DIII–D HYBRID DISCHARGES

1. DIII–D HYBRID DISCHARGES Presented byJ.C. DeBoo In collaboration with J. Jayakumar and C. Petty Presented at the CDBM ITPA Meeting Princeton, NJ April 24-27, 2006

2. Hybrid Scenario on DIII-D • Hybrid regime on DIII-D is a long duration, high performance • regime with favorable fusion and neutron fluence characteristics • for ITER • typical parameters: bN ~ 2.8, H89P ~ 2.5, q95 ~ 3 - 5 • Hybrid scenario is distinct from the Advanced Tokamak scenario • inductively driven • bootstrap driven fractions of 35% to 50% • fully penetrated current profile with q(0) ~ 1

3. Two Types of Hybrid Discharges on DIII-D • Standard hybrid discharge with high performance and reduced • ohmic flux consumption leading to extended plasma duration • volt-sec reduction comes from lower Ip and substantial fBS • provides maximum neutron fluence per pulse for ITER • projection for ITER is Q ≈ 10 for flat-top length of ~ 4000 s (q95 ≈ 4) • Advanced inductive hybrid mode extends the hybrid regime to • larger fusion gain by maximizing Ip but with shorter duration • projection for ITER is Q ≈ 40 for flat-top length of ~ 1500 s (q95 ≈ 3) • these plasmas have greater disruption risk owing to larger Ip

4. Improved Performance of Hybrid Scenarios is Due to Broader Current Profile Formed by Moderate Heating During Ip Ramp • Central flat magnetic shear • region forms spontaneously, no • specific noninductive current • drive profile is required • This q profile is less susceptible to • onset of 2/1 NTM, allowing higher • bN operation • This q profile also has theoretically • lower turbulent growth rate • characteristics, allowing higher • confinement

5. Hybrid Discharges On DIII-D To Be Contributed To Global Database • DIII-D data from the advanced inductive dataset has been collected for contribution to the global H-mode database • 97 discharges, one time slice • per discharge [Wade et al., Nucl Fusion 45 (2005) 407]

6. bN Is Held Fixed By Feedback Control With PNBI • Because of the feedback control • of bN, essentially stored energy, • changes in confinement are • manifested in changes in PNBI • required to hold stored energy • constant • The feedback response is fast, • so PNBI must be integrated to • be meaningful • Careful consideration of time of • analysis and integration time of • PNBI is required • PNBI for DIII-D discharges are • back-averaged (causal) over • 50 ms, typically 1–2 fast ion • slowing down times

7. Regression Analysis To Fit Confinement Time Is Limited Since Most Parameters In The Dataset Have Little Variation • tth varies by about +/- 25% due predominately to PNBI variations required to hold b fixed • <PNBI> = 4.2 ± 0.9 MW • <bT> = 4.1 ± 0.3% , <bN> = 2.73 ± 0.15 • <Ip> = 1.19 ± 0.02 MA, <BT> = 1.30 ± 0.08 T • <q95> = 3.3 ± 0.2 • <d> = 0.53 ± 0.02, <k> = 1.82 ± 0.01, <a> = 0.606 ± 0.002 • Largest Parameter Variations Occur in PNBI and ne • <ne> = 4.7 ± 0.7 x 1019 m-3 • Zeff and PNBI are well correlated, inversely • <Zeff> = 2.2 ± 0.4

8. Confinement Time is Correlated With Density, Power and Zeff • A strong correlation between • PNBI and Zeff produces the • correlation between Zeff and t • Zeff decreases with increasing PNBI • Zeff is not strongly coupled to ne

9. Confinement In Hybrid Discharges Is Better Than Predicted By the ITER98Y2 Scaling Expression For H-mode Discharges

10. Hybrid Confinement Scales Somewhat Different From H-mode • The slopes of hybrid and • H-mode data are not well • matched • A simple multiplier times • ITER98Y2 does not agree • well with the hybrid data

11. Most Data Has Low Fast Ion Content, Wf/WMHD ≤ 13% • Two separate datasets have higher fast ion content, due to high power • and low density operation

12. MHD Activity Is Inversely Correlated With Confinement Time • n=2 MHD activity • is strongest • Highest WMHD values • correlated with lower • n=2 levels • Highest n=2 levels • correlated with lower • WMHD values

13. Thermal Confinement Time Degrades Strongly With Pth • Degradation with power is very clear • when plot all data, indicating range in • other parameters is rather small • Regression analysis gives • tth n0.57Pth-0.99

14. Power Degradation Is Too Strong • tth Pth-1  stored energy is not a function of power • Problem: would not have been able to regulate fixed bN with power • Issues to consider: • probably not because range of variation in PNBI is too small (minimum • to maximum ratio is about a factor 2) • bad or poor breakdown discharges included? If so, too few to matter. • problem related to modulation of PNBI? • due to correlation between PNBI and other “hidden variables”? • ?????

15. Summary • DIII-D advanced scenario hybrid discharges with q95 < 4 have been • identified for contribution to the H-mode confinement database • 97 discharges, one time slice per discharge • Confinement time displays P-1 dependence • too strong, impossible since bN was held constant by varying power • search for possible correlations between PNBI and “hidden variables” • Plan to work on a better understanding of the dataset a bit longer • before submitting the data to the global H-mode database.