effect of divertor nitrogen seeding on the power exhaust ...€¦ · effect of divertor nitrogen...

Effect of divertor nitrogen seeding on the power exhaust channel width

in Alcator C-Mod

B. LaBombard, D. Brunner, A.Q. Kuang, W. McCarthy, J.L. Terry and the Alcator Team

Presented at the International Conference on Plasma Surface Interactions in Controlled Fusion Devices Princeton University, NJ, USA, 17-22 June 2018

This work was supported by US DoE cooperative agreements DE-SC0014264 and DE-FC02-99ER54512 on Alcator C-Mod, a DoE Office of Science user facility.

Effect of divertor nitrogen seeding on power exhaust channel width in Alcator C-Mod B. LaBombard, PSI 2018

•  Scrape-off layer power channel widths (λq) are projected empirically to be ~0.5 mm in power reactors (Bp ~ 1.2 T), based on data from attached divertor plasmas with low levels of volumetric dissipation.

Question: Does ‘upstream’ power exhaust width depend on conditions in the divertor? – radiation, detachment, electrical disconnection?

•  At high levels of volumetric dissipation and/or partial detached target conditions – electrical connection can be weak or broken

•  Theory indicates that electrical connection to the target can play a role in SOL turbulence: Ø  parallel currents to target can reduce blob

polarization/transport [e.g. O.E. Garcia PoP 2006] Ø  sheath potentials can impose ExB shear in near SOL – regulating turbulence [e.g. F. Halpern NF 2017]

Experiments were performed on C-Mod to examine this question ... •  Does ‘upstream λq’ change under these conditions? T. Eich 2013, D. Brunner 2018

2

new database probe-based sensors

old database IR thermograpic analysis


Experiments enabled by real-time heat flux measurements (via surface thermocouples) to feedback control divertor N2 seeding [1]

Experiment: Systematically vary divertor dissipation (via N2 seeding) in otherwise identical plasmas; study SOL response.

Ramped Tiles

B-field

Surface Thermocouples

Divertor N2 seeding

Scanning Mirror Langmuir Probe

Experiment •  Produce a series of ohmic L-mode plasmas with

constant core plasma conditions •  Systematically lower ‘set point’ of divertor heat flux,

causing divertor to change from sheath-limited to high-recycling to partially detached.

•  Record divertor response with Surface TC array and with a high resolution ‘Rail’ Langmuir probe array [2]

•  Record upstream profiles with Scanning Mirror Langmuir Probe [3]

=> examine SOL response and heat flux widths

[1] Brunner, RSI 87 (2016); [2] Kuang,RSI (2018); [3] LaBombard, PoP 21 (2014)

21 ‘Rail’LangmuirProbes

3


•  Divertor surface heat fluxes reduced by factor of ~10, approaching (but not triggering) complete divertor detachment

•  Scanning MLP samples ‘upstream’ profiles during this evolution Divertor N2 seeding

Real-time surface heat flux measurements are used to feedback control divertor N2 seeding – allows divertor heat fluxes to be precisely specified

Scanning Mirror Langmuir Probe

4

1160629029

0.0 0.5 1.0 1.5 2.00.00.20.40.60.81.01.2

MA

PlasmaCurrent

01

23

1020

m-3

Density

0.0 0.5 1.0 1.5 2.00.00.51.01.52.0

MW

PsolPrad Ptot

0.0 0.5 1.0 1.5 2.002468

MW

m-2

Ave. Surface Heat Flux3+4+5+6+7

050100150200

%

Gas Valve

0.0 0.5 1.0 1.5 2.0seconds

050

100150

mm

Scanning MLPInsertion

Set Point


Fluctuations in data are not noise! MLP bias system tracks turbulence with high fidelity.

High resolution SOL profiles are deduced from time-averaging MLP data

Scanning Mirror Langmuir Probe: Electrode Geometry Langmuir-Mach Probe

High heat-flux geometry

Raw data consist of 100,000 measurements of each parameter from each electrode (NE, SE, SW, NW)

5



Langmuir-Mach Probe


Step 1: average data from four electrodes Step 2: time average over 200 µs

Scanning Mirror Langmuir Probe: Electrode Geometry


6



Langmuir-Mach Probe


Step 1: average data from four electrodes Step 2: time average over 200 µs Step 3: fit smooth spline curves to data Step 4: shift profiles in ‘rho’ to satisfy SOL power balance

Data from spline-fitted curves are shown in subsequent slides

Scanning Mirror Langmuir Probe: Electrode Geometry


7


What might we expect?

8

Myra and D’Ippolito plasma collisionality parameter, Λ , increases, possibly enhancing cross-field blob transport

[1] Myra, PoP 13 (2006) 112502

Reduction of radially sheared ExB flows allows enhanced cross-field transport

Blob Propagation Model

Plasma Potential and ExB Shear Layer Model

[1] Halpern, NF 57 (2017) 034001

Φ/Te

As N2 divertor seeding is increased ... ⇒ reduction in divertor target electron temperature ⇒ increase in divertor target density ⇒ increase in divertor plasma collisionality

⇒ increase in SOL width

⇒ decrease in sheath potential ⇒ decrease in SOL plasma potential ⇒ decrease in ExB shear

⇒ increase in SOL width


What did we find? Look at four L-mode cases at three plasma currents ...

Ohmic L-mode Toroidal field: 5.4 tesla Greenwald fraction: ~ 0.2 Sheath-limited divertor conditions prior to N2 injection

Plasma Current: 1.1 MA ne ~ 1.7x1020 PSOL ~ 1.1 MW Max measured q// ~ 350 MW m-2



Case 1 (2016) Case 2 (2015)

Case 3 (2016)

Case 4 (2016)

9






Case 1 (2016) Case 2 (2015)

Case 3 (2016)

Case 4 (2016)

Examine 1.1 MA data first: 21 shot/time slices

10

What did we find? Look at four L-mode cases at three plasma currents ...


Divertor N2 seeding reduced divertor surface heat fluxes by a factor of ~10 with core plasma relatively unperturbed

1.1 MA datasets: 2015 + 2016

Line average density held ~constant

Power into SOL ~constant

Divertor conditions near strike point change from sheath-limited to high-recycling to near detached (~ 5 eV)

11

0 50 100 150 200 2500.00.5

1.0

1.5

2.02.5

1020

m-3

1.1 MA : STC ave q|| > 100: |SSEP| < 10 mm1.1 MA : 25 < STC ave q|| < 100 : |SSEP| < 10 mm1.1 MA : STC ave q|| < 25 : |SSEP| < 10 mm1.1 MA : STC ave q|| > 1001.1 MA : 25 < STC ave q|| < 1001.1 MA : STC ave q|| < 25

0 50 100 150 200 2500.0

0.5

1.0

1.5

Psol

(MW

)

0 50 100 150 200 250Parallel Heat Flux Density on Divertor Surface (MW m-2, averaged over multiple Surface TCs)

05

10152025

Te (e

V)

Rail probe data

Line-Averaged Density

Power into Scrape-off Layer

Rail probe dataDivertor Electron Temperatureρ = 2 mm


05

1015202530

Te (e

V)

Divertor Electron Temperature

-0.2

0.0

0.2

Jgnd

(A m

m-2 )

RAIL probe array data

0 5 10 15Rho (mm)

050

100

150

200250

q || (MW

m-2

)

Surface Thermocouple Data

Net Current Densityto Divertor Surface

Parallel Heat Fluxat Divertor Surface


1.1 MA dataset:

Divertor Upstream Scrape-off Layer 2016

12

10

100

Te (e

V)

MLP spline-fit profile data

0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar


Midplane Electron Temperature

Midplane Density - normalized to core line-averaged


05

1015202530

Te (e

V)


-0.2

0.0

0.2

Jgnd

(A m

m-2 )


0 5 10 15Rho (mm)

050

100

150

200250

q || (MW

m-2

)





10

100

Te (e

V)


0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar




1.1 MA dataset:


Divertor Response

13

• Divertor Te approaches ~5 eV; attains partial detachment

• Net current densities on divertor surface reduced by factor of ~10

• Parallel heat fluxes on divertor surface reduced by factor of ~10

Divertor conditions change dramatically with N2 seeding


05

1015202530

Te (e

V)



0 5 10 15Rho (mm)

050

100

150

200250

q || (MW

m-2

)


Divertor CollisionalityParameter, Λdiv - Myra


RAIL probe array data0

50

100

150

Lam

bda M

yra 10

100

Te (e

V)


0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar




1.1 MA dataset:


Divertor Response




14

• Divertor collisionality (Λ – Myra[1]) increases factor of ~50 near strike

[1] Myra, PoP 13 (2006) 112502



10

100

Te (e

V)


0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar




05

1015202530

Te (e

V)


-0.2

0.0

0.2

Jgnd

(A m

m-2 )


0 5 10 15Rho (mm)

050

100

150

200250

q || (MW

m-2

)





Upstream Te, ne profiles steepen slightly near LCFS and reduce in far SOL in response to divertor N2 seeding

1.1 MA dataset:

Upstream SOL Response


• Te

15

Divertor Response

[1] Myra, PoP 13 (2006) 112502

and ne reduced in far SOL(!)

⇒  Near SOL width becomes slightly narrower (!) with increased divertor dissipation (N2)





Note log scale

Note log scale


1

10

LTe (

mm

)

0 5 10 15Rho (mm)1

10

LNe (

mm

)



Midplane Electron TemperatureGradient Scale Length

Midplane DensityGradient Scale Length

05

1015202530

Te (e

V)


-0.2

0.0

0.2

Jgnd

(A m

m-2 )


0 5 10 15Rho (mm)

050

100

150

200250

q || (MW

m-2

)





Upstream Te, ne profiles steepen slightly near LCFS and reduce in far SOL in response to divertor N2 seeding

1.1 MA dataset:


Divertor

2016

• Te

16

[1] Myra, PoP 13 (2006) 112502

and ne reduced in far SOL(!)

Upstream Scrape-off Layer

⇒  Near SOL width becomes slightly narrower (!) with increased divertor dissipation (N2)

Divertor Response






10

100

Te (e

V)


0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar




10

100

Te (e

V)

0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar





17

2015 2016

Upstream Temperature and Density Profiles

Question: Drop of Te & ne in far SOL and narrowing of Near SOL with N2 Reproducible? => yes, also seen in 2015 investigation


0.0

0.1

0.2

0.3

0.4

RMS

Te/<

Te>

0 5 10 15Rho (mm)0.0

0.1

0.2

0.3

0.4

RMS

n/<n

>



Te Fluctuation Amplitude(RMS Te/<Te>)

Density Fluctuation Amplitude(RMS n/<n>)

0.0

0.1

0.2

0.3

0.4

RMS

Te/<

Te>

0 5 10 15Rho (mm)0.0

0.1

0.2

0.3

0.4

RMS

n/<n

>





⇒  Something specific to the Unseeded cases systematically had factor of ~2 higher fluctuation levels at LCFS!

18

1.1 MA, 2015 1.1 MA, 2016

Correlated with: Reduction in plasma fluctuations (and transport)

Upstream Temperature and Density Fluctuation Profiles

Question: Drop of Te & ne in far SOL and narrowing of Near SOL with N2 -- What caused it?

But, is this effect caused by the change in divertor conditions? ...



Case 1 (2016) Case 2 (2015)

Case 3 (2016)

Case 4 (2016)

0.8 MA dataset: 16 shot/time slices




Next, look at 0.8 MA dataset ....

19






0.8 MA dataset:

20

• Same as 1.1 MA case

0 20 40 60 800.0

0.5

1.0

1.5

2.0

0.8 MA : STC ave q|| > 600.8 MA : 25 < STC ave q|| < 600.8 MA : STC ave q|| < 25

0 20 40 60 800.0

0.4

0.8

1.2

0 20 40 60 800

10

20

30

1020

m-3

Psol

(MW

)

Parallel Heat Flux Density on Divertor Surface (MW m-2, averaged over multiple Surface TCs)

Te (e

V)Line-Averaged Density


Rail probe dataDivertor Electron Temperatureρ = 2 mm


0

10

20

30

40

-0.4

-0.2

0.0

0.2

0.4

0 5 10 15Rho (mm)

-50

0

50

100

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







0.8 MA dataset:

Divertor Response Divertor




21

• Same as 1.1 MA



0

10

20

30

40

-0.4

-0.2

0.0

0.2

0.4

0 5 10 15Rho (mm)

-50

0

50

100

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







0.8 MA dataset:

Divertor Upstream Scrape-off Layer

22


Upstream Te, ne profiles ~flatten slightly near LCFS; No change in far scrape-off layer with divertor N2 seeding

• Te and ne unchanged in far SOL • Different from 1.1 MA

10

100

Te (e

V)

0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar





Divertor Response





⇒  A hint that near SOL width becomes less narrow with increased divertor dissipation (N2)


1

10

LTe (

mm

)

0 5 10 15Rho (mm)1

10

LNe (

mm

)





0

10

20

30

40

-0.4

-0.2

0.0

0.2

0.4

0 5 10 15Rho (mm)

-50

0

50

100

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







0.8 MA dataset:

Divertor

23






Divertor Response






0

10

20

30

40

-0.4

-0.2

0.0

0.2

0.4

0 5 10 15Rho (mm)

-50

0

50

100

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







0.8 MA dataset:

Divertor

24



NO SYSTEMATIC CHANGE in SOL fluctuations with N seeding




2

0.0

0.1

0.2

0.3

0.4

RMS

Te/<

Te>

0 5 10 15Rho (mm)0.0

0.1

0.2

0.3

0.4

RMS

n/<n

>





Divertor Response







0.55 MA dataset: 32 shot/time slices

Case 1 (2016) Case 2 (2015)

Case 3 (2016)

Case 4 (2016)




25

Next, look at 0.55 MA dataset ....






0.55 MA dataset:

26

• Same as 1.1 and 0.8 MA cases

0 10 20 30 40 500.0

0.4

0.8

1.2

0.55 MA : STC ave q|| > 300.55 MA : 10 < STC ave q|| < 300.55 MA : STC ave q|| < 10

0 10 20 30 40 500.0

0.2

0.4

0.6

0 10 20 30 40 500

10

20

30

1020

m-3

Psol

(MW

)

Parallel Heat Flux Density on Divertor Surface (MW m-2, averaged over multiple Surface TCs)

Te (e

V)

Rail probe data

Line-Averaged Density


Divertor Electron Temperatureρ = 2 mm


010

20

30

4050

||

-0.2-0.10.00.10.2

0 5 10 15Rho (mm)

0

20

40

60

80

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







Divertor

0.55 MA dataset:

27

Divertor Response




• Same as 1.1 & 0.8 MA



010

20

30

4050

||

-0.2-0.10.00.10.2

0 5 10 15Rho (mm)

0

20

40

60

80

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







Divertor Upstream Scrape-off Layer

=> Profiles are unchanged, within statistical uncertainties

0.55 MA dataset:

28

NO CHANGE in SOL fluctuations with N seeding


• Te and ne unchanged in far SOL

• Similar to 0.8 MA case

Upstream Te, ne profiles are not affected by divertor N2 seeding (within experimental uncertainties)

2

10

100

Te (e

V)

0 5 10 15Rho (mm)

0.1

1.0

Dens

ity/N

eBar





Divertor Response




• Same as 1.1 & 0.8 MA


=> Profiles are unchanged, within statistical uncertainties

0.55 MA dataset:

29


• Te and ne unchanged in far SOL

• Similar to 0.8 MA case


010

20

30

4050

||

-0.2-0.10.00.10.2

0 5 10 15Rho (mm)

0

20

40

60

80

Te (e

V)Jg

nd (A

mm-

2 )q || (M

W m

-2)







Divertor

2

1

10

LTe (

mm

)

0 5 10 15Rho (mm)1

10

LNe (

mm

)





Upstream Te, ne profiles are not affected by divertor N2 seeding (within experimental uncertainties)

Divertor Response




• Same as 1.1 & 0.8 MA

NO CHANGE in SOL fluctuations with N seeding


But - Upstream Plasma Potential Profile is strongly affected by divertor N2 seeding

0.55 MA dataset:

30

020406080

100120

Phi (

V)

010

20

30

4050

Te (e

V)

|| > 30

0 5 10 15Rho (mm)

0.1

1.0

10.0

100.0

Lam

bda M

yra


Upstream Plasma Potential





Plasma potential at LCFS drops by ~ 50 V, roughly consistent drop in divertor Te and corresponding sheath potential

Divertor collisionality in near SOL increases by two orders of magnitude with N2 seeding


But - Upstream Plasma Potential Profile is strongly affected by divertor N2 seeding

0.55 MA dataset:

31

020406080

100120

Phi (

V)

010

20

30

4050

Te (e

V)

|| > 30

0 5 10 15Rho (mm)

0.1

1.0

10.0

100.0

Lam

bda M

yra


Upstream Plasma Potential





Plasma potential at LCFS drops by ~ 50 V, roughly consistent drop in divertor Te and corresponding sheath potential

Message: Near SOL density and temperature profiles (~heat flux widths) are robustly insensitive to: •  Time-averaged potential, ExB flows and their

shear •  Divertor conditions (e.g. collisionality)

Divertor collisionality in near SOL increases by two orders of magnitude with N2 seeding


Summary

•  Factor of ~10 reduction in divertor target plate heat fluxes (with core plasma unchanged)

•  Divertor target conditions varying from sheath-limited to high-recycling, approaching detachment; divertor collisionality (Λdiv – Myra) changing by factor of ~100.

•  DC current densities to the target plate reduced by over an order of magnitude

message: beware of confounding influences when doing ‘controlled’ experiments

32

Upstream Te, ne profiles near the LCFS (~heat flux widths) are robustly insensitive to divertor plasma conditions

Upstream Te, ne profiles near the LCFS (~heat flux widths) are insensitive to plasma potential profile (and ExB shear details)

Upstream Te, ne profiles are sensitive to plasma fluctuations (e.g. shoulder formation in unseeded 1.1 MA cases)

•  Mechanism that generates fluctuations seen in 1.1 MA unseeded cases is unknown


Why do these results matter?

Indicates that divertor dissipation does not reduce peak heat flux densities entering into the divertor volume via an increase in λq

Theoretical

Implication for Advanced Divertors

Practical

Indicates that divertor plasma conditions, including divertor sheath boundary conditions, and plasma potential profiles (~equilibrium ExB shear) do not play a defining role in the physics of cross-field transport in the near SOL region – contrary to some notions

Indicates that: (1) upstream λq will be largely unaffected by divertor details – length of divertor leg, flux expansion, embedded x-points, ... (2) divertor should be designed to accommodate empirical upstream λq

33

effect of divertor nitrogen seeding on the power exhaust ...€¦ · effect of divertor nitrogen...

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