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

3 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.

4 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 (D 2, 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.

5 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 Array 7x6 array of Coil Triplets 6 Flux Loops on Solenoid Flux penetration through liner Spoke Probes Triplets at 5 radial locations Probes at 8 toroidal angles Midplane magnetic perturbations in B R and B Z Triple Probe Fast Camera

6 FRC Capabilities Recently Upgraded, Including Ohmic Solenoid

7 FRC Sustainment Merging Spheromaks Have Oppositely Directed Toroidal Fields

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

9 Sustainment Visible in Fast Camera Images Fast Camera Images, Argon, White Light

10 Ohmic Sustainment for ~300  s Demonstrated 275  s 325  s 375  s450  s 550  s

11 Peaked Pressure Profile Evolves to Sustain FRC Equilibrium 275  s 325  s 375  s450  s 550  s Electron Pressure Triple Langmuir Probe Radial Scan Red: Sustained Blue: Decaying

12 Increased Ohmic Voltage Leads to More Flux, Longer Sustainment Capacitance and Inductance Fixed for Longest Ohmic Waveform Ohmic Voltages 5kV-9kV Input Powers: kW Trapped Flux (mWb) Surface Voltage (V) Current Density (A/m 2 ) Solenoid Current (kA) Maximum Ohmic voltage limited by null radius expansion, not instability.

13 Lighter Gasses Demonstrate Rapid Instability Helium Example Shot 65788

14 Lighter Gasses Demonstrate Co-Interchange Instabilities Helium Example Shot Trapped Flux (mWb) BZBZ BZBZ BZBZ BRBR BRBR BRBR

15 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. Trapped Flux (mWb) BZBZ BZBZ BZBZ BRBR BRBR BRBR 1 Belova et al, Phys Plasmas 2006; M. Yamada et al, Plasma and Fusion Research

16 Flux & Lifetime Best for Argon Average Flux During Ohmic Discharge Peak Flux Lifetime (  s) Average Flux During Ohmic (mWb) Lifetime/Resistive Time Steady Sustainment Transient Helium Deuterium Nitrogen Neon Argon Krypton Mass    s) Ohmic Waveform Limit

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

18 Helium and Neon Spheromaks Often Tilt Helium Example: Pure n=1 “tilt” spectrum Flux B R, n=1 B R, n=2 B R, n=3

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

20 Increased EF and higher fill pressure can suppress the n=1 tilt …but n=2 kink develops to terminate the discharge. 10 mT 8.2 mT 6.77 mT 5.4 mT 4.9 mT time (  s) Poloidal Flux (mWb) B R, n=1 (T) B R, n=2 (T) -q 0 All Neon

21 Spheromak  FRC Conversion Observed in Argon Plasmas With Ohmic

22 Toroidal Field Decays As Poloidal Flux is Sustained & Poloidal Flux (mWb) Toroidal Flux (mWb) Taylor Eigenvalue ( ) time (  s)

23 “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 Poloidal Flux (mWb) Taylor Eigenvalue Solenoid Current (A) time (  s)

24 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 q 0 <0.5, a terminal n=2 mode appears. Similar to previous results: –In S-1, non-uniform T e 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).

25 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 D 2, 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

26 Extra stuff, for reference and for poster

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

28 Sustainment Visible in Fast Camera Images 200  s 280  s 330  s380  s 480  s Argon Counter-Helicity Merging with Sustainment

29 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

30 B Z 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

31 Lighter Gasses Demonstrate Rapid Instability Shot Many toroidal mode numbers simultaneously unstable to co-interchange. Illustrates the importance of nearby conducting structures. Trapped Flux (mWb) Helium Example

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