The effect of plasma shaping on plasma performance on NSTX

1
The effect of plasma shapingon plasma
performance onNSTX
Columbia U Comp-X General Atomics INEL Johns
Hopkins U LANL LLNL Lodestar MIT Nova
Photonics NYU ORNL PPPL PSI SNL UC Davis UC
Irvine UCLA UCSD U Maryland U New Mexico U
Rochester U Washington U Wisconsin Culham Sci
Ctr Hiroshima U HIST Kyushu Tokai U Niigata
U Tsukuba U U Tokyo JAERI Ioffe
Inst TRINITI KBSI KAIST ENEA, Frascati CEA,
Cadarache IPP, Jülich IPP, Garching U Quebec

• Presented by D. A. Gates
• At the NSTX Physics Meeting
• May 2, 2005

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Proposed APS Outline

• Spherical Torus (ST) overview
• Machine Improvements
• Control system upgrades
• PF1A upgrade
• Widened operating boundaries
• Early H-mode operation
• Pulse length extension
• Effect of increased d at high k
• Effects of shaping on
• Stability (ELMS, Tearing modes, External kinks)
• Transport
• Effect of shaping on divertor
• Integrated Scenario modeling
• Summary

3
Reduced latency expands achievable elongation

• Control latency reduced to 1/4 previous value
• Plasma elongation increased 25 (at fixed li)
• Increased elongation has broadened operating
  space (pulse length, b)
• Achieved sustained k 2.6 for many wall times

From EFIT - entire NSTX database
2004 2002-3 2001
4
High elongation extends pulse at high Ip/Irod

• 25 more Ip, 10 less toroidal field, 5 longer
  pulse in NBI heated discharges

Boundary from 109063 overlaid (green)
Shot 112581 - 2004 Shot 109063 - 2002
5
Pulse averaged toroidal b increased

• Pulse average b calculated over the plasma
  current flat top
• Clear correlation of increase the increase in b
  with increase the in k
• Will increase in d achieve change this plot?

6
New coil enables detailed d scan
Scan of d at fixed k and aspect ratio (calculated
using ISOLVER code)

• Detailed scan of plasma boundaries using new PF1A
  indicates much better shape control possible
• High priority XP (S. Kaye, boundary)

7
Shape evolution at high b on NSTX
High k,d
High d
High k
2004
2002-3
2005?
8
Scenario modeling identifies 100 non-inductive
case with bt 40
Predicted 100 non-inductive current sustainment
(TSC)

• Scenario modeled using TSC code
• Transport coefficients based on measured profiles
  with values scaled using H-mode scaling laws
• Plasma stable to n1-3 ideal kink modes (with
  perfectly conducting wall)
• Stability requires modified plasma shaping
  capability

Steady state current profiles (TSC)
9
Summary

• NSTX has made substantial progress towards steady
  state operating goal
• High k has been crucial
• Real-time reconstructions now routine
• HHFW studies have demonstrated current drive as
  well as ion heating issues
• EBW is a promising candidate for current drive in
  the ST
• Strong collaboration with MAST on EBW
• Planned upgrades should lead to continued
  improvements in performance

10
Magnetic pulse length increased

• Simulataneous doubling of bt (pulse averaged) and
  50 increase in normalized pulse length (tpulseN
  ?Ipdt/ltIrodgt)
• Improvement correlates strongly with high k

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High bt achieved at Ip/Irod gt 1

• IN Ip/aB 7 (MA/mT) - Ip/Irod 1.1
• bt 39 - uncertain within 10
• MSE available but not yet analyzed

Shot 114465, 1.4MA, Itf 3635.5kA
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High b regime extended
EFIT data from 2004 EFIT data from 2001-3

• Many shots with bt gt 35
• Troyon scaling confirmed with bN 6.3 (wall
  stabilized)
• Highest b also at high k 2.3

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Minimizing Aspect Ratio Maximizes Good Curvature
region

• Improved stability to
• Ideal pressure driven kink modes
• Neoclassical tearing modes
• m-instabilities
• High ExB shearing rates
• Up to 100 bootstrap fraction
• Simple construction
• Lower capital cost

Spherical Torus
Tokamak
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NSTX is addressing key ST Issues

• ST requires non-inductive startup and sustainment
• Coaxial Helicity Injection, High Harmonic Fast
  Wave and Electron Bernstein Wave Current Drive,
  Bootstrap Current
• Increased recirculating power due to copper TF
  coils
• High b operation
• Increased divertor power loading
• Natural inboard divertor, large flux expansion
• High b, and enhanced tE are required for high Q
• Passive conducting structure, high ExB shearing
  rates, active RWM feedback (future)
• Profile Control (rtEFIT,J(r))

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NSTX explores ST physics in a midsized device
Parameters Design Achieved Major Radius
0.85m Minor Radius 0.68m Elongation 2.2
2.5 Triangularity 0.6 0.8 Plasma Current 1MA
1.5MA Toroidal Field 0.6T 0.6T Heating and
Current Drive Induction 0.7Vs 0.7Vs NBI
(100keV) 5MW 7 MW RF (30MHz) 6MW 6
MW CHI 0.5MA 0.4MA Pulse Length 5s 1.0 s
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Control system block diagram
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rtEFIT/isoflux controls boundary precisely
Four consecutive rtEFIT shots- Boundaries from
EFIT

• Digital control of plasma boundary based on
  real-time inversion of the Grad-Shafranov
  equation
• Isoflux control -VPFiGidyi where dyi is the flux
  error between requested and actual boundary along
  control segment
• New capability - used for 40 of shots in 2004

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Wall stabilization physics key to sustained
operation at high b

• High bt 39, bN 6.8 reached

• Operation with bN/bNno-wall gt 1.3 at highest bN
  for pulse gtgt twall

bN/li 12
6
8
10
112402
wall stabilized
4
bN
wall stabilized
bN
core plasma rotation (x10 kHz)
0
n1 (wall)
dW
10
n1 (no-wall)
20
DCON
EFIT
0.6
0.7
0.5
0.4
0.3
0.2
0.1
0.0
li
t(s)

• Global MHD modes can lead to rotation damping, b
  collapse
• Physics of sustained stabilization is applicable
  to ITER

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NSTX is investigating High Harmonic Fast Wave
Heating (HHFW)

• 12 Strap antenna connected to 6 1MW RF sources at
  30MHz
• w 10-15wci
• Relative antenna strap phasing controllable in
  real-time
• Can vary current drive in real time
• Heats electrons by Landau damping

20
HHFW has been observed to damp on neutral beam
particles

• Effect observed in measured neutron rate and in
  lost fast neutral particle distribution
• Important issue for the applicability of HHFW to
  fusion plasmas

Measured and predicted neutron rate for 2
otherwise identical plasmas Red-RF on, Solid Blue
RF off, dashed blue TRANSP prediction
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Edge thermal ion heating observed during HHFW
heating

• Consistent with decay of high harmonic fast wave
  into ion Bernstein wave and ion cyclotron
  quasi-mode
• Decay modes detected with RF probe

Edge HeII spectrum during HHFW
RF Probe measurements during HHFW
22
EBW emission measured

• Electron cyclotron heating not possible in ST
• High ne, low Bt ? ECH below cut-off
• Electron Bernstein Wave (EBW) can propagate in
  the plasma
• Coupling via O-X-B conversion scheme has been
  investigated using the inverse B-X-O emission
  mechanism

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Modified PF1A coil being installed
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PF1A upgrade will allow stronger shaping

• PF1A coil is being modified for better control of
  triangularity (d 0.8) at high elongation (k
  2.5)
• High triangularity combined with high elongation
  will permit 40 more current for fixed q
• Alternatively higher q for the same current
• 100 non-inductively sustained scenario has been
  identified for target double null shape
• Assumes functioning EBW current drive
• Will also test if error field control can raise
  bN
• Important for increased bootstrap current

25
Early H-mode reduces initial flux consumption
Shot 112546 - early H-mode Shot 111964 - No early
H-mode

• Ip flat-spot induces early H-mode
• Lowers internal inductance
• delays MHD onset (presumably due to increased
  qmin)
• Raises elongation/ bootstrap current (for fixed
  field curvature)

H-mode transition
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High Harmonic Fast waves have been used to drive
current in NSTX

• Observed absorption efficiency is strongly
  dependent on k
• Absorption weakest where current drive is
  expected to be strongest

Results of a controlled experiment comparing co-
and counter-current drive phasings with HHFW with
fixed Te
Measured Te profiles at different times in the
discharges
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Plasma control crucial to progress on steady state

• High bootstrap plasmas will require integrated
  plasma control with simultaneous
• Vertical control
• Shape control
• Real-time current profile reconstructions
• RF source control
• Must also avoid b limits
• rotation control
• n1 feedback

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Increased elongation enables increased sustained
bt

• Define new figure of merit
• bsus 0.5e0.5bpbt fbsbt
• Balances trade-off between high bootstrap
  fraction and high bt
• Increased k results in 50 increase in bsus
• bsus bN2(1k2)
• (elliptical plasma approximation)

Plot of bsus parameter vs. (1k2) for entire NSTX
database (each point represents 1 discharge
averaged over current flat-top)
bsus 1.6(1k2)