Impact of dilution of deuterium ions on energy transport and turbulence in C-

Mod Ohmic plasmas.M. Porkolab, P. Ennever, E. Edlund, J. Rice, J. C. Rost, D. Ernst, C. Fiore, A.

Hubbard, J. Hughes, J. Terry, S. G. Baek, and the Alcator C-Mod Team, MIT; M. L. Reinke, ORNL; G. Staebler and J. Candy, General Atomics.

APS DPP 2015, Savannah, GA, Poster CP12.00008Work supported by US DOE awards

DE-FG02-94-ER54235, DE-FC02-99-ER54512, and DE-AC02-05CH1123 (NERSC).

Outline

- Motivation for studying main-ion dilution

- Controlled nitrogen seeding experiments

- Effect of nitrogen seeding on intrinsic rotation

- Effect of nitrogen seeding on experimental and GYRO simulated ion energy transport

- Effect of nitrogen seeding on PCI density fluctuations

- Summary & Conclusions

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Ohmic plasmas have two confinement regimes: linear and saturated ohmic confinement (LOC and SOC)

- At low values of neq95, energy confinement time scales linearly with density

- At high values of neq95, energy confinement time is independent of density (follows L-Mode scaling)

- LOC and SOC plasmas also have different intrinsic rotation profiles, LOC plasmas are flat and co-current, while SOC plasmas are hollow and counter-current 3

LOC

SOC

neq95 [1020 m-3]

Dilution was needed in TGLF get agreement with experimental ion transport in ohmic plasmas

From: Porkolab, M et al. Plasma Physics and Controlled Fusion 54.12 (2012): 124029. A similar Zeff scan assuming Zimp was molybdenum showed no effect on transport

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Dilution from low-Z impurities will affect transport in tokamak plasmas

- Metal walled tokamaks (C-Mod, JET, ASDEX-U) need low-Z seeding to mitigate the influx of high-Z impurities- Reactor plasmas will also have helium ash

- Low-Z impurities have been found to reduce turbulent energy transport in experiments and simulations1,2,3,4

- The effects of dilution from low-Z impurities were investigated through controlled seeding experiments and nonlinear GYRO simulations

[1] McKee, G et al. Phys. Rev. Lett. 84.9 (2000): 1922–1925.[2] Porkolab, M et al. Plasma Physics and Controlled Fusion 54.12 (2012): 124029.[3] Giroud, C et al. Nucl. Fusion 43.11 (2013): 113025[4] Tardini, G., et. al. Plasma Physics and Controlled Fusion, 55.1 (2013): 015010.

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Ohmic plasmas on C-Mod at a range of q95 and ne values were seeded with nitrogen while ne was held fixed using a cryopump

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- q95 was changed by changing Ip, BT = 5.4T for all shots

- Changing Ip changed POH, Te, Ti

- Seeding increased Zeff by about 1

- Seeding decreased nD/ne by 10-20%

Density scans span the low-density LOC and high density SOC regimes. Seeding did not change overall

E, so increase in Prad offset decrease in transport

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Unseeded Seeded

q95 = 4.5

q95 = 3.9

q95 = 3.4

Nitrogen seeding caused reversals of intrinsic rotation for SOC plasmas near the LOC-SOC transition, no effect with seeding for LOC plasmas

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Seeded plasmas have a different critical eff for intrinsic rotation direction compared to unseeded plasmas, implies that collisionality may not be responsible for seeding induced rotation reversal

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Core rotation direction in both seeded and unseeded plasmas display a critical nDq95 ~ 4 for all q95 cases

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Unseeded Seeded

q95 = 4.5

q95 = 3.9

q95 = 3.4

Stiffness and critical gradient define the relationship between the energy flux and the temperature profile

Critical Gradient

Stiffness

Plot of experimentally measured Qe/QGB using ECH to locally change 1/LTe (a similar relationship is expected for ion energy flux and temperature gradient)

From: Hillesheim, J C et al. Physics of Plasmas 20.5 (2013). Grey lines have been added for reference.

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Nitrogen seeding increased normalized ion temperature gradients, but reduced normalized ion energy flux at r/a = 0.8; implies that turbulence was being stabilized

12q95 = 3.9, SOC, nD/ne = 0.95 (unseeded), 0.84 (seeded)

Seeding increased inverse Ti gradient scale lengths, while normalized ion energy fluxes decreased - implies that seeding changed ion stiffness or critical gradient

Unseeded and Seeded Ion Fluxes And Gradients at r/a = 0.8

a/LTi = a ▽Ti / Ti13

Qi /Q

GB

The nonlinear gyrokinetic code GYRO was used to investigate the theoretical effects of the seeding and dilution on ion energy flux

- GYRO simulation parameters:

- k ρs ≤ 1 (ITG and TEM, not ETG)

- Only electrostatic (Because < 0.1%)

- ExB shear not included (tested in a few cases, but was found to be negligible)

- Average impurity ion species used that matched Zeff and nD/ne

- Electron-ion collisions included

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In q95 = 3.9 SOC case seeding increased critical gradient and reduced stiffness; GYRO reproduced change in transport with seeding

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Unseeded Seeded

GYRO

Fit to GYRO

Crit. Grad.

Exp. Flux

r/a = 0.8, q95 = 3.9, ne = 1.4 x 1020 m-3(SOC), nD/ne = 0.95, 0.84

Qi /Q

GB

At ne = 0.7 x 1020 m-3 (LOC regime), the seeding increased the critical gradient and increased the stiffness in GYRO, and experimental fluxes and gradients agree with GYRO

r/a = 0.8, q95 = 4.5, ne = 0.7 x 1020 m-3(LOC), nD/ne = 0.88, 0.77

Unseeded Seeded

GYRO

Fit to GYRO

Crit. Grad.

Exp. Flux

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Qi /Q

GB

Scans of linear GYRO simulations at r/a = 0.8 varying a/LTi and a/LTe show seeding strongly decreased ITG growth rates and slightly decreased TEM growth rates

17q95 = 3.9, SOC, a/Ln ~ 2, nD/ne = 0.95 (unseeded), 0.84 (seeded)

TEM TEM

ITG ITG

Unseeded Growth Rates Seeded Growth Rates

Line integrated ne fluctuations with kR ϵ [0.5 cm-1, 20 cm-1] were measured with phase contrast imaging (PCI) both before and after nitrogen seeding- ne fluctuations were measured

with a 50 W CO2 laser beam passing through the plasma vertically and detected with a 32 channel cryogenically cooled HgCdTe detector array

- Present geometry lacks ability to localize

- Measures kR of turbulence, which is approximately k in the core

- Absolutely calibrated 18

PCI Chords

Low q95 (high Ip) PCI spectra show high frequency (f > 150 kHz) feature that is significantly decreased by seeding, unrelated to change in the plasma rotation

19q95 = 3.4, SOC case

Nitrogen seeding significantly reduced high frequency (f > 150 kHz) PCI spectral feature in both LOC and SOC q95 = 3.4 cases

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~

Reflectometer channels measuring at r/a ~ 0.8 showed a strong decrease in ne fluctuations with seeding, implies high-f PCI feature is localized near r/a ~ 0.8; r/a ~ 0.6 channel shows a moderate decrease with seeding as well

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Normalized energy fluxes at r/a = 0.85 are large (Q/QGB ≳ 10 as a rule-of-thumb), which implies that turbulence there is well above marginal stability

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q95 = 4.5, SOC Case

Reflectometer shows large decrease in fluctuations with seeding

Reflectometer shows moderate decrease in fluctuations with seeding

ITERRegime

* = 0.2 @ r/a = 0.6;* = 0.5 @ r/a = 0.8;

Local nonlinear GYRO at r/a = 0.6 significantly over-predicts experimental ion and electron energy fluxes using nominal experimental temperature profiles

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Unseeded Seeded

q95 = 4.5 GYRO

q95 = 4.5Exp.

q95 = 3.9 GYRO

q95 = 3.9 Exp.

At ne = 0.8 x 1020 m-3 (SOC regime) and r/a = 0.6

GYRO shows the seeding decreasing the slope, but GYRO over-predicts the experimental fluxes

r/a = 0.6 ne = 0.8 x 1020 m-3(SOC), nD/ne = 0.89, 0.74 Q

i/QG

B

a/LTi

Unseeded and Seeded Critical

Gradients

Unseeded and Seeded GYRO

Simulations

Unseeded and Seeded Experimental

Flux and Gradient Unseeded and Seeded offset

linear fit

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In addition to dilution, reducing a/LTi about 20% reduces both the ion and electron energy fluxes to within a factor of 2 of the experimental values in the range r/a ∈ [ 0.55, 0.75]

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Decreasing nD/ne reduces GYRO simulated ITG and ETG linear growth rates, despite no change in the Zeff in the simulation

26q95 = 3.9, SOC case at r/a = 0.6

Summary & Conclusions

- Dilution has been shown experimentally to reduce ion transport by changes to the flux-gradient relationship, resulting in higher Ti gradients for the same ion energy flux

- This is reproduced in the GYRO simulations as increases in critical gradient and decreases in stiffness

- Nitrogen seeding also affected the intrinsic rotation independent of the collisionality, and seemed to be dependent on nDq95

- Dilution also is predicted to have a strong effect on ETG linear growth rates at k ρs > 2, although its impact on the experimental results cannot be assessed without nonlinear GYRO runs

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Implications- The present results may provide partial explanation of the RI mode

observations on DIII-D and Textor wherein low-Z seeding increased energy confinement in beam heated ITG dominant plasmas

- Dilution could also be related to the observed degradation in performance when changing from carbon to metallic walls in ASDEX-U and JET, which is partially recovered through low-Z seeding

- Dilution will also likely have an impact on future fusion devices such as ITER due to both helium ash buildup and extrinsic impurity seeding that will be necessary to reduce divertor heat loads

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