Dynamo Current Drive

1. Dynamo Current Drive ICC/1-1Ra Progress on HIT-SI and Imposed Dynamo Current DriveT. R. Jarboe, C. Akcay, D. A. Ennis, C. J. Hansen, N. K. Hicks, A. C. Hossack, G. J. Marklin, B. A. Nelson, R. J. Smith, B. S. Victor University of Washington, Seattle, WA 98195, USA ICC/1-1Rb Flow and Magnetic Field Profiles in the HIST Spherical Torus Plasmas Sustained by Double Pulsing Coaxial Helicity InjectionM. Nagata, T. Higashi, M. Ishihara, T. Hanao, K. Ito, K. Matumoto, Y. Kikuchi, and N. Fukumoto, T. Kanki*Univerisity of Hyogo, Himeji, Hyogo 671-2201, Japan *Japan Coast Guard Academy, Kure, Hiroshima 737-8512, Japan 24th IAEA Fusion Energy Conference, San Diego, USA October 8-13, 2012

2. Outline • Explanation of dynamo current drive • Prediction of model • Experimental data • Possible implications for tokamaks • Summary

3. Both experiments drive edge current directly and produce current and flux amplification by dynamo current drive. HIST HIT-SI

4. Dynamo current drive can be understood based on well known concepts. • Electrons are frozen to magnetic fields. • Magnetic fluctuations can be considered separately from equilibrium (mean dynamo theory). • Field lines have tension. • Massless electrons transmit force to ions. • In the externally driven regions dynamo force brakes electrons so the force is in the direction of the current giving ion velocity in that direction. • In the dynamo driven region the force is with the electron flow resulting ion flow against the current.

5. Fluctuations crossing flux surfaces give cross-field current drive effect. • Fluctuations drives mean current. • BBll tension on the flux surface provides the force that drives current inside the flux surface.

6. Three conditions must be met for dynamo current drive to form and sustain an equilibrium. • Externally driven current must be higher speed than dynamo driven equilibrium current. • Cross-field fluctuations must be strong enough to drive the current. • Can be produce by instability. • Can be imposed externally. • The driven current must be in a region that supports an equilibrium.

7. Progress on Coaxial Helicity Injection Coaxial Helicity Injection Transient-CHI Steady-state CHI Non-inductive current start-up Dynamo current drive (ST: HIT-II) CHI start-up and OH coupling ramp-up (ST: HIT-II, NSTX-CHI; See EX-P2-10) Muti-pulsing CHI (better time averaged confinement) ST: HIST Spheromak: SSPX HIST (U.Hyogo,Japan) NSTX-CHI (PPPL) HIT-II (UW) SSPX (LLNL)

8. HIST sustains current and flux amplification in mean-field closed flux surfaces by dynamo Flux amplification Current amplification Dynamo Fluctuation 2nd CHI pulse HIST

9. MHD and Hall dynamo electric fields balance in the parallel mean-field Ohm’s law Electron dynamics MHD dynamo Hall dynamo Anti-dynamo Dynamo ExB drift Diamagnetic drift Core MHD dynamo Edge Core Hall dynamo Two fluid effect is large HIST (Driven region) Separatrix

10. Poloidal shear flow is generated by CHI Self-Generation of radial electric field Er Observation of Zonal flow–like poloidal flow profile Poloidal flow Diamagnetic drift is significant Itf Toroidal flow Poloidal shear flow layer ion It Density . It Er Steep gradient Reduction of dB HIST Separatrix

11. Imposed Dynamo Current Drive (IDCD)is a major advance toward fusion power. • Current driving fluctuations are imposed (not produced by instability). • A stable equilibrium can be sustained. • Current profile can be controlled. • The following data validates model: • Toroidal current versus time • Current profile • Injector impedance scaling with j/n of spheromak.

12. On HIT-SI the injectors drive the edge and impose the fluctuations. Imposed dynamo model predicts current time dependence. Data also validate 2-fluid XMHD simulation by NIMROD. HIT-SI T.R. Jarboe et al., Nucl. Fusion 52 (2012) 083017

13. IDCD equation models the toroidal current with the same fitting parameter across a range of shots. 13 HIT-SI

14. j/B a r Imposed dynamo model predicts current profile • The required B decreases as the minor radius decreases. • Inside the dynamo driven region the imposed B increases as the minor radius decreases. • The excess drive results in a constant j/B in the dynamo driven region*. For more on NIMROD simulation see PSI-Center poster TH/P2-02 transition *T.R. Jarboe et al., Nucl. Fusion 52 (2012) 083017 HIT-SI

15. Data validate imposed-dynamo prediction* that injector impedance  spheromak j/n • IDCD is engaged during the time between the vertical blue lines. *T.R. Jarboe et al., Nucl. Fusion 52 (2012) 083017 HIT-SI

16. Two-Fluid resistive MHD seems to capture the physics of HIST and HIT-SI HIST: • MHD and Hall dynamo electric fields balance in the parallel mean-field Ohm’s law. • Poloidal shear flow generated by CHI is consistent with two-fluid dynamo. HIT: • Imposed dynamo model is two-fluid. • Two-fluid simulations show good agreement: • n=1 to n=0 transition rate correct • Current gain only 30% low • Profile shape agrees with data as well as might be expected considering: • Injectors modeled as boundary conditions • Assumed constant density, resistivity, and viscosity

17. On a reactor and ITER the required injector parameters and fluctuation levels are quite modest. • Dynamo action should have the following effects on tokamaks: • Fluctuations flatten the j/B profile. • Locked RWM can stop plasma current. • RMPs change the edge equilibrium and rotation along with confinement. • Fluctuations that change rotate also change the current profile. • Low level uncontrolled fluctuation can randomly modify the equilibrium.

18. Summary of dynamo current drive • Dynamo current drive is a power efficient and cost effective method of sustaining plasma current. • Mean closed flux has been sustained by coaxial helicity injection and by imposed dynamo current drive. • With imposed dynamo current drive the current driving fluctuations are imposed externally (not produced by instability) • A stable equilibrium can be sustained • Current profile can be controlled • The amplitude of the fluctuations required for current drive in a reactor is quite low B/B  10-4. • Method may be compatible with sufficient confinement • Normal uncontrolled fluctuations not only affect confinement but also the equilibrium profile

19. The Taylor state that includes the injector fields can be used to predict the imposed B Br B vs. Time Data: Taylor:

20. HIT-SI imposes excess fluctuation inside the separatrix separatrix

21. HIT-SI3 provides a definitive test of profile control B2 required for constant  B2 produced (Taylor) B2 Injectors 120o out of phase Injectors in phase B2 HIT-SI3 

22. Injector requirements are nearly constant. rplas/rgi decreases with machine size Parameters for sustaining various plasmas with IDCD. The injector frequency is 80 kHz and a Zeff= 2 is used. alfv is the wavelength of the Alfvén wave at 80 kHz. On the two tokamaks the gyro-radius is the poloidal gyro-radius to approximate the banana width. The plasma oscillates about the mean flux surface by rplas because of the first order term jB in mean dynamo theory. It time averages to zero but has finite amplitude. [T.R. Jarboe et al., Nucl. Fusion 52 (2012) 083017 ]

23. Place three injectors on one side. Drives plasma rotation for stability Injectors have same preferred direction Injectors easy to shield from DC spheromak fields Thicker plate gives better injector opening Using higher power surface treatment Try perforated plate backed by a pumped chamber for density control Future Plans HIT-SI3

24. Limitations of HIT-SI • Density builds in time and spoils performance. • Limits the temperature because of pressure limit • Excessive wall loading can damage insulator. • Limits the time of a shot and the power  • Limits the spheromak current  • Limits Temperature and density • rplas/rgi too large for a confinement experiment (fundamental problem with all previous spheromak experiments).

25. HIT-PoP is designed to be TF-coil free, high-beta (>10%), and DCON stable with an IDCD sustained equilibrium Cryro-pumping Experiment to test and develop IDCD with sufficient confinement

26. The science is now advanced to the point it is ready for a new Dynomak device • A TF-coil free, high-beta (>10%), and DCON stable with an IDCD sustained equilibrium seems possible. • Thus, formation, sustainment, equilibrium, stability, and confinement are all in hand. • The goal of this dedicated current drive experiment will be to demonstrate good confinement while sustaining by IDCD.

27. Achieving sufficient confinement is expected • A stable equilibrium will be sustained having better confinement than previously sustained unstable configurations. • The amplitude of the plasma motion due to dynamo will be about an ion gyro radius. In previous experiments the amplitude was greater then the minor radius, unacceptably high. • Decaying spheromaks and RFPs have shown good confinement. • The fluctuations will be controlled to be only as large as necessary. Presently they are too large near the magnetic axis. • The fluctuations needed for the reactor seem acceptably low. • A larger machine will allow the development of good confinement. • Validated two-fluid NIMROD simulations show transient flux surface formation with only a current gain of 6 for an IDCD sustained equilibria. The experiment will have over 40 gain. With more optimal fluctuation profile sufficient confinement likely. • Required fluctuations at the magnetic axis are much less than for a transformer driven RFP. (The largest fluctuations are required in the externally driven region, which is around the magnetic axis on the RFP) • Will use much better (than previous spheromaks) density control • 1 second pulse will allow time for good confinement to develop

28. The Dynomak experiment Ip = 850 kA a = 0.6 m T = 1 keV n = 3.3 x 1019 m-1 tpulse= 1 s 3 m

29. Current amplification of 3 achieved at 36.8 kHz Figure shown on same scale as results from 14.5 kHz Still working on increasing power at this frequency

30. Density evolution similar at 14.5 and 36.8 kHz Fueling rate of 36.8 kHz shot is ~50% of the 14.5 kHz shot Similar ratio of δne/ne at both frequencies

31. Two fluid MHD simulations results are similar to data

32. B (unfiltered) Fluctuations are too large at smaller r to expect good confinement Bpol (averaged over a cycle) Btor (averaged over a cycle)

33. Injector requirements are nearly constant. rplas/rgi decreases with machine size

34. Helicity and power always injected