S. Lisgo 1, G. Counsell 2, A. Darke 1, G. De. Temmerman 1, J. Huang 3, G. Maddison 1 1 UKAEA 2 F4E 3...
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Transcript of S. Lisgo 1, G. Counsell 2, A. Darke 1, G. De. Temmerman 1, J. Huang 3, G. Maddison 1 1 UKAEA 2 F4E 3...
S. Lisgo1, G. Counsell2, A. Darke1, G. De. Temmerman1, J. Huang3, G. Maddison1
1UKAEA2F4E3ASIPP
EU-PWI SEWG: FUEL RETENTION
MAST Activities for 2008-2009MAST Activities for 2008-2009
EU-PWI SEWG Fuel Retention Wednesday, July 23, 2008 Culham
EU-PWI SEWG RETENTION, JULY 2008Overview
(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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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)
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST vessel design: vessel parameters
2.1 m
4.5
m
An economy of coils…– up-down symmetry (almost)
– flexible: LSN, DN, USN, both Ip and Bt can be reversed at the same time
Plasma volume 8 m3, tank volume 60 m3
– 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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)
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST vessel design: wall composition
SSGRAPHITE
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 0th 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTMAST 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)
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTWall 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 1-210-8 mbar, dominated by H20
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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTNeutral 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
EU-PWI SEWG RETENTION, JULY 2008Overview
(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
EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTIONMAST 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?)
EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTIONKeeping 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
EU-PWI SEWG RETENTION, JULY 2008: MAST CONTRIBUTIONKeeping 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
EU-PWI SEWG RETENTION, JULY 2008: INTRODUCTION TO MASTOverview
(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
EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGEGas 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)
EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGEGas 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)
EU-PWI SEWG RETENTION, JULY 2008: SAMPLE DISCHARGEExperimental 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 D2+ and He+ peaks
– try D imaging during the glow
Evaluate dependencies (piggyback): boronisation, GDC, disruptions, fuelling location, confinement regime, equilibrium geometry, Ip, <ne>, input power, wall temperature (within reason), ELM coils, …
..?
EU-PWI SEWG RETENTION, JULY 2008Overview
(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
EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODELOSM-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
EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODELAutomated 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
EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODELEIRENE 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
Tene nD nD29.51018 m-3 max 30 eV max 0.61018 m-3 max 0.61019 m-3 max
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 Dmeasurements
HU12DIVCAM1 (D)
HL07DIVCAM2 (D)
HM07FAST CAMERAWITH 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
EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODELAttempt 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
EU-PWI SEWG RETENTION, JULY 2008: INTERPRETIVE MODELAttempt 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
EU-PWI SEWG RETENTION, JULY 2008Summary
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 Da 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