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Sustained Compact Toroids in MRX

Sustained Compact Toroids in MRX. A First Look At Our Recently Completed Ohmic Campaign. S.P. Gerhardt, M. Yamada, H. Ji M. Inomoto 1 , E. Belova, R. Maqueda 2 Y. Ren, B. McGeehan, & C. Jacobsen. 1: Osaka University 2:Nova Photonics.

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Sustained Compact Toroids in MRX

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  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 2:Nova 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 n=1 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 >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.

  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

  5. FRC Capabilities Recently Upgraded, Including Ohmic Solenoid

  6. FRC Sustainment Merging Spheromaks Have Oppositely Directed Toroidal Fields

  7. Ohmic Sustainment for ~300s Demonstrated No Ohmic With Ohmic Flux Plots From Magnetic Probe Array

  8. Sustainment Visible in Fast Camera Images Fast Camera Images, Argon, White Light

  9. Ohmic Sustainment for ~300s Demonstrated 275 s 375 s 450 s 325 s 550 s

  10. 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

  11. 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

  12. Lighter Gasses Demonstrate Rapid Instability Helium Example Shot 65788

  13. Lighter Gasses Demonstrate Co-Interchange Instabilities Helium Example Trapped Flux (mWb) BZ BZ BZ BR BR BR Shot 65788

  14. Lighter Gasses Demonstrate Co-Interchange Instabilities Trapped Flux (mWb) • 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. BZ BZ BZ BR BR BR 1Belova et al, Phys Plasmas 2006; M. Yamada et al, Plasma and Fusion Research 2007..

  15. 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 Deuterium 2 2-4 4-6 Helium 4 4-10 6-11 Nitrogen 14 10-20 15-30 Neon 20 10-20 20-32 Krypton 84 35-50 50-100 Argon 40 25-35 40-90 Mass s)

  16. Spheromak Sustainment Merging Spheromaks Have Toroidal Fields Which Point in the Same Direction

  17. Flux BR, n=1 BR, n=2 BR, n=3 Helium and Neon Spheromaks Often Tilt Helium Example: Pure n=1 “tilt” spectrum

  18. Helium and Neon Spheromaks Often Tilt Poloidal Field Vectors Toroidal Field Colors Nova Photonics Fast Camera White Light, 100kHz N=1 amplitude Poloidal flux Helium Example

  19. Increased EF and higher fill pressure can suppress the n=1 tilt 10 mT 8.2 mT 6.77 mT 5.4 mT 4.9 mT Poloidal Flux (mWb) All Neon BR, n=1 (T) …but n=2 kink develops to terminate the discharge. BR, n=2 (T) -q0 time (s)

  20. SpheromakFRC Conversion Observed in Argon Plasmas With Ohmic

  21. Poloidal Flux (mWb) Toroidal Flux (mWb) Taylor Eigenvalue () Toroidal Field Decays As Poloidal Flux is Sustained time (s) 66536 & 66523

  22. “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)

  23. 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 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).

  24. …and Implications Conclusions • FRCs sustained for >300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 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.

  25. Extra stuff, for reference and for poster

  26. Co-Helicity Merging Forms a Spheromak • Initial spheromaks have the same polarity of toroidal field • Merging results in a new spheromak.

  27. Sustainment Visible in Fast Camera Images 200 s 330 s 380 s 280 s 480 s Argon Counter-Helicity Merging with Sustainment

  28. Tilt Shows Field Opening 184 s 300 s 242 s 416 s 358 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)

  29. BZ Profile Used to Estimate Flux, Plasma Size, Taylor Eigenvalue N=0 Component from N-Probes Inner Separatrix Radius Outer Separatrix Radius Null Radius Separatrix Flux Solenoid Surface Flux Null Flux

  30. 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

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

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