# Presentation: 109885

## Divertor heat flux reduction and detachment in NSTX

_**V. A. Soukhanovskii**_**1****,  R. Maingi****2****, R. Raman****3****, C. E. Bush****2****,**
**R. E. Bell****4****, R. Kaita****4****, H. W. Kugel****4****, C. J. Lasnier****1****, **
**B. P. LeBlanc****4****, J. Menard****4****, S. F. Paul****4****, A. L. Roquemore****4****, **
**and the NSTX Team**
***1 ******Lawrence Livermore National Laboratory, Livermore, CA***
***2 ******Oak Ridge National Laboratory, Oak Ridge, TN***
***3 ******University of Washington, Seattle, WA***
***4 ******Princeton Plasma Physics Laboratory, Princeton, NJ***
**Poster EX-P4-28**

**Divertor heat flux reduction and detachment in NSTX**

**Supported by**

## Abstract

- This research was supported by the U.S. Department of Energy under Contracts W-7405-Eng-48,            DE-AC02-76CH03073,  DE-AC05-00OR22725,  and  W-7405-ENG-36.

**Acknowledgements**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

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

**Low aspect ratio magnetic configuration leads to**
- 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*
**Divertor heat flux mitigation solutions:**
- Poloidal flux expansion at outer strike point (OSP)
- Strike point sweeping
- Radiative divertor: outer SOL in high-recycling
- 	regime with enhanced radiation at divertor plate
- Dissipative divertor (detachment)
**These scenarios must be compatible with good core plasma performance (H-mode confinement, MHD, ELM regime, density)**

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

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Outline

**NSTX background: parameters, divertor heat and particle fluxes, divertor asymmetries **
**Naturally occurring divertor regimes in NSTX**
**Study of outer strike point (OSP) heat flux reduction and detachment with D****2 ****injection**
**radiative divertor (RD) regime **
**	****compatible with high performance **
**	****H-mode plasmas**
**partially detached divertor (PDD) **
**	****regime**
**OSP heat flux reduction by flux expansion**
**Summary**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Divertor heat flux mitigation in NSTX: background

**NSTX divertor**
- open, horizontal targets, graphite tiles, no active pumping
- 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
**Goal - study divertor heat flux reduction and detachment**
- in lower single null ****** = 0.4-0.5, ******** = 1.8-2.0**
- 	configuration with **D****2**** puffing**
- in lower single null ****** = 0.7-0.8, ******** = 2.2-2.5**
- 	configuration by **flux expansion**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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
**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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## A range of core and divertor conditions obtained at various divertor D2 puffing rates

|  | 1.85 |
| --- | --- |
|  | 0.4 |
| Drsep (cm) | 1.0-1.5 |
| qedge | 13 |
| LII, inner (m) | 8.0 |
| LII, outer (m)  | 6.0 |
| M 
(Mirror ratio) | 5.0 |
| f inner
(Flux expansion) | 2-3 |
| f outer | 2-3 |

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Radiative divertor obtained using moderate rate divertor gas puffing; OSP peak heat flux reduced

**Radiative divertor regime**
- Obtained by steady-state D2 injection in PFR or at ISP at
- 	80-160 Torr l /s
- OSP heat flux reduced by 2-5
- ISP heat flux practically unaffected
- X-point (“divertor”) MARFE developed during gas puffing
- No sign of volume recombination at OSP until later phase of gas injection
- Divertor bolometer signal increased from 10-15 W/m2 to
- 	20-30 W/m2
- Generally compatible with H-mode
- Gas puffing eventually caused confinement degradation, locked mode, large MHD modes and low *m,n* modes
- RD is suitable for heat flux reduction in long high-performance H-mode plasmas

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Core confinement properties are unaffected by moderate flow rate divertor D2 injection

**Core confinement properties are unaffected by moderate flow rate divertor D****2**** injection**

- Core plasma parameters practically unaffected
- Gas puffing eventually leads X-point MARFE onset
- Discharge duration is limited by -limiting instability (both reference and RD)

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Radiative Divertor leads to OSP peak heat flux reduction by 2-5, development of X-point MARFE

**Radiative Divertor leads to OSP peak heat flux reduction by 2-5, development of X-point MARFE**

- Outer peak heat flux reduced
- 	by x 2-5, but no sign of recombination
- X-point MARFE develops during gas puffing
- At highest D2 puffing rate D/D ratio at OSP transiently increases

**Plasma TV:**
**D******** emission,**
**similar RD**
**discharge**

- 1D CCD
- camera
- viewing
- geometry

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

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

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## OSP partial detachment obtained using pulsed divertor D2 injection at high flow rates 

**Inner**

**Inner**

**Outer**

**Outer**

**Partially detached divertor regime (OSP detached, outer SOL attached)**
- Obtained by pulsed D2 injection at > 200 T l /s at ISP
- OSP heat flux reduced by 4-5
- ISP heat flux did not change
- Broadened D and D brightness profiles, increases outer leg D / D ratio - volume recombination at OSP
- Divertor bolometer signal increased from 10-15 W/m2 to 15-20 W/m2
- Correlation of *I**sat* drop in Langmuir Probe close to OSP and D / D  increase in OSP region
- Detachment extent: 2-3 cm at OSP, detached only during gas pulse
- H-L transition within 20-100 ms

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Core confinement properties are affected by high flow rate deuterium injection in PDD

**Core confinement properties are affected by high flow rate deuterium injection in PDD**

- Core plasma parameters degrade quickly
- X-point MARFE develops within 20-30 ms
- Discharge duration is limited by large MHD modes, confinement degradation

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## OSP partial detachment evident from peak qout reduction and onset of volume recombination

**Pulsed D****2**** injection from ISP divertor region**
- Decreased peak qout by 4-5, with peak shift outward by up to 3 cm
- Broadened D and D brightness profiles, increased outer leg D / D ratio at OSP
- Divertor C II profile also broadened during gas pulses
- X-point MARFE onset during gas pulses

**OSP partial detachment evident from peak *****q******out***** ****reduction and onset of volume recombination**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Langmuir probe Isat and D/D brightness ratio at OSP indicative of partial detachment

**Langmuir probe *****I******sat***** and D********/D******** brightness ratio at OSP indicative of partial detachment**

**Langmuir**
**probe**

-  Langmuir Probe *I**sat** *drop correlates with D/D increase at separatrix
-  X-point MARFE develops during gas pulses, disappears in between

**Outer**

**Plasma TV:**
**D********emission,**
**PDD discharge**

**before gas**
**pulse**

**X-point **
**MARFE**
**during gas**
**pulse**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

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

- High-performance long-pulse H-mode plasmas (Menard OV-2-4)
- Poloidal flux expansion at OSP 20-25
- ISP on vertical target (detached), OSP on horizontal target
- OSP detachment threshold to be investigated

-  CHI
- gap

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

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

-  Scaling depends on fueling location and gas injection rate
-  *P**SOL* is determined from measured and TRANSP-calcualted quantities as

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## 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**c* =20 m, *q**II* = 25-30 MW/m2

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Why is it difficult to obtain OSP detachment?

**Why is it difficult to obtain OSP detachment?**

- 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Observed midplane and PFR pressure trends are due to open divertor geometry 

- 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, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Summary and Future work

- Divertor heat flux reduction and detachment in low aspect ratio geometry are studied in high input power H-mode plasmas in NSTX
- OSP peak heat flux was reduced x 2-5 using high recycling radiative divertor and partially detached divertor regimes in low  LSN 4 MW H-mode plasmas with divertor D2 puffing
- Significant (x 2-3) reduction in peak divertor heat flux has been achieved in 4 MW high  LSN configuration using high flux expansion
- Future work will focus on
- dissipative divertor regimes with **CD****4**** puffing**
- dissipative or radiative divertor regimes in **high ****** H-mode plasmas relevant to **Component Test Facility**

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China

## Sign-up sheet

- V. A. Soukhanovskii, IAEA FEC,  Poster EX-P4-28, 16-21 October 2006, Chengdu, China