NSTX CSU Preliminary Assessment of PFCs Art Brooks December 8,

Overview Project has chosen to use isotropic graphite (ie ATJ or equivalent) based on cost savings over CFCs Structural response of PFC tiles and supports analyzed subject to Surface Heat flux and EM loading (eddy and halo currents interacting with background fields) Goal is to determine operating limits for existing design and investigate alternative geometries that can increase performance 2

GRD Requirements – Heat Flux Heat Flux applied to Plasma Facing Surface of Tiles For IBDhs this includes vertical surface 3

Requirements – EM Loads Eddy Currents SPARK Scan of above disruptions yielded Max dB/dt = 520 T/s Radial, 460 T/s Vertical at diverter 4

dB/dt scan from Plasma at Horizontal Inboard Diverter During Disruptions Based on 2 MA for NSTX CSU 5 Max Radial dB/dt 520 T/s Max Vertical dB/dt 460 T/s

Requirements - Halo Excepted from Disruption_scenario_currents_v2.xlsx For IBDhs, Halo = 35 kA per 15 deg Tile ( 2MA/24Tiles*.35HCF*1.2TPF) Halo current assumed to take longest path across TF for worse case loading unless justification can be made not to. 6

Requirements – Peak Background Fields PF Configuration from NSTX_CS_Upgrade_ xls Scan of 96 scenarios in same spreadsheet used to establish max fields: Max Br = 0.5 T Max Bz = T Avg Btf ~ 2 T at IBDhs Max Btf ~ 3 T at CS Btf = 1T at m 7

ATJ Graphite Properties ATJ very brittle – Yield strength close to Ultimate Representative Tensile Stress-Strain Curve from GRAPHITE DESIGN HANDBOOK GA 1988 (for 2020 graphite) 8

Structural Design Criteria Per NSTX Structural Design Criteria –Design Tresca Stress Values (Sm) = 2/3 Sy or ½ Su General primary membrane 1.0 KSm Local primary membrane 1.5 KSm Primary membrane plus bending stresses 1.5 KSm Total primary plus secondary stress 3.0 KSm For ATJ – Sm (tensile) = 13 MPa – Sm (compression) = 33 MPa Criteria if applied to ATJ (questionable for brittle material with poor ductility) would allow Tensile stresses of 39 MPa for thermal plus EM loading Performance herein assumes peak tensile stress < Su = 26 MPa 9

Heat up of IBDhs Tile subjected to 5 MW/m2 for 5 s 10

11 1 st Pulse Heat Flux/Pulse Length Capability Single pulse without ratcheting with ATJ Graphite ~DN avg 1D analysis in good agreement with 3D away from corner

Eddy Current Distribution 12 Eddy current loop ~ 3 kA Net Moments on Tile are small: Mr = 50 N-m Mth = 14 N-m Mr = 3 N-m

Halo Current Distribution kA flowing from inner to outer radius, crossing TF Field produces forces of Fr =-223 N Fth = 3160 N Fz = N

Compressive Stress on Tile Surface MPa Compression Where Temp = 1435C

15 Peak ATJ Stress 85.4 MPa >> Su (26 MPA) Supports restricting free expansion Tensile Stress at Support too high

High Z Stress Reduced Modestly by deleting contact in region Sz=43 MPa Sz=56 MPa Note: Results for previous analysis with vertical halo

2D Study of T-Bar Size 17 Includes Thermal, Eddy & Halo Loading

Alternate Geometry – Single, Larger T-Bar 18 Peak Tensile Stress 27.7 MPa in slot

2D Estimate of Split Tile (ie 48 vs 24 Tiles Toroidally) 19 Peak Tensile Stress in slot Drops from 27.7 MPa to 9.5 MPa

General Observations A tile free to expand thermally will perform better than one rigidly clamped –Grafoil is very compliant if not fully compressed –Eliminating cross T-bar also allows freer expansion A free tile must still resist EM loading (moments about T- bar and launching forces) –Eddy current forces are modest due to low electrical conductivity of graphite but Halo forces are significant Slot size has large impact on tensile stresses –Slot radius can increase with size reducing peak stress Tile width impacts not just thermal stress thru thickness but mitigates effect of EM forces Cross T-bar does not appear to be effective or desired 20

Ongoing work Following Slides are a first pass thru thermal and structural response for the other tile types on the CS –Center Stack Angled Section (CSAS) –Inboard Diverter Vertical Section (IBDvs) EM Loading from Eddy and Halo Currents not yet included T-Bars assumed bonded to Tile –As shown leads to high thermal stresses in tile –Need to redo assuming unbonded – found to help for IBDhs Mounting Hardware still needs to be assessed for all tiles CSAS and IBDvs tiles are thinner and may not permit (or need) larger slots –CSAS will benefit from splitting in half vertically - TBD 21

Center Stack Angled Section (CSAS) Temperature Response – Single Pulse Heating much lower on CSAS and IBDvs

Center Stack Angled Section (CSAS) Structural Response Max Peak Stress Intensity at slots and sharp corners 246 MPa Tensile Stress 101 MPa For assumed bonded (needs to be rerun) Assumes bonded along red section above

Inboard Diverter Vertical Section (IBDvs) Temperature Response – Single Pulse Heating much lower on CSAS and IBDvs (than IBDhs)

Inboard Diverter Vertical Section (IBDvs) Structural Response IBDvs Max Peak Stress Intensity at Slots 46MPa Max Tensile Stress At slots 14 MPa (not shown) Assumes bonded along red section above

Inboard Diverter Vertical Section (IBDvs) Structural Response – T-Bar IBDvs T Bar Stresses fairly low for inconel or SS – 56.8 MPa (8.1 ksi) (without EM loads)