ISTW2006 11-13 October Chengdu CDX-U The Lithium Tokamak – Results from CDX-U and Progress towards...

ISTW200611-13 October

Chengdu

CDX-U

PRINCETON PLASMA PHYSICS LABORATORY

PPPL

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, USAR. Doerner, University of California at San Diego, CA, USA

Dick Majeski, PPPL

and


Chengdu

CDX-U


PPPL

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 2 - 3» Record confinement enhancement for an ohmic tokamak

LTX description and status

– Implications of the CDX-U results for LTX


Chengdu

CDX-U


PPPL

Three lithium, two gas fueling systems available on CDX-U in 2005

R0=34 cma = 22 cm

1.6BT(0) 2.1 kG

IP 80 kAdisch<25 msec

Te(0)~100 eVne(0)<6x1019 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

R0= 34 cm, width = 10 cm6 mm deep

Up to 1000Å of lithium coatings between discharges600 cm2 of liquid lithium forms lower limiter


Chengdu

CDX-U


PPPL

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, 300 - 400 mA typ. 300 - 400 sec. run typical Total power modest: <1.6 kW Power density high: < 60 MW/m2

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 cm2 >> beam spot

•Maximum evaporation rate: 600 mg/minute


Chengdu

CDX-U


PPPL

QuickTime™ and aCinepak decompressor

are needed to see this picture.

QuickTime™ and aCinepak decompressor

are needed to see this picture.

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 rampLocalized heat deposition (and/or beam current) induces lithium flows

–Marangoni effect; temperature-dependent surface tension

IR image

Visible image

Centerstack

Lithium in tray


Chengdu

CDX-U


PPPL

Recycling coefficient reduced to ~0.3 with full liquid lithium tray + evaporative coatings

~3 reduction in D for full- tray liquid lithium operation (2000 cm2) Te(a)~28 eV with lithium

– ~20 eV without» ~17% Demission 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~0.5 - 0.6– 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, 2003-2004)

Resistive evaporator,e-beam (2005)


Chengdu

CDX-U


PPPL

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 cm2 liquid lithium limiter + 3000 cm2 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)


Chengdu

CDX-U


PPPL

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 lithium24 eV

Hot lithium71 eV


Chengdu

CDX-U


PPPL

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 Zeff < 1.2

– But no direct measure of Zeff available

Liquid lithium

Bare SS tray

Oxygen


Chengdu

CDX-U


PPPL

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


Chengdu

CDX-U


PPPL

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

0

0.001

0.002

0.003

0.004

0.005

0.006

0.00E+00 2.00E-03 4.00E-03 6.00E-03

ITER98P(y,1) (sec)

_e ( )sec

Active Lievaporation

No Li evaporation for 2 weeks

61kA <Ip <78kA

2.1 kG Identical loop voltage waveforms 0.5 < ne < 1 1019 m-3

Gas puffing terminated several msec before peak in plasma current

– “Pellet fueling” simulation

Pre-lithium confinement times: 0.6 - 1.1 msec (kinetic)

– New magnetics not available

All discharges:


Chengdu

CDX-U


PPPL

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

0

0.001

0.002

0.003

0.004

0.005

0.006

-3.E+21 -2.E+21 -1.E+21 0.E+00 1.E+21 2.E+21

dne/dt (sec -1 m-3)

_e ( )sec

High recycling

Low recycling


Chengdu

CDX-U


PPPL

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– Li ~ 0.7– Very low ohmic power input: 45 kW– Low ohmic power a future concern

» Lithium area 600 cm2 for the discharges for which reconstructions (& surface loop voltage) are available

» External loop voltage was lower with a full (2000 cm2) tray (2003, 2004)

q0>1 in all analyzed lithium discharges– No sawteeth– No significant MHD

Li tray


Chengdu

CDX-U


PPPL

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 Ip>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


Chengdu

CDX-U


PPPL

Follow-on to CDX-U, LTX, will have 5 m2 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


Chengdu

CDX-U


PPPL

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 performancePrelithium 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


Chengdu


Chengdu

CDX-U


PPPL

LTX operation at Ip > 100 kA requires new PF coilsNew coil set designed for Ip ≤ 400 kA CDX-U

New PF coils

Equilibrium modeling shows that CDX plasmas with Ip ≥ 70-80 kA scraped-off on outer limiter– Vertical field too low– One major factor limiting Ip

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


Chengdu

CDX-U


PPPL

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


Chengdu

CDX-U


PPPL

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 (8 - 12 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 121.6 nm is very low All other spectroscopic diagnostics to be transferred from CDX-U


Chengdu

CDX-U


PPPL

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


Chengdu

CDX-U


PPPL

Summary In 2005 CDX-U simultaneously employed 600 cm2 liquid lithium limiter

+ 1000 Å between-shots lithium wall coatings

– Higher recycling than 2003-04 full-tray (2000 cm2) 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

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ISTW2006 11-13 October Chengdu CDX-U The Lithium Tokamak – Results from CDX-U and Progress towards...

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