1 Plasma Start-up In NSTX Using Transient CHI R. Raman, T.R. Jarboe 1, D. Mueller 2, B.A. Nelson 1, M.G. Bell 2, M. Ono 2, T. Bigelow 3, R. Kaita 2, B. Leblanc 2, R. Maqueda 4, J. Menard 2, S. Paul 2, L. Roquemore 2 and the NSTX Research Team 1 University of Washington, Seattle, USA 2 Princeton Plasma Physics Laboratory, USA 3 Oak Ridge National Laboratory, Oak Ridge, TN, USA 4 Nova Photonics, USA 21 st IAEA Fusion Energy Conference October 2006 Chengdu, China Supported by Office of Science 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 *Work supported by US DOE contracts DE-FG03-9ER54519 and DE-AC02-76CH03073.

2 Primary results For the first time, 160 kA of closed flux current is produced in the National Spherical Torus Experiment (NSTX), without using a solenoid. A process called Transient Coaxial Helicity Injection (CHI), previously demonstrated on the HIT-II experiment at the University of Washington [1] is now used on NSTX to generate the self-contained equilibrium. This is an important step in the production of a starting equilibrium for solenoid free operation. Until now, almost all tokamaks and spherical torus plasma confinement devices have relied on a solenoid, through the center of the device, to produce the initial plasma current needed to confine the plasma. An alternate method for plasma startup is essential for developing a fusion reactor based on the spherical torus concept and could also reduce the cost of a future tokamak reactor as well. [1] R. Raman, T.R. Jarboe, B.A. Nelson et al., “Demonstration of plasma startup by coaxial helicity injection,” Phys Rev. Lett (2003)

3 Outline Motivation for solenoid-free plasma startup Implementation of Coaxial Helicity Injection (CHI) in NSTX Requirements for Transient CHI Experimental results from NSTX –Brief summary of HIT-II results Summary and Conclusions

4 Implementation of CHI in NSTX In the CHI method, a plasma current is rapidly produced by forming a discharge between coaxial electrodes connected to an external power supply in the presence of toroidal and poloidal magnetic fields. The initial poloidal field configuration is chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes, with the toroidal plasma current forming closed flux surfaces. CHI is implemented in NSTX by driving current along field lines that connect the inner and outer lower divertor plates (the injector region). For experiments reported here, a 15 or 40 mF capacitor bank was used at up to 1.75 kV to provide the injector current.

5 Implementation of CHI in NSTX Transient CHI: Expect axisymmetric reconnection at the injector to result in formation of closed flux surfaces

6 Implementation of CHI in NSTX The standard operating condition for CHI in NSTX uses the inner vessel and inner divertor plates as the cathode while the outer divertor plates and vessel are the anode. The operational sequence for CHI involves first energizing the toroidal field coils and the poloidal field coils to produce the desired flux conditions in the injector region. The CHI voltage is then applied to the inner and outer divertor plates and a pre-programmed amount of gas is injected in a cavity below the lower divertor plates. These conditions cause the gas in the lower divertor region to ionize and result in current flowing along helical magnetic field lines connecting the lower divertor plates. The applied toroidal field causes the current in the plasma to develop a strong toroidal component, the beginning of the desired toroidal plasma current. If the injector current exceeds a threshold value, the resulting ∆B tor 2 (J pol  B tor ) stress across the current layer exceeds the field-line tension of the injector flux causing the helicity and plasma in the lower divertor region to extend into the main torus chamber.

7 Requirements for optimizing Transient CHI Bubble burst current* Volt-seconds to replace the toroidal flux –For 600 mWb, at ~500V need ~1 ms just for current ramp-up Energy for peak toroidal current Energy for ionization of injected gas and heating to 20eV (~50eV/D) –For 2 Torr.L injected, need ~2kJ * T.R. Jarboe,"Formation and steady-state sustainment of a tokamak by coaxial helicity injection," Fusion Technology 15, 7 (1989).

8 Capacitor bank used in Transient CHI Experiments 50 mF (10 caps), 2 kV Operated reliably at up to 1.75kV Produced reliable breakdown at ~ 1/10th the previous gas pressure (20 Torr.Liter used in 2004) –Constant voltage application allowed more precise synchronization with gas injection –EC-Pi and gas injection below divertor used for Pre-ionization assist

9 Improved pre-ionization to a level that results in injected gas 10 times less than in 2004 Novel pre-ionization system –Injects gas and 10-20kW of 18GHz ECH in a cavity below the lower divertor gap –Successfully tested, achieved discharge generation at injected gas amount of < 2 Torr.Liter Fast Crowbar system –Rapidly reduces the injector current after the CHI discharge has elongated into the vessel. The small glow shown by the arrow is in the gap between the lower divertor plates and it is produced solely by EC- Preionization of the gas injected below the lower divertor plates. No voltage is applied. Shot Shot EC-Pi glow along the center stack Divertor gap ECH: T. Bigelow (ORNL)

10 Closed flux current generation by Transient CHI Plasma current amplified many times over the injected current. The sequence of camera images shows a fish eye image of the interior of the NSTX vacuum vessel. The central column is the center stack, which contains the conventional induction solenoid. The lower bright region seen at 6ms is the injector region. 6 ms8 ms10 ms 12 ms15 ms17 ms Hiroshima University (N. Nishino) Camera Images: R. Kaita (PPPL)

11 Discharges without an absorber arc show high current multiplication ratios (I p / I inj ) of 60

12 Dramatic improvement in closed flux current generation from 2005 LRDFIT (J. Menard) 2006 discharges operated at higher capacitor bank voltage and higher toroidal field

13 Electron temperature and density profiles become less hollow with time : Black: 8ms, Red: 12ms : Black: 8ms, Red: 10ms Thomson (B. LeBlanc) Profile becomes less hollow with time Plasma and Injector current

14 Thomson scattering data indicates Te drops to 50% in 3- 5ms  TauE ~ 4ms Zero-D estimates indicate 200kW ECH would increase Te ~ 60eV in 8ms and 100eV in 20ms, assuming TauE does not increase. Consistent with Radiated power levels of <100kW Consistent with low electron densities of ~2x10 18 m -3, for impurity burn through. Li a possibility for controlling Oxygen. Data indicates that ~200kW of ECH would increase Te to ~100eV

15 Some discharges persist for as long as the equilibrium coil currents are maintained Fast camera: R. Maqueda

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

17

18 Favorable scaling with machine size Attainable current multiplication is given as, For similar values of B T, So current multiplication in NSTX should be 10x HIT-II, which is observed Next step STs would have about 10x the toroidal flux in NSTX, Which means current multiplication ratios in excess of 100 is not unrealistic in larger STs Potential for high current multiplication in larger STs

19 Allowable injector currents determined by maximum voltage Assuming constant, For similar values of, at the same voltage, in HIT-II is about 10 times higher than in NSTX Consistent with ~15-20kA on HIT-II vs ~2kA in NSTX Also consistent with the bubble burst relation, Which requires 10x more current in HIT-II than in NSTX 10x more injector flux of that in present NSTX 60kA experiments with 10x more injector flux leads to >2MA startup currents with 20kA injector current in future larger machines.

20 Full 2kV capability in NSTX would increase Ip ~ 300kA Best results from NSTX 2005 and 2006 HIT-II data: R. Raman, T.R. Jarboe et al., Nuclear Fusion, 45, L15-L19 (2005) Voltage, flux optimization allowed HIT-II to increase closed flux current as capacitor charging voltage was increased

21 Record non-inductive plasma startup currents in a tokamak (160kA in NSTX) verifies high current feasibility of CHI for plasma startup applications The significance of these results are: 1)demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor, 2)a remarkable multiplication factor of 60 between the injected current and the achieved toroidal current, compared to six in previous experiments, 3)results were obtained on a machine designed with mainly conventional components and systems, 4)indicate favorable scaling with machine size. NSTX high current discharges not yet optimized –Extension to ~300kA should be possible at 2kV Future experiments to explore coupling to OH –200kW ECH to heat the CHI plasma –Coupling to RF and NBI