1 Boundary Physics Five Year Plan R. Maingi, H. Kugel* and the NSTX Team Oak Ridge National Laboratory * Princeton Plasma Physics Laboratory Five Year Planning Workshop Princeton Plasma Physics Laboratory Dec. 2002

2 Boundary physics activities have enabling technology goals and science goals Enabling technology - tailor edge plasma to optimize discharges –evaluate power handling needs and solutions –assess fueling and particle pumping needs –develop and evaluate wall conditioning techniques Science goals –characterize edge power and particle transport regimes parallel vs. cross-field do conventional aspect ratio models fit? –understand effect of ST features on boundary physics, e.g. large in/out B t ratio –pedestal asymmetry –target heat flux profiles large SOL mirror ratio and short connection length

3 Boundary goals are linked to IPPA goals 5-Year Objective: Make preliminary determination of the attractiveness of the spherical torus (ST), by assessing high-  T stability, confinement, self-consistent high bootstrap operation, and acceptable divertor heat flux, for pulse lengths much greater than energy confinement times. Implementing Approach : –Disperse Edge Heat Flux at Acceptable Levels –Study ST specific effects high mirror ratio high flux expansion

4 Boundary physics talk outline Fuel and impurity particle control –sources and sinks –wall conditioning techniques Power handling and mitigation –heat flux scaling and power balance –steady heat flux mitigation and off-normal event study H-mode, pedestal and ELM physics –profile shapes, gradients and widths –ELM characterization and control Edge, SOL, divertor and wall conditioning physics –nature of cross-field transport and “classical” parallel transport –ST kinetic effects

5 Fuel and impurity particle control background Center stack gas injection enabled routine H-mode access –higher  due to lower pressure peaking factor –long pulse due to low volt-second consumption rate, owing to high bootstrap fraction from edge pressure gradient Limited control leads to uncontrolled density rise Location of gas fueling source affects H-mode access Goal: make density independent of time Staged plan –improve control of sources, aiming toward higher efficiency –improve control of sinks via wall conditioning and active pumping

6 Uncontrolled (non-disruptive) density rise in long pulse H-modes I p [MA] P NBI /10 [MW] n e [10 19 m -3 ] D  [au] W MHD *10 [MJ] H98pby2  n e    m    sec  sec  sec  sec  Inner and outer ‘pedestal’ n e show similar trends Profile requires ~ msec to flatten out

7 Density rise in long pulse H-modes may be reduced with improved gas injector control I p [MA] P NBI /10 [MW] n e [10 19 m -3 ] D  [au] W MHD *10 [MJ] H98pby2  Center stack injector has a long tube and pumpout time “Shoulder” injector should have better control (FY03)

8 Improved source control –more poloidal locations for gas (FY03-04) –supersonic gas nozzle (FY03-04) –Lithium pellets (FY03) –D 2 pellet injection (FY05) –CT injection? (FY07 ) New diagnostics –D  cameras for core fueling –pellet diagnostics –more edge Thomson channels –divertor Langmuir probe upgrade Fuel and impurity source control plan ?

9 Fuel and impurity sink control plan Improved sink (density) control –improved boronization (FY03) high temp. boronization morning boronization between shots TMB –lithium pellets (FY03) –in-vessel cryopumps (FY06,FY04 ) –lithium module(FY08, FY06 ) New diagnostics –n C (r) f/CHERs, Z eff f/MPTS (FY03) –D  cameras for core fueling –divertor SPRED –upgraded Langmuir probe array –fast pressure gauges ? ?

10 Preliminary neutral transport studies show possible plenum pressure sensitivity to strike point Same model and n e, T e, P e profiles for DIII-D as in Maingi, et. al., Nucl. Fusion 39 (1999) 1190 Ion current = 5kA, used for normalization Leakage conductance = 20 m^3/sec Menon, ORNL

11 Power handling and mitigation background ST’s can have high heat flux because of high P heat /R – NSTX: P NBI ~ 7 MW, P RF ~ 6 MW, P heat /R ~ 15.3 Highest q peak in NSTX ~ 10 MW/m 2 –  T div ~ C in LSN –extrapolates to ~ 3 sec. pulse length limit (  T div ~ C) For T e < 2 keV, current diffusion time << 3 sec. –If T e increases, current diffusion time could approach few seconds Goal: assess power balance and survey heat flux in many scenarios (vs. shape, input parameters, etc.) Staged plan –quasi-steady power balance over next few years –transient events in future

12 Peak heat flux increased with NBI power in lower- single null configuration Good power accountability:P div in+out ~ 70% of P SOL

13 Peak heat flux increased decreased in double-null (outer strike point)

14 Power handling and mitigation plan Experimental plan –Power balance studies (FY02+) divertor heat flux vs core and divertor radiation parallel vs perpendicular transport –Detailed comparison between single-nulls and double-nulls (FY03+) –Heat flux reduction studies (FY03+) –Impact of fast events, e.g. ELMs, IREs, and disruptions (FY05) New Diagnostics –Cross-calibrate core bolometry with platinum-foils (FY03) –Add and optimize divertor bolometer channels (FY03-04) –Add two slow and one fast infrared cameras (FY04, FY05, FY07)

15 H-mode, pedestal and ELM physics background L-H transition physics (common with transport group) –P LH higher than scalings - trapped particles, poloidal damping? –I p and maybe B t dependence of P LH appears different –role of E X B shear? Pedestal: profile shapes, gradients and widths –density pedestal high and in/out asymmetry –pedestal T e < 400 eV, below predictions ELMs –Tokamak-like ELMs in double-null –Usually ELM-free or very large events in single-null –Fueling affects ELM type strongly Plan: characterization studies over next few years and optimization studies over longer term

sec sec sec sec   sec sec sec High n e and relatively low T e pedestal observed n e profile hollow after transition and fills in ms Moderate in/out n e asymmetry usually observed T e profile flattens initially and peaks later in time sec

Hz LSN DN  W/W 0 : 5-25%  W/W 0 : 1-4%   Time (s) D  (arb.) ELM behavior depends on operating conditions Higher Fueling Lower Fueling Bush (ORNL)

18 H-mode, pedestal and ELM physics plan Continue L-H mode transition parametric studies Pedestal studies –height, width, and maximum gradient scalings –role of fueling profile in setting density width ELM studies –role of shape and fueling –conductive vs. convective losses –optimization for density and impurity control New Diagnostics –extra edge Thomson channels –improved time resolution in CHERs –edge Helium line ratio being considered

19 Edge, SOL, divertor and wall conditioning physics background Classical divertor heat flow regimes applicable? –high divertor T e, sheath-limited heat flow –low divertor T e, conduction-limited heat flow –detachment Impact of unique ST features –high SOL mirror ratio, short connection length, high flux expansion –in/out B t ratio and target; E X B flows Cross-field transport: diffusive vs. intermittent –intermittent transport appears strong in both L and H-modes –any cross-correlation with ELMs? Wall conditioning technique assessment Plan: near-term focus on nature of cross-field transport, and longer term on ST kinetic effects

20 Density is intermittent Rate of bursts is 2E3 Poloidal field is much less intermittent Instantaneous particle flux ~10 19 m -2 s -1 H-mode # L-mode # Intermittent behavior observed in L and H-mode Boedo, UCSD Time [sec] n e [cm -3 ] Time [sec]

21 # GPI view Gas Puff Imaging shows a narrower, more quiescent emission pattern in H-mode than L-mode Radial vs. poloidal imaging Gas manifold Side-viewing re- entrant window Image distributed gas puff of Helium (or Deuterium) View looks directly at field lines Zweben (PPPL), Maqueda (LANL) sec (L) sec (H) 12 cm core SOL

22 UEDGE modeling needed to estimate transport rates D  = 1 m 2 /sec,   = 3 m 2 /s Porter (LLNL) LLNL using diffusive cross- field transport UCSD examining convective cross-field transport

23 Progress made in 2-D modeling of L-modes Pigarov (UCSD) UEDGE with convective cross- field transport model Able to match L-mode well Concluded that main chamber recycling plays role in NSTX

24 Edge, SOL, and divertor physics plan Do classic divertor heat flow models apply? (FY03-04) –need UEDGE simulations for interpretation Continue GPI studies (FY03+) –simulation and coupling with theory crucial (e.g. BOUT, DEGAS-2) Cross-field transport and SOL scaling with reciprocating probe (FY03+) Experiments to test ST kinetic effects (FY05+) New Diagnostics –divertor Langmuir probe array with improved spatial resolution (FY04) –divertor Thomson scattering (FY06) –X-point reciprocating probe (FY06) –energy extract analyzer (FY06) –imaging spectrometer (FY07)

25 Wall conditioning physics plan Wall conditioning technique assessment (FY03+) –how does boronization change wall physics? –how do those changes improve the core plasma? New Diagnostics –quartz crystal deposition monitors for real time flux –divertor SPRED for impurities –divertor materials probes (like DiMES on DIII-D) –plus all other divertor diagnostics...

26 Power Handling FY weeks/year IPPA: 5 year IPPA: 10 yr Fast CHERs Power balance, heat flux scaling Divertor swept Langmuir probes X-point Reciprocating probe Helium Line ratio? MPTS upgrades ST Edge Physics Improved TMB Divertor SPRED Particle Control and Fueling Li/B pellets Cryopumps and baffles Liquid li module ? Fast pressure gauges D  cameras Multi-pulse CT injector ? Fueling studies Supersonic gas injectors D  pellets Divertor bolometer, upgrade Add’l IR camera Fast IR camera PFC upgrade? Energy Extract Analyzer DiMES probe Imaging Spectrometer Add’l IR camera Pedestal and ELM characterizationEdge Optimization Studies Heat flux reduction scenarios Off-normal event studies Cryo-pumps ? MPTS divertor chans Particle balance Long pulse with density control Long pulse with power handling Comprehensive edge physics studies

27 High-Field Side Gas Injector Fueling Allows Early L-H Transition and Longer H-mode Duration