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Sustained Compact Toroids in MRX

S.P. Gerhardt, M. Yamada, H. Ji

M. Inomoto1, E. Belova, R. Maqueda2 Y. Ren, B. McGeehan, & C. Jacobsen

A First Look At Our Recently Completed Ohmic Campaign

1: Osaka University 2:Nova Photonics

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Ohmic Sustainment a Step Toward the “SPIRIT” Oblate FRC Concept

4 main components of the SPIRIT concept.– Spheromak merging to form large-flux FRCs.– Ohmic system to heat the plasma and further increase the

flux for energetic ion confinement.– Conducting shells to stabilize n=1 modes.– NBI to sustain the plasma and stabilize low-n co-

interchange modes via FLR effects.

“Self-Organized Plasma With Induction, Reconnection, and Injection Techniques”

Developing Ohmic system is an important step towards realizing this concept in an MRX-scale device.

Three month campaign of sustained CTs……machine now again devoted to basic reconnection science.

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This talk…

• The MRX facility modified for Ohmic sustainment of Compact Toroids (CTs).

• Ohmic sustainment of FRCs– FRC equilibrium maintained for >300 s.– Pressure profile peaks to maintain equilibrium– Only Argon plasmas provide sufficient stability in the

present experiments.

• Ohmic sustainment of spheromaks– Light gasses (D2, He, Ne)n=1 tilt or n=2 kink typically

terminate the configuration.– Argon, no tilt or kink is observed, and the spheromak

plasma transitions to an FRC equilibrium during Ohmic.

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MRX Modified for CT Sustainment Campaign

• 68 turn Ohmic solenoid, Inconel liner• Three capacitor banks for 4 coils (TF, PF, SF, Ohmic)….

…reduced shape control• New shaping coils with reduced field errors.• No nearby passive stabilizers.

New 2D Probe Array7x6 array of Coil Triplets

€

ψ R,Z( ) = 2π RBZdR∫

€

φ ψ( ) = BT∫∫ dA

€

p R,Z( ) = jZBT − jTBZ( )∫ dR

6 Flux Loops on SolenoidFlux penetration through liner

Spoke Probes• Triplets at 5 radial locations• Probes at 8 toroidal angles

• Midplane magnetic perturbations in BR and BZ

Triple ProbeFast Camera

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FRC Capabilities Recently Upgraded, Including Ohmic Solenoid

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FRC Sustainment

Merging Spheromaks Have Oppositely Directed Toroidal Fields

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Ohmic Sustainment for ~300s Demonstrated

No Ohmic With Ohmic

Flux Plots From Magnetic Probe Array

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QuickTime™ and a decompressor

are needed to see this picture.

Sustainment Visible in Fast Camera Images

Fast Camera Images, Argon, White Light

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Ohmic Sustainment for ~300s Demonstrated

275 s 325 s 375 s 450 s 550 s

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Peaked Pressure Profile Evolves to Sustain FRC Equilibrium

275 s 325 s 375 s 450 s 550 s

Electron Pressure Triple Langmuir Probe Radial Scan

Red: SustainedBlue: Decaying

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Increased Ohmic Voltage Leads to More Flux, Longer Sustainment

Capacitance and Inductance Fixed for Longest Ohmic Waveform

Ohmic Voltages 5kV-9kV

Input Powers: 300-800kW

Tra

pp

ed

Flu

x(m

Wb

)

Su

rfa

ce V

olta

ge

(V)

Cu

rre

nt

De

nsi

ty(A

/m2)

So

len

oid

Cu

rre

nt

(kA

)

Maximum Ohmic voltage limited by null radius expansion, not instability.

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Lighter Gasses Demonstrate Rapid Instability

Helium Example

Shot 65788

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Lighter Gasses Demonstrate Co-Interchange Instabilities

Helium Example

Shot 65788

Tra

pped

Flu

x(m

Wb

)

BZ BZ BZ

BR BR BR

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Lighter Gasses Demonstrate Co-Interchange Instabilities

• Co-Interchange: pressure driven modes.• ~1• Bad curvature everywhere •MHD predicts instability…correctly

• Many toroidal mode numbers simultaneously unstable.

• Configurations have been identified with stability to all co-interchange modes via conducting shells and NBI (SPIRIT).1

• SSX experiment to test oblate FRC with complete set of nearby conductors.

Tra

pped

Flu

x(m

Wb

)

BZ BZ BZ

BR BR BR

1Belova et al, Phys Plasmas 2006; M. Yamada et al, Plasma and Fusion Research 2007..

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Flux & Lifetime Best for Argon

Average Flux During Ohmic

Discharge Peak Flux

Life

time

(s)

Average Flux During Ohmic (mWb)

Life

time/

Res

istiv

e T

ime

Steady SustainmentTransient

Helium4

4-106-11

Deuterium2

2-44-6

Nitrogen14

10-2015-30

Neon20

10-2020-32

Argon40

25-3540-90

Krypton84

35-5050-100

Mass

€

ρi (cm)

s)

Ohmic Waveform Limit

€

R =μ0a

2

η Sp

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Spheromak Sustainment

Merging Spheromaks Have Toroidal Fields Which Point in the Same Direction

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Helium and Neon Spheromaks Often Tilt

Helium Example: Pure n=1 “tilt” spectrum

Flux

BR, n=1

BR, n=2

BR, n=3

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Helium and Neon Spheromaks Often Tilt

Helium Example

Nova Photonics Fast CameraWhite Light, 100kHz

N=1 amplitudePoloidal flux

Poloidal Field VectorsToroidal Field Colors

QuickTime™ and a decompressor

are needed to see this picture.

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Increased EF and higher fill pressure can suppress the n=1 tilt

…but n=2 kink develops to terminate the discharge.

10 mT8.2 mT6.77 mT5.4 mT4.9 mT

time (s)

Pol

oid

al F

lux

(m

Wb

)B

R,

n=

1 (

T)

BR,

n=

2 (

T)

-q0

All Neon

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SpheromakFRC Conversion Observed in Argon Plasmas

With Ohmic

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Toroidal Field Decays As Poloidal Flux is Sustained

66536 & 66523

Pol

oida

l Flu

x (

mW

b)T

oroi

dal F

lux

(m

Wb)

Tay

lor

Eig

enva

lue

()

€

ψ R,Z( ) = 2π RBZdR∫

€

φ ψ( ) = BTInside Separatrix

∫∫ dA

€

"Poloidal Flux"=ψ null −ψ sep

€

∇×B = λB

⇓

λ ∝2πRBTψ

time (s)

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“Conversion” To FRC is Robust in Argon

Transition Occurs:• in Argon and Krypton• over a wide range of

fill pressures and

Ohmic voltages. • never in He or Ne

Pol

oida

l Flu

x (

mW

b)

Tay

lor

Eig

enva

lue

Sol

enoi

d C

urre

nt (

A)

time (s)

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Instability Suppression is Key to “transition”

• Tilt/shift instabilities can terminate plasma even before Ohmic is energized.

• Ohmic adds poloidal flux to the system, while toroidal flux decaysdrops q.

• In He and Ne, when q0<0.5, a terminal n=2 mode appears.

• Similar to previous results:– In S-1, non-uniform Te profile leads to a drop in q and nonuniform , with

n=2 mode restoring Taylor state.1

• In Ar and Kr, the kink is suppressed, the toroidal flux decays to zero, and an FRC equilibrium is formed.

1: Ono et al. Phys. Plasmas B 3, 1452 (1991); 2: Knox et al, PRL 56, 843 (1986).

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Conclusions

• FRCs sustained for >300s using Ohmic current drive.– Evidence that an equilibrium suitable for NBI can be prepared with Ohmic.– Need larger Ohmic bank, additional EF coils to realize full potential.

• Argon utilized to stabilize both merging and sustainment phases.– Nearby passive stabilizers are essential for oblate FRCs.

– FLR stabilization by NBI will be necessary.

• Spheromaks in D2, He, and Ne show violent n=1 and n=2 instabilities with Ohmic.– Nearby passive stabilizers are essential…well known from S-1, CTX,…

• Argon Spheromaks can be driven to an FRC-equilibrium with Ohmic– Under these conditions, the FRC may be a preferred state.

…and Implications

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• Extra stuff, for reference and for poster

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Co-Helicity Merging Forms a Spheromak

• Initial spheromaks have the same polarity of toroidal field• Merging results in a new spheromak.

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Sustainment Visible in Fast Camera Images

200 s 280 s 330 s 380 s 480 s

Argon Counter-Helicity Merging with Sustainment

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Tilt Shows Field Opening

T. Hayashi, T. Sato, F. Wysocki, D.D. Meyerhofer, & M. Yamada, JPSF 54, 4172 (1985)

• Magnetics and images show clear tilt.• Field lines appear to open as in simulation by Hayashi.• Plasma cannot tilt beyond 90 degrees.

184 s 242 s 300 s 358 s 416 s

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BZ Profile Used to Estimate Flux, Plasma Size, Taylor Eigenvalue

Separatrix Flux

Null Flux

N=0 Component from N-Probes

Inner Separatrix Radius

Outer Separatrix Radius

Null Radius

Solenoid Surface Flux

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Lighter Gasses Demonstrate Rapid Instability

Shot 65788• Many toroidal mode numbers simultaneously unstable to co-interchange.• Illustrates the importance of nearby conducting structures.

Tra

pped

Flu

x(m

Wb

)

Helium Example

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Increased Ohmic Voltage Leads to More Flux, Longer Sustainment

Ohmic Voltages From 5kV-9kVInput Powers: 300-600kW