Sustained Compact Toroids in MRX

1
Sustained Compact Toroids in MRX
A First Look At Our Recently Completed Ohmic
Campaign

• S.P. Gerhardt, M. Yamada, H. Ji
• M. Inomoto1, E. Belova, R. Maqueda2
• Y. Ren, B. McGeehan, C. Jacobsen

1 Osaka University 2Nova Photonics
2
Ohmic Sustainment a Step Toward the SPIRIT
Oblate FRC Concept
Self-Organized Plasma With Induction,
Reconnection, and Injection Techniques

• 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 n1 modes.
• NBI to sustain the plasma and stabilize low-n
  co-interchange modes via FLR effects.

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.
3
This talk

• The MRX facility modified for Ohmic sustainment
  of Compact Toroids (CTs).
• Ohmic sustainment of FRCs
• FRC equilibrium maintained for gt300 ?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)?n1 tilt or n2 kink
  typically terminate the configuration.
• Argon, no tilt or kink is observed, and the
  spheromak plasma transitions to an FRC
  equilibrium during Ohmic.

4
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.

6 Flux Loops on Solenoid Flux penetration through
liner
New 2D Probe Array
7x6 array of Coil Triplets

• Spoke Probes
• Triplets at 5 radial locations
• Probes at 8 toroidal angles
• Midplane magnetic perturbations in BR and BZ

Triple Probe
Fast 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 300?s Demonstrated
No Ohmic
With Ohmic
Flux Plots From Magnetic Probe Array
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Sustainment Visible in Fast Camera Images
Fast Camera Images, Argon, White Light
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Ohmic Sustainment for 300?s Demonstrated
275 ?s
375 ?s
450 ?s
325 ?s
550 ?s
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Peaked Pressure Profile Evolves to Sustain FRC
Equilibrium
275 ?s
375 ?s
450 ?s
325 ?s
550 ?s
Electron Pressure Triple Langmuir Probe Radial
Scan Red Sustained Blue Decaying
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Increased Ohmic Voltage Leads to More Flux,
Longer Sustainment
Trapped Flux (mWb)
Ohmic Voltages 5kV-9kV Input Powers 300-800kW
Surface Voltage (V)
Maximum Ohmic voltage limited by null radius
expansion, not instability.
Current Density (A/m2)
Solenoid Current (kA)
Capacitance and Inductance Fixed for Longest
Ohmic Waveform
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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
Trapped Flux (mWb)
BZ
BZ
BZ
BR
BR
BR
Shot 65788
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Lighter Gasses Demonstrate Co-Interchange
Instabilities
Trapped Flux (mWb)

• Co-Interchange pressure driven modes.
• ?1
• Bad curvature everywhere
• MHD predicts instabilitycorrectly
• 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.

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
Steady Sustainment
Transient
Ohmic Waveform Limit
Lifetime/Resistive Time
Lifetime (?s)
Average Flux During Ohmic (mWb)
Average Flux During Ohmic
Discharge Peak Flux
Helium 4 4-10 6-11
Deuterium 2 2-4 4-6
Nitrogen 14 10-20 15-30
Neon 20 10-20 20-32
Argon 40 25-35 40-90
Krypton 84 35-50 50-100
Mass
?????s)
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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 n1 tilt spectrum
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Helium and Neon Spheromaks Often Tilt
Poloidal Field Vectors Toroidal Field Colors
Nova Photonics Fast Camera White Light, 100kHz
N1 amplitude Poloidal flux
Helium Example
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Increased EF and higher fill pressure can
suppress the n1 tilt
10 mT 8.2 mT 6.77 mT 5.4 mT 4.9 mT
Poloidal Flux (mWb)
All Neon
BR, n1 (T)
but n2 kink develops to terminate the discharge.
BR, n2 (T)
-q0
time (?s)
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Spheromak?FRC Conversion Observed in Argon Plasmas
With Ohmic
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Toroidal Field Decays As Poloidal Flux is
Sustained
time (?s)
66536 66523
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Conversion To FRC is Robust in Argon
Poloidal Flux (mWb)

• Transition Occurs
• in Argon and Krypton
• over a wide range of fill pressures and Ohmic
  voltages.
• never in He or Ne

Taylor Eigenvalue
Solenoid Current (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 decays?drops q.
• In He and Ne, when q0lt0.5, a terminal n2 mode
  appears.
• Similar to previous results
• In S-1, non-uniform Te profile leads to a drop in
  q and nonuniform ?, with n2 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
and Implications

• FRCs sustained for gt300?s 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 n1 and
  n2 instabilities with Ohmic.
• Nearby passive stabilizers are essentialwell
  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.

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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
330 ?s
380 ?s
280 ?s
480 ?s
Argon Counter-Helicity Merging with Sustainment
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Tilt Shows Field Opening
184 ?s
242 ?s
300 ?s
358 ?s
416 ?s

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

T. Hayashi, T. Sato, F. Wysocki, D.D. Meyerhofer,
M. Yamada, JPSF 54, 4172 (1985)
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BZ Profile Used to Estimate Flux, Plasma Size,
Taylor Eigenvalue
N0 Component from N-Probes
Inner Separatrix Radius
Outer Separatrix Radius
Null Radius
Separatrix Flux
Solenoid Surface Flux
Null Flux
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Lighter Gasses Demonstrate Rapid Instability
Helium Example
Trapped Flux (mWb)

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

Shot 65788
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Increased Ohmic Voltage Leads to More Flux,
Longer Sustainment
Ohmic Voltages From 5kV-9kV Input Powers
300-600kW