# Presentation: 838526

## Edge Turbulence Imaging on NSTX 

- •  Summary of C-Mod GPI results
- •  GPI diagnostic set-up in NSTX
- •  GPI data from NSTX ‘01 run
- •  Interpretation of GPI signals
- •  Tentative conclusions
- •  Plans for 2002 run

_**Edge Turbulence Imaging on NSTX **_
_**and Alcator C-Mod**_
- S.J. Zweben1, R.A. Maqueda2, J.L. Terry3, B. Bai3, C.J. Boswell3, C. E. Bush4, D. D’Ippolito5, E.D. Fredrickson1, M. Greenwald3, K. Hallatschek6, S. Kaye1, B. LaBombard3, R. Maingi4,
- J. Myra5, W.M. Nevins7, B.N. Rogers8, D. Stotler1, and X.Q. Xu7
- 1 *Princeton Plasma Physics Laboratory             *2*Los Alamos National Laboratory *
- 3*MIT Plasma Science and Fusion Center            **4**Oak Ridge National Laboratory*
*5** Lodestar Research Corp                          **6 **Max-Plank-Institut fuer Plasmaphysik *
- 7*Lawrence Livermore Laboratory*                                                            8 *Dartmouth University*

## Motivations for Studying Edge Turbulence

- •  Very likely determines radial power and particle flow through
-        SOL onto first wall (“scrape-off length”)
- •  Probably controls L / H transition (turbulence suppressed ?)
- •  Maybe determines density limit (turbulence increased ?)
- •  Maybe affects RF power coupling through edge
- •  Good place to study physics of tokamak turbulence, since
-       both measurements and modeling are *relatively* simple

## Gas Puff Imaging Diagnostic

_**Gas Puff Imaging Diagnostic**_

- •  Gas puff imaging (GPI) telescope
-      views neutral line emission from
-      He or D2  gas puff ≈ along B field
-      at the plasma edge (like BES but
-      uses neutral gas instead of NBI)
-       S(photons/cm3) = no f(ne,Te) A
-      where the radiative decay rate is
-      A >> 107 sec-1 for these lines.
- •   Space and time variation of neutral light emission is measured with fast-
-       fast gated cameras and PMs or PDs on discrete chords to determine
-       edge turbulence structure (assumes kII << k)
- •  Gas puff changes plasma density by ≈ 1% in C-Mod and ≤10%
-       in NSTX, but this is unlikely (based on previous experience) to perturb
-       edge turbulence

## Summary of C-Mod GPI Results

- •  Edge turbulence imaged over ≈ 6 cm poloidally x 3 cm radially at outer midplane
- •  Poloidal k-spectra from GPI compared with DBM code and BOUT for one case
- •  Videos at 250,000 frames/sec often show “blobs” moving radially and poloidally

## GPI Diagnostic Set-up in NSTX

**Gas manifold**

**Side-viewing re-entrant window**

**Bay B**

**Local magnetic field**

- •  Similar to C-Mod system but using re-entrant port instead of coherent fiberbundle
-       and elongated gas manifold instead of single-point gas nozzle
- •  Generally have used He puffs in NSTX and Deuterium puffs in C-Mod (although
-       results from both are similar in each machine)

## Location of GPI View (2001 Run)

- •  Kodak camera views ≈ 30 cm x 30 cm area just above outer midplane
- •  Fast chords view 2 cm diameter “spots” in 7 channel radial array with PM tubes
- 	(bandwidth ≈ 100 kHz, digitized at 500 kHz)

- 12 cm

- GPI
- view

## NSTX GPI Diagnostic Issues

- •  He puff itself is unlikely to affect the local edge turbulence
- 	-  see similar edge turbulence in natural recycling light
- 	-  see same turbulence in GPI over a range of x5 in He puff level
- 	-  in C-Mod, see same frequency spectrum in LP (with or without puff)
- 	-  search for such an effect in PBX-M was negative (Pedrosa, PoP ‘95)
- 	-  need to compare GPI turbulence signals with LP signals in NSTX
- •  Spatial resolution due to angle between GPI sightlines and
- 	local turbulence filaments probably ≈ 2-3 cm (est).
- •  Time resolution set by HeI radiative decay time ≈ 0.014 μsec
- •  Relative effect of ne and Te fluctuations on GPI not yet known
- 	- in theory these should be in phase and have a similar spectrum, with
- 	  (Te/Te) / (ne/ne) ≤ Ln / LT, where “ < “ may be due to parallel electron
- 	  heat conduction (e ≈ 1/kIIve ≈ 20 μsec), depending on magnetic flutter
- 	  [BOUT got (Te/Te) / (ne/ne) ≈ 4 in SOL and ≈ 2 in edge of NSTX DN]3

## GPI Results from NSTX in 2001

- •  Typical 2-D GPI images using LANL Kodak Camera
- •  Typical signals from fast chords (#105637, 0.7 MA, 3.0 T)
- •  GPI spectra, fluctuation levels, correlation lengths, etc.
- •  H-L transitions from ELM-free case (#105710, 0.7 MA, 4.5 T)
- •  Results from XP#34 (SN vs. DN, Ip / BT scans) -  LATER

## Typical 2-D Images vs. Time  from Kodak

- #105627
- (0.7 MA, 3 kG)
- density scan

- #105710
- (0.7 MA, 4.5 kG)
- H-L transition

- #105711
- (0.7 MA, 4.5 kG)
- no-H mode

## Typical Shot with GPI data

- •  Moderate density shot from Stan’s density scan 7/3/01 (no NBI or ICRH)

## Typical Signals from Fast GPI Chords

- Time (sec)

- Time (sec)

_**Typical Signals from Fast GPI Chords**_

- •  Signals from 7 radial chords digitized for 0.128 msec @ 500 kHz
- •  Near outer wall see “intermittant” fluctuations with ~ 100% modulation
- •  Nearer center, see “Gaussian” fluctuations with ≈ 20% modulation

- outer

- inner

- 1 msec

- 100 msec

## Comparison of GPI and Mirnov Data

- •  GPI power spectrum is broadband with most of power below ≈ 50 kHz
- •  Broadband GPI spectrum exists without coherent MHD present
- •  Coherent MHD or ST cause small modulation in GPI signals

- Time

- Time

- Frequency spectrum of GPI

- Frequency spectrum of Mirnov

- E. Fredrickson

- 250 kHz

## Statistics of Inner GPI Chord

_**Statistics of Inner GPI Chord**_

- •  Looks like the usual “Gaussian” plasma turbulence

## Statistics of Outer GPI Chord

- no puff

- •  Looks like the usual “intermittent” SOL turbulence

## Radial Profile of GPI Fluctuations

- •  Relative fluctuation level goes from ≈10% @ r/a≈0.85 to ≈100% outside separatrix
- •  Autocorrelation times ≈10-20 μsec @ r/a≈0.85 to ≈30-100 μsec outside separatrix
- •  Radial correlation lengths ≈ 4-7 cm in all cases (FWHM)
- =>  These are all at least qualitatively similar to edge turbulence measured elsewhere

**Relative fluctuation level**

**Autocorrelation time**

**Cross-correlation w/#4**

## Fluctuations vs. Time (shot #105637)

- •  Relative fluctuation levels, autocorrelation times, and and cross-correlation
-         coefficients are approx. constant over 0.16-0.20 sec
- •  GPI signal levels rise by ≈ x5  during this time due as He gas enters,
-         while plasma density rises by only ≈ 10%
- •  Discharge changes after ≈ 0.2 sec as ne increases and Te drops (maybe
-         in part due to MHD after this time)

**GPI Signal levels **

**Relative fluctuation level**

**Autocorrelation time**

## Radial Propagation of Edge Turbulence

- •  Often (not always) a radially outward group velocity, especially in the outer
-      channels, at Vg ≈ 105 cm/sec (seen with LP in many machines)
- •  Also consistent with radially outward “blob” motion seen in GPI in C-Mod SOL

**Raw data**

**Cross-correlation functions**

- Outer

- Inner

- Outer

- Inner

## Edge Turbulence Signals at L-H Transition

_**Edge Turbulence Signals at L-H Transition**_

- Time (sec)

- Time (sec)

- •  L-H transition occurs within ≈ 1 msec on most channels
- •  Perhaps preceeded by large “bursts” in outermost channels
- •  Data quality for L-H transitions is poor due to low signal levels

**H-mode**

**L-mode**

- Outer

- Inner

- Outer

- Inner

**L-mode**

- L-H

- He puff

## Edge Turbulence Signals at H-L Transition

_**Edge Turbulence Signals at H-L Transition**_

- Time (sec)

- Time (sec)

- •  H-L transitions occurs within ≈20 μsec in most channels
- •  Transition seems to propagate outward at ≈106 cm/sec
- •  Transition seems to start with coherent mode (ELM ?)

**H-mode**

**L-mode**

- ELM ?

- Outer

- Inner

- Outer

- Inner

## Fluctuation Profiles in H- vs. L-mode

_**Fluctuation Profiles in H- vs. L-mode**_

- •  Fluctuation level seems to be lower in H-mode (but non-zero) in #105710 etc,
- 	as expected from appearance of “flat” H-mode emission in images
- •  Only one Thomson Scattering point within range of GPI data

**T****e****=13.3 eV**
**n****e****=1.1x10****12**** cm****-3**

**T****e****=8.9 eV**
**n****e****=2.2x10****12**** cm****-3**

- sep.

## DEGAS-2 Simulation of GPI in NSTX

- •  Average HeI emission cloud can be simulated given edge profiles
- •  For Te ≈ 10 - 50 eV, HeI line emission  ne0.5 to1.0 and Te1.5 to -0.5
- •  H-mode emission is narrower due to narrower edge profiles

- D. Stotler

## Tentative Conclusions

- •  Edge turbulence as seen by GPI in NSTX looks qualitatively
- 	similar to that in tokamaks, at least near outer midplane
- •  Most dramatic variation comes with H-mode, where edge
- 	turbulence level seems to be reduced by x2 or so
- •  GPI fluctuation levels need to be corrected for the variation of
- 	HeI light emission vs. ne and Te  derived from edge profile
- 	measurements and neutral modeling (similar to BES)
- •  Autocorrelation times and correlation lengths from GPI are
- 	probably close to correct, since the atomic physics just
- 	multiplies small fluctuations by ≈ some constant

## Plans for 2002

- 1.    Add fast channels to make simultaneous 7 radial x 7 poloidal array  DONE
- Set up new Phantom fast camera for GPI imaging @ 1000 frames/sec
-        (more sensitive and more pixels than Kodak camera)  DONE
- Swap Phantom with Princeton Scientific Instruments camera @ 1 million
-         frames/sec for about 1-2 months (SBIR with PSI)  EST. MID-APRIL
- Get GPI data for the “Edge Characterization” XP along with good edge
-         profiles (TS, LP, reflectometry), and cross-check GPI data with other
-         edge fluctuation data (LP, reflectometer, USXR, etc.)  MAY ?
- Feed edge profiles to BOUT and DEGAS 2  to calculate expected GPI
-         signals from theoretical simulations and compare with GPI data
- 6.   Repeat (4)-(5) for other conditions, especially L/H mode and density limit