Progress on Rotation Effects in MGI

Simulations of DIII-D

V.A. Izzo

NIMROD Team Meeting

14 November 2015

Savannah, GA

Motivation

• NIMROD MHD modeling has successfully predicted several features of

disruption mitigation by massive gas injection (MGI):

- Correlation of n=1 mode phase with location of radiation toroidal peak

- Orientation of initial mode phase 180⁰ away from gas injector

- Value of instantaneous (peak power) toroidal peaking factor (TPF) in

DIII-D (~1.4)

• Some differences between modeling and experiment may be important when

extrapolating to ITER

- Toroidal spreading of impurities (experiment appears more uniform)

- Rotation of n=1 mode phase prior to TQ-onset

- Value of time integrated TPF

Pre-MGI plasma rotation may play an important role in all of these

discrepancies and has recently been included in NIMROD modeling

DIII-D experiments: Initial n=1 phase corresponds to

NIMROD prediction, then phase rotates

Initial n=1 phase

Phase of n=1 mode when it first appears (prior to the TQ) is 180 degrees from gas jet location, in

agreement with NIMROD prediction

Between initial appearance and TQ, n=1 phase rotates.

Higher pre-MGI plasma rotation more pre-TQ

mode rotation

D. Shiraki et al, Nuclear Fusion 55 (2015) 073029

DIII-D experiments: n=1 phase at TQ can be controlled

with error fields (particularly at low rotation)

Use of error fields to control final phase of mode is useful to measure radiation toroidal peaking

factor with limited diagnostic set. Same TPF found at 90 and 210 degrees suggests impurity distribution not a large factor in TPF (uniform?)

D. Shiraki et al, Nuclear Fusion 55 (2015) 073029

3. NIMROD simulations with rotation

Two cases differ in rotation profile, viscosity profile, and

toroidal resolution

Separatrix

Case 1 has large viscosity in the

“vacuum region” to suppress flows

Toroidal

resolution:

Case 1:

11 toroidal

modes (n=0-10)

Case 2:

22 toroidal

modes (n=0-21)

Neglect of MGI source inertia may make suppressed

flow case more physical

VBJpt

V

∇∇∇- ρρ

Momentum Equation

Inclusion of a volumetric source (assumed to have no significant

momentum) requires explicit inclusion of source inertia term

VBJpt

Vt

V

source

∇∇∇- ρρ

This term can only be neglected when V=0 in the injection

region (e.g. Case 1)

Case 1 likely under-resolved: role of n>1 modes differs

with toroidal resolution

Case 1 Case 2

n=10n=2

n=8

n=21

4. Preliminary Simulation Results

Addition of rotation clearly affects impurity spreading

Rotating (Case 2)Rotating (Case 1)

Toro

idal angle

(deg.)

Non-rotating

MGI

Time (ms)Time (ms) Time (ms)

3D view of impurity spreading with rotation

Case 2: Flows in vacuum region

not suppressed – t = 3.0 msCase 1: Flows in vacuum region

suppressed – t = 3.0 ms

15⁰15⁰

Injection location

Ionized Ne density

Radiation peak rotates during pre-TQ, continuing into TQ

Radiated power (Case 1) Radiated power (Case 2)

180⁰

Toro

idal angle

(deg.)

Time (ms) Time (ms)

Toro

idal angle

(deg.)

MGI

Rotation of m=2/n=1 mode tracks with rotation of peak

radiated power

MGI

Case 2: Time = 3 ms

2/1 is

dominant

n=1

component

for most of

simulation

Radiated power (Case 2)

m

Norm

aliz

ed p

olo

idal flux

In both cases, drop in core velocity precedes

significant mass penetrationToro

idal ro

tation (

m/s

)

Toro

idal ro

tation (

m/s

)

Case 1 Case 2

Edge angular

momentum rises as

core drops, but more

meaningful results will

depend on correct

treatment of edge

viscosity, MGI source

inertia

Summary

• Goal is to understand role of plasma rotation in impurity

spreading and radiation peaking during MGI

• Inclusion of rotation in MGI simulations changes impurity

spreading (follows rotation direction)

• Rotating peak in radiated power is seen in two simulations, but qualitatively different behavior at TQ observed in two cases

(with more than one difference between them)

• Rotating radiation peak tracks with 2/1 mode rotation until

appearance of 1/1 mode

Future work:

• Correct treatment of MGI source inertia term along with sufficient toroidal resolution and realistic initial rotation profile