# Presentation: 933116

## NSTX Physics Results and Opportunities

- S. M. Kaye
- Head, NSTX Physics Analysis
- PPPL, Princeton University
- Princeton N.J. 08550
- Presented at: U. of Maryland
- 17 October 2001

*Lodestar *
*NYU *
*Princeton Sci. Instruments*
*UC Irvine*
*UKAEA*
*Tokyo U *
*Kyushu Tokai U *
*Himeji Inst. Science Tech Hiroshima*
*Niigata U*
*Tsukuba U*
*Ioffe Inst*
*Triniti (RF)*
*KBSI (Korea)*

## Outline

- Potential benefits of low aspect ratio
- Role of NSTX
- Physics program, results and opportunities
- Confinement and transport
- MHD
- Non-inductive current startup and sustainment
- Conclusions and future plans

## The Spherical Torus (ST) is a low aspect ratio tokamak

- • ST maximizes the field line length in the good curvature
- (stable) region - strong toriodicity
-      - Potential benefit to stability and confinement
- • Strong natural shaping
-      - Enhance stability, relax magnet requirements

## NSTX Mission

- • 	Extend the understanding of toroidal physics to high-, low-collisionality regimes at low aspect ratio (R/a ≤ 1.4)
- •	Develop technologies necessary for a Spherical Torus (ST) power plant
- Essential elements of mission
- High-t (≤ 40%), large bootstrap current fraction (fbs ≤ 0.7)
- Non-inductive current generation and sustainment
- Adequate power and particle handling
- Integration of individual elements in long-pulse discharges

## National Spherical Torus Experiment (NSTX)

_**Baseline Parameters**_
- 	(Achieved)
- Major Radius 0.85 m
- Minor Radius 0.68 m
- Elongation ≤ 2.2 (2.5)
- Triangularity ≤ 0.6 (0.7)
- Plasma Current
-     1 MA  (1.4 MA)
- Toroidal Field
-    0.3 to 0.6 T (≤0.45 T)
- Heating and CD
-    5 MW NBI (5 MW)
-    6 MW HHFW (6 MW)
-    CHI
- Pulse Length
-    ≤ 5 sec (0.5 sec)

## National Spherical Torus Experiment (NSTX)

## U.S. National NSTX Research Team Collaboration and International Research Cooperation

- 	R. Budny, D. Darrow, P. Efthimion, D. Ernst, J. Foley, G. Fu, D. Gates, L. Grisham, N. Gorelenkov, R. Kaita, H. Kugel, K. Hill,
- 	J. Hosea, H. Ji, S. Jardin, D. Johnson, B. LeBlanc, Z. Lin, R. Majeski, J. Manickam, E. Mazzucato, S. Medley, J. Menard, D. Mueller,
- 	M. Okabayashi, H. Park, S. Paul, C.K. Phillips, N. Pomphrey, M. Redi, G. Rewoldt,  A. Rosenberg, C. Skinner, V. Soukhanovskii,
- 	D. Stotler, B. Stratton, H. Takahashi, G. Taylor, R. White, J. Wilson, M. Yamada, S. Zweben
***Oak Ridge National Laboratory:**** *M. Peng, R. Maingi, C. Bush, T. Bigelow, S. Hirshman,* W. Houlberg, M. Menon,* D. Rasmussen,* P. Mioduszewski, P. Ryan, P. Strand, D. Swain, J. Wilgen
***University of Washington:*** R. Raman,* *T. Jarboe*, *B. A. Nelson, A. Redd, D. Orvis, E. Ewig
***Columbia University:***** **S. Sabbagh, F. Paoletti, J. Bialek, G. Navratil, W. Zhu
***General Atomics:***** **J. Ferron, R. Pinsker, M. Schaffer, L. Lao, B. Penaflor, D. Piglowski
***Johns Hopkins University:***** **D. Stutman, M. Finkenthal, B. Blagojevic, R. Vero
***Los Alamos National Laboratory:***** **G. Wurden, R. Maqueda, A. Glasser*
***Lawrence Livermore National Laboratory:***** **G. Porter, M. Rensink, X. Xu, P. Beiersdorfer,* G. Brown*
***UC San Diego:***** **T. Mau, J. Boedo, S. Luckhardt, A. Pigarov,* S. Krasheninnikov*
***UC Davis:***** **N. Luhmann, K. Lee, B. Deng, B. Nathan, H. Lu
***UC Los Angeles:***** **S. Kubota, T. Peebles, M. Gilmore
***Nova Photonics:***** **F. Levinton
***Massachusetts Institute of Technology:***** **A. Bers, P. Bonoli, A. Ram, J. Egedal*
***UC Irvine:***** **W. Heidbrink
***Sandia National Laboratory:***** **M. Ulrickson,* R. Nygren,* W. Wampler*
***Princeton Scientific Instruments:*** J. Lowrance,* S. von Goeler*
***Lodestar:*** J. Myra, D. D’Ippolito
***NYU:***** **C. Cheng*
***University of Maryland:*** W. Dorland*
***Dartmouth University:***** **B. Rogers*
***U.K.,*** ***EURATOM UKAEA Culham: ***A. Sykes, R. Akers, S. Fielding, B. Lloyd, M. Nightingale, G. Voss, H. Wilson
***JAPAN,***** *****Univ. Tokyo:***** **Y. Takase, H. Hayashiya, Y. Ono, S. Shiraiwa; ***Kyushu Tokai Univ.:***** **O. Mitarai; ***Himeji Inst of Science & Technology:***** **M. Nagata; ***Hiroshima Univ.*****: **N. Nishino; ***NIFS.:**** *T. Hayashi; ***Niigata Univ.:**** *A. Ishida; ***Tsukuba Univ.:**** *T. Tamano
***Russian Federation, Ioffe Inst.:***** **V. Gusev, A. Detch, E. Mukhin, M. Petrov, Y. Petrov, N. Sakharov, S. Tolstyakov, Dyachenko, A. Alexeev; ***TRINITI:*** S. Mirnov, I. Semenov,
***Korea,***** *****KBSI:*** N. Na
*______________________________________________*
- 		*In cooperation with DOE OFES Theory, OFES Technology, Astrophysics, or SBIR programs

**U.S. National NSTX Research Team Collaboration and International Research Cooperation**

## Confinement and Transport

- Moved quickly from global confinement to local transport studies
- Local studies require well-calibrated profile diagnostics
- 	(Te, ne, v, Ti, ...)
- Global confinement well-enhanced over L-, and H-modes
- Local transport often shows good ion confinement
- Power balance puzzle – anomalous ion heating?
- How good is classical collisional framework?
- H-mode studies a big part of last run period
- Operational scenarios for accessing H-mode routinely
- L-H threshold studies
- Power threshold 10x that predicted by ITER scaling
- Edge parameters show little ordering with respect to various theories

## Global Confinement Scaling

- 2x

- 1x

- 0.5x

- 2x

- 1x

- 0.5x

-  • Generally similar parametric scalings as higher R/a
- 	- Stronger Ip scaling (thermal + fast ion effect?)
- 	- Enhanced confinement observed
-  • 15-20% discrepancy between external magnetics and
-       kinetic stored energy
- 	- Kinetic EFITs being prepared

- 

## Well-Calibrated Profile Diagnostics Key to Local Analysis

- Ti, V: development of direct background measurement
- Required for time-dependent measurements
- Required cross-calibrations performed; analysis techniques being developed

- Observed

- Background

- LeBlanc, R. Bell; Bitter, Hill

- R. Bell

- Fitted line

## Diagnostic Validation (cont’d)

- S. Medley, R. Bell

- Kubota, Gilmore, Peebles (UCLA); LeBlanc, R. Bell

- TS high throughput, small error bars. ne consistent with reflectometry
- Ti: CHERS, NPA consistent with each other

## Generally good agreement between kinetic and magnetic stored energies

- • Kinetic stored energy determined from measured profiles,
-    guiding center calculation with FLR (TRANSP)
- • Magnetic stored energy determined from EFIT with external
-    magnetics (flux loops, Bpol probes)

## Rotation speeds near Mach 1!

*What is role of rotation on equilibrium reconstruction?*

## Power Balance Indicates Electrons as PrimaryLoss Channel

## Ion Thermal Diffusivity Can Be Close to Neoclassical

## Power Balance Is Sometimes Puzzling!

## Astrophysics and observed MHD may hold one clue to the power balance puzzle

*Fredrickson (PRL 2001)*

- Time (sec)

- 103932/103936

- TF

- TF

- neL (1015 cm2)

- Frequency (MHz)

- ne

- Tesla

- •  Being investigated:
-     Compressional
-     Alfven
-     Eigenmodes
- •  Modes excited
-     by fast ions;
-     waves transfer
-     energy to
-     thermal ions

- Beam-driven Compressional Alfven Waves may heat ions on NSTX

- Simulations of compressional Alfven modes give stochastic ion heating.
- e.g.-- with 20 modes centered at half Alfven frequency
- Possible relevance to interpretation of ion-heating on NSTX

*D. Gates, N. Gorelenkov, R. White*
*(PRL, 2001)*

- (keV/s)

## Measured Ti is matched with neoclassical transport and majority of beam power channeled to ions

- • i assumed to be i,Chang-Hinton ~ i, NCLASS
- • 85% of heating power channeled to the ions
- 	- Typical electron:ion heating is 2:1

## Measured and Calculated Neutron Rates Are in Rough

- Agreement

*•* *Fast ion confinement appears to be near classical (L. Roquemore)*

*• **Fast ion loss probes show *_*much*_* lower loss than expected (D. Darrow)*

## Impurity transport rates are small in the core

- After puff, almost no neon penetrates the core until MHD event near 260 ms
- Modelling suggests core diffusivity < 1 m2/s, rivaling neoclassical theory
- Low ion particle transport consistent with low ion thermal transport

- D. Stutman, JHU

*Signals from difference of*
*similar plasmas w/ and w/o neon*

## Long wavelength modes may be suppressed

- Long wavelengths: growth rate lower than **E**x**B** shear rate
- Large associated with ion thermal transport
- Short wavelengths: growth rates large
- Responsible for electron thermal transport?
- Non-linear simulations begun

- C. Bourdelle (PPPL), W. Dorland (U. MD)

## Electron ITB formation in HHFW Heated Plasmas

- Te

- ne

- Pressure

## Preliminary

- Core e drops as high Te develops
- Steep gradients due to transport changes, not source
- Conductivity still very high!
- Heating source from HPRT ray tracing (Rosenberg)

- B. LeBlanc

- 105830a03

- 0.1

- 0.3

- 0.5

- 0.7

- 0.9

- r/a

- 1

- 10

- 100

- 1000

- m2/s

## Turbulence theory suggests testable trend for transport experiments

- Conduct a theory experiment: vary Ti/Te, keeping other profiles constant

- ETG

- ITG

- C. Bourdelle

- Shearing rate

- Growth rates

- measured

- Theory indicates:
- High Ti/Te stabilizes ion modes
- High Te/Ti *stabilizes* *electron modes*
*Do we see signatures of this in measured confinement trends?*

## Both ELM-free and ELMy H-modes have been observed in NSTX  

- HHFW and NBI
- Exploration for scenario development, transport, and boundary physics

- ELMing

- ELM-free

- EBW mode conversion efficiency increases at L-H transition when edge density increases

- Mode conversion of EBW from core occurs in scrapeoff in H mode
- Measured EBW conversion efficiency agrees relatively well with calculated values
- On CDX-U, limiter inserted to control Ln demonstrates conversion efficiency can be controlled
- Possible tool for plasma startup, island healing
- Joint research with U. Wisconsin

## H-Modes with RF

- LSN
- Lower current (350 - 500 kA)
- He and D
- ELMy, ELM-free
- p = 1 observed
- Large bootstrap?
- Large dip in surface voltage

## Transitions to H mode state exhibit typical signatures but high power thresholds

*Visible light, false color*

*Maingi, Bush (ORNL); LeBlanc; PRL (to be published)*

*Fast camera: Maqueda (LANL)*

- NBI: Power required > 10x that predicted from empircal scaling laws:
- Strong magnetic shear?
- Poloidal damping? Wall neutrals?

*Movie not available*

## Imaging of edge reveals qualitative differences in H- and L-mode turbulence

- He0 emission observed with a fast-framing, digital, visible camera
- 1000 frames/sec, 10 s exposure each frame

**Los Alamos**
- NATIONAL LABORATORY

*Maqueda, LANL; Zweben*

*Movie not available*

## Theory indicates complex turbulent structures may exist in NSTX edge

- 2-fluid,3D Braginskii equation code

*Xu (LLNL)*

## MHD: beta-limiting

- Range of operations
- Towards assessing physics of RWM’s
- Locked mode coils/error field detection
- NTM’s
- Current driven kinks

## Profile variations have been performed to map out operational space for stability studies

- Maximum N increases, then saturates with increased current profile peaking
- Fast beta collapses observed at all values of li
- High pressure peaking
- Beta saturation coincident with tearing activity at higher N, p ~ 0.45

- N = 4*l**i*

## Candidates for RWM have a fast collapse when rotation frequency is below a small value

- “Critical” rotation frequency < 2 kHz at the plasma core at time of collapse, consistent with DIII-D observations of RWM and theory
- Rotation decrease occurs across the entire plasma (preliminary CHERS analysis)

## USXR data shows a mode structure resembling a global kink in RWM candidate shots

- RWM Candidate
- Fast collapse
- No core islands  and no 1/1 mode before reconnection event (RE)
- More likely a kink mode onset

- D. Stutman

- 1/1

*midplane*

*down*

- RE

*up*

- t (s)

- t (s)

- Plasma with island activity
- Reconnection event leads to 1/1 mode at 237 ms
- Less likely a kink mode onset

- 106168

- 106169

- Wall-plasma gap scan, B r and rotation measurements reveal clues regarding RWM

- Smaller gap
- 	 longer time between start of rapid growth phase of Br and beta collapse
-  stronger wall mode interaction
- With clear tearing activity, no observed Br precursor to collapse

*At least two possible complications*
- Theory suggests Br largest on inboard side for these N’s,  weaker wall interactions compared to DIII-D
-  Locked mode coils  large field error from PF 5

- Error field is localized to lower PF5 coil
- 30-50 Gauss *n*=1 error field for typical PF5 current and 0 cm gap
- 20-30 Gauss peak error field for 10cm gap

- 10cm gap

- 0 cm gap

******=1.8 plasma**
** ****boundary**

**Gauss**

- PF5 is apparently responsible for a large *n*=1 field error

- Slow-growing, -limiting internal modes are likely NTM’s

- Coincident with saturation of beta
- Limits p ~ 0.4
- Modes appear near crit
- Further analysis required

*D. Gates, E. Fredrickson*

## Co-Axial Helicity Injection (CHI) Generates Toroidal Current Non-Inductively

- • Inject poloidal current on open
-    field lines in lower divertor
- • Plasma moves up into main chamber
- • Toroidal current develops to maintain
-    force-free configuration
- • Flux surfaces may close through
-    magnetic reconnection

- Plasma Startup w/CHI

- R. Raman

- High currents, strong MHD observed in CHI start-up studies

- Up to 390 kA of toroidal current was produced, with 14 times current multiplication
- Discharges sustained for 330 ms
- Strong *n* = 1 oscillations observed

- Spectroscopy used to measure Ti and edge plasma rotation in CHI plasmas

- Direction of rotation is clockwise (same as HIT-II)
- Ti ~ 30 - 50 eV, midplane view
- V ~ -10 to -20 km/s

- Flux closure is being assessed via further analysis of magnetics and spectroscopy

- MFIT consistently shows modest closed flux regions when CHI-driven current is sufficiently high
- Not seen in absence of n=1 MHD
- Encouraging, but is not proof of closed mean-field surfaces

- USXR emission increases at the higher CHI currents

*Schaffer, GA*

*Stutman, JHU*

- Transport: confinement picture intriguing and potentially important
- New heating physics specific to the ST?
- Electron thermal channel may dominate
- Edge turbulence measurements made; comparison to modelling begun
- HHFW: effective heating in all regimes
- Fast ion interactions observed
- CD studies begun; no obvious signatures of HHFW-CD yet
- MHD: Beta-limiting modes being investigated
- RWM studies underway: well along towards ‘02 Milestone
- Importance of locked mode uncovered
- NTM’s, current-driven kinks both being studied

## Physics central to the ST mission is being investigated (con’t)

- Boundary physics extended to heat and particle flux studies
- Comparisons to modelling begun
- Enabling techniques extended: plasma boronization
- H mode power threshold established for one condition: ~ 10x usual scaling
- CHI research produced record toroidal current, observed critical MHD activity
- Ti of 30 - 50 eV measured
- MFIT analysis consistent with closed flux, not conclusive
- Absorber arcs reduced, but continue to be an issue
- First CHI-to-ohmic, ohmic-to-CHI experiments performed; technical exploration stage