1. S. Lisgo 1, G. Counsell 2, A. Darke 1, G. De. Temmerman 1, J. Huang 3, G. Maddison 1 1 UKAEA 2 F4E 3 ASIPP MAST Activities for 2008-2009 EU-PWI SEWG: FUEL RETENTION MAST Activities for 2008-2009 EU-PWI SEWG Fuel Retention Wednesday, July 23, 2008 Culham

2. EU-PWI SEWG RETENTION, JULY 2008 Overview (Brief) introduction to MAST: vessel and operation Possible contribution to EU retention studies (reality check) Data from a sample discharge Planned interpretation effort: diagnostics + modeling

3. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel: (relatively) unrestricted diagnostic access Tin can vessel design – 20 mm thick steel vacuum vessel provides the structural integrity – remote wall means limited need for periscopes or re-entrant ports simplifies diagnostic design, particularly for cameras

4. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: PF coils inside the vessel Poloidal field coils suspended inside the vessel, each with its own vacuum sealed coil can – partial wall tokamak – implications for maximum bake-out temperature – very little plasma contact on the coils except during large transients (and startup, typically)

5. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: no/partial wall (Very) open divertor – fine grain graphite targets and center-column armour – no main chamber recycling during standard operation – low aspect ratio and remote wall allow camera views to cover a large fraction of the plasma-wall interaction

6. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: vessel parameters 2.1 m 4.5 m An economy of coils… – up-down symmetry (almost) – flexible: LSN, DN, USN, both I p and B t can be reversed at the same time Plasma volume 8 m 3, tank volume 60 m 3 – plasma to vessel volume ratio of 7:1 – large neutral reservoir surrounding the plasma No cryo-pumping – 5 Leybold turbos attached to the lower divertor – 2500 L s -1 manufacturer pumping speed, 1600 L s -1 measured Flexible fuelling options – in/out, top/bottom gas puffs – vertical/radial pellets

7. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: strike-point sweeping Strike-points sweep with the solenoid current ramp due to the strong fringing field – 1 mm per ms on the outer targets – stationary strike-point moving x-point (typically)

8. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: wall composition GRAPHITE Fine grain graphite armour on center-column and divertor – EK986 (10 um grain size) – total amount of graphite: 1.5 metric tonnes

9. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: wall composition SS GRAPHITE Stainless steel everywhere else – 304LN for main vessel, 316L for coil cans – graphite paint applied to outer wall near midplane to reduce reflections – (deposits elsewhere of course 0 th order sample analysis gave expected results, no quantitative studies) Fine grain graphite armour on center-column and divertor – EK986 (10 um grain size) – total amount of graphite: 1.5 metric tonnes

10. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST MAST vessel design: imbricated outer target plates Imbrications outer divertor co-deposition in the shadowed regions – co-deposits have not been analysed – done to give diagnostic views up through the divertor All targets are typically (always?) attached – (efforts underway to develop a detached target scenario via impurity injection)

11. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST Wall conditioning: GDC and boronisation, operation with cold walls Regular He GDC: 5-10 minutes between each shot – vessel pressure 10 -3 mbar – 4 antennas (3 currently in use): 400-600 V to strike, 250 V during glow Boronisation – every 4-6 weeks, 5-10 g per deposition – deuterated trimethyl boron (TMB) – deposition during standard He GDC with 5% TMB Baking: CC armour to 120 C (solenoid is actively cooled), coil cans get to 90 C (cooled), 160 C everywhere else – typically bake for 2 weeks – viton caskets in the top and bottom end plates can go to 200 C (twin gaskets, pumped interspace) – base pressure is 1-2 10 -8 mbar, dominated by H 2 0 Everything near room temperature during operations – target surface temperatures get to 50 C during L-mode, inter-ELM H-mode (IR) – 200-300 C during ELMs – 500 C during disruptions

12. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST Neutral beams Recently upgraded to 2 JET-style PINI sources: regular 4 MW operation (max. 5 MW) – previously, limited to 300 ms beam duration (unreliable as well) – still commissioning the new beams, completion by October (hopefully) Titanium gettering in the beam line Deuterium neutral beams on sectors 6 and 8 MIDPLANE SLICE

13. EU-PWI SEWG RETENTION, JULY 2008 Overview (Brief) introduction to MAST: vessel and operation Possible contribution to EU retention studies (reality check) Data from a sample discharge Planned interpretation effort: diagnostics + modeling

14. EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTION MAST operation: limitations with respect to PWI studies So, the main MAST issues are for PWI studies: short pulse, regular disruptions – list of non-disrupting shots is short (currently) limited ability to repeat shots in order to build up measurable fluence onto material probes for post-mortem analysis – not well suited for bulk erosion/deposition studies Typical flat-top current duration of 200-400 ms for reasonable plasma currents (very) short pulse device – typically operate close to double-null (lowest L-H transition threshold) – note: lower SND shot, off-axis beams (plasma shifted downward), regular sawteeth 600 ms Plasma disrupts with some regularity – predominately locked modes – first sawtooth can be a problem… – (the claim has recently been made that density control is lost without regular disruptions, with the standard GDC need thermal flash to fully deplete walls inter-shot?)

15. EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTION Keeping these limitations in mind: The Plan for retention studies Motivation for retention studies: – contribute meaningfully to overall EU-PWI effort, if possible, but also… – understand MAST fuel cycle to try and improve performance: optimise conditioning, quantify role of disruptions, provide input to the upgrade study

16. EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTION Keeping these limitations in mind: The Plan for retention studies Motivation for retention studies – contribute meaningfully to overall EU-PWI effort, if possible, but also… – understand MAST fuel cycle to try and improve performance: optimise conditioning, quantify role of disruptions, provide input to the upgrade study Objectives / work plan: Routine gas balance calculations during the shot and evaluation of dependencies, if any Routine analysis of fuel recovery between shots Semi-routine 2D simulations of neutral particle transport throughout the discharge, in an effort to refine the wall pump model (long project) Preliminary exposure of material probes in the divertor and at the outer midplane for post-mortem analysis (particularly challenging in MAST) Post-mortem analysis of divertor tiles, in particular co-deposits in shadowed regions (gaps, back of tiles) in the outer divertor? – films likely disturbed by semi-regular disruptions makes meaningful campaign- averaged post-mortem analysis difficult/impossible? – some new tiles installed in the outer divertor for present campaign

17. EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MAST Overview (Brief) introduction to MAST: vessel and operation Possible contribution to EU retention studies (reality check) Data from a sample discharge Planned interpretation effort: diagnostics + modeling

18. EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGE Gas balance for representative Ohmic pulse: 18296: 0-300 ms 0 - 300 ms All gate valves closed (turbos + NBI) Shot repeated but no plasma breakdown initiated – gives more accurate measure of injected gas than from the gas valve calibration done at the start of the campaign 20% discrepancy due to piezo valve drift and/or valve reconfigurations – more frequent gas valve calibrations may avoid the need for the no plasma shot, but not sure… Retention: 80% (Note: the fast ionisation gauge is relatively new)

19. EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGE Gas balance for representative Ohmic pulse: 18296: entire discharge 0 - 600 ms All gate valves closed (turbos + NBI) Shot repeated but no plasma breakdown initiated – gives more accurate measure of injected gas than from the gas valve calibration done at the start of the campaign 20% discrepancy due to piezo valve drift and/or valve reconfigurations – more frequent gas valve calibrations may avoid the need for the no plasma shot, but not sure… Retention: 80% (Note: the fast ionisation gauge is relatively new)

20. EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGE Experimental activities intended for 2008-2009 See if the torus gate valves can remain open during the pulse greatly simplifies routine studies (should be OK…) Bid has been made to try H and He plasmas (D beams) Analysis of post-shot D recovery – currently analysing the available RGA data – considering a high resolution RGA to resolve D 2 + and He + peaks – try D imaging during the glow Evaluate dependencies (piggyback): boronisation, GDC, disruptions, fuelling location, confinement regime, equilibrium geometry, Ip,, input power, wall temperature (within reason), ELM coils, …..?

21. EU-PWI SEWG RETENTION, JULY 2008 Overview (Brief) introduction to MAST: vessel and operation Possible contribution to EU retention studies (reality check) Data from a sample discharge Planned interpretation effort: diagnostics + modeling

22. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL OSM-EIRENE dedicated interpretive code package OSM dedicated interpretive model plasma reconstruction from experimental data – impose plasma data directly onto the simulation – attached targets makes life easier… – and very little main chamber recycling… OUTER MIDPLANE THOMSON LOWER INNER LPs Upstream inner SOL and PFRs less constrained than the outer SOL – working on He line ratio analysis to address this

23. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL Automated data collection and setup of the code Want to follow the plasma all the way through the shot – temporal resolution of plasma evolution timescale, not particle transport timescales Need automated code setup for routine application to a large number of shots/timeslices work underway… – automated computational grid generation – semi-automated data collection and processing OUTER MIDPLANE THOMSON LOWER INNER LPs

24. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL EIRENE kinetic Monte-Carlo simulations Once the plasma is defined, use EIRENE [D. Reiter] to calculate the neutral particle distributions throughout the vessel – 3D wall and plasma will be implemented shortly… – refine particle inventory calculations based on FIG TeTe nene nDnD n D2 9.5 10 18 m -3 max 30 eV max 0.6 10 18 m -3 max 0.6 10 19 m -3 max

25. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL Wall pump calibration from D measurements Initially, just using a crude any neutral hitting the wall can be pumped model – move to something more sophisticated only pump atoms and/or include co- deposition with C in the divertor (eventually, complicated) 4% 0% Unlikely to ever provide any detailed spatial information on where the deuterium is being retained, but can perhaps discriminate between the dominant processes/regions (and interesting to see if this code exercise works at all…) If the plasma is specified correctly, then the principal free parameter remaining in the model is the wall pump constrained by D measurements

26. HU12 DIVCAM1 (D ) HL07 DIVCAM2 (D ) HM07 FAST CAMERA WITH D FILTER EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL Extended D measurements for constraining the model Full-vessel high spatial resolution 2D D will be available for the 2008-09 experimental campaign – commissioning almost complete… – mega-pixel cameras in the divertors and main chamber 5 mm spatial resolution – main chamber camera at 3 kHz, divertor cameras at 100 Hz Reconstruction of the poloidal emission profile from the camera images is performed to facilitate comparison with the model

27. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL Attempt to fold in post-mortem analysis of well characterised samples Divertor Science Facility (DSF) big name, little probe (3x3 cm) – vacuum-lock probe system commissioned by end of 2008, with luck… – need to develop a good scenario so that a significant fluence can be built-up quasi stationary strike-point, no disruptions… – perhaps only local transport studies, i.e. no input to global retention picture – can also expose samples on an existing outer midplane probe system

28. EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODEL Attempt to fold in post-mortem analysis of well characterised samples Divertor Science Facility (DSF) big name, little probe (3x3 cm) – vacuum-lock probe system commissioned by end of 2008, with luck… – need to develop a good scenario so that a significant fluence can be built-up quasi stationary strike-point, no disruptions… – perhaps only local transport studies, i.e. no input to global retention picture – can also expose samples on an existing outer midplane probe system TESTS OF MOCK-UP PROBE HEAD AND SHAFT

29. EU-PWI SEWG RETENTION, JULY 2008 Summary MAST operational limitations need to be careful when identifying appropriate PWI studies – but, ST geometry and the open vessel represent a different corner of operational space, which is usually interesting Fuel retention studies will focus on gas balance calculations in the near-term – supported by a detailed interpretive modelling effort – full Langmuir probe and D a measurements, soon… – maybe some campaign-averaged post-mortem analysis during the next engineering break Plans for post-mortem analysis of well characterised samples, but only preliminary work in 2009 – Divertor Science Facility, midplane materials probe