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Solenoid-free Plasma Start-up In NSTX Using Transient Coaxial Helicity Injection

Office of Science. Supported by. Solenoid-free Plasma Start-up In NSTX Using Transient Coaxial Helicity Injection Roger Raman, Univ. of Washington For the NSTX Team University of Maryland 9 May 2006 College Park, MD Acknowledgments

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Solenoid-free Plasma Start-up In NSTX Using Transient Coaxial Helicity Injection

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  1. Office of Science Supported by Solenoid-free Plasma Start-up In NSTX Using Transient Coaxial Helicity Injection Roger Raman, Univ. of Washington For the NSTX Team University of Maryland 9 May 2006 College Park, MD Acknowledgments T.R. Jarboe, D. Mueller, B.A. Nelson, M.G. Bell, M. Ono, M.J. Schaffer Institutions Univ. of Washington, PPPL, ORNL, GA, Nova Photonics College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAERI Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep

  2. Outline • Motivation for solenoid-free plasma startup • Helicity Injection & Implementation of CHI • Requirements for Transient CHI • Experimental results from NSTX • Brief summary of HIT-II results • Brief summary of other plasma-startup methods • Summary and Conclusions

  3. Solenoid-free Plasma Startup is Essential for the Viability of the ST Concept • Elimination of the central solenoid: • Simplifies the engineering design of tokamaks (Re: ARIES AT & RS) • Allows access to lower aspect ratio (high β) • CHI is capable of both plasma start-up and edge current in a pre-established diverted discharge - Edge current profile for high beta discharges

  4. 160 kA of closed flux current produced in NSTX, without using a solenoid • Transient Coaxial Helicity Injection (CHI), previously demonstrated on HIT-II* at U- Washington • Important step in the production of a starting equilibrium for solenoid free operation • Conventional tokamak uses solenoid • ARIES-AT has no solenoid • ST has advantages of high β and good E • ST reactor cannot use solenoid • Alternate method for plasma startup is essential for ST concept • Could also reduce the cost of a future tokamak reactor *R. Raman, T.R. Jarboe, B.A. Nelson et al., Phys Rev. Lett. 90 075005 (2003)

  5. To inject helicity one must apply voltage to a flux tube

  6. NSTX incorporates toroidal insulation breaks to enable CHI operation Transient CHI: Axisymmetric reconnection leads to formation of closed flux surfaces. Driven (Steady State) CHI: Non-axisymmetric modes needed for closed flux generation Fast Camera (R. Maqueda, Nova Photonics & L. Roquemore, PPPL)

  7. NSTX CHI Builds on Progress made on HIT-II HIT-II (R/a = 0./0.2m) NSTX: (R/a = 0.86/0.68m, 30x HIT-II Vol)

  8. Simultaneous Requirements for Transient CHI • Bubble burst current*: = injector flux = flux foot print width = current in TF coil • Time needed to displace toroidal flux • For typical voltage at the injector after breakdown ~500V need ~1 ms to displace 600 mWb • Energy for peak toroidal current: • Exceed Energy for ionization and heating to 20eV (~50eV/D) • For 2 Torr.L injected, need ~2kJ * T.R. Jarboe Fusion Tech.15, 7 (1989)

  9. Upgrades that enabled progress in Transient CHI • Replaced rectifier PS with capacitor PS • Added MOV and Snubber • Gas & microwave injection from lower divertor region • Improved Absorber insulator design • Discharge resistor across capacitor bank

  10. Improved pre-ionization to a level that results in injected gas 10 times less than in 2004 Location of center stack Injected gas amount now same as that used for inductive discharges Divertor gap ECH: T. Bigelow (ORNL)

  11. Closed flux current generation by Transient CHI Abs. Arc R. Raman, B.A. Nelson, MG.Bell, et al., PRL 97, 175002 (2006)

  12. Discharges without an absorber arc show high current multiplication ratios of 60 Zero injector current

  13. Improvement in high current generation due to operation at higher voltage • 2006 discharges operated at higher toroidal field and injector flux • EFIT is done when no injector current is present • Magnetic sensors and flux loops used in reconstruction LRDFIT (J. Menard)

  14. 8 ms 12 ms Electron temperature and density profiles become less hollow with time 120814: Black: 8ms, Red: 12ms Profile becomes less hollow with time Plasma and Injector current Thomson Scattering (B. LeBlanc)

  15. Some discharges persist for as long as the equilibrium coil currents are maintained. These discharges are sensitive to gas pressure Fast camera: R. Maqueda

  16. Movie of a high current discharge. Fast Camera: R. Maqueda & L. Roquemore

  17. Highest ever plasma current on HIT-II (300 kA) obtained using CHI start-up • No transient coil currents • Discharges initiated & maintained in diverted configuration • CHI is more tolerant to field errors • Discharges can be initiated when -VLoop is applied to CHI

  18. Current multiplication needed for larger machines achieved on NSTX • Attainable current multiplication (CM) is given as: • For similar values of BT, toroidal flux, • So CM in NSTX should be 10x HIT-II, which is observed • CM is understood from HIT-II and NSTX results • No reason to believe CM will get worse in a larger machine (may get better)

  19. Maximum injector currents determined by system voltage limits • Assuming constant resistivity on field lines, For similar values of , at the same voltage, in HIT-II should be ~10 x NSTX • Consistent with observed ~15-30kA in HIT-II vs ~2-4 kA in NSTX • Also consistent with the bubble burst relation, which requires 10x more current in HIT-II than in NSTX • With 10-20kA of injector current MA levels of startup current should be possible in larger STs

  20. Full 2kV capability in NSTX would increase Ip ~ 300kA. Voltage, flux optimization allowed HIT-II to increase closed flux current as capacitor charging voltage was increased HIT-II data: R. Raman, T.R. Jarboe et al., Nuclear Fusion, 45, L15-L19 (2005)

  21. Plasma Startup Using Outer Poloidal Field Coils Outer PFs has been used to startup the plasma: • MAST (START) - poloidal field coils + radial compression • JT-60U - Aggressive application of RF heating and current drive Three important conditions for “Inductive” plasma startup: Satisfy the “Lloyd condition” for plasma startup.With strong ECH (in DIII-D), ET·BT / BP ≥ 0.12  kV/m. The field null must be maintained for a sufficient duration 2 – 3 ms in the presence of wall eddy currents. 3. Sufficient available flux for subsequent current ramp up to ~ 500 kA.

  22. Creation of High-Quality Field-Null with Significant Poloidal Flux is Possible with NSTX PF Coils Radial profile of flux Mod-B contours (Gauss) 24 kA Flux contours - 24 kA 3.4 kA • ~0.15 Vs available on NSTX • 1 MA achieved with ~0.3 Vs from solenoid • Sugggests possibility of ~ 500 kA in NSTX - ~20 kA produced in FY 2005 - Improvements to pre-ionization are needed - e.g. plasma injectors, ECH W. Choe, J. Kim and M. Ono, NF 45 (2005) 1463

  23. Solenoid-free Start-up with Outer Poloidal Field Coils Three scenarios being investigated J. Menard Improved preionization and PF coil waveforms to be optimized

  24. Pegaus – Plasma Gun Injector Produces Current FilamentsRelaxed plasmas can be formed • At low IInj and/or high pitch angle, helical current filaments form • M = Ip/Iinj = Geometric stacking • Filaments merge into cylindrical sheet as IInj increased, pitch decreased • M > Geometric stacking • At low fields (Bf ≈ 0.01 T, Bq ≈ 0.005 T), tokamak-like relaxation occurs • M >> Geometric stacking Filaments Reconnected Sheet Relaxed GDG, ISTW 2006, Chengdu Greg Garstka (Pegasus)

  25. 160kA closed flux current in NSTX validates feasibility of CHI for high current generation • NSTX has addressed the plasma startup issue for the ST by demonstrating a viable alternate plasma startup method • World record for non-inductively generated closed flux current • Other Solenoid-Free plasma start-up methods are also being tried (Synergistic effects) • demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor, • a remarkable multiplication factor of 60 between the injected current and the achieved toroidal current, compared to six in previous experiments, • results were obtained on a machine designed with mainly conventional components and systems, • indicate favorable scaling with machine size • HIT-II results indicate efficient coupling to other current drive methods is feasible • Future experiments in NSTX to explore coupling to OH • 200kW ECH to heat the CHI plasma • Coupling to RF and NBI

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