# Presentation: 294579

## Results and analysis of Gas Puff Imaging experiments in NSTX: turbulence, L-H transitions, ELMs and other phenomena

- R.J. Maqueda
*Nova Photonics*
- S.J. Zweben, T. Munsat, T. Biewer
*Princeton Plasma Physics Laboratory*
- C. Bush, R. Maingi
*Oak Ridge National Laboratory Laboratory*
- J. Boedo
*UC-San Diego*
- N.A. Crocker, S. Kubota, X.V. Nguyen, W.A. Peebles
*UC-Los Angeles*
- and the NSTX Team

## Abstract

- The 2-D structure of the plasma edge is measured in the National Spherical Torus Experiment (NSTX) by imaging its spectral line emission with 4-10 s time resolution. In the Gas Puff Imaging (GPI) diagnostic a poloidally elongated gas puff is used to localize the emission region in the poloidal-radial plane. The typical L-mode edge is turbulent with toroidally elongated filaments moving poloidally and radially. This is contrast with the H-mode edge where a more quiescent emission is observed. By using a PSI-5 ultra-fast framing camera developed by Princeton Scientific Instruments, capable of frame rates of up to 250000 frames/sec, the transitions between L- and H-modes are recorded. In addition, ELMs and MHD activity with high poloidal modes numbers are also observed with this diagnostic. Analysis of data obtained in recent GPI experiments will be presented, as well as comparison with other edge diagnostics. This includes spatial mode number, time series and velocity field analysis. Plans for future research/experiments will also be presented.
- Work supported by DoE grant DE-FG02-04ER54520.

## Motivation

- There are very many reasons why the edge plasma is important...

- Core confinement depends on edge gradients and gradient driven instabilities.
- The transport at the edge and across the SOL depends on the level and characteristics of the local edge turbulence, either directly or through their effect on the edge profiles.
- The edge plasma interacts with the wall (divertor or limiters), liberating impurities.
- Impurity transport along SOL and across the edge determines impurity concentration in the core.
- Edge plasma interacts with injected auxiliary RF heating power.

**Gas Puff Imaging**** (GPI) provides access to many of the important phenomena at the edge: turbulence, L-H transitions, ELMs, RF coupling, etc.**

## GPI Diagnostic

- Imaging camera used to view visible emission from edge.
- Gas puff is injected to increase image contrast and brightness.  Gas puff _does not perturb_ local (nor global) plasma.
- Emission filtered for D (or HeI) light from gas puff:              **I ******** n****o****n****e ****f(n****e****,T****e****)**
- Da emission only seen in range          ~ 5 eV < Te < 50 eV
- View aligned along B field line to see 2-D structure  B.  Typical edge phenomena has a long parallel wavelength, **filament ****structure**.
- For more details: **“Gas puff imaging of edge ****turbulence”****,** R.J. Maqueda et al., Rev. Sci. Instrum. 74(3), p. 2020, 2003.

## GPI Diagnostic (cont.)

- Viewing area located just above midplane on outer edge of NSTX.
- 25 cm x 25 cm area imaged with 1-2 cm resolution.
- PSI-5 camera used to record 300 frames, 64 x 64 pixels/frame, typically at 250,000 frames/s           -> 1.2 ms of discharge coverage
- Measurement complemented with 13 discrete chords having 200 kHz resolution and 128 ms of discharge coverage, each viewing a 2 cm spot of the emission cloud.

## Typical NSTX parameters

- Shot 108322 at 0.18s

**Core**
- B(0) = 3.5 kG
- I = 900 kA
- Te(0) = 1 keV
- ne(0) = 2.5 x 1013 cm-3
- <ne> = 2 x 1013 cm-3

**Outer edge **(Rmid = 1.46 m)
- ne ~ 5 x 1012 cm-3
- Te ~ 13 eV ~ 10-3
- Ln ~ 2 cm
- s ~ 0.2 cmei ~ 6 x 106 s-1
- Lc ~ 5 m (connection length to divertor)ei/Lc ~ 0.05
- q = (BT/Bpol)(R/A) ~ 2
- LRBM ~ 1 cm

**“****High-speed imaging of edge turbulence in NSTX”**, S.J. Zweben et al., Nucl. Fusion 44, p. 134, 2004.

## GPI Movie Clips

-               http://www.pppl.gov/~szweben/NSTX04/NSTX_04.html

**GPI Movie Clips**

- Gas injection

- Gas injection

## Movie clips

- Shot 113830	L-mode
- Shot 113139	H-mode
- Shot 113420	H-mode
- Shot 113732	L-H transition
- Shot 113079	L-H transition
- Shot 113735	L-mode, just before L-H transition
- Shot 113062	H-L transition – dither
- Shot 113074	H-L transition – dither
- Shot 113409	Type II/III ELM
- Shot 113470	Type V ELM
- Shot 113012	MHD – “Breathing”
- Shot 113371	MHD – “Bouncing”
- Shot 113757	MHD – “Blinking”

## Image analysisL-mode vs. H-mode comparison

- Turbulence is qualitatively similar in Ohmic and L-mode discharges.
- No differences have been (yet) observed in lower single null, upper single null, double null and limited discharges.

## Pre-LH transition dithers

**Pre-LH transition dithers**

**t-t****LH**** (ms)**

**Filament****frequency****(filaments/ms)**

**113737****113741****113743****113744**

**Transient H-mode like periods can occur up to 10 ms before the main L-H transition**

- Although turbulence is much reduced in H-mode, with quiet periods lasting up to 100 ms, filaments (“blobs”) and waves can be occasionally seen.

**dithers**

- Filaments on 2 cm diameter spot

## L-H transition in other diagnostics

- Fluctuations in 30 GHz reflectometer decrease rapidly during L-H transition.
- For details see **Crocker et al. JP1.022**

## Transition observations

- L-H transition “dithers” can precede the main transition.
- The main L-H transition appears like a continuous evolution from turbulent filaments to the characteristic quiescent state in H-mode.  This evolution lasts < 100 s without any apparent new spatial features or flows.  Filaments just appear to “drain out”.
- Similar suppression of turbulence observed in other diagnostics during L-H transition.
- The H-L back transition generally appears as a high-n poloidal perturbation that evolves into radially moving filaments.

## New analysis tool

**Velocity fields*****Tobin Munsat***

- Do shear or zonal flows develop at the time of the L-H transition? ...work in progress.

## ELMs

**Type I**
- Mid WMHD with very fast or no pre-cursor, Pheat >> PL-H
- GPI images best described as a momentary transition back to L-mode characteristics.
**Type II/III**
- Small WMHD with low frequency, long lived pre-cursor
- Increased filament activity respect to quiescent H-mode discharges.
**Type V**
- Very small WMHD with n=1 pre-cursor
- Modest difference (if any) with background H-mode turbulence (filament frequency).
- ELMs in NSTX: **Maingi et al., CO3.005**

## Compound ELMs

**Divertor****D******** (a.u.)**

**GPI fast****chord****(a.u.)**

**Auto-**
**correlation**

**Power****spectrum**

**H-mode**

**L-mode**

**113737**

**Compound**
**ELM**

- No substantial differences observed between L-mode turbulence and that during compound ELMs.

## Summary and open questions

**Measurements**

**Open Questions**

- Turbulence is similar in Ohmic and L-mode discharges.  (Differences are observed during H-modes.)
- H-modes might be preceded by short quiescent phases.
- H-modes have short periods of L-mode type activity (filaments).
- H-L transitions (generally) appear first as high-n poloidal modes.

- Are there differences in Ohmic, L-mode, upper single null, lower single null, double null, and inner wall limited?
- Is the main transition, and any pre-transition dithers, associated with poloidal flows (shear or zonal)?.
- What triggers filament formation during H-modes?
- Is there a consistent instability pattern leading to the H-L transition?

## Summary and open questions

**Open Questions**

- Fast divertor D light appears to start decreasing before the midplane GPI images show H-mode characteristics.
- ELMs are seen with similar characteristics as L-mode blobs.
- “Breathing”, “bouncing” and “blinking” are seen associated with MHD activity?

- Where are the first signs of the L-H transition observed?  Candidate: **X-point region**.
- Are ELMs and L-mode filaments similar entities? Are the sporadic H-mode filaments minute ELMs?
- What causes breathing, bouncing and blinking?

**Summary and open questions**

## Next steps

- Use _new fast framing camera_, capable of recording > 100000 frames/s and whole discharge coverage.
- Observe emission from X-point region during L-H and H-L transition..
- Use wide view of NSTX plasma to determine location of **L-H transition and ELM birthplace**.
- Study effects of low Z (lithium) and medium Z (neon), **radiatively cooled edge**, on the edge turbulence.  Is there a relation with impurity enhanced modes?
- Search for **poloidal flows** (shear, zonal, etc.) during L-H transition, both at midplane and at the X-point region.