# Presentation: 041586

## Progress on NSTX towards steady state at low aspect ratio 

**Progress on NSTX towards steady state at low aspect ratio***** ***

- D. A. Gates, Princeton Plasma Physics Laboratory
*on behalf of the* NSTX Research Team

** Work supported by US DOE Contract No. DE-AC02-76CH03073 *

- ST Requires high bootstrap fraction, *f**bs*, simultaneous with high ***t*

- High *f**bs* and high ***t** * competing requirements (at fixed shape and ***N*)
- Progress for ST and advanced tokamak given by ***sus** **** f**bs****t** ~ S****N**2*        *[S = q**95**(I**p**/(aB**t**))]*
- If ***Nmax** = C**Troyon*, then only shape improves ***sus*
-  ***sus* increases linearly with increasing *S [S = q**95**(I**p**/(aB**t**))]*
- Component Test Facility requires ***t* ~ 20% and *f**bs* ~ 50%, ***sus* = 10%
- ARIES-ST requires ***t* ~ 40% and *f**bs* ~ 90%
- Optimized shaping with new PF coils for high triangularity and elongation
- NSTX has achieved record values of elongation and shape factor
- Leads directly to record values of the ***sus* for the ST
- For NSTX 100% non-inductive operation with ***N* ~ 7 only with strong shaping

- New inboard divertor coils increase accessibility of high-triangularity, high-elongation shapes

- Highest  now obtained at highest ≈0.8  S  q95 IP/aBT = 41 MA/m·T
- Small (Type V) ELM regime recovered at high  > 2.5 with new coils
- Previously observed onset of large ELM-like events when  > 2.2

*D. Gates , J. Menard *

- Highest elongation =3.0 (transient)

- Sustained elongation =2.7 (0.1s)

- Record pulse-lengths achieved at high current by operating with sustained H-mode

- H-mode with small ELMs  lower flux consumption, slow density rise

- Long duration discharges reach ~70% non-inductive current

- TRANSP model agrees with measured neutron rate during high-phase
- Model includes anomalous fast ion diffusion during later phase when low-m MHD activity is present
- 85% of non-inductive current is p-driven
- Bootstrap + Diamagnetic + Pfirsch-Schlüter
- 1/1 mode onset causes ** drop, fast ion diffusion (Menard, PRL)

- Expected performance improvements observed as shaping has increased

- Reduction in control system latency increases elongation (2004)

- Plasma shaping enhancements

- Upgrade of PF1A coil enables simultaneous achievement of high ** and **

- 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

- Implementation of rtEFIT improves shape control reproducibility

- Shot 121241 (record **)
- All shots 2001-2003
- All shots 2004-2006

- 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

** = 3, **= 0.8, S ~ 41

- 2006

- Pulse averaged approximate sus versus S (S is averaged over the same time window as sus)

***N**H89* vs. ***pulse**/****E*

- Pulse averaged t versus pulse length
-  Data are sorted by year and by S

- 2001-2004
- 2005-2006

- External kink mode ultimate limit on ***t *-long pulse discharges above no-wall limit

- Troyon diagram showing ***tmax** vs. I**p**/aB**t*

- PEST eigenfunction Shot 117707

- Time history of ***t* and n=1 (no-wall) kink mode growth rate, **, for Shot 117707, calculated by PEST using LRDFIT equilibrium including MSE

- Alfven

*** **%*

- Each point in the above plots represents an EFIT equilibrium reconstruction
- Data span entire NSTX database and are filtered against rapid plasma motion

- Effect of modifications provide clear increase in NSTX operating space

## Encouraging results from both EBW emission and HHFW current drive experiments

- Long pulse discharges have elevated q(0) without low frequency MHD modes

***Experiment***
***(116313)***

***Target***

- The time history of shot 120001 showing:
- 1) the components of non-inductive current as indicated in the plot legend,
- 2) The plasma current (black) and the surface voltage (red),
- 3) the reconstructed *q(0)* (black) and *q**min* (red) and q(0) as determined by a TRANSP magnetic diffusion calculation (blue)
- 4)  ***t* (black) and the neutral beam power (red). The profiles in preceding figure are calculated over the time interval indicated in green.

- A spectrogram of magnetic fluctuations as measured by a Mirnov coil for shot 120001.
- The colors represent toroidal mode numbers as indicated in the legend.
- Notice the period of time after 0.6s where there are only small amplitude high-n MHD modes present.

- Measured

- EBW uses efficient Ohkhawa current drive
- Data on efficiency of EBW emission from identical plasmas for which the EBW antenna pointing angle is varied.
- The colors represent measured efficiencies of
- 81-90% (red)
- 71-80% (orange),
- 61-70% (green)
- 51-60% (blue)
- 41-50% (purple),
- 31-40% (black).
- The ellipses are contours of theoretically predicted emission efficiency

- Measured EBW emission angle matches
- theoretical predictions

-  Achieved high Te= 3.6keV in current drive phasing for first time using high BT = 5.5kG
- Improvement consistent with reduced PDI and surface waves expected at higher BT
- Expect similar improvements from **higher k**||
- Useful for HHFW-CD during ramp-up
- Useful for HHFW heating at high-

- HHFW heats efficiently in current drive phasing

- Steady state scenario predicted with 100% non-inductive current using only NBICD and pressure driven currents

- Plasma shape and profiles for predicted 100% non-inductive scenario

- IP  = 750kA
- N < 5.6, P < 1.5, T < 17%
- li   = 0.6, qmin=1.3, BT=4.5kG
-  = 2.3, X-L = 0.75, q*=3.9

***Present high-f******NI****** long-pulse H-modes:***

***Target scenario:***

***Inductive current drive is replaced by:******   ******	******Higher J******NBI****** from higher T******e***
***	******	******	******	******	******Higher J******BS****** from higher ************P-thermal***

- Need 60% increase in T, 25% decrease in ne
**Lithium for higher ********E**** & density control?**
**20% increase in thermal confinement**
**30% increase in HH****98**
**Core HHFW heating**
- Want q0  qmin  2.4  higher with-wall limit

- Higher  for higher q, P, fBS
- High  for improved kink stability
- (shape parameters already achieved in other discharges)*** ***

- IP  = 700kA
- N = 6.7, P = 2.7, T =15%
- li   = 0.5, qmin = 2.4, BT=5.2kG
-  = 2.6, X-L = 0.85, q*=5.6

- Neoclassical

- 1)

- 2)

- 3)

- 4)

- 2