ISTW October Chengdu CDX-U The Lithium Tokamak – Results from CDX-U and Progress towards LTX R. Kaita, H. Kugel, T. Gray, D. Mansfield, J. Spaleta, J. Timberlake, L. Zakharov, Princeton Plasma Physics Laboratory, Princeton, NJ, USA V. Soukhanovskii, Lawrence Livermore National Laboratory, Livermore, CA, USA R. Maingi, Oak Ridge National Laboratory, Oak Ridge, TN, USA R. Doerner, University of California at San Diego, CA, USA Dick Majeski, PPPL and

ISTW October Chengdu CDX-U Outline  CDX-U lithium and fueling systems for 2005 –Final results from the CDX-U lithium program  Recycling and particle confinement time –~30% recycling coefficient (record for magnetically confined plasmas) –Temperature, impurity effects  New magnetic diagnostics, equilibrium reconstructions  Plasma confinement results –Approximate order of magnitude increase in confinement times –Exceeds ITER98P(y,1) ELMy H-mode scaling by  »Record confinement enhancement for an ohmic tokamak  LTX description and status –Implications of the CDX-U results for LTX

ISTW October Chengdu CDX-U Three lithium, two gas fueling systems available on CDX-U in 2005 R 0 =34 cm a = 22 cm  1.6 B T (0)  2.1 kG I P  80 kA  disch <25 msec T e (0)~100 eV n e (0)<6x10 19 m -3 CDX-U:  Lithium tray limiter –300 g of lithium in a toroidal tray –Half Li inventory liquid  New electron beam lithium coating system –Used lithium in tray as source  New resistively heated lithium evaporator –NSTX prototype  Gas injection systems –Wall mounted piezo valve –Supersonic gas injector R 0 = 34 cm, width = 10 cm 6 mm deep  Up to 1000Å of lithium coatings between discharges  600 cm 2 of liquid lithium forms lower limiter

ISTW October Chengdu CDX-U High power density electron gun intended to “spot heat” lithium Charging of probe tip insulator disturbs beam Gaussian beam profile, width 3 mm  Converted commercial gun  4 kV, mA typ.  sec. run typical  Total power modest: <1.6 kW  Power density high: < 60 MW/m 2  Objective: 1000Å lithium wall coatings –TF + VF used to guide beam »Can be pulsed to 600G; typ. 200 G –Lithium tray fill (~3 mm deep)used as evaporation target. »Lithium area ~600 cm 2 >> beam spot Maximum evaporation rate: 600 mg/minute

ISTW October Chengdu CDX-U  Electron beam heating induces flow  Flow very effectively inhibits localized heating  25 sec movies (visible, IR) 150 sec. into an e-beam run  Yellow denotes +55°C, red denotes +110°C  Field ramps from 200 G to 400G 10 sec into clip  If only conduction were active, area under beam would heat to 1400ºC in 0.1 sec. Beam spot. Note NO local heating 1000 Å wall coating in < 1 min, 30 sec before discharge Framing pauses, white flag at field ramp  Localized heat deposition (and/or beam current) induces lithium flows –Marangoni effect; temperature- dependent surface tension IR image Visible image Centerstack Lithium in tray

ISTW October Chengdu CDX-U Recycling coefficient reduced to ~0.3 with full liquid lithium tray + evaporative coatings  ~3  reduction in D  for full- tray liquid lithium operation (2000 cm 2 )  T e (a)~28 eV with lithium –~20 eV without »~17% D  emission correction  Bare tray: deuterium prefill only –Liquid lithium operation required 8  increase in gas fueling  Lithium reduces recycling coefficient R from ~1 to ~0.3 –Overestimate (background light)  Lowest R ever obtained for a magnetically confined plasma  Thinner coatings from resistive evaporator produced ~5% recycling reduction  2005 global R~ –1/2 tray + evaporator  D  emission at the centerstack –Lithium coated (solid) –Primary plasma contact Bare SS tray - R~1 Full liquid lithium tray, coatings (no confinement data, ) Resistive evaporator, e-beam (2005)

ISTW October Chengdu CDX-U Liquid lithium + coatings produce strong pumping  CDX-U lithium systems: –Electron beam heating, evaporation of the lithium tray limiter –Second resistive oven to coat centerstack »600 cm 2 liquid lithium limiter cm 2 solid lithium coatings  Exceeds wall pumping rate in a TFTR supershot by 2  –Active wall area is two orders of magnitude smaller High recycling Low recycling (Discharge duration 25 msec)

ISTW October Chengdu CDX-U Impurity ion temperature increases by 3  with lithium  Carbon impurity level (signal magnitude) drops by over an order of magnitude  No profile information (no radial localization)  No Thomson scattering available No lithium 24 eV Hot lithium 71 eV

ISTW October Chengdu CDX-U Lithium operation eliminated impurities  Oxygen emission was reduced to nearly the noise level  Water lines were eliminated from the RGA  Modeling (TSC, S. Jardin) indicated Z eff < 1.2 –But no direct measure of Z eff available Liquid lithium Bare SS tray Oxygen

ISTW October Chengdu CDX-U New magnetic diagnostics permited reconstructions, measurement of  E in 2005  Magnetic probes, compensated diamagnetic loop added  Equilibrium and Stability Code (ESC) modified to include vessel eddy currents –Response function approach –Calibrated with “step function” coil pulses –Compensation for nonaxisymmetric eddy currents B-dot probes Flux loop Rogowski coil Flux loop Diamagnetic loop, compensation coil

ISTW October Chengdu CDX-U Measured confinement times significantly exceed ELMy H-mode scalings  ITER98P(y,1) included START data (slightly larger “small” ST)  Confinement in CDX improved by 6  or more with lithium wall coatings, partial liquid lithium limiter  Exceeds scaling by 2-3   Largest increase in ohmic tokamak confinement ever observed Active Li evaporation No Li evaporation for 2 weeks  61kA <I p <78kA  2.1 kG  Identical loop voltage waveforms  0.5 < n e < 1  m -3  Gas puffing terminated several msec before peak in plasma current –“Pellet fueling” simulation  Pre-lithium confinement times: msec (kinetic) –New magnetics not available All discharges:

ISTW October Chengdu CDX-U Confinement time is correlated with recycling  Density pumpout rate (dn/dt) is a measure of recycling suppression  Strong pumping (low recycling) results in high confinement High recycling Low recycling

ISTW October Chengdu CDX-U Lithium discharges exhibit long confinement times, very low loop voltage  Reconstruction of centerstack limited plasma from ESC  Total coating of 13,000 Å (4 g) of lithium had been applied during preceding 2 hrs –1000 Å applied <1 min. before discharge   E for this discharge 6 msec  Surface voltage at current peak < 0.5V –300 J stored energy –L i ~ 0.7 –Very low ohmic power input: 45 kW –Low ohmic power a future concern »Lithium area 600 cm 2 for the discharges for which reconstructions (& surface loop voltage) are available »External loop voltage was lower with a full (2000 cm 2 ) tray (2003, 2004)  q 0 >1 in all analyzed lithium discharges –No sawteeth –No significant MHD Li tray

ISTW October Chengdu CDX-U LTX status  CDX-U has been disassembled –Vacuum vessel undergoing modifications  Shell support structure being modified to withstand disruptive forces  Poloidal field coil set is being upgraded –Present set incapable of holding equilibria with I p >80 kA  Thomson scattering system will be rebuilt –Based on existing ruby system –Will incorporate full amplifier set from PBX-M (~15J output) –12 spatial channels  Interferometer will utilize 1 fixed, 2 movable channels to provide a 5 point profile in 2 shots  Upgrade magnetics (more flux loops)  Reinstall spectroscopy diagnostics from CDX-U –But: shift from D-  to Lyman-  to reduce stray light problem  Schedule is for first plasma in Spring 2007 –Only a limited diagnostic set will be available

ISTW October Chengdu CDX-U Follow-on to CDX-U, LTX, will have 5 m 2 wall of liquid lithium film  Two e-beam lithium evaporators  Heaters will maintain shell temperature ~300ºC Shell fitted into vessel Stainless steel inner surface Inner & outer poloidal gaps Shell interior 1 cm copper with heater fixtures Toroidal gaps

ISTW October Chengdu CDX-U  Projections from recent renewal submission (Spring 05) –L-mode scaling  TSC, ASTRA projected confinement time for LTX at 3.8 kG, 250 kA was <3.1 msec  Observed confinement time for CDX-U at ~70 kA, 2.1 kG is already 2  higher  Existing tokamak scalings are not good predictors for lithium tokamak performance Prelithium CDX-U LTX (orig proj) - 2 KG, 250 kA LTX (orig proj) - 4 kG, 300 kA CDX-U performance has already exceeded code predictions for LTX START data CDX-U lithium (measured, ~70 kA, 2 kG) (M. Walsh, APS-DPP98) 5 msec ISTW October Chengdu

ISTW October Chengdu CDX-U LTX operation at I p > 100 kA requires new PF coils New coil set designed for I p ≤ 400 kA CDX-U New PF coils  Equilibrium modeling shows that CDX plasmas with I p ≥ kA scraped-off on outer limiter –Vertical field too low –One major factor limiting I p  New PF set, rearrangement of existing power supplies address this problem for LTX  Toroidal field will increase to 2.4 kG  Poloidal field coil set designed for a 400kA equilibrium –Higher current may be necessary to offset lowered loop voltage  Have power supplies for all external coils –New internal coil requires fast capacitor-driven IGBT supply

ISTW October Chengdu CDX-U Shell support structure redesigned for significant disruptive forces  Shell is electrically isolated from vessel –Ceramic breaks required for in-vessel mounting  Disruptive forces modeled by Zakharov  Maximum force ~ 5 kN –Overturning moment on shell quadrant –Impulse rules out internal ceramic supports  Load will be transferred to mounting points exterior to the vessel –Compliant G10 insulating supports –In-vacuum supports fully welded

ISTW October Chengdu CDX-U Diagnostics and auxilliary systems for LTX  Fueling with supersonic gas jets –HFS and LFS –Pellets on hold; ohmic input power in CDX is very low –Proposing low-energy neutral beam fueling ( kV)  Expanded, upgraded magnetic diagnostics  Upgraded Thomson scattering –Single pulse, 10-15J ruby –12 spatial points –Additional edge channels to be implemented  Multiple interferometer chords  Lyman-  detectors for recycling –Lithium reflectivity at nm is very low  All other spectroscopic diagnostics to be transferred from CDX-U

ISTW October Chengdu CDX-U LTX schedule  CDX-U vessel has been disassembled, being modified  Rework of shell supports underway  Shell will be assembled into the vessel at the PPPL shop facility in September for fitting –Removed for final reassembly, with all internal diagnostics and coils –Installation of new PF set in late 2006  First pumpdown in early 2007  New OH supply available in early spring 2007  First plasma (no lithium) in late spring 2007  First lithium in summer 2007 –Very limited diagnostic set in 2007

ISTW October Chengdu CDX-U Summary  In 2005 CDX-U simultaneously employed 600 cm 2 liquid lithium limiter Å between-shots lithium wall coatings –Higher recycling than full-tray (2000 cm 2 ) operation –But: new diagnostics, analysis for equilibrium, confinement  Particle removal rates produced in CDX-U sufficient to pump a TFTR supershot  Recycling coefficients of ~30% are the lowest ever achieved in a magnetically confined plasma  6-10  enhancement in low recycling discharge confinement times over high recycling case –Largest increase in ohmic tokamak confinement ever observed –Empirical tokamak scalings appear irrelevant to lithium tokamaks  CDX-U now being disassembled, converted to LTX –25  increase in liquid lithium surface over best-case CDX-U