DIII–D HYBRID DISCHARGES

Presented byJ.C. DeBoo

In collaboration withJ. Jayakumar and C. Petty

Presented at the CDBM ITPA MeetingPrinceton, NJ

April 24-27, 2006

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: N ~ 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

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

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 N operation

5

4

3

q

Conventional

AdvancedInductive

2

1

00.0 0.2 0.4 ρ 0.6 0.8 1.0

Hybrid• This q profile also has theoretically lower turbulent growth rate characteristics, allowing higher confinement

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]

N Is Held Fixed By Feedback Control With PNBI

• Because of the feedback control of N, essentially stored energy, changes in confinement are manifested in changes in PNBI required to hold stored energy constant• 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

• The feedback response is fast, so PNBI must be integrated to be meaningful

Ip

N

NH89P/q2

PNBI

Regression Analysis To Fit Confinement Time Is Limited Since Most Parameters In The Dataset Have Little Variation

• th varies by about +/- 25% due predominately to PNBI variations required to hold fixed

– <PNBI> = 4.2 ± 0.9 MW– <T> = 4.1 ± 0.3% , <N> = 2.73 ± 0.15– <Ip> = 1.19 ± 0.02 MA, <BT> = 1.30 ± 0.08 T – <q95> = 3.3 ± 0.2– <> = 0.53 ± 0.02, <> = 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

Confinement Time is Correlated With Density, Power and Zeff

• A strong correlation between PNBI and Zeff produces the

correlation between Zeff and

• Zeff decreases with increasing PNBI

• Zeff is not strongly coupled to ne

ne

PNBI

th

Zeff

Ip

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

Hybrid Confinement Scales Somewhat Different From H-mode

• A simple multiplier times ITER98Y2 does not agree well with the hybrid data

• The slopes of hybrid and H-mode data are not well matched

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

MHD Activity Is Inversely Correlated With Confinement Time

• n=2 MHD activity is strongest

WMHD

PNBI

N2ave

PNBI

N1ave

PNBI

N3ave

PNBI

• Highest WMHD values correlated with lower n=2 levels

•Highest n=2 levels correlated with lower WMHD values

Thermal Confinement Time Degrades Strongly With Pth

• Regression analysis gives

th n0.57Pth-0.99

PLTH

TAU_fit

RMSE = 7.6%

• Degradation with power is very clear when plot all data, indicating range in other parameters is rather small

Power Degradation Is Too Strong

• th Pth-1 stored energy is not a function of power

• Problem: would not have been able to regulate fixed N 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”?–?????

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 N 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.