Confinement and heating studies on
the National Spherical Torus
Experiment (NSTX)
E.J. Synakowski
Princeton Plasma Physics Laboratory
Princeton, New Jersey USA
for the NSTX Research TeamNOVA PHOTONICS, INC.

12th International Toki Conference and the 3rd General
Scientific Assembly of the Asia Plasma & Fusion Association
December 11 - 14, 2001

A program goal is to understand the physics
specific to high beta and low aspect ratio• Overview of operating scenarios, tools• Neutral beam heating & transport• Electron heating & transport• The edge
Change the aspectratio, increase beta:
what physics
changes?

NSTX operational capabilities increasing, and allow
confinement studies to begin
Baseline(Achieved)

Major Radius 0.85 m
Minor Radius 0.68 m
Elongation ≤ 2.2 (2.5)
Triangularity ≤ 0.6 (0.7)
Plasma Current
1 MA(1.4 MA)
Toroidal Field
0.6 T(≤0.45 T)
Heating and CD
5 MW NBI(5 MW)
6 MW HHFW (6 MW)
0.5 MA CHI(0.4 MA)
Pulse Length≤ 5 sec(0.5 sec)Local transport studies focus on understanding global trends
Core transportphysics
NBI:

Thermaltransport

Ion thermal energy
higher than expectedwith neoclassicalionsNBI & HHFW: Electronchannel dominates

HeatingHeating puzzle with NBI

Impurity ionparticle
transport

Close to neoclassicalwhen measured

τ Emeasured (ms)8060x1x1x2x240

L mode20maximum stored energy
pre-mhd0NSTX

02040

τ

60

80

89P(ms)E

Kaye; Sabbagh (Columbia)

Theory ofinstabilities

NBI: kθρi >> 1 dominantkθρi < 1 stable orsuppressedNSTX Systems, Diagnostics, Analysis Tools
Enable Study of Local Transport
NEUTRAL BEAMINJECTION• NBI:80 kV, deuterium• HHFW: 30 MHz,12 strap antennaCHERSBackground
Sightlines

CHERS
Sightlines
Viewing Beam

• MPTS: Te(R,t), Ne(R,t)
10 ch., 60 Hz• CHERS: Ti(R,t),Vφ(R,t)

Beam
Trajectory

16 ch., 20 ms
HHFW
AntennaThomson
Scattering• EFIT:
Equilibrium• TRANSP: Transport Analysis• GS2:
Gyrokinetic Analysis

Neutral beam heating yields high ion
temperatures in high current plasmas
106382

Ti

11E

01
RADIUS (m .6)

E

0.9

1
RADIUS (m .6)

Vφ

250 km/s• Ti broad)
(s 0M
TI

19(s)

IM• Large Vφ:co-directed
NBI

RADIUS (m)1.6000.9TIME(RADIUS (m)0.
2500.9E

1.600.
2542• Edge VφpedestalTNe-36 10 ms)• Ti >TeLeBlanc

0.
2520.
25

200.9

Te R. Bell, B.

303M
TI(s)

L mode1.2 MA, 0.33 T
4.8 MW NBI

Ion Thermal Confinement Better Than Electrons• Ti > Te• Classical PNB 2:1
electrons:ions• Peaked NB deposition

1I P (MA)05

PNB (MW)00.0
0.00.1
0.1

0.2
0.2

TIME (s)2.5 106382

Te (keV), T i (keV)

0.30.3

βT ~23%2.0
1.5

1/3 PNB

1.0Te
2/3 PNB

0.50.020Ti

406080100 120 140 160

RADIUS (cm)Power Balance Points To Puzzles
Power Source/Sink

ELECTRONS (MW)IONS (MW)NET (MW)OHMIC Heating

1.20

1.2

BEAM Heating

2.77

1.42

4.19

i-e Coupling

2.73

-2.73

0dW/dt

0.11

-0.54

-0.43

Other*

-0.66

0.26

-0.4

NET POWER IN

6.15 MW

-1.59 MW4.56 MW“Misplaced Heating”< -1.59 MW>1.59 MW

0.0 MW

NET POWER OUT< 4.56 MW> 0.0 MW4.56 MWTRANSP 106382A01 @ 0.25 s*Beam Thermalization,Rotation, Radiation, ConvectionMore Power Outof Ions Than In!

Need Extra Ion Heatingto Balance PowerSummary of power balance with NBI• Ti consistently larger than Te , despite expected large
fraction of electron heating by beams⇒ electron conduction is the dominant loss channel• Power balance makes sense if–

χi is exceptionally low– Ions get more heat from fast ions than expected classically• Diagnostic validation ongoing• Non-classical effects in heating and Qie being exploredAstrophysics and observed MHD may hold one clue
to the power balance puzzle
103932/103936

0.5

6

neL (1015 cm2)

• Being investigated:
4Compressional
Alfven2Eigenmodes

Fredrickson

Tesla

0.4

TF

00.0
2.5

Frequency (MHz)• Modes excitedby fast ions;waves transfer
energy tothermal ions

TF

ne

0.2

0.3

0.2
0.4

2.0
1.5

•

Theory of stochastic wave
heating of corona
developed (White)•Application of theory to ST
has begun•Vbeam > VAlfven key

1.00.5
0.0
0.1

0.2

0.3

Time (sec)However, initial study suggests:
low Ebeam ⇒ no CAE modes observed,
but ion stored energy is still too highGates, Gorelenkov, WhiteLow ion particle transport consistent with
low ion thermal transport•

After neon puff, almost
no neon penetrates thecore until MHD eventnear 260 ms•

Modelling suggests
core diffusivity < 1 m2/s,
near neoclassicaltheorySignals fromdifference ofsimilar plasmas
wtih and withoutneon puff
D. Stutman

USXR
measurements ofHe-, H-like neon lines

Theory: short wavelength modes may dominatetransport, long wavelength modes may be suppressed•

Long λ, low k (ITG, TEM):
growth rate < ExB shear rate
– Large λ associated with ion
thermal transport– Low aspect ratio: Analysis
suggests ∇β strongly stabilizing•

Low λ, high k (ETG):
growth rates large

Growth rates,ExBshearingrate(Short λ)=0.31(Long λ)– Responsible for electron thermaltransport?
– Non-linear simulations begun toestimate possible fluxes

C. Bourdelle (PPPL), W. Dorland (U. MD)

High Harmonic Fast Wave (HHFW) Heats Electrons
11IP (MA)0505PNB, PPRF
RF (MW)

00.00
0.00

0.10
0.10

0.20
0.20

TIME (s)

0.30
0.30

1.4 106194
Ne(0) = 4.0 ∞ 10 m
Ip = 760 kA-3

2.5

Te (keV), Ti (keV)

Te (keV), Ti(keV)

1.2
1.0TTee

0.8

TTi i

0.6
0.4

0.10
0.10

4060

80

100120RADIUS (cm)

140

160

0.20
0.20

0.30
0.30

0.40
0.40

TIME (s)105822
Ne(0) = 1.5 ∞ 1019 m-3
Ip = 380 kA

2.0TTee

1.5
1.0
0.5

0.2
0.020

PNB, P
PRF
RF (MW)

00.00
0.00

0.400.40

3.019IP (MA)0.020TTi
406080100120

RADIUS (cm)• Te > Ti with auxiliary power to electrons

140160Electron Loss Channel Also Dominant withHHFW Heating106194 0.19 sγγ••Power deposition: HPRT raytracing code
χi ~ 2-2.5 χi NC , χe >> χi

•
•

ρi

ρi

ETG unstableLow kθ modes ITG +TEM

Power balance analysis reveals that reduced
electron transport is correlated with high Te•Core χe drops as high Tedevelops

10 0115 msm2/s– Steep gradients due totransport changes, notsource•105830a01

Heating source fromHPRT ray tracing(Rosenberg) 3

10190 ms

Electronthermalconductivity

190 ms2Te1
0. 1

0. 2

0. 3

0. 4

0. 5

0. 6

0. 7

0. 8

normalized minor radius

1LeBlanc
115 ms

0
0. 0

0. 2

0. 4

0. 6

0. 8

1. 0

0. 9

1. 0

Bifurcations to enhanced plasma confinement
state observed with both NBI and HHFW
Visible light, false color
Before transition

After transition

LeBlanc••

NBI: Power required ~ 10x thatpredicted from empirical scaling
laws:– Strong magnetic shear?
– Poloidal damping? Wall neutrals?
Fast camera: Maqueda (LANL)

Change in edge •transport evident
in density profile

Fluctuations
reduced atH modetransition

Edge reflectometry:
Peebles, Kubota (UCLA)

H mode: Maingi, Bush (ORNL); LeBlanc

Imaging of edge reveals qualitative
differences in H- and L-mode turbulence
Xu, LLNL

l.
po•

•

Helium puffed; emission
viewed along a field line
• He0 emission observed
witha fast-framing,
BOUT
code:
turbulence digital,
visible camera
modeling– 1000 frames/sec, 10 µs– 2-fluid,3D Braginskii
exposure each frame
equation coded.
raMaqueda, LANL; Zweben Los Alamos
NATIONAL LABORATORYImaging of edge reveals qualitative
differences in H- and L-mode turbulence
During H mode••

Helium puffed; emission
viewed along a field line
He0 emission observed witha fast-framing, digital, visible
camera– 1000 frames/sec, 10 µs
exposure each frame

After H-L
transition

Maqueda, LANL; Zweben Los Alamos
NATIONAL LABORATORYHHFW-driven H modes found•

LSN•

Lower current (350 - 500 kA)•He and D•ELMy, ELM-free•βp = 1 observed– Large bootstrap?
– Large dip in surface voltage

IpHHFWpowerLoopvoltage
DαβpStarting scenario for future CD work?

Studies of underlying physics of ST transport
has begun• Kinetic profiles are enabling initial local transport analysis• NBI: Ti > Te, despite prediction that 2/3 PNBI goes to electrons– Electrons are the dominant loss channel– Ion heating not understood– Low particle transport correlated with low ion thermal confinement– ExB shear suppression of low k modes seen in analysis at high beta
– Exploring role of ETG• HHFW: Te > Ti
– Electrons are the dominant loss channel– Reductions in χe with strong central Te peaking– Possible role of Te/Ti in determining χe to be investigatedSummary (2)• L-H transitions observed with NBI and HHFW
– Turbulent structures observed in L mode state; modellingunderway– Pth ~ 850 kW for NBI; ≈ similar-sized tokamaks, >> scaling• Role of strong poloidal damping at low aspect ratio?

• Near-term transport goals and plans– Understand ion heating– Turbulence measurements to be extended into core
– Scans of beta: is beta or ∇β favorable for transport?

• Long-term goals– Establish a physics-based understanding of the underlying
causes of ST transport trends• Comparison with moderate-aspect-ratio trends will reveal newphysics relevant to all