m.g. bell princeton plasma physics laboratory on behalf of the nstx research team
DESCRIPTION
Toroidal Magnetic Plasma Confinement at the Limit: Recent Results from the National Spherical Torus Experiment. M.G. Bell Princeton Plasma Physics Laboratory on behalf of the NSTX Research Team. “Spherical Torus” Extends Tokamak to Extreme Toroidicity. - PowerPoint PPT PresentationTRANSCRIPT
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
MIT seminar / 040514 / MGB 3
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)
MIT seminar / 040514 / MGB 4
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
MIT seminar / 040514 / MGB 5
View of NSTX, Heating Systems and Diagnostics
MIT seminar / 040514 / MGB 6
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
MIT seminar / 040514 / MGB 7
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
MIT seminar / 040514 / MGB 8
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
MIT seminar / 040514 / MGB 9
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)
MIT seminar / 040514 / MGB 10
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
MIT seminar / 040514 / MGB 11
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
MIT seminar / 040514 / MGB 12
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
MIT seminar / 040514 / MGB 13
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
MIT seminar / 040514 / MGB 14
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)
MIT seminar / 040514 / MGB 15
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)
MIT seminar / 040514 / MGB 16
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
MIT seminar / 040514 / MGB 17
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
MIT seminar / 040514 / MGB 18
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)
MIT seminar / 040514 / MGB 19
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
MIT seminar / 040514 / MGB 20
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
MIT seminar / 040514 / MGB 21
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
MIT seminar / 040514 / MGB 22
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
MIT seminar / 040514 / MGB 23
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
MIT seminar / 040514 / MGB 24
Experimental Uncertainties Propagated Through TRANSP Power Balance
• Increased confidence in analysis for confinement zoneLeBlanc
MIT seminar / 040514 / MGB 25
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
MIT seminar / 040514 / MGB 26
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
MIT seminar / 040514 / MGB 27
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 %
MIT seminar / 040514 / MGB 28
With Gas Fueling, Densities are Approaching the Greenwald Limit
• Observe little degradation in confinement at high density
MIT seminar / 040514 / MGB 29
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
MIT seminar / 040514 / MGB 30
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
MIT seminar / 040514 / MGB 31
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.
MIT seminar / 040514 / MGB 32
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
MIT seminar / 040514 / MGB 33
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
MIT seminar / 040514 / MGB 34
• 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
MIT seminar / 040514 / MGB 35
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
MIT seminar / 040514 / MGB 36
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)
MIT seminar / 040514 / MGB 37
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
MIT seminar / 040514 / MGB 38
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
MIT seminar / 040514 / MGB 39
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
MIT seminar / 040514 / MGB 40
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)
MIT seminar / 040514 / MGB 41
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
+
-