Controlling density rise through helium conditioning.

NSTX

NSTX

Controlling density rise through helium conditioning.

C H Skinner, H Kugel, R Maingi, D Mueller,...

• Surface physics

• TFTR, DIII-D, NSTX results

• XP: Controlling density rise through limiting divertor

temp. rise and through He conditioning.

PPPL Jan 7th, 2003

NSTX

Surface Physics: Complex non-linearly coupled plasma - wall system.

TFTR

tile

KC-22

200 µm

Not a simple homogeneous system on any scale: - processes on wall include:

Chiu et al., codeposit original tile

convoluted, highly porous structure,codeposit poorly thermally connected

Microscopic-scale perspective

NSTX

≈10 mm

D release

Chemical

Sputtering

Temp: 1841 °C left, 1181 °C right

Same heat flux (from laser spot)

Microhotspots - surface temperature not single value (varies x2)

Temperature effects:

Doyle et al., Roth & Moller

Vietzke &

Haasz.

Skinner et al.,

Radiation

Enhanced

Sublimation

(RES) as

temp. rises

NSTX

0

250

500

750

0 20 40 60 80 100

Edge temperature (eV)

Sheath potential (eV)

Plasma ions approaching the material surfaces areaccelerated by the sheath potential to an energy of

E≈ 2T +3ZT,

where T is the plasma temperature adjacent to the material and Z is the ion charge.

He++ impacts at higher energy than D+

Doyle et al.,

Interaction Depth:

z = 2

z = 1

High performance shots access depths into wall untouched by He glow GDC.

Wampler et al.,

NSTX

TFTR He conditioning:

Strachan et al., Mansfield et al.,

Strachan concluded that on TFTR both L & Supershot plasma transport increased with D influx through deeper heated layer in PFCs at increased beam power and increased plasma wetted area. [NF 39 (1999) 1093].

NSTX

HELIUM DISCHARGES USEFUL FOR REDUCING RECYCLING AND IMPURITIES

• Deuterium reference discharges(12 Helium discharges between)

• D reduced

• Carbon reduced

• li slightly reduced

• Stored energy unchanged102300

102317

Ip [MA]

D [au]

CIII emission

li

WMHD [kJ]

R Maingi, P Efthimion et al., DPP APS 2000 “Wall conditioning and impurity control in NSTX”

NSTX

Density Rise: DIII-D

Rensink et al., modeling showed H-mode density rise due to longer confinement time and thinner scrape off layer more transparent to incoming particles.

Ip [MA]

PNBI/10 [MW]

ne [1019 m-3]

D [au]

WMHD*10 [MJ]

H98pby2

Maingi et al., 5 yr forum

Density Rise: NSTX

The IPPA 5 year goal (for end FY 2005) includes “Determine the ability for managing intense energy and particle fluxes in the edge geometry and for increasing pulse durations…”

NSTX

Outline of XP:

• Prequisite: well boronized, NBI available

• D fiducial e.g. 108741

• Change triangularity to jog strikepoint to reduce divertor

temperature rise

• Compare density rise and divertor temperature.

• Repeat D fiducial

• Run He NBI discharges to deplete D in divertor.

• Repeat D fiducial

• Compare USL discharges if time permits.

NSTX

NSTX EXPERIMENTAL PROPOSAL XP 304

Controlling density rise through helium conditioning

C. H. Skinner, D Mueller, H Kugel, R Maingi…..

1. Overview of planned experiment

One run day:

• Goal 1: Assess role of divertor temperature in density rise and impurity generation by strike point jog.

• Goal 2: Reduce density rise by depleting D in divertor with He conditioning with NBI.

• Goal 3: (time permitting) Compare conditioned LSN fiducial to same discharge but USN run on un He-conditioned upper divertor.

run Wednesday January 29th...

NSTX

≈10 mm

D release

Chemical

Sputtering

Temp: 1841 °C left, 1181 °C right

Same heat flux (from laser spot)

Microhotspots - surface temperature not single value (varies x2)

Temperature effects:

Doyle et al., Roth & Moller

Vietzke &

Haasz.

Skinner et al.,

Radiation

Enhanced

Sublimation

(RES) as

temp. rises

NSTX

Overview of run day:

Problems:

• machine control software (2.25 h)

• CA to fix Mirnov (0.75 hr)

• Software loads wrong TF - CA to check rectifier (2.25 hr)

• TF trip gives 26% Beta toroidal (#109941)

XP time (3.75 hr)

• insufficient time for He conditioning, focus on strike point jog

• develop discharge with 220ms long, reasonably quiescent H-mode by adjusting CS gas feed

• jog strike point with PF2 (PF1 ineffective)

• 2.5 jog/no jog comparisons show roll over in density before loss of H-mode (caveat emptor)

•

NSTX

H-mode from 267 - 418 ms

effect on density rise ?

Jog shot 109947:

109947 had PF2L increase from -7 to -8.5 from 380 to 415 ms, hold until 530 ms then ramp back down to -7

NSTX

Strike point jog

L-mode

109946109947

Indication of density rollover after strike point jog, before L-mode, but...

outer strike point position

D-alpha

Line average density (1e13)

109947 had PF2L increase from -7 to -8.5 from 380 to 415 ms, hold until 530 ms then ramp back down to -7

...H-mode...

MPTS comparison next VG

NSTX

... but motion away from CS causes major part of density reduction.

NSTX

NSTX

Strike point jog

L-mode

109950109952

Indication of density rollover after strike point jog, before L-mode, but...

outer strike point position

D-alpha

Line average density (1e13)

109952 had PF2L increase from -7 to -8.5 from 380 to 420 ms,

MPTS comparison next VG

NSTX

... but motion away from CS causes major part of density reduction.

NSTX

Strike point jog

L-mode

109955no time for no jog shot

Indication of density rollover after strike point jog, before L-mode, but...

outer strike point position

D-alpha

Line average density (1e13)

109955 had PF2L increase from from 320 ms,

MPTS comparison next VG

NSTX

NSTX

NSTXSummary:• Density rollover observed after strike point jog, but causality complicated due to change in

plasma shape.

- jog duration before loss of H-mode too short for IR camera (33 ms frame time)

- H-alpha camera data .... ?

• Some data gained on high Beta and H-mode behavior vs plasma shape

• Need more discharge development time to stay in H-mode longer and control inner gap

( RTEFIT ?) for definitive experiment on effect of jog on density rise.

• Still need to run helium conditioning part of XP to address

access to discharges > 1 s without exceeding density limit.