# Presentation: 170611

## The effect of shaping on plasma performance on NSTX

- Presented by D. A. Gates
**47****th**** annual meeting of the APS – DPP Meeting**
- Oct 24-28, 2005
- Denver, CO

***Culham Sci Ctr***
***U St. Andrews***
***York U***
***Chubu U***
***Fukui U***
***Hiroshima U***
***Hyogo U***
***Kyoto U***
***Kyushu U***
***Kyushu Tokai U***
***NIFS***
***Niigata U***
***U Tokyo***
***JAERI***
***Hebrew U***
***Ioffe Inst***
***RRC Kurchatov Inst***
***TRINITI***
***KBSI***
***KAIST***
***ENEA, Frascati***
***CEA, Cadarache***
***IPP, J******ü******lich***
***IPP, Garching***
***ASCR, Czech Rep***
***U Quebec***

***College W&M***
***Colorado Sch Mines***
***Columbia U***
***Comp-X***
***General Atomics***
***INEL***
***Johns Hopkins U***
***LANL***
***LLNL***
***Lodestar***
***MIT***
***Nova Photonics***
***New York U***
***Old Dominion U***
***ORNL***
***PPPL***
***PSI***
***Princeton U***
***SNL***
***Think Tank, Inc.***
***UC Davis***
***UC Irvine***
***UCLA***
***UCSD***
***U Colorado***
***U Maryland***
***U Rochester***
***U Washington***
***U Wisconsin***

## Plasma shaping an important determining factor for performance of burning plasmas

- Reactor designs aim at the highest elongation that technology can support
- High elongation (**) requirement for AT-like scenarios comes from self-sustainment at high ***t *
- Define figure of merit:		*	**	**	**	**	**	**	**	**	*
- (high ***t* is required to compete with TF ohmic losses)

- 2b

- Shape factor definitions:
*** **= a/R**0*
*** **= b/a*
***top** = (R**top**-R**0**)/a*

- ITER

- ARIES-ST

- CTF

- 2a

*R**0*

*R**top*

- High ** and triangularity (**) modify ELMs
- For NSTX TSC indicates 100% non-inductive operation with ***N** ~ 8 *for strong shaping

**See C. Kessel Invited talk UI2.00004 Fri. 11AM**

## Plasma shaping on NSTX is evolving toward future reactor requirements

- Machine improvements have increased steady state shaping factor

- 2001

- 2005

- 2004

- 2002-3

** = 2.75, **= 0.8, S ~ 37

** = 1.8, ** = 0.6, S ~ 22

** = 2.0, ** = 0.8, S ~ 23

** = 2.3, ** = 0.6, S ~ 27

## Upgrades to the NSTX control system have enabled high 

- Upgrades to the NSTX control system have enabled high **

- System latency was reduced by factor of 4 from 3ms to 0.75ms
- Have reached maximum ** ~ 2.75
- roughly equal to tokamak record
- High ** achieved in NSTX without internal coils

- Vertical stability space for NSTX. The upper boundary of the data is roughly a curve of constant vertical growth rate.

- Normalized Internal Inductance = li

**

## Inner divertor poloidal field coil modified

- Can now simultaneously achieve high ** and **

- Modified PF1A Coil

- Old PF1A Coil

****

****

***2004***

***2005***

## Real-time EFIT enables precise boundary control

- Real-time equilibrium reconstruction used for shape control on NSTX
- Collaboration with General Atomics
- Longest 1MA discharge on NSTX made using rtEFIT/isoflux control
- Precise control makes detailed shape variations possible - important research tool

- Overlay of EFIT boundaries from shots 117707 and 117814 between 0.3 and 0.9s

- ~3cm includes MHD perturbations

- Control points

- Boundary overlay time window

## Plasma performance improves measurably with increased shaping

- Shaping is associated with both increased time averaged ***t** (**** <****t**>)* and increased pulse length
- Pulse length has been extended to ~50****E* while maintaining high confinement and N
- N*H89 saturates with pulse extension, similar to tokamak performance

- <***t*> versus pulse duration (***t* is averaged over ***flat*) sorted by shape factor *S **** q**95***I/aB**t*

## Sustained  scales with increasing 

- TRANSP calculations verify *f**bs* scales according to approximate relationship
- Define approximate expression for sustained **
- Approximate expression shows expected scaling with plasma elongation
- Gives confidence in scaling to larger devices at higher **

- Pulse averaged approximate ***sus*
- versus *1+****2* (** is averaged over
- the same time window as* ****sus*)

## ELM characteristics improved in long pulse discharges at high 

- ELM characteristics improved in long pulse discharges at high **

- Systematic study of dependence of divertor geometry on ELM characteristics is required (planned for 2006)
- Does improved shaping improve edge stability?
- How do vessel conditions affect ELM?
- Is ELM change due to reduced beam power?

- Shot 112581, 2004

- Shot 117707, 2005

- Shot 112581

- Shot 117707

- 1.0 0.0

## Changes in X-point balance affect ELM characteristics

- Very small changes in the plasma boundary reproducibly lead to large differences in ELM behavior
- ELMs have a major impact on plasma performance, controlling them is crucial
- Precise plasma control provides an important tool for controlling ELMs - highly ITER relevant

***r**sep* (mm) for 117424 (black)
- and 117425 (red)

- Shot 117424

- Shot 117425

- Shot 117424

- Shot 117425

***r**sep* is the radial separation of the flux surfaces which pass through the x-points measured at the outboard midplane

- 1.0 0.0

## Magnetic balance associated with improved confinement regime

- Improved confinement regime is observed when magnetic balance is achieved during the plasma current ramp
- Edge temperature pedestals increases dramatically
- H89P ~ 2.6
- MSE indicates very low central current density
- current hole
- Plasma terminates when N ~4.5

- MSE pitch angle
- t=0.2s

- Current hole

- H-mode

- EC-mode

- EC-Mode

## Improved ELM characteristics contribute to increased bootstrap fraction

- Same *q**95* as CTF
- Long pulse discharge lasting ~1.6s
- Sustains for ~50***E* and ~ 6***R*
- Achieves 65% non-inductive fraction with 85% of non-inductive current pressure driven
- Onset of 1/1 mode reduces confinement, *q(0)*  1
- Pulse length determined by toroidal field coil temperature limits

- Shot 116318

- Onset of 1/1 mode

***R **(=****0****nc**a**2**/6)*

## 20% t sustained for ~ 2R

- 20% t sustained for ~ 2R

- Similar *q**95* to ARIES-ST
- Long pulse discharge with ~50% non-inductive current, ~80% pressure driven
- Reproducible performance
- Plasma is not MHD limited
- limited by available OH flux
- Higher current (1.4MA) discharge achieves record stored energy ~430kJ

- Shot 117707

***R*

## Double null plasma reduces divertor power loading by factor of ~ 2
Flux expansion varied from 2 to 10 with shaping capability
Flux expansion further reduces power loading by factor of ~3

- Double null plasma reduces divertor power loading by factor of ~ 2
- Flux expansion varied from 2 to 10 with shaping capability
- Flux expansion further reduces power loading by factor of ~3

## Boundary shaping affects all aspects of plasma performance

- Improvements to NSTX shape control capability has achieved:
- Record plasma shape factor ~37 MA/(mTesla)
- ST record plasma elongation ** = 2.75
- ST record stored energy 430kJ
- Simultaneously achieved at CTF *q**95*
- Non-inductive current fraction ~ 65%
- Pressure driven current fraction ~ 50%
- 1.6s pulse length *~ *6***R **~ *50***E*
- At ARIES-ST *q**95* achieved 20% ~ ***t* for ~ *2****R*
- Precise boundary control has enabled access to important ITER relevant research on ELM behavior
- New high confinement regime discovered with current hole with true double null during current ramp
- Divertor shape control can change power loading significantly through flux expansion

## Other NSTX talks/posters

- Invited Talks:
- S. Zweben		CI1b	Mon PM
- R. Maingi		CI1b	Mon PM
- E. Fredrickson	QI1	Thursday 11:30AM
- C. Kessel		UI2	Friday 11:00AM
- Contributed
- Orals			GO3	Tuesday 2-5PM
- Posters			RP1	Thursday 2-5PM