NSTX Center Stack Upgrade Workshop Requirements & Design Point

1. NSTX Center Stack UpgradeWorkshopRequirements & Design Point C Neumeyer Jan 22, 2009

2. Background • Design point spreadsheet studies initiated April ‘08 • Initial approach was aggressive (e.g. 2kV TF, 10kV OH, 2 MG) at A ~ 1.5-1.6 to understand possible envelope • Found that maximum usage of center stack area from simple spreadsheet stress/thermal considerations would allow Ip=3.1MA with 5 sec flat top at B=1.4T • Thought about targeting even higher performance for shorter pulse! • Detailed ANSYS modeling initiated by R Woolley mid-July • Decided to limit to 1kV TF, 8kV OH, 1 MG and round down to Ip=2MA with 5 sec flat top at Bt=1T and investigate design concepts in detail, end of August • Involvement of Mech Eng Div early Oct • PPPL managerial reviews related to mission statement mid-Oct • Small design point iterations followed, no changes since Nov, posted to web at http://www.pppl.gov/~neumeyer/NSTX_CSU/Design_Point.html • Draft GRD circulated mid-Nov

3. Basic Parameters of Upgrade (1) Aspect ratio* increase to 1.5 from the original value of 1.26 increases the cross sectional area of the center stack by a factor of 3 and makes possible higher levels of performance and pulse duration: * A= R0/a, ratio of plasma major radius R0 to minor radius a

4. Basic Parameters of Upgrade (2)

5. Basic Parameters of Upgrade (3) • New PF inner coils • PF1a,PF1b, Pf1c • Symmetric about midplane • Retain PF outer coils • Increased currents

6. Basic Parameters of Upgrade (4) • Magnitudes • TF field and current • Bt increases to 1T from 0.6T • R0 increases to 0.934m from 0.854m • Itf increases to 130kA from 71kA • Ip increases to 2MA from 1MA • OH central field increases to 7.3T from 6.9T • PF • - New PF1A/B/C, to replace old PF1a and PF1b, current • - PF2,3,4 currents increase to 24kA from 20kA • - PF5 current increases to 36kA from 20kAT • Approximate force ratios • TF inner leg torque ratio ~ (130/71*7.3/6.9)=1.9 • TF outer leg • in-plane force ratio (130/71)2=3.3 • out-of-plane force ratio ~ (130/71*36/20)=3.3 • - PF axial force ratio ~ (24/20*36/20)=2.2 • - Passive plate & OB divertor force ratio ~ (2/1*36/20)=3.6 • Durations • T_pulse = 5 second plasma flat top • T_rep = 1800 second (30 minute) repetition period • Any combination of T_pulse/T_rep ≤ 5/1800 as long as T_pulse ≤ 5 • TF flat top for full duration when Ip≠0

7. Basic Desgin Point Assumptions & Limits • Completely replace center stack • New TF same dZ as average turn of original • New OH same dZ as old • Retain existing TF outer legs • TF at flat top for full duration of Ip • Provide OH flux sufficient for Ip ramp in 1st swing • conservative Ip_dot (2MA/sec) • use OH 2nd swing if thermal/stress allows • Retain existing PF outer coils • Coil temperature range 12-100C, assume adiabatic, allow for L/R decay • Simple formulae for TF von Mise stress* and OH hoop stress** (peak) • VM allowable 133 MPA • peak allowable 200MPA • 1kV TF, 8kV/24kA OH, 1 MG • Two TFTR NBI systems imposing MG loads * neglects tension due to force from outer leg ** neglects interaction with PF coils and plasma

8. Spreadsheet Methology (1) • Includes allowance for details of center stack radial build • Includes allowances for conductor cooling hole, electrical insulation • Adiabatic conductor heating models • Simple formulae for TF inner leg von Mise stress and OH hoop stress • Models not included for TF inner leg torsion, TF outer leg, VV, etc. • Simplified linear power supply models • TF and OH L/R circuit models V = L*I_dot+I*R • MG power and energy models • Plasma loop voltage and flux requirement modeled • OH waveform and flux model accounts for plasma initiation and Ip ramping • XL solver… • finds radius of TF necessary to meet Bt and pulse length requirement • designs OH coil to meet flux requirement of 1st swing, maximizes 2nd swing until thermal

9. Spreadsheet Methology (2) • “Base” worksheet contains main calculations • Other worksheets provide summary data from “Base” • - results given in both SI and English units • side-by-side comparison of base NSTX to Upgrade • Spreadsheet is protected

10. Spreadsheet Results (1) • TF bundle requires ~ 70% of center stack radius available to TF and OH • - temperature limited (100oC in case of L/R decay) • - VM stress ~ 30MPA • OH optimizes based on thermal considerations • peak hoop stress at 1st swing 146MPA • temperature limit reached at end of 2nd swing • 1.4 Wb 1st swing flux, bonus 2nd swing flux of 0.5 Wb • Peak MG loading • peak power 289MW/133MVAR/318MVA (47MVA rating) • peak energy 1274MJ (2250MJ available) • start pulse at f>= 77Hz (87.5Hz rating)

11. Spreadsheet Results (2) Radial Build • Increase in thickness of: • electrical insulation • clearance gaps • CS casing • PFC tiles

12. Spreadsheet Results (3) TF Coil • Increase in thickness of: • electrical ground insulation • Increase in final temperature of outer legs • Same cooling hole as base (needs assessment) • Allowance for central hole needs to be increased

13. Spreadsheet Results (3) OH Coil • Increase in thickness of electrical insulation • Similar conductor cross section • Copper mass per winding approx. 2.3x previous, may not be feasible w/o jointing • Same cooling hole as base (needs assessment, cools down in 24 minutes, not 20) • Decreased packing factor • Need to do a trade-off study of 48kA/turn to reduce cooling time, increase packing

14. Next Steps in Req’ts & Design Point Development • Calculate disruption loads • Determine PFC heat loads • Determine CHI requirements • Issue GRD • Perform OH optimization study • Make other adjustments and refinements as necessary