Langmuir Probe Peer ReviewLangmuir Probe Peer Review

Josh Kallmanfor the LLD Team

February 13, 2009

NSTXNSTX Supported by

College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU Illinois UCU RochesterU WashingtonU Wisconsin

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

NSTXNSTX Langmuir Probe Peer Review (Kallman) 2Feb 13, 2009

Physics requirements for the Langmuir probe array

• Poor coverage of edge diagnostics necessitates a new method for measuring plasma parameters (ne, Te, Γ) in the divertor region, especially with the LLD installation

• Heat flux profile at outer strike point has FWHM of 10 cm

– current IR camera resolution is 16 data points over this region

– higher spatial resolution could allow more accurate particle flux measurements

• ELMs occur on a time scale of several ms

– temporal resolution should be sufficient to operate during transient events (single tip probes would be limited by voltage sweep rate)

– triple probes would provide instantaneous data

129019

0.455 0.460 0.465 0.470 0.475

Time (seconds)

ELMs in lower divertor, 129019

0.2 0.4 0.6 0.8 1.0 1.2 1.4

Radius (m)

2

4

6

Q [

MW

/m^2

]

Heat flux, 129019

2

4

6

t = .33st = .36st = .39s

Arb

itrar

y un

its

Strike pointlocation

0.45

NSTXNSTX Langmuir Probe Peer Review (Kallman) 3Feb 13, 2009

• 33 radially arrayed triple-probes will provide edge temperature and density characterization on a continuous basis

• Probes based on MAST design involving a macor cassette of closely spaced probes embedded in a carbon tile

– tile mount with radial coverage of divertor (Bay B)

– electronics to be provided by UIUC

• Close spacing of probes will provide better resolution in high-gradient (strike point) regions

– each probe covers 3.5 mm radially, including spacing

– probe heads are 2.5mm radial x 7mm toroidal rectangles

Triple Langmuir probe array will address edge diagnostic needs

carbon tile and endcap

NSTXNSTX Langmuir Probe Peer Review (Kallman) 4Feb 13, 2009 4

Cassette design will allow for ease of probe mounting and will include channels for wire transport

• Macor cassette features grooves that will allow probes to slide in easily without necessitating screws to secure probes vertically

– screwless design reduces mechanical stresses on the probes

• Wiring channels allow for the wires from each group of probes to exit independently– wires will exit on sides of edge probes and through base of central probes

macor cassette

wire channels

NSTXNSTX Langmuir Probe Peer Review (Kallman) 5Feb 13, 2009 5

Probe design includes features to protect underlying surfaces and uses novel materials to facilitate assembly

• Probes will be shaped so as to minimize direct exposure of macor to plasma or lithium – probe bend prevents direct line of sight for lithium or plasma down to cassette– probe widening at top allows for smaller gaps and greater shielding of surfaces below

• Macor spacers between probes will provide insulation and structure to probe assembly– widened probe and spacer bases couple to cassette grooves

• Probe fabrication concept has been developed– material is molded graphite cement to allow precise shaping– wires can also be attached with graphite cement, obviating need for mixed materials or additional

screws

graphite

probemacor spacer

wire

exits

NSTXNSTX Langmuir Probe Peer Review (Kallman) 6Feb 13, 2009

Probe material is vacuum compatible after curing

NSTXNSTX Langmuir Probe Peer Review (Kallman) 7Feb 13, 2009

In-vessel wiring

• Maximum current through probes is estimated at ~1 A (transient)– based on Isat of existing probes

and comparison of probe area

• 24-gauge wires will be used, unshielded within probe tile– capacitive coupling test on

representative twisted pair shows minimal cross-talk

• Wire bundles for each radial row of triple probes will be run out through channels in cassette to channels in outboard tiles and eventually under passive plate to feedthrus

NSTXNSTX Langmuir Probe Peer Review (Kallman) 8Feb 13, 2009

New probe electronics will permit continuous triple-probe operation

a

M. Jaworski, UIUC

• Triple probe interpretation will be supplemented by single-tip tile probe sweep characteristics– single tip probes can provide ion

saturation current data

– existing probe hardware can be migrated to new rack to be used with single tip probes

• Initial implementation provides for 10 triple-probe sets– future upgrades will expand

probe electronics

– wiring scheme will allow external (to vessel) selection of which probes to operate

NSTXNSTX Langmuir Probe Peer Review (Kallman) 9Feb 13, 2009

Ex-vessel wiring

• Langmuir probes will be cabled through 4 Bay J feedthrus

• Instrumentation will be located in the LLD diagnostic rack– rack will be placed next to LLD

control rack on east mezzanine

F. Jones

NSTXNSTX Langmuir Probe Peer Review (Kallman) 10Feb 13, 2009

Backup

NSTXNSTX Langmuir Probe Peer Review (Kallman) 11Feb 13, 2009

Probe material RGA before 2nd cure