# Presentation: 590954

## Heat flux amelioration in highly shaped plasmas in NSTX

**R. Raman****4****, R. E. Bell****3****, C. E. Bush****2****, R. Kaita****3****, H. W. Kugel****3****, **
_**B. P. LeBlanc**_**3****, S. F. Paul****3****, A. L. Roquemore****3****, and the NSTX Team**
***1 ******Lawrence Livermore National Laboratory, Livermore, CA***
***2 ******Oak Ridge National Laboratory, Oak Ridge, TN***
***3 ******Princeton Plasma Physics Laboratory, Princeton, NJ****** ***
***4 ******University of Washington, Seattle, WA***
**Poster P2.023**

**Heat flux amelioration in highly shaped plasmas in NSTX**

**Supported by**

## Summary

- Steady-state divertor heat flux amelioration in low aspect ratio geometry is studied in 0.8-1.2 MA 4-6 MW NBI **highly shaped**      H-mode plasmas in NSTX
- Significant (x 2-3) reduction in divertor peak heat flux is achieved by using **high flux expansion divertor** (in comparison with low  lower single null configuration)
- Divertor peak heat flux is reduced further by x 2-3 using **partially detached divertor regime** with divertor D2 injection

**in low **
****
**shape**

**in high**
** ******
**shape**

**Partial Detachment **
**in high ********shape**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Goal: develop steady-state divertor heat load mitigation scenarios in a large Spherical Torus

**Divertor heat flux mitigation solutions:**
- Poloidal flux expansion at outer strike point
- Strike point sweeping
- Radiative divertor: outer SOL in high-recycling
- 	regime with enhanced radiation at divertor plate
- Dissipative divertor (detachment)
**Goal - study divertor heat flux reduction and detachment**
- in lower single null ****** = 0.4-0.5, ******** = 1.8-2.0**
- 	configuration with divertor **D****2**** injection**
- in lower single null ****** = 0.7-0.8, ******** = 2.2-2.5**
- 	configuration by **flux expansion**
- in lower single null ****** = 0.7-0.8, ******** = 2.2-2.5**
- 	configuration with divertor **D****2**** injection**
**Study ***********q***** scaling with *****I******p***** and *****P******NBI***** - R. Maingi, Poster P2.020**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Is divertor physics different at low aspect ratio?

**NSTX divertor**
- Open geometry enables much flexibility
- One horizontal target & one 24o tilted target plate,
- Graphite 2.5-5.0 cm thick tiles
- No active pumping (recently experimenting with lithium coatings)
- Typical divertor tile temperature in ~ 1 s NSTX pulses *T* < 300 C. Engineering limit is *T *= 1200 C. Long pulses will require steady-state heat flux mitigation solutions
*q**out* < 10 MW/m2, P/R < 9
**Low aspect ratio configuration**
- small divertor volume
- small plasma wetted area
- high *q**II*
- short connection length *L**II*
- High SOL mirror ratio *M=|B**min**| / |B**max**|*
- SOL area factor: *A**out** > A**in*

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Radiative divertor in low  LSN configuration reduced peak heat flux by up to 70 %

**Radiative divertor in low ******** LSN configuration reduced peak heat flux by up to 70 %**

**Inner wall**
**gas puff**

**Radiating**
**lower**
**divertor**

- Outer peak heat flux reduced
- 	by x 2-5, but no sign of recombination
- X-point MARFE develops during gas injection
- At highest D2 puffing rate D/D ratio at strike point transiently increases
**Generally compatible with high performance and H-mode confinement**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Partially detached divertor in low  LSN configuration incompatible with long H-modes

- High D2 injection rate ( < 400 Torr l / s)
- Core plasma parameters degrade quickly, X-point MARFE develops
**Peak heat flux reduced  by x 4-5**
- Volume recombination in outer divertor

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Significant peak heat flux reduction can be achieved by poloidal flux expansion 

- High-performance long-pulse H-mode plasmas (Menard OV-2-4, IAEA FEC 2006)
- Poloidal flux expansion at OSP 16-24
- Inner strike point detached, outer strike point on horizontal target - attached

-  CHI
- gap

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## More favorable scaling of peak heat flux with input power is obtained in higher  plasmas 

-  Scaling sensitive to fueling location and gas injection rate
-  *P**SOL* is determined from measured and TRANSP-calculated quantities as

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Study radiative divertor and detachment in highly shaped plasmas

- Highly shaped plasmas mean high performance
- higher bootstrap current fraction at higher 
- thermal confinement scales as 
- higher edge pressure gradient with higher 
- small ELM regimes at higher 
- Highly shaped plasmas have high flux expansion divertor
- Reduced peak heat flux - favorable for detachment threshold
- Higher neutral penetration (?)
- Higher “plasma plugging” (?) - Important in open divertor
- Higher isothermal radiating volume (?)
- Sputtering yield (?)
- Parallel and poloidal connection lengths are shortest

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Proper diagnostic configuration and rtEFIT shape control were essential for success of experiment

- rtEFIT control was used for *drsep*, *R**OSP*, and gap control

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Partial strike point detachment is obtained using D2 divertor injection 

- D2 injected for 200 ms
- Peak heat flux reduced x 2-3
- Core confinement and performance were not affected
- Core carbon concentration reduced x 1.5
- Clear signs of divertor radiated power increase and neutral pressure increase
- No signs of X-point MARFE (?)
- Volume recombination in outer divertor
- Same PDD picture in 1 MA, 4 MW; **1 MA, 6 MW;** 1.2 MA, 6 MW cases

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Divertor peak heat flux significantly reduced during partial detachment phase

- Peak heat flux reduced by 2-3
- Profileq changes from 5-10 to 20-20 cm
- PDD zone is 10-15 cm (0.01-0.02 in )

**1 MA, 6 MW **

**1 MA, 4 MW **

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Divertor D profiles are indicative of partial detachment

- Camera signal saturated during PDD (indication of X-point MARFE ?)
- Increase consistent with low *T**e*, high *n**e* plasmas
- Outside of PDD zone - high-recycling SOL
- PDD zone is 10-15 cm (0.01-0.02 in )

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Significant volume recombination is observed at outer strike point during detachment

- Balmer series lines 2-6...11
- Sign of strong volume recombination
- Stark broadening yields *n**e*
- Intensities determined by Saha-Boltzman level population distribution
*n**e** *~ 2-3 x 1020 m-3 in inner divertor (bottom panel)
*n**e** *~ 4-6 x 1020 m-3 at outer strike point (top panel)
*T**e** *< 1.5 eV
- Interim calibration used
**Soukhanovskii et. al., Rev. Sci. Instrum. 77, 10F127 (2006)**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Large momentum and power losses are needed for divertor detachment according to 2PM-L

-  Two point model with losses
-  *f**p*, *f**m* scanned, *f**cond*=0.9
-  *n**u*, *q**II*, *L**c* from experiment

**Detachment**

*L**x* =6-8 m, *q**II* = 25-30 MW/m2 - low  configuration
*L**x* =2-3 m - high  configuration

- P. C. Stangeby. The plasma boundary of
- Magnetic Fusion Devices. IoP, Bristol, 2000.

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Magnetic field line path is different in highly and low shaped plasmas

- Figure courtesy of Dr M. Bell (PPPL)

**low **
****
**shape**

**high**
** ******
**shape**

- Different power and momentum losses
- Different sensitivity to MARFE onset

**high**
** ******
**shape**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## UEDGE modeling guided detachment experiments

**UEDGE modeling guided detachment experiments**

- Model divertor conditions *vs* Pin, nedge with UEDGE to guide experiment
- Generic low  LSN equilibrium used
- Diffusive transport model
- Impurities (carbon) included
- Outer midplane *n**e**, T**e* profiles matched, D and IRTV not matched

**G. Porter, N. Wolf**

- Parallel momentum and power balance:

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Why is it difficult to obtain detachment in low  configuration?

**Why is it difficult to obtain detachment in low ******** configuration?**

- NSTX
- SOL

- Connection length decreases to very short values within radial distance of        1-3 cm (both midplane to plate and        X-point to plate)
-  SOL temperature 10-40 eV (rather low)
-  Weak dTe/dsII in high-recycling outer SOL
-  Carbon cooling rate max at *T**e* < 10 eV
- Recombination time:
- 	***rec* = 1./(*n**e** R**rec*) ~ 1−10 ms at *T**e* =1.3 eV
- 	Ion divertor residence time:
- 	***ion* = *L**d*/*v**ion* ~ 0.8 ms (with *v**ion* ~ 104 m/s)
-  Open divertor geometry - high detachment threshold is expected
-  Neutral compression ratio is 5-10 (low)

**Outer**
**midplane**

**Coronal**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Sign-up sheet

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Abstract

- References
- [1] V. A. Soukhanovskii et al., Divertor Heat Flux Reduction and Detachment in NSTX, Paper EX/P4-28, IAEA FEC 2006, Chengdu, China, 2006
- [2] J. E. Menard et al., Recent Physics Results from NSTX, Paper OV/2-4, IAEA FEC 2006, Chengdu, China, 2006

**Abstract**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## NSTX reference data

**NSTX eng. and plasma parameters**
- 	R = 0.85 m, a = 0.67 m, A = R/a > 1.27, PNBI < 7 MW, PHHFW < 6 MW, Bt < 0.6 T
**NSTX fueling****	****	****	****	****	****_x0005_*** 1 Torr l/s = 7e19 s-1
- Gas injection: low field side (LFS, top + side) and high field side (HFS, midplane + shoulder).  D2, He, injected at  S = 20 - 80 Torr l /s.
- Neutral beam injection system: three beams,  80 - 100 keV, 0.8-7 MW,
- 	fueling rate: S < 4 Torr l / s
- Supersonic gas injection: S = 30 - 65 Torr l / s
**NSTX wall conditioning**
- Between shots He GDC, He conditioning plasmas
- TMB and  Plasma TMB
- Bake out at 350o C
- Li coatings deposited by Li evaporator
- Li pellet injector
**NSTX pumping**
- Turbomolecular pump (3400 l / s)
- NBI cryopump ( 50000 l / s, in NBI plasmas only)
- Conditioned walls, Li coatings
**Plasma Facing Components**
- ATJ graphite tiles on divertor and passive  plates
- ATJ and CFC tiles on center stack
- Thickness 1” and 2”

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## NSTX diagnostic set enables divertor studies

**IRTV**: two Indigo Alpha 160 x 128 pixel microbolometer cameras, 7-13 mm range,  30 ms frame rate
**D********, D********, C III filtered cameras**: four Dalsa
- 	1 x 2048 pixel CCDs, filter FWHM 10-15 A, frame rate 0.2 - 1 ms
**Neutral pressure gauges**: four micro-ion gauges on top and at midplane, two Penning gauges in lower and upper divertor,  time response 5-10 ms
**High-resolution spectrometer** (“VIPS 2”): ARC Spectro-Pro 500i, three input fibers (channels), time response 15-30 ms,
- 	FWHM > 0.6 A
**Bolometry**: midplane (AXUV radiometer array), divertor - ASDEX-type four channel bolometer, time response 20 ms
**Langmuir probes**: midplane - fast probe tile LPs - *I**sat*, *T**e  *measurements
- Midplane **Multi-point Thomson** scattering with 2-4 points in SOL

**NSTX diagnostic set enables divertor studies**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## NSTX divertor regimes

- Heat flux asymmetry always *q**out**/q**in* > 1, typically *q**out**/q**in*= 2-4. Typical peak heat flux *q**in** *<  0.5-1.0 MW/m2, *q**out** *<  2-7 MW/m2 in 2-6 MW NBI-heated plasmas
- Recycling in-out asymmetry up to 15 from divertor D profiles
- Divertor D observed in inner divertor only, typical ratio D/ D about 0.020 - 0.12 - sign of volume recombination
- High divertor neutral pressure (0.1-0.2 mTorr), neutral compression ratio is 5-10 (open divertor)
- Inner divertor leg is naturally detached throughout most of operational space, similarly to conventional tokamak divertors operating w/o pumping. Outer divertor leg is always attached, being in sheath-limited and high-recycling regime up to *n**e** < n**G** *

**ISP**

**OSP**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## High ne, low Te inferred from Balmer and Paschen emission lines measured in inner divertor leg

-   In dense low temperature plasmas 3-body recombination rate is high - Lyman (FUV), Balmer (UV), Paschen (NIR) series lines are prominent
-  Stark broadening due to plasma electron and ion statistical microfield
* **n**e *= 0.6-6 x 1020 m-3 from Stark broadening (Model Microfield Method calculations)    	**Soukhanovskii et. al., Rev. Sci. Instrum. 77, 10F127 (2006)**
-  *T**e *= 0.3-1.3 eV from line intensity ratios (Saha-Boltzman population distribution, ADAS data)

**Upper traces - attached, lower traces - detached**

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland

## Open divertor geometry determines observed midplane and PFR pressure trends at low 

- In reference discharges, *n**u* independent of *P**mp*, but a strong linear function of *P**PFR*
- X-point MARFE critical PFR pressure is 0.5-0.6 mTorr
- Reference discharges never reach PFR critical pressure
- PDD discharges reach MARFE onset PFR pressure faster than RD discharges
*P**mp* similar in ref. and RD discharges
*P**mp** *higher in PDD discharges (stronger gas puffing)

- V. A. Soukhanovskii, EPS 2007, 3 July 2007, Warsaw, Poland