high-field-side mhd activity during local helicity injection · high-field-side mhd activity during...

PEGASUS Toroidal Experiment

University of Wisconsin-Madison

59th Annual Meeting of the APS Division of Plasma Physics

Milwaukee, WI

26 October 2017

High-Field-Side MHD Activity During Local Helicity Injection

Jessica L. Pachicano M.W. Bongard, R.J. Fonck, J.M. Perry, J.A. Reusch, N.J. Richner

2

Layout Slide (Include for Posters)

US Legal 8.5 x 14”

US Letter 8.5 x 11”

Panel size: 8’ x 4’ 12:1 scale

MHD transition

PEGASUS Magnetic

Diagnostic Locations

NIMROD Simulated n=1 Activity Present

in Experiment on Low-Field-Side

(LFS)

NIMROD Provides

Simulation of Injected Current

Streams

Overview/LHI Introduction

Troubleshooting Noise Source

Noise Suppression

Solution

Results of implemented

solution

Motivation for Noise

Reduction

Transition Behavior Differs Greatly between the LFS and HFS

Large Changes in MHD Auto-Power

at Transition: LFS

vs. HFS

Little Correlation

between HFS and LFS

HFS LHI MHD Transition Behavior

Toroidal Mode Number: LFS

vs. HFS

High Frequency

Spectra of HFS

Summary of high MHD on

HFS

Coherence vs. poloidal angle

Toroidal Mode Number on

HFS

High Frequency

Spectra of HFS

Summary of low MHD on

HFS

Coherence vs. poloidal angle

Title Slide

Elimination of EMI-driven switching

noise

High amplitude MHD behavior

on HFS

Low amplitude MHD behavior

on HFS

Local Helicity Injection Current Drive

4

Local Helicity Injection (LHI)

J.L. Pachicano, APS-DPP 2017

•  Non-solenoidal startup on PEGASUS •  High-Field-Side (HFS) current injection •  Reduced-MHD regime discovered

Local Helicity Injectors

Injected Current Stream

0.25

0.20

0.15

0.10

0.05

0.00

I p [M

A]

28242016Time [ms]

Plasma CurrentInjector Current

Ip ≤ 0.2 MA (Iinj ≤ 8 kA)

5

PEGASUS Magnetic Diagnostic Locations Poloidal Inboard High Resolution Mirnov Coils: “HighRes1-12”

Poloidal Outboard Mirnov Coils: “PDX 1-13”

Toroidal Outboard Mirnov Coils: “Otor 1-6”


6

NIMROD Simulated n=1 Activity Present in Experiment on Low-Field-Side (LFS)

•  NIMROD simulates mechanism for current growth during LHI

–  Build toroidal current and poloidal flux –  Induce large magnetic bursting phenomena

1. Streams follow field lines

2. Adjacent passes attract

3.Reconnection pinches off current rings

10-16

10-14

10-12

10-10

LFS

B z A

uto-

Pow

er [

T2 /H

z]

140120100806040200Frequency [kHz]

n=0 n=11st n=1Harmonic

•  LHI exhibits large Alfvénic, n=1 MHD activity

–  Both continuous and bursting behavior

–  Structure consistent with island coalescence and/or kinking of injected current streams


7

NIMROD Provides Simulation of Injected Current Streams

•  Simulations indicate coherent streams and reconnection activity on both LFS and HFS

•  Experiment has shown evidence of coherent injected streams in plasma edge at LFS

–  Outboard magnetic signature

•  Do the current streams exist, and how do they behave on the Inboard side during experiment?

–  Upgrade Inboard magnetic diagnostics to view magnetic activity down the center column


Elimination of EMI-driven Switching Noise

9

Motivation for Noise Reduction on Core Magnetics •  Switching power supply systems

introduce EMI-driven switching noise –  Many Mirnov coils along the center column

were unusable

•  Core magnetics needed to study HFS magnetic activity

•  Mitigate noise on the HFS Mirnov coils

–  Characterize where noise is originating –  Determine if coil, connections, or signal

processing are problem areas

-20

-10

0

10

20

CTor

2 [V

]

806040200

Time [ms]

-20

-10

0

10

20

CTor

2 [V

]

SN 91413

Plasma shot

SN 91412

Vacuum shot

Example of noisy Mirnov coil:


10

Troubleshooting Noise Source

•  Looked at noise level on Mirnov coils –  Noise-free triax cable run used with properly executed signal

processing

•  Results: –  Decreased noise level from 800mV to 150mV on CTor’s –  Coils with bad signals are recoverable, not damaged

50Ω Digitizer Anti

Aliasing

Digitizer Anti

Aliasing

•  Tested stand alone triax cable through clean, properly shielded signal processing

–  Terminated cable with 50Ω resistor –  Typical noise level ≈ 400 mV

-10

-5

0

5

10

Tria

x Ca

ble

[mV]

806040200

Time [ms]

-1.0

-0.5

0.0

0.5

1.0

CTor

1 [V

]

50Ω terminated cableVacuum Shot

BEFORE (SN 93488) AFTER (SN 91565)

SN 91308

CTor1 Vacuum shot


11

Noise Suppression Solution •  New shielded cable runs from the

machine to the screen room –  Cat7A Ethernet cable is Shielded Shielded

Twisted Pair (SSTP) –  Low cost, industry standard to 1GHz

•  Modify anti aliasing module –  Input SSTP –  Output shielded LEMO cables –  Output differentially driven


12

Results of Implemented Solution •  EMI-driven switching noise reduced by an

order of magnitude –  From 400mV to 40mV

•  Plasma signal no longer polluted with noise –  Accurate enough to use for detailed spectral analysis

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

High

Res7

[V]

806040200

Time [ms]

BEFORE SN 91565 AFTER SN 94198 Ethernet cable noise floor SN 93490

Vacuum shots

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

High

Res

12 [

V]

806040200

Time [ms]

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

High

Res

12 [

V]

BEFORE

AFTER

Plasma SN 91309

Vacuum SN 91308

Plasma SN 94193

Vacuum SN 94198


MHD Transition

14

HFS LHI MHD Transition Behavior

10080604020

0

Ip [k

A]

2422201816

Time [ms]-8-4048

P8Pr

b1 [V

]

SN 89533 SN 89532

•  Abrupt MHD transition can occur: –  Low-f n=1 activity reduced by over 10x

–  Current growth continues after transition

•  Understanding and controlling this MHD activity may be central to increasing Ip

200

150

100

50

0

I p [k

A]

543210

Geometry Normalized VLHI

Low MHD High MHD


15

Transition Behavior Differs Greatly between the LFS and HFS

•  Inboard magnetics show minimal reduction

•  Outboard magnetics consistently experience abrupt MHD transition

600400200

0-200-400

High

Res1

1 [T

/s]

323028262422201816

Time [ms]

600400200

0-200-400

OTor

2 [T

/s]

SN 95161 Inboard

Outboard


16

Large Changes in MHD Auto-Power at Transition: LFS vs. HFS

•  Inboard Auto-Power: –  Little changes in power after transition –  Displays comparable magnetic activity at higher frequencies

•  Outboard Auto-Power: –  Dramatic drop after transition

120

80

40

0

Ip [k

A]

3028262422201816

Time [ms]-400

-200

0

200

400

PDX

7 [T

/s]

SN 95161

10-6

10-4

10-2

100

Auto

-Pow

er [

(T/s

)^2/

Hz]

4003002001000

Frequency [kHz]

10-6

10-4

10-2

100

Auto

-Pow

er [(

T/s)

^2/H

z]

SN 95161

High MHD Low MHD

Inboard

Outboard


17

Little Correlation between HFS and LFS

•  Large n=1 mode on LFS strongly reduced after transition •  In general, decrease in activity at all frequencies •  For f > 15 kHz, no correlation between LFS and HFS activity •  High correlation toroidally on LFS

1.0

0.8

0.6

0.4

0.2

0.0

Cohe

renc

e

250200150100500

Frequency [kHz]

High MHD10-6

10-4

10-2

100

Cros

s-Po

wer

[(T/

s)^2

/Hz] High MHD

OTor 2 PDX 7 HighRes 11

Referencedto OTor2:

1.0

0.8

0.6

0.4

0.2

0.0

Cohe

renc

e

250200150100500Frequency [kHz]

Low MHD10-6

10-4

10-2

100

Cros

s-Po

wer

[(T/

s)^2

/Hz] OTor2

PDX 7 HighRes 11

Low MHDReferencedto OTor2:


High Amplitude MHD Behavior on HFS

19

Coherence of n=1 MHD activity traced from the outboard side, around the machine, and down the center column

•  n=1 activity at 30 kHz –  Coherent on LFS –  Dramatic drop in coherence

on HFS

1.2

1.0

0.8

0.6

0.4

0.2

0.0Co

here

nce

150100500-50-100-150Poloidal Angle [degrees]

High

Res1

High

Res2

High

Res3

High

Res5

High

Res6

High

Res7

High

Res8

High

Res9

High

Res1

0Hi

ghRe

s11

High

Res1

2

PDX_

01

PDX_

03

PDX_

05

PDX_

06

PDX_

07

PDX_

09

PDX_

11

Coherence at 30 kHz PDX 7 reference Profile Bias Error

SN 95161


20

Toroidal Mode Number: LFS vs. HFS

•  LFS toroidal mode number is consistently n=1 before the transition

•  HFS instead sees n=0 mode before and after the transition

-200

-100

0

100

200Ph

ase

Angl

e [d

egre

es]

3002001000

Toroidal Physical Angle Ø[degrees]

fpeak = 31.532 kHz n = 1 reference ( X2 ) n = 1 reference ( |Diff| )

Outboard SN 97651

-200

-100

0

100

200

Phas

e An

gle

[deg

rees

]

3002001000


fpeak = 31.5 kHz n = 0 reference ( X2

)

Inboard SN 97651

0.001

0.01

0.1

1

Cros

s-Po

wer

[(

T/s)

^2/H

z]

3002001000

Frequency [kHz]

InboardHigh MHD


21

High Frequency Spectra of HFS Before Transition

•  Top of the HFS exhibits magnetic activity at low-f

•  Activity increases in frequency down the center column

0.6

0.5

0.4

0.3

0.2

0.1

0.0Auto

-Pow

er [

(T/s

)^2/

Hz]

1.00.80.60.40.20.0

Frequency [MHz]

SN 97460

HighRes2 Highres7 Highres11

Turbulence Characteristics 0.12

0.10

0.08

0.06

0.04

0.02

0.00

Cros

s-Po

wer

[(T

/s)^

2/Hz

] SN 97460 HighRes12referenced to:

Highres2 Highres6 Highres12

1.0

0.8

0.6

0.4

0.2

0.0

Cohe

renc

e

1.00.80.60.40.20.0

Frequency [MHz]


22

High Amplitude MHD Summary •  Strong n=1 mode on the LFS does not appear on the HFS

–  Little to no coherence –  Toroidal mode number is n=0 on the HFS

•  No obvious n=1 activity at higher frequency on HFS

•  Indicates large-scale fluctuations of current streams may not be present on center column


Low Amplitude MHD Behavior on HFS

24

Coherence of magnetic activity traced from the outboard side, around the machine, and down the center column

•  Low power magnetic activity at 15 kHz

•  Good coherence between LFS midplane and HFS –  Reduced coherence at

the top and bottom

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Cohe

renc

e

150100500-50-100-150Poloidal Angle [degrees]

High

Res1

High

Res2

High

Res3

High

Res5

High

Res6

High

Res7

High

Res8

High

Res9

High

Res1

0Hi

ghRe

s11

High

Res1

2

PDX_

01

PDX_

03

PDX_

05PD

X_06

PDX_

07

PDX_

09

PDX_

11SN 95161

Coherence at 15 kHz PDX 7 reference Profile Bias Error


25

Toroidal Mode Number on HFS After Transition

•  HFS toroidal mode number is also n=0 after the transition

-200

-150-100

-500

50

100150

200

Phas

e An

gle

[deg

rees

]

350300250200150100500


fpeak = 15 kHz n = 0 reference

InboardSN 96949

2

4

0.0012

4

0.012

4

0.1

Cros

s-Po

wer

[(T/

s)^2

/Hz]

350300250200150100500

Frequency [kHz]

InboardSN 96949


26

High Frequency Spectra on HFS After Transition •  Apparent vertical correlation

length is about a quarter of the center column length

•  Dominant magnetic activity occurs at higher frequencies

–  150-350 kHz

•  Related to slinking motion of streams around the core?

40

30

20

10

0

Cros

s-Po

wer

[m(T

/s)^

2/Hz

] HighRes 12 referenced to:

Highres2 Highres6 Highres12

SN 97491

1.0

0.8

0.6

0.4

0.2

0.0

Cohe

renc

e

1.00.80.60.40.20.0

Frequency [MHz]


27

Low Amplitude MHD Summary •  Low power magnetic activity coherent on the LFS and HFS

–  Ip continues to increase after transition with lack on strong n=1 feature on LFS

•  Toroidal mode number on HFS is consistently n=0 during the transition

•  Higher Frequency magnetic activity seen after the transition –  Coherent along the center column


high-field-side mhd activity during local helicity injection · high-field-side mhd activity during...

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