Experimental Techniques at ASDEX Upgrade for Validation of …€¦ · Experimental Techniques at ASDEX Upgrade for Validation of Gyrokinetic Simulations A.J. Creely1 G.D. Conway,2

Experimental Techniques at ASDEX Upgrade for Validation of Gyrokinetic Simulations

A.J. Creely1 G.D. Conway,2 S.J. Freethy,1,2 T. Görler,2

T. Happel,1 A.E. White,1 and the ASDEX Upgrade Team2

1 MIT PSFC 2 Max Plank Institute for Plasma Physics – Garching This work is supported by the US DOE under Grants DE-SC0006419 and DE-FC02-99ER54512-CMOD and by the US DOD under the NDSEG Fellowship

1A.J.CreelyTTF2017,Williamsburg,VA

Validation of Gyrokinetic Codes Requires

Detailed Comparison to Experiment

A.J.CreelyTTF2017,Williamsburg,VA 2

n  Gyrokinetic simulations must be validated with experimental results before they can be used predictively

n  Ion-scale simulations contain reduced physics, but run much faster than multi-scale simulations (~107 CPU hours) [Howard PoP 2016].

n  Would like to use ion-scale simulations, but recent results on Alcator C-Mod suggest that ion-scale GYRO simulations are insufficient for some L- and I-mode plasmas [Creely PoP 2017].

n  New techniques on ASDEX Upgrade enable further investigation of these discrepancies, as well as cross-machine and cross-code comparisons:

n  Long Wavelength Electron Temperature fluctuations (CECE)

n  Perturbative Thermal Diffusivity (Partial Sawtooth Heat Pulses)

Ion-Scale Simulations of Alcator C-Mod

L-mode Plasmas Motivate Further Work


n  Ion Heat Flux (Matched)

[Creely PoP 2017]



n  Electron Heat Flux (Under)

[Creely PoP 2017]






Ion Heat Flux-Matched Simulation

χGYROpert = 0.4m2 sχExp

pert = 4.0m2 s



n  Perturbative Diffusivity (Under)

[Creely PoP 2017]






pert = 4.0m2 s




n  Temperature Fluctuations (Under)

Sensitivity Limit

[Creely PoP 2017]






pert = 4.0m2 s





Sensitivity Limit

[Creely PoP 2017]






pert = 4.0m2 s





Sensitivity Limit

Done on ASDEX Upgrade Same Result

Done on ASDEX Upgrade Different Results

In Progress on ASDEX Upgrade

In Progress on ASDEX Upgrade

[Told PoP 2013]

[Creely PoP 2017]

[Told PoP 2013]

Perturbative Thermal Diffusivity Governs

the Propagation of Heat Pulses


€

χePB =

1ne

Q∇rTe

χepert =

1ne

∂Q∂(∇rTe )

n  Standard power balance electron thermal diffusivity, governs steady state diffusion.

n  Perturbative, or incremental, thermal diffusivity governs the diffusion of perturbations and is related to stiffness [Tubbing NF 1987].

n  Should not be directly compared with one another [Cardozo PPCF 1995].

€

ne∇rTe€

Qe

Perturbative Diffusivity is Measured

Experimentally through Heat Pulses


n  Heat pulses generated with partial sawteeth [Creely NF 2016], modulated ECH [Ryter PPCF 2010], etc.

n  Partial sawteeth avoid non-diffusive ‘ballistic’ transport associated with full sawteeth [Fredrickson PoP 2000].

n  Perturbative diffusivity calculated here with Extended-Time-to-Peak Method for partial sawteeth [Tubbing NF 1987, Creely NF 2016].

n  Can then compare to gyrokinetics [Smith NF 2015].

χepert ~ VHP

αVelocity of Peak: VHP Pulse Damping: α

Alcator C-Mod Partial Sawtooth Heat Pulse

Perturbative Thermal Diffusivity Found to

Correlate with Various Plasma Parameters


n  Previous work investigated the correlation between perturbative diffusivity and various plasma parameters (ne, Te, a/LTe, etc.)

C-Mod data from [Creely NF 2016]

Perturbative Thermal Diffusivity Found to

Correlate with Various Plasma Parameters


C-Mod data from [Creely NF 2016]

n  Previous work investigated the correlation between perturbative diffusivity and various plasma parameters (ne, Te, a/LTe, etc.)

n  Initial work on ASDEX Upgrade shows trends consistent with those found on Alcator C-Mod

Future Work Will Compare to Perturbative

Diffusivity Measured with Modulated ECH


n  All previous work with partial sawteeth has been on Alcator C-Mod, which doesn’t have ECH.

n  ASDEX Upgrade can also measure perturbative diffusivity with modulated ECH [Ryter PPCF 2001].

n  Allows for cross-machine gyrokinetic validation (GENE and GYRO), and multi-scale simulations

[Adapted from Ryter PPCF 2001]

Modulated ECH on ASDEX Upgrade

Future Work Will Compare to Perturbative

Diffusivity Measured with Modulated ECH


n  All previous work with partial sawteeth has been on Alcator C-Mod, which doesn’t have ECH.

n  ASDEX Upgrade can also measure perturbative diffusivity with modulated ECH [Ryter PPCF 2001].

n  Allows for cross-machine gyrokinetic validation (GENE and GYRO), and multi-scale simulations

[Adapted from Ryter PPCF 2001]

Modulated ECH on ASDEX Upgrade

Preliminary Data Falls in this Range

Correlation Electron Cyclotron Emission

(CECE) Measures Temperature Fluctuations


n  CECE measures low-k (kθρs<0.4) electron temperature fluctuations, key to understanding ITG/TEM turbulence [Freethy RSI 2016, This Conference]

n  Correlating two closely spaced ECE radiometer channels allows detection of fluctuations below thermal noise of single channel

n  Statistical noise floor depends on length of the measurement time period.

Statistical Noise Floor

ASDEX Upgrade Shot 33995

ρtor= 0.66

New Hardware on ASDEX can Measure

Radial Profiles and Correlation Length


n  Recent upgrade from 10 channels on two systems to 30 channels on one system

n  Allows for fine radial profile measurements

n  Preliminary analysis of L-modes shows that temperature fluctuation levels increase with radius

n  Future work will look at radial correlation lengths

Conclusions and Future Work


n  Would like to use ion-scale gyrokinetic simulations, but recent results on Alcator C-Mod suggest that ion-scale GYRO simulations are insufficient for some L- and I-mode plasmas [Creely PoP 2017].

n  New techniques on ASDEX Upgrade enable further investigation of these discrepancies, as well as cross-machine and cross-code comparisons:

n  New CECE enables fine radial profile and correlation length measurements

n  Partial Sawtooth Heat Pulses enable passive measurement of perturbative thermal diffusivity

n  Currently in the process of comparing GYRO and GENE on Alcator C-Mod and utilizing new techniques on ASDEX Upgrade for GENE comparisons.

References [1] N.T. Howard et al., Phys. Plasmas 23, 056109 (2016). [2] A.J. Creely et al., Phys. Plasmas, 24, 056104 (2017). [3] Told, D. et al., Physics of Plasmas 20, 122312 (2013). [4] Tubbing, B.J.D. et al., Nucl. Fusion 27, 1843 (1987). [5] Lopes Cardozo, N.J., Plasma Phys. Controlled Fusion 37, 799 (1995). [6] A.J. Creely et al., Nucl. Fusion 56, 036003 (2016). [7] Ryter, F. et al., Plasma Phys. Control. Fusion 52, 124043 (2010). [8] Smith, S.P. et al., Nucl. Fusion 55, 083011 (2015). [9] Fredrickson, E.D., et al., Phys. Plasmas 7, 5051 (2000). [10] Ryter, F. et al., Plasma Phys. Controlled Fusion 43, A323 (2001). [11] S.J. Freethy et al., Rev. Sci. Instum., 87, 11E102 (2016). [12] S.J. Freethy, This Conference. [A1] F. Jenko et al., Phys. Plasmas 7, 1904 (2000). [A2] A. Casati et al., Phys. Rev. Lett. 102, 165005 (2009). [A3] Citrin, J. et al., Nucl. Fusion 54, 023008 (2014).

18A.J.CreelyTTF2017,Williamsburg,VA

19

Backup Slides

A.J.CreelyTTF2017,Williamsburg,VA

Radial Profile Resolution Greatly Expanded


n  In 2017 – Greater flexibility and increased number of channels to allow finer δTe/Te radial profiles (below) and radial correlation lengths

2015/16 radial profile

2017 radial profile

Perturbative Thermal Diffusivity is Related

to Gyrokinetic Temperature Profile Stiffness

21

n  In gyrokinetic simulations, can measure slope of heat flux against a/LTe above the critical gradient [Citrin NF 2014].

n  Run gyrokinetics with different a/LTe to map out slope.

n  Can compare to experimentally measured perturbative diffusivity [Smith NF 2015]

High and low stiffness plasmas, with same critical gradient.

A.J.CreelyTTF2017,Williamsburg,VA

∂Qe

∂(a / LTe )= χe

HP ⋅neTea

a/LTe

Qe (MW/m2)

Higher Stiffness

Lower Stiffness

LTe =Te∇Te


Te Before Sawtooth


Te

r Inversion Radius

Mixing Radius

Before Sawtooth

After Full Sawtooth

Ballistic Effect

Partial Sawtooth


Te

r Inversion Radius

Mixing Radius

Before Sawtooth

After Partial Sawtooth




n  Propagation of diffusive heat pulses in plasma can be used to measure perturbative diffusivity.

n  Heat pulses generated with partial sawteeth crashes, modulated ECH, etc. [Creely NF 2016, Cardozo PPCF 1995, Ryter PPCF 2010]


χeHP ~ VHP


ASDEX Partial Sawtooth Heat Pulse

ECE Radiometer







χeHP ~ VHP









χeHP ~ VHP









χeHP ~ VHP






Waveguide

Local Oscillator

Mixer

Amplifier

Power Divider

Amplifier

Detector

Digitizer

Mirror Bandpass

Filter

Antenna

Plasma

24 Fixed Frequency Channels 6 Tunable Frequency Channels




Sensitivity Limit

Noise

n  Recent upgrade from 10 channels on two systems to 30 channels on one system

n  Preliminary analysis of L-modes shows that temperature fluctuation levels increase with radius

!TT = 0.48%!TT = 0.63%!TT = 0.80%

Alcator C-Mod GYRO parameters


n  Will compare experimental Qi , Qe , fluctuations and perturbative diffusivity to global (r/a = 0.65 - 0.9) nonlinear GYRO simulations.

n  Simulations exhibit high physics fidelity: n  All inputs were obtained from experiment n  3 kinetic species (deuterium, electrons, impurities) n  Realistic geometry (Miller parameterization) n  Electrostatic turbulence (E&M effects neglected due to low beta) n  Rotation effects (ExB shear, etc.) n  e-i, and i-i collisions are included

n  Simulation box size of approximately 105 x 120 ρs n  28 toroidal modes ; ~500 radial grid points n  Captures long wavelength (ITG/TEM) up to kθρs up to ~ 1.35

[Candy PRL 2003]

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Experimental Techniques at ASDEX Upgrade for Validation of …€¦ · Experimental Techniques at ASDEX Upgrade for Validation of Gyrokinetic Simulations A.J. Creely1 G.D. Conway,2