution for divertor total radiation (profiles and inverted data) good enough? 2 What is the minimum required time resolution and precision for target plate heat flux measurements? 3 Given the measurement difficulties, what are the resolution requirements for target plate erosion? 4 The question of T e, n e, ion flux measurement at the target plates. 5 How serious is it if T e, n e measurements have relatively low spatial resolution in the outer divertor leg? How serious is it if there are no measurements in the inner leg? 6 Is visible spectroscopy sufficient in the divertor (given the difficulty of doing UV spectroscopy)? * A. Costley & G. Vayakis, 14th Divertor and SOL EG meeting, Naka, 9-11 July 2001

Discussion point #1: radiated Power A spatial resolution of 5 cm in the radiated power is all that is likely to be available. Is it enough? What are the consequences if it is > 5 cm, say 10 cm?

Current proposed LOS: main chamber/divertor Upper Port 60 lines of sight X 0 500 mm A Upper Port 60 lines of sight H F C B G E D Equatorial Port 80 lines of sight Div Bolom/LAMx Divertor Cassette 120 lines of sight Current design has ~340 lines of sight. Chordal resolution: ~5 cm, spatial resolution after inversion > 10 cm. Concern about lifetime of divertor cameras and strong effect of neutrals. Only very coarse resolution possible if too many divertor chords are lost. Performance improvements presently limited by cash.

From G. Janeschitz, L. de Kock, A. Kukushkin et al., Diagnostic Requirements for the ITER Divertor in Proc. Int. Workshop on Diagnostics for ITER, Varenna, 1997 Partially attached case div. inner main & SOL div. outer 1%C (#136) 80 total 60 40 carbon neon helium Carbon + 0.2% Ne seeding 10% He at coreedge interface 2D radiated power profile per unit volume (MW/m 3 ) from all C charge states 20 neutral A.Kukushkin, H.D.Pacher 5/97 0 0 1 2 0 10 20 2 1 0 target X X target length [m] Cumulative poloidal radiation integral in inner, outer divertor and SOL Most of the radiation comes from the divertors, but otherwise well spread out along the divertor legs In general, proposed spatial resolution (say 10x10 cm pixel) in an inverted image should be acceptable except in strike point zones (especially outer target). Within a tight budget, performance improvements should concentrate on improving resolution in the target zones if possible.

Discussion point #1: Radiated Power - conclusion Presently envisaged potential resolution offered by the divertor (in combination with main chamber LOS) acceptable. If possible, performance enhancements to be directed toward improving resolution near the targets. Is it worth spending some time investigating the possibility of complementing the system with dead layer AXUV diodes if radiation hardness can be demonstrated?

Discussion point #2: time resolution and precision of target plate surface temperature/power flux We have been asked to consider increasing the time resolution in the target plate measurement [of heat flux] to 20 µs (from 2 ms). On this timescale, we expect the accuracy [of the present system] to be poor ( T > 200 C). Is this serious? What is the minimum requirement?

Current proposed target IR measurement 1.0 m z 4780 nm P6 P5 P4 4630 nm 4340 nm P3 3915 nm P6 P5 P4 P3 P2 P1? P2 3510 nm Horizontal Viewing Duct 0.4 m Ellipsoidal M irror Grating 200 L/mm Slit1.6 mm x 40 mm 3210 nm 7.0 m 7.4 m R Concept unchanged from ITER-1998 design. Figures here from DDD 5.5.G.06 P1 A system exists for both target plates Spatial/time resolution: 3 mm/2 ms temperature range: 200-2500 C, wavelength range: 3-5 µm, accuracy, 10% Combined with wide-angle viewing from equatorial ports Wide viewing angle makes borescope optics difficult and mulit-element - use wavelength multiplexing collection optics Inverse Rowland circle spectrometer - spatial information encoded into wavelength and recovered with a second spectrometer outside biological shield

Requirement for monitoring effects of ELMs Temperature,ûC 5000 4000 3000 2000 1000 6000 5000 4000 3000 2000 1000 Transient ELMs CFC (20 mm) (0.8 MJ/m2-200µs, initial heat flux=10 MW/m2) Vaporised thickness Temperature 100 0 0.1 0.00001 0.0001 0.001 0.01 0.1 Time, s Transient ELMs W (15 mm) (0.8 MJ/m2, initial Heat Flux=10 MW/m2) Melted layer Vaporised thickness Temperature 100 0.1 0 0.0001 0.00001 0.0001 0.001 0.01 0.1 Time, s 10 1 10 1 0.01 0.001 Vaporised/Melted Thickness (µm) From G. Federici, 14th Divertor SOL EG meeting, Naka, July 2001 Simulations show that a 200 µs ELM depositing 0.8 MJ/m 2 raises T surf to melting (sublimation) limit of W (CFC) in a time < 100 µs Depending on inter-elm power flux, starting temperature could be as low as ~400 C. ITER FDR instrumental resolution would permit time resolution of the order of several µs for T surf exceeding 1000 C with high accuracy. Any events leading to T surf rise above ~1000 C can be monitored with high accuracy Poorer performance at low T surf not a serious problem - time resolution can be sacrified if no transients. Need more simulation (and understanding) of exactly what to expect for ITER ELMs: Instrument could be in difficulty for low starting T surf and smaller ELMs Sensitivity can also be strongly affected by changes in surface ε.

Discussion point #2: Surface temperature - conclusion Quoted spatial resolution (~ 3 mm) adequate. Very high time resolution (µs) and sensitivity for high T surf near divertor target operating limits. Perfectly adequate for ELM monitoring. Some potential problems if low starting T surf and smaller ELMs. Potential difficulties under conditions when surfaces change radically (and quickly) under erosion/ redeposition (common to any system). Need to use any new ITER ELM simulations as they come to re-evaluate instrument performance. More emphasis should be placed on the use of tile thermocouples (but water cooling makes it harder).

Discussion point #3: target plate erosion It is suggested that the resolution in the measurement of target plate erosion should be 0.1 µm for a single Type I ELM in real time. It will not be possible to meet this; 100 µm is more likely. How serious is this?

Quiescent case: no ELMs no disruptions ( semidetached, most recent case )* Peak net erosion rate: ~6 (< 0.0005) nm/s for C (W) Tritium co-deposition rate: 1-5 (~0) mg/s for C(W) If C used in the divertor, ~3 µm per 500s pulse eroded and ~500g T retained after 200, 8 min. pulses. Situation unknown but probably much worse in case of ELMs, disruptions etc. (depending on the ELMs ITER will have). Erosion measurements are difficult and so important to specify as carefully as possible the minimum requirements What should be the functionality of the system(s)? *From G. Federici, 14th Divertor SOL EG meeting, Naka, July 2001

Function 1: tread wear: signs of divertor end of life. Function 2: Real time capability: avoid dangerous regimes for the divertor when they are encountered, monitor ELM induced erosion etc. For C targets, erosion measurement can also be an indirect indicator of approximate T-retention. Real time would allow erosion budgets to be set for the operators - could even be one of the control categories. Function 1: Range finding system forseen (periscopic insertion between pulses) covering ~80% of the targets and first wall with spot size ~1mm and depth resolution ± 0.1-0.5 mm. Function 2: Interferometric/range finding techniques might be extendable to 10 s µm and could conceivably be made real time (several sec. time res.) but this seems unlikely (and will be toroidally localised).

A full remote handling inspection system is forseen for ITER. This system can, if enough modules are equipped, image almost the entire first wall and divertor surfaces with 1 mm spatial resolution and ± 0.5 mm resolution for metrology (range finding, interferometry etc). It is possible that these systems could go down as low as ± 0.1 mm But not really an inter-pulse option.

Feasibility studies etc. required to find limitations of any proposed technique for real time (or at least inter-pulse), divertor localised observations. What minimum realistic measurement requirements can physics impose?

Discussion point #4: target plate probe measurements It will probably not be possible to measure reliably the plasma parameters (n e and T e ) at the target but only the ion flux with Langmuir probes. How serious is this? What are the consequences?

Proposed system has 3 groups of ~80 CFC single probes with ~1 cm pitch arranged as triple probes (as in JET) Probe is of JET design Potential problems with RIC and RIED Unknown how long these will survive in full performance discharges (or even before then). Survival only realistically possible in partially detached, cold divertors Probes an excellent indicator of detachment (via ion flux), with no problems of interpretation (except changes in A proj ). The usual problems of interpretation of absolute value of T e in high recycling - appears difficult to solve, but relative changes in T e could be usefully employed. System should be more than adequate and should be included. Lifetime issues impossible to judge for ITER using only JET experience.

Discussion points #5,6: T e and spectroscopy in the divertor Measurement of T e along the outer divertor leg is very difficult. There will be high resolution measurements in the upper SOL region (main chamber) and there could be some across the X- point. How serious is it if there are no measurements along the divertor leg? If there are, what should be the spatial resolution? It is difficult to make UV measurements in the divertor - will visible be good enough if good bolometry measurements are available. Presently no provision for measurements along the inner leg. How serious is this?

12 23 Divertor (imaging) and X-point (LIDAR) Thomson Centre de Recherches en Physique des Plasmas Divertor impurity monitor UPPER-PORT (2 port apart Toridally) EQUATORIAL-PORT (next Troidal port) X-POINT IV IH OV OH DIVERTOR-PORT 3.194 DIVERTOR-CASSETTE BIO-SHIELD Two systems forseen: outer leg and X-pt. LIDAR system expected to provide 5 cm spatial resolution, target for divertor system is ~10 cm (both with 1 ms time res.) Inner leg viewing practically impossible (3 potential sightlines along inner leg could be provided by divertor reflectometry). Divertor TS cannot be swept poloidally, at least not in real time. Poloidal res. ~ 1 cm Combined viewing from main chamber and divertor give resolution along legs of ~ 4 cm and possibility for crude inversion giving 5-10 cm resolution for main impurities (C, W, Be) and D,T (200-500 nm) Number of sightlines reduced in going from FDR to FEAT (shorter divertor legs and cost).

What divertor Thomson needs to do 2 10 21 40 n e (sep.) 20 T e (sep.) T e.5 10 21 00 1 10 21 80 60 5 10 20 x-point target 40 x-point target 20 0 0 9 9.2 9.4 9.6 9.8 10 10.2 9 9.2 9.4 9.6 9.8 10 10.2 Poloidal distance [m] Poloidal distance [m] From A. Kukushkin, P SOL = 100 MW, n sep = 3.2x10 19 m -3, carbon divertor, no additoinal seeding Would ideally require better spatial resolution (say ~ 2 cm) within 20 cm of the target, more uniform (say 5-10 cm) away from the target. But must be some flexibility to vary the poloidal position of the main sightline once measurements begin (strike pt. sweeps difficult in ITER) This system extremely important as an aid toward interpretation of spectroscopic influxes but there are serious concerns for lifetime of front end components.

Regarding the need for UV spectroscopy: If Carbon is in the machine, then can use visible C spectroscopy in combination with bolometry to compute contributions to total radiation both diagnostics must have similar spatial resolution. C spectroscopy can also be used as an ionisation front position detector. If only W targets and no C elsewhere, UV spectroscopy probably essential if details of W transport required. Recommendation is that divertor (and X-pt.) Thomson essential, divertor UV not essential depending on impurities present. Divertor Thomson spatial resolution needs to be < 5 cm near plate, but can be lower further from target. Chordal resolution of impurity monitor ~ 4 cm along legs adequate but should not be lower.

Summary of revisions to ITER-FEAT FDR divertor requirements (changes in red)* MEASUREMENT PARAMETER RANGE or COVERAGE 16. Divertor operational parameters 37. Radiation profile 38. Heat loading profile in divertor 41. Divertor electron parameters 41. Divertor ion temperature RESOLUTION Temporal Spatial ACCUR ACY Max. Surface Temp. 200-2500 C 2 ms - 10% Erosion rate 0.1-1 µm/s 2 s 1 cm 30% Net Erosion 0-3 mm Per pulse 1 cm 12 µm Ionis. front position 0 - TBD m 1 ms 10 cm - X-pt./MARFE TBD - 300 region P rad MWm -2 10 ms a/15 20% Divertor P rad TBD - 100 MWm -2 10 ms 5 cm 30% Surface Temp. 200-1000 C 1000-2500 C Power load (default) 2 ms 20 µs 3 mm 10% 10% TBD - 25 MWm -2 2 ms 3 mm 10% n e 10 19-10 22 m -3 1 ms 5 cm along leg, 3 mm across leg T e 0.3-200 ev 1 ms 5 cm along leg, 3 mm across leg T i 0.3-200 ev 1 ms 5 cm along leg, 3 mm across leg 20% 20% 20% * Minutes of 1st ITPA Diagnostics group N CX MI 6 02-01-11-F1

Summary Centre de Recherches en Physique des Plasmas Divertor bolometry probably adequate as is. Very fast T surf excursions (eg. ELMs) could be followed with the proposed IR thermography if rise in T surf large enough. More work required to quantify effect of lower starting temp. and lower transient amplitude. Erosion measurement is the hardest. We need to discuss in more detail exactly what to specify here and to justify the need for real time measurements. Proposed target Langmuir probe diagnostic perfectly adequate (spatial resolution). Lifetime is another issue. Some changes required to divertor TS spatial resolution, but this diagnostic is essential and must be retained. Visible spectroscopy resolution should not be decreased below proposed (already reduced) specs. Could live without divertor UV if resources are limited.