# Presentation: 503938

## Three-dimensional structure of MARFEs and ELMs in the National Spherical Torus Experiment

**Three-dimensional structure of MARFEs and ELMs in the National Spherical Torus Experiment**

**R. J. Maqueda**
*Nova Photonics Inc., New Jersey, USA*
**R. Maingi, C. E. Bush***Oak Ridge National Laboratory, Tennessee, USA*
**K. Tritz***Johns Hopkins University, Maryland, USA*
**K. C. Lee***University of California at Davis, California, USA*
**E. D. Fredrickson, J. E. Menard, ****A. L. Roquemore, S. J. Zweben***Princeton Plasma Physics Laboratory, New Jersey, USA*
**S. A. Sabbagh***Columbia University, New York, USA*
**and the NSTX Research Team**

**17****th**** PSI Meeting**
- May 22-26, 2006
- Hefei, Anhui, China

## Outline

- ELM structure and dynamics- comparison with “blobs”
- MARFE structure and dynamics

**Typical NSTX parameters**
- R ~ 0.85 m  a ~ 0.67 m   ~ 2-2.3
- Baxis = 4.5 kG
- Ip = 0.8-1.0 MA
- PNBI < 7 MW
- Te(0) ~ 1 keV
- ne(0) ~ 8 x 1019 m-3

**Center stack**

**Carbon tiles**

**Other edge studies in NSTX**
- Divertor heat loads/reductionR. Maingi, V. Soukhanovskii
- Lithium coatingsH. Kugel
- Dust generation and dynamicsC. Parker, L. Roquemore, C. Skinner
- Blobs and neutral transportD. Stotler

## Phantom 7 camera

**Phantom 7 camera**
- Frame rate:	≤68000 frames/s		at 128 x 128 pixels
- 	≤120000 frames/s		at 64 x 64 pixels
- Minimum frame exposure: 2 s
- Digitization: 12-bit
- Full discharge coverage with 2 GB of on-board memory

**Complex filament structure and dynamics**

- Clip:	**no filter**	2 s exposures
- 	7 ms at 100000 frame/s	playback at 150 s/s

## Divertor fast tangential imaging

**Phantom camera image**

**NSTX poloidal cross-section**

## Type V ELM: Propagating perturbation

- Increased luminosity in the divertor region, including secondary band outside outer strike point.
- Single heat pulse propagates downward on inner separatrix, reaching X-point region.
- Downward propagation along field lines (~20 km/s) too slow to correspond to ion accoustic speed with Te≈Te,ped~200 eV.

- Clip:	**CII filter (657.8 nm)**	5 s exposures
- 	5 ms at 100000 frame/s	playback at 150 s/s

- 800 kA
- 4.2 MW NBI
- Lower single null

## Cross-field structure of filament: ribbon like

**Cross-field structure of filament: ribbon like**

- 800 kA
- 6.5 MW NBI
- Lower single null
- Type V ELMs

**23 cm poloidal**

**23 cm radial**

- Clip:	**D******** filter**	3 s exposures
- 	5 ms at 120000 frame/s	playback at 125 s/s

**separatrix**

**limiter****shadow**

- Type V ELM filament ribbon

**Tangential edge imaging**

## Type V ELM filament characteristics

- Well aligned with local magnetic field.
- Toroidal velocity: ~8 km/s (~0.9 kHz at R~1.45 m)  **...counter I****P**** and plasma rotation**
- Radial velocity: ≤0.2 km/s
- Crossfield widths: ~12 cm poloidal, ~3-4 cm radial
- Current: ~400 A (~100 kA/m2)  **...co-I****P**
- Some ELMs show 2 filaments, separation might indicate n = 3-4.  But... no full n = 3 or 4 structure seen on diagnostics (Mirnov array).
- Lifetime: 0.5 to 1 ms ...longer than the peeling-ballooning “detonation” time for NSTX.

## Edge turbulence and “blobs” compared to ELM activity

**Gas puff  imaging**
**Field of view****23 cm x 23 cm**

**L-mode**
- 800 kA
- 2 MW NBI
- Lower single null

**H-mode**
- 1 MA
- 4.7 MW NBI
- Lower single null

- Clip:	**D******** filter**	3 s exposures
- 	5 ms at 120000 frame/s	playback at 125 s/s

- Blob characteristics during H-mode different from small ELMs
- After large ELMs (~1 ms) edge similar to L-mode edge

**poloidal**

**R**

**D****2**** puff**

## MARFE dynamics

**MARFE dynamics**

- 800 kA
- 6.0 MW NBI
- Double null

- Clip:	**D******** filter**	9 s exposures
- 	10 ms at 68000 frame/s	playback at 220 s/s

- Toroidally symmetric MARFE moves downwards (ion B-drift direction).
- ELM activity in divertor region coincides with burn through of most of MARFE.
- Toroidally localized MARFE remnants move upwards, following field line.
- Upward movement stagnates and MARFE precursor expands into a toroidally symmetric band.
- Type I ELM (at ~666 ms) burns through MARFE.

## The ELM cycle gives rise to MARFE precursor and dynamics

**Time (ms)**

**Divertor D******** (a.u.)**

**High sensitivity scale**

**Low sensitivity scale**

**Medium sensitivity scale**

**2 m**

**Midplane**

**Lower divertor**

**Upper divertor**

**117125**

- Wide-slit “streak” composition

- ELM cycle and MARFE cycle closely linked.
- Nevertheless, behavior and dimensions varies.
- Precursor of Type I ELM first reverses MARFE movement and then burns through MARFE.

## Summary and conclusion

- Type V ELMs in NSTX are characterized by elongated filaments that rotate counter Ip, NBI and plasma rotation and have lifetimes of 0.5 to 1 ms.
- Filaments are “ribbon like”, wider in the cross-field (poloidal) than radial direction, and carry a substantial amount of current.
- As filament propagate poloidally the ionization front is shifted and, possibly as well, the magnetic surfaces shift.
- Coincident with the ELM cycle the MARFE moves up/down the center stack.
- The toroidally symmetric MARFE is, in some cases, born from a precursor resulting from partial burnthrough of the preceding MARFE in the cycle.