M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team

NOVA PHOTONICS, INC.PHOTONICS, INC.

Supported by

Columbia UComp-X

General AtomicsINEL

Johns Hopkins ULANLLLNL

LodestarMIT

Nova PhotonicsNYU

ORNLPPPL

PSISNL

UC DavisUC Irvine

UCLAUCSD

U MarylandU New Mexico

U RochesterU Washington

U WisconsinCulham Sci Ctr

Hiroshima UHIST

Kyushu Tokai UNiigata U

Tsukuba UU TokyoIoffe Inst

TRINITIKBSI

KAISTENEA, Frascati

CEA, CadaracheIPP, Jülich

IPP, GarchingU Quebec

Toroidal Magnetic PlasmaConfinement at the Limit:Recent Results from the

National Spherical Torus Experiment

M.G. BellPrinceton Plasma Physics Laboratory

on behalf of theNSTX Research Team

MIT seminar / 040514 / MGB 2

“Spherical Torus” Extends Tokamak to Extreme Toroidicity

• Motivated by potential for increased (Peng & Strickler, 1980s)

max (= 20p/BT2) = C·Ip/aBT C·Aq

BT: toroidal magnetic field on axis;p: average plasma pressure;Ip: plasma current; a: minor radius;

: elongation of cross-section;A: aspect ratio (= R/a); q: MHD “safety factor” (> 2)C: Constant ~3%·m·T/MA

(Troyon, Sykes - early 1980s)

• Confirmed by experiments– max ≈ 40% (START - UK, 1990s)

SphericalTorusA ≈ 1.3

ConventionalTokamak

A ≈ 3

Fieldlines


In Addition to High , New Physics Regimes

Are Expected at Low Aspect Ratio

• Intrinsic cross-section shaping (BP/BT ~1)

• Large gyro-radius (a/i ~ 30–50)

• Large fraction of trapped particles √r/R))

• Large bootstrap current (>50% of total)

• Large plasma flow & flow shear (M ~ 0.5)

• Supra-Alfvénic fast ions (vNBI/vAlfvén ~ 4)

• High dielectric constant ( ~ 30–100)


NSTX Designed to Study High-Temperature Toroidal Plasmas at Low Aspect-Ratio

Aspect ratio A 1.27

Elongation 2.5

Triangularity 0.8

Major radius R0 0.85m

Plasma Current Ip 1.5MA

Toroidal Field BT0 0.6T

Pulse Length 1s

Auxiliary heating:

NBI (100kV) 7 MW

RF (30MHz) 6 MW

Central temperature 1 – 3 keVExperiments started in Sep. 99


View of NSTX, Heating Systems and Diagnostics


Now Operating With a New Center Bundle for TF Coil

• Original irrepairably damaged by joint failure in February ‘03– Operated for > 4500 plasma pulses, including 140 with BT ≥ 0.5T

• New bundle constructed after redesign, modeling and review

• Joints are now continuously monitored and are operating stably at 0.45T– Maintaining contact resistances below industrial norm


The 2003 Five-Year Plan for NSTX Aims to Assess the ST for Fusion Energy Development

Extend knowledge base

of plasma science

Establish physics basis & tools for

high performance for ∆t >> tskin

Develop control strategies for

toroidal systems

Use high & low A to deepen understanding through measurement, theory & comparison with conventional A

Physics exploration & passive limits- Identify needed control tools

Optimization & integration

• high T and JBS near with-wall limit for ∆ >> skin

‘03

‘04

‘07

‘08

‘09

‘05

‘06 Advanced control & High-physics

• high T & E, ∆t > E • high N & E, ∆t >> E

• establish solenoid-free

physics & tools


High-Performance Steady-State Operation

Poses Many Scientific Challenges

• MHD Stability at High T and N

– High pressure at low toroidal field with high bootstrap current fraction

– T ~ 40%, N ~ 8 for a ST power plant

• Good confinement at small size, a/i

– HITER-98pb(y,2) ~ 1.4 – 1.7 with good electron confinement for ignition

• Power and Particle Handling

– Small major radius increases Ploss/R by factor 2 – 3

• Solenoid-Free Start-Up and Sustainment

– Eliminate solenoid for compact reactor design

• Integrating Scenarios

– Achieve all this together


NSTX Rapidly Achieved High- With NBI Heating

• Data from 2001 – 3

Bootstrap fraction

up to 50%N = 5-6.2 at IP/aBT0 3

Pulse-length up to 1s

T = 30 - 35 %N = 5-6 at IP/aBT0 = 5-6Pulse-length = 0.3-0.4s

T 20p / BT02

Menard, Sabbagh (Columbia)


Measured Dependence of Beta-Limit in 2001–3 Motivated Shaping Enhancements

• Capability for higher Ip at high contributes to strong dependence

• Planning to modify inboard PF coils to increase at higher

• Reducing error fields and routine H-modes (broader profiles) improved performance in 2002

2003 data2002 data2001 data

Menard, Gates


Reduced Latency in Vertical Position Control & Earlier H-modes Opened Operating Window

• Propagation latency through digital control system reduced to ~700µs• Pause in current ramp & lower gas puffing promoted early H-mode

– Lower internal inductance allows higher elongation– Reduced flux consumption extends pulse

Gates, Menard

20012002-32004

High T 1.2MA High P 0.8MA

li


Long H-modes Provided a Route toHigh in Recent Experiments

• Use He pre-conditioning to control recycling

• Initiate plasma at high BT for most quiescent ramp-up

• Early NBI and pause in Ip ramp trigger H-mode

• High Te & low li allow high , Ip

• Stage PNBI increase to avoidN limit & excessive density

• Ramp down TF to increase T

• ELMs develop as TF falls

– reduce edge density

• T maximum as It = Ip (q95 = 4)Menard, Wade (GA)

0.00.10.20.30.40.50.6Time (s)PNBI [MW]WEFIT, WTRANSP [MJ] ,T EFIT, ,T TRANSP[%]ne(0),<ne>[1020m-3]It,Ip[ ]MAHα[ ]arbTi(0),Te(0)[ ]keV0.00.52100500.30204001


Pre-conditioning, Minimal Gas & ELMs Reduce Edge Density and Allow Good NBI Penetration

• High-resolution CHERS confirms large gradients in Ti, vi

• Control of ELMs will be critical to optimizing

• A = 1.5

• = 2.3

• av = 0.6

• q95 = 4.0

• li = 0.6

• N = 5.9%·m·T/MA

• T = 40% (EFIT)34%

(TRANSP)

112600, 0.55s112600, 0.55s

R.Bell, LeBlanc, Sabbagh, Kaye


High v/vA Affects MHD Equilibrium; Relevance to Stability Being Explored

• Density shows in-out asymmetry

– Effect of high Mach number of driven flow

Menard, Park, LeBlanc, Stutman

• Experiment: kinks saturate

MA = 0.3

0 1 2

Time

MA = 0

• Theory (M3D): for fixed momentum input, growth rate reduced by factor 2 - 3

Measured (MPTS) (assuming density

is a flux function)

(with centrifugal effects)


Installed Internal Sensors for Detecting Resistive Wall Modes

BR

BZ

• 24 each large-area internal BR,

BZ coils installed before ‘03 run

– Mounted on passive stabilizers

– Symmetric about midplane

• External n=1 detector signal lags the internal sensor by ~ wall

• Internal sensor signal greater than external by factor of 5

• Internal sensors reveal clear up/down mode asymmetry

Menard, Sabbagh

0.40 0.45 0.50 0.55 0.60

Bp

(G)

02468

Time (s)


RWM Sensors Detect Mode in High T Plasma

Sabbagh, Bell, Menard

for toroidal mode number:

0.2 0.4 0.6Time (s)

0

20

40

60

80

100

Shot 112600wB(w) spectrum1 2 3 4 5

Time (s)

Fre

quen

cy o

f Mirn

ov a

citiv

ity (

kHz)

n=1 frequencyincreases with edge rotation

during collapse

RWMsensorcutoff

Bp[n=1] (G), lower sensors

0

2

4

6

8

Radius (cm)C

HE

RS

rot

atio

n fr

eque

ncy

(kH

z)

0.535s0.545s0.555s0.565s0.575s

• Global rotation collapse consistent with neoclassical viscosity due to ideal RWM perturbation


Variety of Kinetic Instabilities Occurs with NBI

• Some modes correlated with fast ion losses

– TAE

– "fishbones"

• “Fishbones” are different at low aspect-ratio

– Possibly driven by bounce-resonance

• All modes interact

Fredrickson


Detailed Stability Analysis Will Benefit from New MSE Measurements of q-Profile

• Eventual aim is to be able to control profiles to optimize stability

• Low field presents severe challenges for MSE technique• First two channels now operating, more being installed this run

Levinton (NOVA)


Developing Capability for Active Control of Resistive Wall Modes

PF5 coils (main vertical field)

• 6 external correction coils being installed during this run

– Operate as opposing pairs driven by three switching amplifiers

• Planning to process sensor data in real-time through plasma control system for feedback control


Conventional Tokamak Confinement Trends Evident in Controlled Single Parameter Scans

• Lower-Single-Null divertor plasmas (small difference in upper, lower X-point flux)– = 2 – 2.2, = 0.5 – 0.7

050.00.10.20.30.40.50100Time (s)05010.51.01.5Radius (m)Ip [MA]PNB

[MW]WMHD [kJ]We [kJ]100200300010ne [1019m-3]Te [keV]

0 1 2

-2

-1

0

1

2

R (m)

Normalized flux

0.40.80.91.1.051.1

Kaye


Global Confinement Shows Similar NB Power and Current Dependence to ITER Scalings

• Data for D-NBI heated H-modes at time of peak stored energy• BT = 0.45T, R0 = 0.84 – 0.92 m, A = 1.3 – 1.5, = 1.7 – 2.5• EFIT analysis using external magnetic data

– Includes up to ~30% energy in unthermalized NB ions– No correction for unconfined orbit losses


Global and Thermal Confinement Exceed

Standard ITER Scalings

• TRANSP analysis for thermal confinement

E<

ther

mal

> (

ms)

E<ITER-H98p(y,2)> (ms)

12080400

120

80

40

0

• Compare with ITER scaling for total confinement, including fast ions• L-modes have higher non-thermal component and are more transient

Kaye


Power Balance During NBI Heating Shows Ions Have Low Transport

• Some shots show anomalously high Ti in region r/a ~ 0.6 – 0.8, yielding i < i<NC>

• Analyze power balance with

TRANSP code

– Use measured profiles of

Te, Ti, ne, nimp, Prad

– Monte-Carlo calculation of

NBI deposition and

thermalization

• i<NC> < i < e


Experimental Uncertainties Propagated Through TRANSP Power Balance

• Increased confidence in analysis for confinement zoneLeBlanc


Low Aspect-Ratio Appears to Provide a Unique Test-Bed for Studying Electron Transport

• GS2 linear analyses of microstability show

– Long- ITG modes stabilized by E B shearing

– Short- ETG modes are not stabilized & may dominate transport

– Usually insignificant modes with tearing parity may play a role

• Non-linear studies of turbulence with GS2 are underway

• Additional studies planned with FULL, GYRO, GTC

• Plan to install high-k microwave scattering diagnostic in 2005 to probe k = 2 – 30 cm-1 fluctuations

Bourdelle, Redi, Rewoldt , Dorland


Studying Fueling With Several Gas Injectors; Other Techniques Are Being

Developed

• Low-field side and lower divertor injectors provide programmable flow rates

• High-field-side injectors fully discharge external plenums through a pipe (1 – 2 m)

Later in this run period two additional methods will be commissioned:

• Room temperature solid pellet injector

– 1 – 8 pellets per discharge, 20 – 400 m/s

– Li, B, C, LiD (~10mg for lithium, ~1021 Li)

• Supersonic gas injector

– Graphite Laval nozzle close to plasma edge

– Gas at ~1.8km/s

– Up to 6 x 1021 D in 300ms


Deuterium Gas Fueling Efficiency is Low but Neutral Beams Fuel Efficiently

• LFS, HFS gas fueling have similar efficiency in matched plasmas• Accumulated significant NBI fueling in recent long-duration H-mode plasmas• Regression analysis of data indicates:

– Incremental gas fueling efficiency of 4 – 6 % – NB fueling efficiency 90 – 105 %


With Gas Fueling, Densities are Approaching the Greenwald Limit

• Observe little degradation in confinement at high density


Planning Additional Methods for

Controling Recycling

• Density control needed for long pulses in “advanced” modes combining

– Transport barriers (H-mode and possible ITBs)

– High fraction of bootstrap current (dominated by density gradient)

– RF current drive (dependent on Te)

• Currently prepare and condition carbon plasma-facing surfaces with

– 350°C bakeout (1 - 2 per run)

– Boronization with glow discharge in He/D-TMB [(CD3)3B] (~2 weeks)

– Between-shots He-GDC (8 – 12 min)

• Helium discharge cleaning - investigation just started

• Effects of “mini” boronization between shots - this week

• Lithium (boron) pellet conditioning - this run

• Lithium evaporator (CDX-U development) - next year

• Divertor cryo-pump - 2006

• Lithium surface pumping module - 2007

400-BarrelRoom-Temperature- Solid Pellet Injector


Gas Puff Imaging is Producing a Wealth of Data on Edge Turbulence

viewing area≈ 20x25 cm

• Looks at Dα(656 nm) from gas puff: I none f(ne,Te)

• View ≈ along B field line to see 2-D structure B

• Use PSI-5 camera at 250,000 frames/sec (4 µs/frame)

Zweben


Clear Differences in Edge Turbulence Between L- and H- Modes in NSTX

QuickTime™ and aVideo decompressor

are needed to see this picture.

• H-modes look quieter in NSTX than in C-Mod, perhaps because GPI sees further in past separatrix in NSTX

QuickTime™ and aGraphics decompressor

are needed to see this picture.


Exploring Methods for Generatingand Sustaining Toroidal Plasma

Current

• STs need non-inductive current

– space for transformer solenoid in center is very limited

• Exploit the neoclassical “bootstrap” current at high

– Requires a collisionless plasma

• Use RF waves which interact with the electrons

– Fast waves at high harmonics of ion cyclotron frequency (HHFW)

– Electron Bernstein Waves (EBW) at low harmonics of electron cyclotron frequency

• Coaxial Helicity Injection (CHI) can initiate toroidal current

– Create linked toroidal and poloidal magnetic flux (helicity) by injecting poloidal current which relaxes to form closed magnetic surfaces


Neoclassical Bootstrap Effect Drives Substantial Fraction of Plasma

Current

• Achieved substantial fraction of NBI-driven and bootstrap current for ~ skin time in diamagnetic plasma

• Vloop ≈ 0.1V for ~0.3s

• Control of profiles of pressure & current needed to maximize stability & bootstrap current together

Diamagnetic

012010.00.10.20.30.40.50.601Time [s]Ip [MA]Vloop [V]109070PNB[MW]/10P01 NBI drivenFαion0.5Bootstrap


• 6 MW at f = 30 MHz

– Pulse length up to 5 s

• 12 Element antenna

– Fed by 6 RF sources

– Active phase control between elements

– kT = ± (3-14) m-1

Field linein edge

High-Harmonic Fast-Wave System Designedto Provide Both Heating & Current Drive

• D = 9 – 13

• Expect little direct wave absorption on thermal ions• Wave velocity matched to thermal velocity of electrons


HHFW Power Can Heat Electrons and Trigger H-modes

• Antenna operated in phasing (0π)6 for slowest waves: kT ≈ 14m-1

0.50.001230500.00.10.20.30.00.10.20.30.40.5Time [s]PHHFW [MW]WMHD, We [MJ]ne(0), <ne> [1020m-3]Ip [MA]Hα[ ]arb0.51.01.5Radius[ ]m0.00.10.20.30.401ne[1020m-3]Te[ ]keV0.243s0.293s0.343s0.393s0.243s0.293s0.343s0.393s

LeBlanc


Evidence for Current Drive by HHFW with kT ≈ ±7m-1 in Co and Counter CD Phasing

0.00.20.40.60.8Time (s)Te(0) [keV]0120.00.5Ip [MA]012PHHFW

[MW]0123ne(0) [1019m-3]107899107907

• Phase velocity matches 2keV electrons• 150 kA driven current from simple circuit analysis• Modeling codes calculate 90 – 230 kA driven by waves

Ryan (ORNL)


Neutral Particle Analyzer Shows Interaction of HHFW with Energetic Ions Produced by NBI

• Ion “tail” above NB injection energy enhances DD neutron rate• Tail reduced at lower B

– Higher promotes greater off-axis electron absorption reducing power available to central fast-ion population

ln (

flux

/ E

n er g

y1/2 )

8

10

12

6

4

2

14012010080604020Energy (keV)

beam injectionenergy

B0=4.5 kG,t = 5.2%B0=4.0 kG, t = 6.6%B0=3.5 kG, t = 8.6%

B0=4.5 kGB0=4.0 kGB0=3.5 kG

NBIHHFW

NP

A

Medley, Rosenberg


He II (Majority) Ions in Edge Region Develop Hot Component During HHFW Heating

• Hot component grows over ~300ms but decays in ~30ms• Possible indication of parametric decay of RF waves

- kT ≈ -7m-1 waves do not interact directly with ions

Hot component

Biewer


Planning 3MW EBW System for Localized Current Drive

• ~28GHz for 3fce launch and Doppler-shifted 4fce absorption• Use O-X-B mode conversion of waves launched below midplane at

oblique angle to toroidal direction

Taylor, Efthimion, Harvey (CompX)

• Current driven by Ohkawa mechanism

–Selective trapping of co-going electrons

• Current can by driven in the critical mid-radius region


NSTX Explores Plasma Confinement in a Unique Toroidal Configuration

• Potential for high already demonstrated

• Confinement with NBI heating exceeds expectations

– Ions are well confined

– Combined NBI-driven and bootstrap current up to 60%

• Challenge is to achieve favorable characteristics

simultaneously with non-inductive current drive

– Self-consistent bootstrap current

– Current sustainment by RF waves

– Current initiation by coaxial helicity injection• Installing a capacitor bank to exploit “transient CHI” (HIT-II)


CHI Has Generated Significant ToroidalCurrent Without Transformer Induction

IinjBT

Jpol X Btor

Tor

oida

l Ins

ulat

ors

01CHI Injector Voltage (kV)0102030CHI injector current (kA)

0100200300400Plasma current (kA)Current multiplication ratio (Ip/ICHI)106488Time (s)0.00.10.20.30.4010155 n=1oscillations

• Goal to produce reconnection of current onto closed flux surfaces– Demonstrated on HIT-II experiment at U. of Washington, Seattle

DCPowerSupply

+

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Documents

M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team