NSTX High-k Scattering System on NSTX: Status and Plan*

1
NSTX High-k Scattering System on NSTX Status and
Plan
H.K. Park1, W. Lee1, E. Mazzucato1, D.R. Smith1,
C.W. Domier2, W. Horton3, S. Kaye1, J. Kim3,N.C.
Luhmann,Jr.2,NSTX Team European Physical Society
Conference July 1-6, 2007 Warsaw, Poland 1
PPPL, Princeton University, NJ 2 UCD, CA 3 U.
Texas, TX
2
National Spherical Torus Experiment
Major Radius, R 0.85 m
Minor Radius, a 0.67 m
Aspect Radio, A 1.27
Elongation, k 2.6
Triangularity, d 0.8
Plasma Current, IP 1.5 A
Toroidal Field at R0, BT .3-.45 T
NB Power, PNB 7 MW
HHFW Power, PHHFW 6 MW
Toroidal Beta, bT 40
Normalized Beta, bN 9
Pulse Length 1 s
3
NSTX plays a key role in extending fluctuation
measurements beyond the present data base

Scale length and Turbulence type

• Full exploitation of turbulence based transport
  physics is the goal
• Capability of investigating turbulence physics up
  to k-?e0.7
• Multi-channel NSTX scattering system ? k-space
  turbulence continuum through simultaneous
  measurement of five wavenumbers
• Various operating regimes were studied H/L
  modes, RS regime, High Te/Ti regime, confinement
  dependence on BT, Alfvén wave study

? NSTX
Hennequin et al. PPCF 46, 2004
4
BWO millimeter wave source and power supply
Thomson CSF BWO
High Voltage Lines

• Thomson CSF BWO
• Model CO 10-1
• O-type backward wave oscillator (BWO)
• High power
• 200 mW
• Frequency tunable
• 275-290 GHz
• 15 MHz/V
• Lifetime 2000 hrs
• Siemel Power Supply
• High voltage 12 kV
• Low ripple lt 15 mV
• Anode current controls BWO power output
• Cathode voltage controls BWO frequency
• Configured to lower BWO filament current between
  shots to conserve BWO lifetime

Siemel Power Supply
Cooling Lines
F-band Waveguide Output
5
Probe beam launching hardware arrangement

• Launcher system has three mirrors
• Microwave power is piped through corrugated
  waveguide system

Translation Across Entrance Window
Motorized Linear Slide
From Input Beam Splitter
Spherical Focusing Mirror
6
Detection system hardware arrangement

• Detection system is piped through corrugated
  waveguides
• Detection array is located in the test cell base

Windows for the Scattered signals
Scattered signals
From Input Beam Splitter
To the wave-guide Array
Spherical Focusing Mirror
To detector array
7
Characteristics of the scattering system on NSTX

• Tangential multi-channel (5) scattering system
• Po 100 mW
• lo 1 mm (280 GHz)
• System NF 5000oK
• System resolution
• Dk a/2 1.0 cm-1

Scattering length (Lv)
Inboard ? 0.05 k-?e up to 0.7
Outboard ? 0.75 k-?e up to 0.4
8
Calibration of the System with A/O cell

• Possible source of errors
• Emissions from the plasma at the probe beam
  wavelength is negligible
• Cross talk between channels is minimized by
  optical isolation
• Calibration of the scattering system
• Verification of the scattering length, relative
  efficiency curvature effect, magnetic shear
  effect and k-matching condition
• Verification of the direction of waves

9
Verification of the propagation direction of the
wave

• Heterodyne detection system
• Edge region outward propagation direction is
  Positive frequency
• Core region outward propagation direction is
  Negative frequency

outward Propagating wave
Inward Propagating wave
Frequency response from the probe beam at the
edge region
10
Fluctuation of the Ohmic discharge (He)
k-?e0.1

• Monotonically decreasing power spectra as a
  function of wave-numbers in OH plasma
• Plasma parameters
• ne (0) 2.5x1013cm-3
• Te (0) 200eV
• r/a 0.85

11
Characteristics of H-mode edge plasma (pedestal)

• Monotonically decreasing power spectra during
  L-mode phase
• Non-monotonic power spectra during H-mode phase

ETG?
12
Reduction of fluctuation is well correlated with
improved confinement

• Reduction of fluctuation at upper ITG/TEM and
  moderate changes at ETG during H-mode
• Bursts of the scattered signal at the highest k
  is noted.
• Ion transport is close to new classical in H-mode
• Electron transport is reduced from L- to H-mode

13
Theoretical calculations Indicate ITG,TEM and ETG
are possible candidates for electron transport

• GS2 calculations indicate lower growth rate at
  lower k during H-mode phase and higher growth
  rates for all wavenumber during L-mode
• Non-linear GTC results indicate ITG modes are
  stable during H-mode phase
• ETG mode is unstable in L-mode and marginal in
  H-mode

glin gtgt gExB during L-phase for all kqrs glin ltlt
gExB during H-phase for ITG/TEM glin gExB
during H-phase for ETG
Experimental results are Consistent with the
growth rate of ETG mode Other types of
fluctuations???
14
Electron confinement dependence on BT

• Confinement is improved at higher BT due to the
  improved electron transport at the edge region
• The core electron thermal diffusivity increases
  at higher BT

15
Core experimental results
Asymmetric spectral feature increases at higher
field.
16
Edge experimental results
Asymmetric spectral feature decreases at higher
field.
17
Highly shifted frequency spectra (inside the
pedestal of the H-mode)
Highly shifted (inward propagating) turbulence
spectra was observed during H-mode phase (inward
propagating?) For the poloidal component
Negative frequency is electron diamagnetic
direction
r/a 0.75 k- 9.3 cm-1 k-re 0.22
18
L-mode discharge (high Te/Ti) by RF
Frequency broadening during RF heating phase
where Te is peaked at 3 keV while Ti is at 1 keV
19
L-mode discharge (Te Ti) by RFNBI
Te is comparable to Ti and no spectral broadening
during RF NBI heating
20
Alfvén wave study using scattering system
21
Electrostatic component of the Alfvén wave
Beta-induced Alfvén Acoustic Eigenmode (BAAE)
22
Summary

• L-mode (k-4.3) and OH (k-3.7) plasmas -
  Monotonically decreasing fluctuation level
  extends the previous data base up to k-?e 0.2
• H-mode plasmas
• Reduction of fluctuations below k-?e ltlt 0.1
  (ITG/TEM) at the edge of the H-mode plasma is
  consistent with the improved confinement (k-3.4)
• Enhanced fluctuations at higher k (above k-?e
  0.15) is observed in H-mode plasma (suppression
  of ETG in L-mode?)
• Electron transport dependence on toroidal field
  strength
• Enhanced fluctuation at the lower field at the
  edge and at the higher field at the core
• Highly shifted frequency spectra inside the
  pedestal region but not in the core
• Alfvén wave studies
• BAAE mode was detected by scattering system
• Highly broadened spectra at high Te/Ti ratio
• Comparison study of discharges with RF alone and
  RFNBI