Progress on NSTX towards steady state at low aspect ratio

D. A. Gates, Princeton Plasma Physics Laboratoryon behalf of the NSTX Research Team

NOVA PHOTONICS, INC.PHOTONICS, INC.Supported by

* Work supported by US DOE Contract No. DE-AC02-76CH03073

ST Requires high bootstrap fraction, fbs, simultaneous with high t

• High fbs and high t competing requirements (at fixed shape and N)

• Progress for ST and advanced tokamak given by sus fbst ~ SN2 [S

= q95(Ip/(aBt))]

– If Nmax = CTroyon, then only shape improves sus

– sus increases linearly with increasing S [S = q95(Ip/(aBt))]

– Component Test Facility requires t ~ 20% and fbs ~ 50%, sus = 10%

– ARIES-ST requires t ~ 40% and fbs ~ 90%

• Optimized shaping with new PF coils for high triangularity and elongation

• NSTX has achieved record values of elongation and shape factor

– Leads directly to record values of the sus for the ST

• For NSTX 100% non-inductive operation with N ~ 7 only with strong

shaping

New inboard divertor coils increase accessibility of high-triangularity, high-elongation shapes

• Highest now obtained at highest ≈0.8 S q95 IP/aBT = 41 MA/m·T

• Small (Type V) ELM regime recovered at high > 2.5 with new coils– Previously observed onset of large ELM-like events when > 2.2

D. Gates , J. Menard

Highest elongation =3.0 (transient) Sustained elongation =2.7 (0.1s)

Record pulse-lengths achieved at high current by operating with sustained H-mode

• H-mode with small ELMs lower flux consumption, slow density rise

0

1

0.998s

0

1

0.998s

0

1

0.996s

0.5 1.0 1.50

100

Radius (m)

0.996s

ne (1020m-3)

Te (keV)

Ti (keV)

v i (km/s)

0

Ip (MA)

0

1

0

1

0.0 0.5 1.00

1

Time (s)

V loop (V)

p

ne/ne< >G

121152

PNBI ( )/10MW

T = 20% N = 5.2%m•T/MA

E = 52ms H89L = 2.0

A = 1.4, = 2.3, L = 0.75, li = 0.49

Long duration discharges reach ~70% non-inductive current

• TRANSP model agrees with measured neutron rate during high- phase

– Model includes anomalous fast ion diffusion during later phase when low-m MHD activity is present

• 85% of non-inductive current is p-driven

– Bootstrap + Diamagnetic + Pfirsch-Schlüter

• 1/1 mode onset causes drop, fast ion diffusion (Menard, PRL)

0.0 0.5 1.0 1.50.0

0.5

1.0

Time (s)

2

1

0

116318A13

Bootstrap(NCLASS)

∇p-NB driven

Current fraction

Measured

Modelled DD neutron

rate (1014 / )s

1

0 2

1

0

Ip ( )MA

p

PNBI ( )/10MW

Ohmic

Expected performance improvements observed as shaping has increased

Reduction in control system latency increases elongation (2004)

Plasma shaping enhancements

Upgrade of PF1A coil enables simultaneous achievement of high and

Overlay of EFIT boundaries from shots 117707 and 117814 between 0.3 and 0.9s

~3cm includes MHD perturbations

Control points

Boundary overlay time window

Implementation of rtEFIT improves shape control reproducibility

Modified PF1A Coil

Old PF1A Coil

Shot 121241 (record )All shots 2001-2003All shots 2004-2006

2001 200520042002-3

= 2.75, = 0.8, S ~ 37 = 1.8, = 0.6, S ~ 22 = 2.0, = 0.8, S ~ 23 = 2.3, = 0.6, S ~ 27 = 3, = 0.8, S ~ 41

2006

Pulse averaged approximate sus versus S (S is averaged over the same time window as sus)

NH89 vs. pulse/E

Pulse averaged t versus pulse length Data are sorted by year and by S

2001-20042005-2006

External kink mode ultimate limit on t -long pulse discharges above no-wall limit

Troyon diagram showing tmax vs. Ip/aBt

PEST eigenfunction Shot 117707

Time history of t and n=1 (no-wall) kink mode growth rate, , for Shot 117707, calculated by PEST using

LRDFIT equilibrium including MSE

Alfven

%

Each point in the above plots represents an EFIT equilibrium reconstructionData span entire NSTX database and are filtered against rapid plasma motion

Effect of modifications provide clear increase in NSTX operating space

117407 LSN117432 DN117424 high- DN

0

2

4

6

8

10

12

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1Radius [m]

Tile Gap

6 MW DN ( L~0.40)

6 ( MW DNL~0.75)

( )outer strike region

#117407: 0.373LSN@ s#117432: 0.316DN@ s#117424: 0.316DN@ s

6 ( MW LSNL~0.40) 117407 LSN

117432 DN

117424 high- DN

Increased triangularity actually reduces peak heat flux to divertor target

• Flux expansion decreases peak heat flux despite reduced major radius• Compare single-null & double-null configurations with triangularity

≈ 0.4 at X-point and high triangularity = 0.8 double-null plasmas– Measure heat flux with IR thermography of carbon divertor tiles

• Peak heat flux decreases as 1 : 0.5 : 0.2• ELM character changes: Type I Mixed Type V

R. Maingi  

Changes in X-point balance affect ELM characteristics

• Very small changes in the plasma boundary reproducibly lead to large differences in ELM behavior

• ELMs have a major impact on plasma performance, controlling them is crucial• Precise plasma control provides an important tool for controlling ELMs - highly ITER relevant

rsep (mm) for 117424 (black)and 117425 (red)

Shot 117424 Shot 117425

Shot 117424

Shot 117425

rsep is the radial separation of the flux surfaces which pass through the x-points

measured at the outboard midplane

1.0 0.0

Encouraging results from both EBW emission and HHFW current drive experiments

Long pulse discharges have elevated q(0) without low frequency MHD modes

Experiment(116313)

Target

The time history of shot 120001 showing:

1) the components of non-inductive current as indicated in the

plot legend,

2) The plasma current (black) and the surface voltage (red),

3) the reconstructed q(0) (black) and qmin (red) and q(0) as

determined by a TRANSP magnetic diffusion calculation

(blue)

4) t (black) and the neutral beam power (red). The profiles in

preceding figure are calculated over the time interval indicated

in green.

• A spectrogram of magnetic fluctuations as

measured by a Mirnov coil for shot 120001.

• The colors represent toroidal mode numbers

as indicated in the legend.

• Notice the period of time after 0.6s where

there are only small amplitude high-n MHD

modes present.

Measured

• EBW uses efficient Ohkhawa current drive

• Data on efficiency of EBW emission from identical

plasmas for which the EBW antenna pointing angle

is varied.

• The colors represent measured efficiencies of - 81-90% (red)

- 71-80% (orange),

- 61-70% (green)

- 51-60% (blue)

- 41-50% (purple),

- 31-40% (black).

• The ellipses are contours of theoretically predicted

emission efficiency

Measured EBW emission angle matches theoretical predictions

• Achieved high Te= 3.6keV in current drive

phasing for first time using high BT = 5.5kG

–Improvement consistent with reduced

PDI and surface waves expected at

higher BT

–Expect similar improvements from

higher k||

• Useful for HHFW-CD during ramp-up

• Useful for HHFW heating at high-

HHFW heats efficiently in current drive phasing

Steady state scenario predicted with 100% non-inductive current using only NBICD and pressure driven currents

Plasma shape and profiles for predicted 100% non-inductive scenario

IP = 750kA

N < 5.6, P < 1.5, T < 17%

li = 0.6, qmin=1.3, BT=4.5kG

= 2.3, X-L = 0.75, q*=3.9

Present high-fNI long-pulse H-modes: Target scenario:

Inductive current drive is replaced by: Higher JNBI from higher Te

Higher JBS from higher P-thermal

• Need 60% increase in T, 25% decrease in ne

–Lithium for higher E & density control?• 20% increase in thermal

confinement• 30% increase in HH98

–Core HHFW heating

• Want q0 qmin 2.4 higher with-wall limit

• Higher for higher q, P, fBS

• High for improved kink stability

• (shape parameters already achieved in other discharges)

IP = 700kA N = 6.7, P = 2.7, T =15%li = 0.5, qmin = 2.4, BT=5.2kG

= 2.6, X-L = 0.85, q*=5.6

Neoclassical

Analysis of current profile information

Full complement of kinetic profile data enables analysis of current profile composition

Full neoclassical calculation of ohmic, and pressure driven currents Shot 120001

•Black total predicted current

•Gray total reconstructed current (MSE)

•Orange - Ohmic current

•Red - Bootsrap

•Blue - Beam driven current (TRANSP)

•Data averaged over 0.7 - 0.8s

• Loop voltage profile calculated from

equilibria constrained with MSE data

• Neoclassical resistivity and bootstrap

current from Sauter, et al., Phys. Plasmas

6 (1999) 2834

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